CHAPTER 47 THE SICKLE CELL DISEASES AND RELATED DISORDERS
CHAPTER 47 THE SICKLE CELL DISEASES AND RELATED DISORDERS
Definition and History
The Sickle Cell Diseases
Etiology and Pathogenesis
Therapy, Course, and Prognosis
Sickle Cell Trait
Definition and History
Etiology and Pathogenesis
Therapy, Course, and Prognosis
Hemoglobin C Disease
Definition and History
Etiology and Pathogenesis
Therapy, Course, and Prognosis
Hemoglobin D Disease
Definition and History
Etiology and Pathogenesis
Hemoglobin E Disease
Definition and History
Etiology and Pathogenesis
Therapy, Course, and Prognosis
Sickle hemoglobin is a mutant hemoglobin in which valine has been substituted for the glutamic acid normally at the sixth amino acid of the b-globin chain. This hemoglobin polymerizes and becomes poorly soluble when the oxygen tension is lowered, and red cells that contain this hemoglobin become distorted and rigid. Sickle cell disease occurs when an individual is homozygous for the sickle cell mutation or is a compound heterozygote for sickle hemoglobin and b-thalassemia, hemoglobin C, or some less common b-globin mutations. Diagnosis depends upon demonstrating the presence of the abnormal hemoglobin(s) in the red cells. The disease is characterized by hemolytic anemia and by three types of crises: painful (vasoocclusive), sequestration, and aplastic. Complications include splenic infarction and autosplenectomy, stroke, bone infarcts and aseptic necrosis of the femoral head, leg ulcers, priapism, pulmonary hypertension, and renal failure. The severity of clinical manifestations varies greatly from patient to patient and the aggressiveness of treatment needs to be modified accordingly. Early diagnosis, immunization against pneumococcal infection, and prompt treatment of infections that do occur has contributed to greatly improved survival of those born with these disorders. Stem cell transplantation, when successful, cures the disease. Treatment with hydroxyurea increases the fetal hemoglobin level and can result in amelioration of crises. Sickle trait, the heterozygous state for sickle hemoglobin, affects some eight percent of African Americans, and with rare exception is entirely benign. Hemoglobin C disease is associated with splenomegaly but minimal hematologic changes, and the rare hemoglobin D disease is essentially asymptomatic. Hemoglobin E is very common in some parts of Asia. This hemoglobin is greatly underproduced, and the homozygous state or compound heterozygous state with b-thalassemia resembles thalassemia.
Acronyms and abbreviations that appear in this chapter include: BPG, bisphosphoglycerate; G-6-PD, glucose-6-phosphate dehydrogenase; MCHC, mean corpuscular hemoglobin concentration; VLA-4, very late activation antigen-4.
DEFINITION AND HISTORY
James Herrick, the astute Chicago physician who is also credited with description of the clinical syndrome of coronary thrombosis, was the first to observe sickled cells in the blood of an anemic African graduate student1 (Fig. 47-1). Emmel2 demonstrated that red cells sickled when blood from such patients was sealed under glass and allowed to stand at room temperature for several days, but the fact that the transformation to sickled cells occurs in response to a fall in oxygen tension was not recognized until the classic studies of Hahn and Gillespie in 1927.3 In 1923 the sickling phenomenon was shown to be inherited as an autosomal dominant trait.4 Much later, Neel5 and Beet6 clarified the genetic basis of sickle cell anemia by demonstrating that heterozygosity for the sickle cell gene resulted in sickle cell trait without significant clinical symptoms, while homozygosity resulted in sickle cell anemia.
FIGURE 47-1 Peculiar elongated and sickle-shaped red corpuscles in a case of severe anemia. (Herrick,1 by permission.)
In 1949 Pauling and his colleagues7 found that all the hemoglobin in patients with sickle cell anemia showed an abnormally slow rate of migration on electrophoresis, while the parents of the these patients had normal as well as abnormal hemoglobin. Soon after, other abnormal hemoglobins were discovered by subjecting hemoglobin to electrophoresis. The biochemical nature of the defect in sickle cell anemia was elucidated by Ingram,8 who digested hemoglobin with trypsin and separated the resulting peptides on paper by electrophoresis in one direction and chromatography in the other. This technique (“fingerprinting”) demonstrated that one of the digestion products of sickle hemoglobin migrated differently from that of normal hemoglobin. Determination of the amino acid composition of this peptide indicated that sickle cell anemia was the result of the replacement of a glutamic acid residue by valine. This discovery established that the substitution of a single amino acid in a polypeptide chain can alter the function of the gene product sufficiently to produce widespread clinical effects. Conley has chronicled the fascinating history of sickle cell disease.9
After the discovery that sickle hemoglobin, or hemoglobin S (Hb S), was electrophoretically altered, additional variants were assigned letters of the alphabet—C, D, E, etc. The letters of the alphabet were rapidly exhausted, however, and subsequent abnormal hemoglobins were named after the geographic location in which they were found (e.g., hemoglobin Memphis, hemoglobin Mexico). If the hemoglobin had the electrophoretic characteristics of one previously described by a letter, the geographic designation was added as a subscript (e.g., hemoglobin MSaskatoon). In this case M indicates an amino acid substitution resulting in a methemoglobin. In a fully characterized hemoglobin the amino acid substitution is designated by a superscript to the globin chain involved, as, for example, hemoglobin S, a2b26 Glu®Val and hemoglobin GNorfolk, a235 Asp®Asnb2. Thus, this notation indicates that hemoglobin S has a substitution of valine for glutamic acid in the sixth position of the b chain and that hemoglobin GNorfolk is a substitution of asparagine for aspartic acid in the thirty-fifth position of the a chain.
The term sickle cell disorder refers to states in which the red cell undergoes sickling when it is deoxygenated. The sickle cell diseases are those disorders in which sickling produces prominent clinical manifestations. Included are sickle cell–hemoglobin C disease (hemoglobin SC disease), sickle cell–hemoglobin D disease (hemoglobin SD disease), sickle cell b-thalassemia, and sickle cell anemia. The latter term is reserved for the homozygous state for the sickle cell gene.
THE SICKLE CELL DISEASES
Sickle cell anemia (SS disease) may be considered the prototype of the sickle cell diseases, and in general the clinical features and treatment of all these disorders are the same and are therefore considered together here. The homozygous state, sickle cell anemia, is the most severe of these disorders, with hemoglobin SC disease and sickle cell b-thalassemia tending to be somewhat milder, and hemoglobin SD disease being the mildest of the group. However, there is a great deal of overlap in the severity of the clinical manifestations of these disorders, and they are therefore described together here. Some patients with sickle cell thalassemia or hemoglobin SC disease may be more anemic and have more severe and frequent crises than some mildly affected patients with sickle cell anemia. A major difference among these diseases is in their laboratory diagnosis.
ETIOLOGY AND PATHOGENESIS
BIOCHEMICAL BASIS OF SICKLING
There are few diseases of man whose etiology can be traced to as basic a level as sickle cell disease. Sickle cell anemia is due to the substitution of thymine for adenine in the glutamic acid DNA codon (GAG®GTG), which results, in turn, in substitution of b6 valine for glutamic acid. As discussed in Chap. 28, hemoglobin exists in two conformations, designated the oxy (relaxed, R) and deoxy (tense, T) states. Deoxygenation of hemoglobin shifts this equilibrium toward the T conformation. Molecules of deoxyhemoglobin S have a strong tendency to aggregate, and such aggregation requires the substitution of valine for glutamic acid in the b6 position, since only those hemoglobin variants with this substitution (e.g., S and Harlem) undergo sickling. Certain other structural features of the molecule are also of importance.10,11
Electron micrographs of deoxygenated sickle hemoglobin show the presence of multiple microtubules consisting of hemoglobin molecules stacked on top of each other (Fig. 47-2). The molecules do not lie directly over one another, so that a helical structure is formed. Fourteen strands of the fiber are organized into pairs,12 giving rise to a fiber that is 21 nm in diameter. Most of the intermolecular contacts that give rise to this structure have been elucidated.12,13
FIGURE 47-2 Electron micrograph of negatively stained fiber of hemoglobin S and the structure deduced by three-dimensional image reconstruction. The reconstructed fiber is presented as ball models, with each ball representing a hemoglobin S tetramer. The models are presented as the outer sheath (left), the inner core (center), and a combination of both inner and outer filaments (right). (Edelstein,452 by permission.)
The deoxygenated hemoglobin solution turns into a firm gel. The distorted sickled red cell is the visible end result of this molecular aggregation. The process is time dependent.14 Initially there is a rate-limiting nucleation process; a few molecules of sickle hemoglobin must aggregate, forming a “seed” on which aggregation of further molecules occurs rapidly. Thus, the sickling process is characterized by a long delay that is strongly dependent on temperature and concentration.15 The delay is inversely proportional to approximately the thirtieth power of the hemoglobin concentration.16 This delay is quite important in protecting the patient from even more dire consequences than might otherwise be anticipated. Even though the oxygen concentration of venous blood is sufficiently low so that at equilibrium about 85 percent of the red cells would contain sickle hemoglobin polymer, kinetic data suggest that about 80 percent of cells are prevented from sickling during their round trip through the circulation because they reach the lungs and become reoxygenated before significant polymerization has occurred.14
When a cell sickles and unsickles repeatedly, the membrane is affected and the cell becomes irreversibly sickled; it remains so even when the oxygen pressure is increased. These are the sickled forms seen on air-dried films. An irreversibly sickled cell has a high hemoglobin concentration and a high calcium and low potassium content, and it may be ATP-depleted.17 These cells appear to be derived directly from reticulocytes18 but have a short intravascular life span, and the severity of the hemolytic process is directly related to the number of these cells in a patient’s circulation.19 However, the relationship between the number of irreversibly sickled cells and the number and severity of painful crises is an inverse one.20,21
MEMBRANE CHANGES IN SICKLE CELLS
Although the primary defect in sickle cell disease is clearly in the hemoglobin, secondary alterations in red cell metabolism and membrane structure and function have also been described. Rapid potassium loss occurs early in the sickling process.22 Abnormalities of sickle cell membrane phosphorylation have been documented.23,24 and 25 The calcium pump is abnormal.26 Although the calcium content of sickle cell membranes, particularly of those cells that are irreversibly sickled, has been found to be increased,17,23,24,25,26 and 27 the location of the excess calcium appears to be in endocytic vacuoles, so that from a functional point of view its location is extracellular.28,29 Increased generation of free radicals may occur in sickle cells,30,31 and 32 and there is abnormal oxidation of thiols in sickle cells.33 Superoxide dismutase activity of sickle cells is slightly reduced,34 and the amount of NAD+ and the NAD++NADH/NADH ratio are increased.35 The binding of glyceraldehyde phosphate dehydrogenase to the membrane is decreased by 35 to 50 percent,36 and there appears to be uncoupling of the lipid bilayer from the submembrane skeleton.37 Macrophages seem to ingest sickle cells more readily than normal cells, and this could be a result of excessive auto-oxidation of membrane components with the acquisition of immunoglobulins on the cell surface38 or to loss of membrane phospholipid asymmetry, which is a constant finding in sickle cells39,40 and may play an important role in their clearance from the circulation as well as in activation of coagulation.
VARIABILITY IN SEVERITY OF SICKLE CELL DISEASE
Because a large number of inherited and acquired factors influence the pathogenesis of clinical symptoms, the sickle cell disorders vary in clinical severity from the virtually symptomless sickle cell trait to the potentially lethal state characteristic of sickle cell anemia. Wide variation in the severity of clinical manifestations also occurs among patients with sickle cell anemia. Some die within the first few years of life, while others have been discovered late in life as a result of a chance survey.
Both intracellular and extracellular factors influence sickling. Included are the types of hemoglobin in the cell and their concentration, the level of 2,3-bisphosphoglycerate (2,3-diphosphoglycerate; 2,3-BPG; 2,3-DPG), and the hydrogen ion concentration. Some of these factors are determined predominantly by genetic factors; others are environmentally modified. The variability of these factors as well as many others that are not understood probably accounts for the natural pattern of this group of diseases—periods of comparative well-being interspersed with periods of clinical deterioration (crises). Longitudinal studies of patients have suggested that an increase in the number of dense and poorly deformable cells precedes the development of a crisis.41 However, calculation of the mean polymer fraction from the 2,3-BPG concentration, the MCHC, the internal pH, and the percent nonsickle hemoglobin did not make it possible to predict clinical course.42 The precipitating circumstances responsible for the development of crises are often not clear. Of those events that appear to be associated with the appearance of crises, infections are probably among the most common.
However, it is not only the extent of sickling that is important but also the interaction of the sickled cells with the endothelium and other blood cells (see “Blood Flow in the Microvasculature,” below).
Concentration of Hemoglobin S in the Red Cell A correlation exists between the concentration of sickle hemoglobin within a red cell and the susceptibility of the cell to sickling. The red cells of the sickle cell carrier, who is virtually symptom-free, always contain less than 50 percent Hb S; the remainder is largely normal adult hemoglobin. The exact proportions vary from one individual to another. It was proposed many years ago that the distribution of the concentration of sickle hemoglobin in the red cells of subjects with the sickle cell trait was bimodal.43 Subsequent studies confirmed the existence of more than a single mode and indicated that the distribution might actually be trimodal.44 The reason for such a discontinuous distribution has become apparent with the recognition of the very high frequency of a-thalassemia in persons of African ancestry. Individuals carrying a-thalassemic genes have a higher ratio of hemoglobin A to hemoglobin S than those who have four normal copies of the a locus.45 Apparently the affinity of a chains for bA chains is higher than its affinity for bS chains,46 possibly because of differences in the charge of the two chains.47 Thus, when the number of a chains becomes limiting in the formation of hemoglobin tetramers, a higher proportion of a2b2A tetramers than of a2b2S tetramers are formed. Interaction of the a-thalassemic gene and the sickle gene also may influence the course of sickle cell disease: the lower corpuscular hemoglobin concentration in the red cells in a-thalassemia would be expected to protect against sickling. It has been suggested that such an interaction may influence the severity of sickle cell disease in African Americans45,48,49 and that it may play an important role in producing the very mild clinical manifestation of sickle cell anemia in Saudi Arabia.50
The Presence of Other Hemoglobins in the Cell Other hemoglobins present in a red cell containing sickle hemoglobin are not inert bystanders in the sickling process.51 Some hemoglobins, such as F, Korle-Bu, and A2, interact less effectively with hemoglobin S than does hemoglobin A in the sickling process. Two common abnormal hemoglobins, Hb C and Hb D, and the relatively rare hemoglobin OArab become involved in the formation of the sickling tubule. The interaction of these hemoglobins with sickle hemoglobin increases the propensity of red cells to sickle. Moreover, the red cells of patients with SC disease characteristically have an increased MCHC, presumably due to a transport defect, and this too greatly increases sickling.52,53
Other hemoglobins do not appear to play an active role in the sickling process, and their presence in the red cell can greatly reduce the clinical severity of sickle cell anemia. Fetal hemoglobin, for example, protects the red cell from sickling.54 It is distributed heterogeneously in the red cells of an SS homozygote,55,56 and those cells with the largest amount are least susceptible to sickling.55,56 The relatively mild clinical manifestations of patients in the Middle East with sickle cell anemia has been ascribed at least in part to the high level of fetal hemoglobin present in their red cells.57,58 and 59 In the United States, however, no significant correlation exists between fetal hemoglobin levels and the severity of the clinical manifestations of sickle cell anemia,60 and even in the Arab population the relationship is not always clear,61 although it may be that the effect is obscured by a threshold phenomenon,62 i.e., that a favorable effect of fetal hemoglobin concentration is observed only above a certain level. In adults who are heterozygotes for hemoglobin S and hereditary persistence of hemoglobin F, hemoglobin S constitutes more than 70 percent of the hemoglobin, but the high concentration of hemoglobin F inhibits sickling because the distribution is such that each cell contains a considerable amount of hemoglobin F, and the patients experience a benign clinical course.63 The presence of the abnormal hemoglobin Memphis (a23Glu®Glnb2) also decreases the clinical severity of sickle cell disease,64 presumably by inhibiting the formation of the sickle tubule.
Interaction of Sickling and Thalassemia The interaction of b-thalassemia with sickling is discussed in Chap. 46, and that with a-thalassemia is considered above.
Glucose-6-Phosphate Dehydrogenase Deficiency It has been suggested that G-6-PD deficiency may have a beneficial effect on the clinical course of sickle cell anemia,65,66,67 and 68 but this correlation has not been confirmed in other studies.69,70,71,72,73,74,75 and 76 It has also been proposed that hemolytic crises are more common in patients with sickle cell disease who are also G-6-PD deficient.77 However, it seems unlikely that the G-6-PD-deficient cells of such a patient would be particularly sensitive in hemolytic stress; G-6-PD A– is very age labile (Chap. 45), and because the erythrocytes are young they have relatively normal G-6-PD activity. In Jamaica,74 the United States,75 and Brazil76 G-6-PD deficiency did not influence parameters of disease severity such as hemoglobin concentration, reticulocyte count, hemoglobin F concentration, irreversibly sickled cell counts, or plasma hemoglobin concentration, and there was no relationship between clinical severity and presence or absence of G-6-PD deficiency.
Pyruvate kinase deficiency is characterized by an increase of red cell 2,3-BPG levels (see Chap. 45). A patient with sickle trait who had inherited pyruvate kinase deficiency manifested sickling similar in severity to that in some patients with sickle disease.78
Deoxygenation Deoxygenation for a sufficient period of time is the most important factor determining the occurrence of sickling in a red cell containing hemoglobin S. The degree of deoxygenation required to produce sickling varies with the percentage of hemoglobin S in the cells. Red cells from patients with sickle cell anemia will begin to sickle at an oxygen tension of about 40 torr.79 Changes that impair adequate oxygenation of the blood may be deleterious to any person whose red cells contain sickle hemoglobin.
An arterial oxygen tension of about 66 torr is found at about 10,000 ft (3000 m). Hypoxemia may also result from flying in unpressurized aircraft; most commercial aircraft, however, maintain an atmospheric pressure in the cabin equivalent to that encountered at an altitude of 5000 to 7000 ft (1500 to 2100 m). Occasional patients with sickle cell anemia or hemoglobin SC disease have been reported to experience painful crises or splenic infarctions under such circumstances.80 However, there is no evidence that a person with sickle cell trait is at risk in a pressurized airplane.81 The oxygen content of the air may also be reduced during anesthesia or when an artificial breathing apparatus is used improperly, as in scuba diving. If pulmonary or cardiac function deteriorates (e.g., in pneumonia or in cardiac failure), any resulting reduction in arterial oxygen tension may prove hazardous to a patient with sickle cell disease.
Vascular Stasis The PO2 level producing in vitro sickling of cells containing Hb S bears only an indirect relationship to clinical measurements of arterial and venous PO2. This is because the PO2 in the larger peripheral vessels does not accurately reflect the oxygen tension in areas of vascular stasis, such as the sinusoids of the spleen, in which hypoxemia is common and sickling is likely to occur. Although a period of 2 to 4 min is required for the development of marked red cell distortion14,82 and rigidity, the red cells normally remain within the venous circulation for only about 10 to 15 s. For this reason, red cells in areas of vascular stasis are more vulnerable to sickling. Once sickling has occurred, increased blood viscosity83 results in further vascular stasis, further sickling, possible vascular occlusion, and infarction. This course of events leads to tissue death, manifested clinically as a painful crisis.
While no organ of the body is immune to vasoocclusion due to in vivo sickling, certain sites notorious for circulatory stasis are characteristically affected. Splenic and marrow infarctions due to vascular stasis are particularly frequent, and priapism may occur in the male. The role of vascular stasis in the development of leg ulcers and of retinal and renal lesions is discussed below under “Clinical Features.” Studies from Jamaica indicated that the incidence of peptic ulcer was greatly increased in patients with sickle cell disease,84 ulceration being identified in 30.5 percent of male patients over the age of 25, but this could not be confirmed in a West African population.85
Temperature Even though cold temperatures retard hemoglobin polymerization, low temperatures tend to precipitate sickle crises, presumably because of the accompanying vasoconstriction.
Acidosis Hydrogen ions produce a right shift in the oxygen dissociation curve (the Bohr effect), presumably by displacing the equilibrium between the high-affinity oxy conformation and the low-affinity deoxy conformation toward the deoxy conformation of hemoglobin. Since it is sickle hemoglobin in the deoxy conformation that aggregates, the lowered pH profoundly affects the sickling of red cells, even when the percent oxygenation is maintained at a constant level.86,87 Alkalosis, on the other hand, by shifting the equilibrium toward the oxy conformation, tends to retard sickling but impairs oxygen release to tissue.
Corpuscular Hemoglobin Concentration The tendency of hemoglobin S solutions to aggregate is proportional to the thirtieth power of the concentration.16,88 Accordingly, sickling of red cells is markedly influenced by the concentration of sickle hemoglobin in the cells. Suspending sickle cells in a hyperosmolar medium increases the intracellular hemoglobin concentration as the cell is dehydrated. This phenomenon may account in part for sickling in renal papillae.89,90 Conversely, any agent that causes increased red cell volume will retard the sickling process by decreasing the MCHC. Marked dehydration results in both vascular stasis and hypertonicity and can precipitate a crisis.
Blood Flow in the Microvasculature In the last analysis, vasoocclusion is the result of a variety of factors on blood flow in the microvasculature. The factors that influence the rheologic properties of blood that contains sickle cells are extremely complex. For example, shear stresses, such as those that occur in the circulation, serve to break down gel structure.91 However, this results in the creation of more nucleation centers and results in a decrease in the delay time. In the circulation, flow properties of blood are influenced not only by factors such as the rigidity of the erythrocytes but also by the adherence of sickle cells to the endothelium,92 which may involve band 3,93 and to each other.94 Variations in such factors, modifying the rheologic consequences of the sickling process, undoubtedly play a role in determining when vasoocclusive episodes will occur. Granulocytes, too, manifest increased adherence to endothelium, and this has been attributed to increased expression of CD64.95
The sequence of events that leads to occlusion of blood vessels by sickle cells is thus complex.96,97 One essential factor is the aggregation of sickle hemoglobin, with the consequent changes in the rheologic properties of the erythrocytes (see “Biochemical Basis of Sickling,” above). The overall viscosity of the blood is a function of the hematocrit, and occlusion is more likely when hematocrit levels are relatively high. Adhesion of sickle cells to the vascular endothelium is an important factor and may be related to exposure of vascular endothelial adhesion molecules such as VCAM-1 (vascular cell adhesion molecule-1) and to the levels of plasma factors that enhance adhesion, including fibrinogen, factor VIII, fibronectin, von Willebrand factor, and thrombospondin. The adhesion receptors VLA-4 and CD36 are found in unusually high numbers on sickle cell reticulocytes, and they help to mediate adhesion of sickle RBC to endothelium.98 Abnormalities in nitric oxide–induced vascular relaxation has also been indirectly implicated. Leukocytes probably also participate in this complex process, perhaps by releasing cytokines that upregulate adhesive endothelial glycoproteins, and it has been suggested that a part of the therapeutic effect of hydroxyurea may be related to reduction of the leukocyte count.99
Infections It is a common clinical observation that vasoocclusive crises may be precipitated by infections. In many cases the mechanism by which infection increases sickling is easily discernible: fever, vomiting, and diarrhea may produce dehydration; lack of food intake may produce acidosis; and hypoxemia may result from pneumonia. It is quite possible that other, more subtle mechanisms may also be responsible for precipitation of crises in patients with sickle diseases with infections.
A patient with sickle cell anemia is homozygous for the gene for sickle hemoglobin and has therefore inherited one abnormal gene from each parent. If 7.8 percent of a population are sickle cell trait carriers,73 as in the African American population, there is a 1:164 chance that two carriers will marry, and the chances that an offspring of such a marriage will have sickle cell anemia is 1:4. In such a population, about 1 in 650 will have sickle cell anemia.
Similarly, persons with hemoglobin SC disease must have one parent with a sickle hemoglobin gene and another with a hemoglobin C gene. Since these genes are allelic b chain mutations, persons with hemoglobin SC disease have no normal b polypeptide chain gene and therefore have no hemoglobin A. The carrier rate for hemoglobin C in African Americans is about 2.3 percent.73 If 7.8 percent of a population carries the hemoglobin S gene, then the probability of a sickle cell trait and hemoglobin C trait mating is about 1 in 280, and therefore 1 in about 1120 newborns will inherit hemoglobin SC disease. The same principles apply for inheritance of sickle cell b-thalassemia, since the b-thalassemia gene is also allelic to the gene for sickle hemoglobin. In African Americans the frequency of b-thalassemia is approximately 0.8 percent,100 so that the expected birth frequency of sickle cell b-thalassemia is about 1 per 3200.
Hemoglobin S occurs with greatest prevalence in tropical Africa; the heterozygote frequency is usually about 20 percent, but in some areas it reaches 40 percent. The sickle cell trait has a frequency of about 8 percent in the African American populations. The sickle cell gene is found to a lesser extent in the Middle East, in Greece, and in aboriginal tribes in India (Fig. 47-3). On occasion sickle cell disease is found in people of European extraction, especially where racial admixture has occurred over the centuries.101
FIGURE 47-3 Distribution of sickle cell gene in Africa and Asia. (Allison,453 by permission.)
The high prevalence of the gene for sickle hemoglobin in areas of the world where malaria has been common suggests that persons with sickle cell trait have a selective advantage over normal individuals when they contract this disease.102 This advantage seems to be restricted to young children with sickle trait and Plasmodium falciparum infection. Although children with sickle cell trait are readily infected by P. falciparum, the parasite counts remain low. It may be that the infected red cell is preferentially sickled and destroyed, probably in the vascular system of the liver or spleen, where oxygen tensions are low and phagocytic cells abound. Whatever the mechanism, the result is that the infection is of short duration and the incidence of cerebral malaria and death is low.
At one time one could only speculate as to whether the sickle cell mutation had arisen only once and had gradually gained a worldwide distribution or whether the same mutation had arisen independently in various populations and then been the subject of selection, presumably through a protective effect against malaria. The ability to detect mutations in nontranscribed portions of DNA adjacent to the b-globin gene (see Chap. 9) has now provided insight into this problem. Such mutations are so close to the b-globin gene that the probability of a crossover (see Chap. 9) is vanishingly small. Thus, the relationship of the two mutations to one another will persist through hundreds of generations, permitting one to trace population movements. When the b-globin gene cluster is digested with restriction endonucleases, five distinct patterns are found in association with the sickle mutation. Four of these occur in Africa and have been designated the Senegal, Benin, Bantu, and Cameroon types.103 An additional haplotype is typical of the Indian subcontinent.104 These findings suggest that the sickle mutation arose independently at least five times.
Hemoglobin DPunjab, now recognized to be identical with hemoglobin DLos Angeles, both having the structure a2b2121 Glu®Gln, also interacts with hemoglobin S in forming aggregates in the deoxy conformation. Hemoglobin SDPunjab/Los Angeles disease is a relatively severe sickle cell disease.105 This hemoglobin is found in frequencies of approximately 3 percent in Northwest India; however, it is relatively rare in populations of African origin, and hemoglobin SD disease is therefore very uncommon.
Although we regard sickle cell anemia as the prototype of the sickle cell diseases, in the African American population only about one-half of the patients with sickle cell diseases have sickle cell anemia (homozygous SS disease). This fact is important from the point of view of genetic counseling: about half of all children with sickle cell disease arise from matings in which only one of the parents carries the sickle cell gene. Moreover, since early mortality rates are probably higher in sickle cell anemia than in the other sickle cell diseases, an even smaller proportion of adults with these sickle cell diseases are actually homozygous for hemoglobin S.
No naturally occurring animal models of sickle cell disease have been described. Some deer have red cells that undergo sickling when oxygenated,106 but not under physiologic conditions of pH and PO2. Cells from patients with sickle cell anemia have been infused into rats,107 and this model system has been used to study the effect of various therapeutic agents. However, this approach is limited by the short time that the cells survive in the circulation of the heterologous species. The development of transgenic technologies (see Chap. 9) has made it possible to produce mice whose red cells carry a high percentage of sickle hemoglobin.108 Notably, by combining the sickle b-globin transgene and human a-globin genes with knockouts of murine globin genes109 or thalassemic mutations,110 mice that have many of the features of human sickle disease have been produced.
The newborn infant is protected by the high level of fetal hemoglobin in the red cells during the first 8 to 10 weeks of life. As the level declines the clinical manifestations of sickle cell disease appear, and the hematologic manifestations of sickle disease are apparent by 10 to 12 weeks of age.111
Many patients with sickle cell anemia are in reasonably good health much of the time, achieving a steady-state level of fitness. This state of relative well-being is periodically interrupted by a crisis that may have a sudden onset and occasionally a fatal outcome. The early recognition and subsequent clinical assessment of sickle crises are greatly facilitated by familiarity with the patient’s steady state.
Various types of crises occur, and these may be classified as follows: vasoocclusive (painful) crisis, aplastic crisis, sequestration crisis, and hemolytic crisis.
Vasoocclusive Crisis The vasoocclusive crisis is the most common and is the hallmark of the patient with sickle cell disease.112 The frequency with which such crises occur varies from almost daily to less than once yearly. The vasoocclusive crises result from complex interactions between endothelium, plasma factors, leukocytes, and rigid, sickled red cells leading to the obstruction of blood vessels (see “Blood Flow in the Microvasculature,” above). Tissue hypoxia occurs and ultimately leads to tissue death and localized pain. It is important to distinguish the pain of a vasoocclusive crisis from the pain caused by other, sometimes more treatable disorders. Appendicitis must sometimes be considered, but it is notable that it has been suggested that the incidence of appendicitis is lower in patients with sickle cell diseases than in the general population.113 Fever is often present, even in the absence of demonstrable infection. Sickle cell crisis is, to a large extent, a diagnosis by exclusion.114 Vasoocclusive crises may affect any tissue, but the pain occurs especially in bones, chest, and abdomen. Infarctions in the spleen, which may be a cause of abdominal pain, are so common in sickle cell anemia that after age 6 to 8 the spleen usually becomes very small because of scarring111 (autosplenectomy). Myonecrosis is unusual but has been documented.115
Infarction of cerebral vessels, leading to stroke, is the most serious type of vasoocclusive complication (see “Other Clinical Manifestations, Central Nervous System,” below).116
Aplastic Crisis Aplastic crises in sickle cell disease are of the type familiar in patients with other hemolytic disorders, in which the reticulocyte count falls to low levels, indicating that red cell production has decreased dramatically. Depression of erythropoiesis is generally associated with infections. Infections with the B19 strain of Parvovirus appear to be by far the most important cause of such crises117,118 and 119 and may be accompanied by extensive marrow necrosis.120,121 Because of the short red cell life span in sickle cell disease, even in the steady state, a temporary depression of marrow activity can cause a catastrophic fall in hemoglobin level, manifesting as an aplastic crisis. Marrow output failure may also result from a deficiency of folic acid, especially during late pregnancy, and this has sometimes been designated a megaloblastic crisis.
Sequestration Crisis The sequestration crisis occurs particularly in infants and young children,122 although it may occur in adults with splenomegaly, particularly those with hemoglobin SC disease or sickle b-thalassemia.123,124 It is characterized by sudden massive pooling of red cells, especially in the spleen. Hypovolemic shock and cardiovascular failure may develop rapidly.122 A major acute sequestration crisis is considered to be one in which the hemoglobin level is less than 6 g/dl and has fallen more than 3 g/dl when compared with the baseline value; a minor acute sequestration crisis is one in which the hemoglobin level is higher than 6 g/dl.125 In a study of children with sickle disease born in Los Angeles in the 1960s and 1970s, such crises were responsible for 10 to 15 percent of deaths in the first 10 years of life.111
Hemolytic Crisis The red cell life span is shortened in all the varieties of sickle cell disease. It may suddenly be further reduced, probably for a variety of reasons. This increased rate of hemolysis is designated a hemolytic crisis. The resulting increase in jaundice is associated with a falling hemoglobin and an elevated reticulocyte count. Such crises are very rare; in most instances changes regarded as due to increased hemolysis represent some other complication of sickle cell disease.126 It has been suggested that concurrent G-6-PD deficiency may be a factor leading to hemolytic crises,77 but it seems unlikely that this is actually the case, since the young red cell population of patients with sickle cell disease has normal or near-normal G-6-PD activity even when G-6-PD deficiency is present.
An increase in the level of jaundice is not necessarily an indication of increased hemolysis (see “Other Clinical Manifestations, Liver,” below). Other causes for jaundice, such as hepatitis, cirrhosis, and gallstones, should be sought. Patients with a chronic hemolytic anemia are especially likely to form bilirubin stones, which may cause extrahepatic biliary obstruction.
OTHER CLINICAL MANIFESTATIONS
Growth Young children with sickle cell anemia tend to be shorter than normal.127,128 Puberty is delayed, but considerable growth occurs in late adolescence, so that adults with sickle cell anemia are at least as tall as normal.128
Bony Abnormalities The chronic hemolytic anemia with erythroblastic hyperplasia will result in widening of the medullary spaces, thinning of the cortices, and sparseness of the trabecular pattern.129 Although these changes are recognizable in the skull, they are usually not as marked as the typical “hair-on-end” appearance characteristic of the patient with b-thalassemia major (see Chap. 48). The vertebral bodies may show biconcavities of the upper and lower surfaces (codfish spine). Pressure from the nucleus pulposus into an area of bone infarction may result in steplike depressions—as if a coin had been pushed into the vertebral body. This x-ray picture is highly suggestive of sickle cell disease.
Crisis with bone pain may be followed by the appearance of periosteal reaction, and irregular areas of osteosclerosis may be seen, representing areas of bone infarction. Bone scans with 99mTc are not helpful in delineating areas involved in painful crisis.130 However, magnetic resonance imaging seems more promising.131,132 and 133
Sickle cell dactylitis is probably due to limited avascular necrosis of marrow. Nearly one-half of children with sickle cell anemia suffer from this painful disorder, manifesting swelling of the dorsal surfaces of the hands and/or feet (Fig. 47-4). Dactylitis occurs almost entirely in the first 4 years of life, with a peak incidence at about 1 year.134 Environmental cold is considered to be an important precipitating factor.
FIGURE 47-4 Sickle cell dactylitis (hand-foot syndrome). Note the swelling of the right hand involving the thumb and first and second fingers. (Diggs,454 by permission.)
In later life necrosis of the head of the femur due to infarction of the nutrient artery is common and may be responsible for severe pain and serious disturbances of gait. Osteonecrosis of the head of the humerus occurs in about 5 percent of patients with sickle disease. Although the incidence in various genotypes is the same, onset tends to be earliest in those with the SS genotype, latest in those with sickle cell b-thalassemia, and intermediate in those with SC disease.135,136 Chondrolytic arthritis has also been observed.137 The bone manifestations of sickle cell disease may closely mimic osteomyelitis or arthritis. Ultrasonography may be helpful in making the distinction between infarction and infection.138
The presence of necrotic marrow may favor the development of infection, especially with Staphylococcus aureus139 and Salmonella140 (Fig. 47-5). Necrotic marrow may also embolize the lung, producing the “chest syndrome” or in some cases sudden death.141
FIGURE 47-5 Salmonella typhimurium osteomyelitis in a patient with hemoglobin SC disease. (River et al,455 by permission.)
Genitourinary System The renal medulla is an area that is particularly susceptible to damage in sickle cell disease.142 Its unique environment, characterized by anoxia, hyperosmolarity, and low pH, predisposes to sickling. Indeed, the kidney is highly susceptible to the effects of the sickling phenomena and is the only organ commonly affected in the generally benign sickle cell trait. The ability to concentrate urine is lost in patients with sickle cell trait as well as those with sickle cell disease.143 Infarctions may also occur, with renal papillary necrosis (Fig. 47-6) both in patients with SS disease and patients with sickle cell trait.144 Approximately 50 percent of patients with sickle cell anemia have enlarged kidneys as judged by radiologic examination, and calyceal abnormalities of various types are common.145 Renal failure is a late complication of sickle cell disease.146 In one study an increased incidence of renal carcinoma was observed in patients with sickle cell disease.147
FIGURE 47-6 Renal papillary necrosis in a patient with sickle cell trait. Note the small medullary cavities in the upper three calyces of the left kidney (arrows). (Harrow et al,456 by permission.)
Priapism is a serious complication of sickle cell disease.148,149 and 150 It is more common in patients with the SS genotype than in other sickle disease genotypes. It often results in permanent impotence in adults. Prepubertal males have shorter episodes and a good prognosis for future erectile function.
Underdeveloped genitalia and hypogonadism may occur, and it has been suggested but not proven that this could be due to zinc deficiency.151,152
Spleen Splenomegaly is prominent in early childhood, but splenic function is impaired,153 and presumably as a result the incidence of bacteremic infections is high.154 Infections of the splenic remnant itself, sometimes with abscess formation, have been documented.155,156 In adults in the United States splenomegaly is uncommon because of splenic fibrosis. Repeated infarctions of the spleen lead to fibrosis, calcifications, and autosplenectomy. However, in U.S. patients with sickle cell diseases other than SS disease, i.e., sickle cell thalassemia or hemoglobin SC disease, splenomegaly commonly persists into adult life. In Africa, probably as a result of infection with organisms such as Plasmodium, splenomegaly is observed in almost one-quarter of patients with SS disease.157
Liver Jaundice and hepatomegaly are common in sickle cell anemia.158 The liver may be enlarged, sometimes extending to the iliac crest, particularly in young children and again in middle age, at which time there may be evidence of hepatic dysfunction. The small number of sickled cells found in the hepatic vein after passage through the liver suggests that the cells most susceptible to sickling are trapped by their rigidity and engulfed by phagocytes during their passage through the hepatic sinusoids, where the oxygen content of the blood is extremely low. The liver may transiently increase in size during a painful crisis.159 Sickle cell intrahepatic cholestasis is a rare, catastrophic complication. Characterized by sudden onset of right upper quadrant pain, progressive hepatomegaly, and a serum bilirubin level that may rise to well over 1700 µM (100 mg/dl), its outcome is usually fatal, although recovery has been reported after exchange transfusion.160 In sickle cell disease, excretion of urobilinogen is usually greater than normal. Some 50 to 70 percent of adult patients may have bilirubin gallstones,161 and gallstones have also been found in children as young as 6 years of age.162 Patients who have received transfusions may develop hepatitis that is sometimes mistaken for a hemolytic crisis. While about one-third of patients with sickle cell disease manifest liver dysfunction,163 the cause is multifactorial.161,163,164 and 165 Excess iron deposition is common, but frank hemochromatosis is only occasionally encountered. 164,165,166,167 and 168 Some patients with chronic jaundice that seems out of keeping with the degree of hemolysis may have inherited a common mutation in the promoter of the UDP glucuronosyl transferase gene that is known to cause Gilbert disease and increases the jaundice found in patients with thalassemia and G-6-PD deficiency.169,170
Cardiopulmonary System The heart is frequently the site of some of the most prominent physical findings in sickle cell disease.171 During crisis, striking tachycardia may occur because of the combination of fever and anemia. The precordium demonstrates the overactivity similar to that seen with marked hyperthyroidism. The point of maximal impulse is usually forceful and pounding in nature, and the heart is frequently enlarged to both the left and the right. Systolic and diastolic flow murmurs are often heard.
The blood pressure of patients with sickle cell anemia and to a lesser degree with SC disease is significantly lower than published norms for age, race, and sex, a difference that increases with age.172 Stroke was associated with higher systolic but not diastolic pressures.
Pulmonary infarctions are common in persons with sickle cell disease and may lead to repeated episodes of chest pain, unexplained dyspnea, or “atypical pneumonia.” A combination of fever, chest pain, rise in the white count, and appearance of a pulmonary infiltrate in patients with sickle diseases is referred to as the acute chest syndrome.173 Age has been found to exert a marked effect on the clinical picture of acute chest syndrome. In children, acute chest syndrome is milder and more likely due to infection, whereas in adults it is more likely to be severe and to be associated with pain and a higher mortality rate. The clinical and roentgenologic features observed in these patients do not aid in differentiating pulmonary infarction from pulmonary infection, but thin section CT may be more helpful.174 Rib infarctions are commonly observed on bone scan, and it has been suggested that they may play a role in the pathogenesis of the acute chest syndrome.175 This disorder is regarded as being multifactorial, with infection, infarction, and pulmonary fat embolism all being factors that may play a role.176
The combination of increased flow rate and pulmonary vascular occlusions may result in increased pulmonary pressure and eventually cor pulmonale.177 Systemic marrow fat embolism has been associated with pulmonary hypertension.178 However, it has been suggested that patients with recurrent episodes of the acute chest syndrome are not particularly prone to develop pulmonary hypertension.179
Eye Retinal vessel obstruction is followed by neovascularization with arteriovenous aneurysms. These may eventually result in hemorrhage, scarring, retinal detachment, and blindness.180 These changes occur at the periphery and may initially be difficult to visualize through an ophthalmoscope, even with a fully dilated pupil. At the early stage of retinal disease, vision is therefore not impaired. The retinal changes, collectively termed “sickle retinopathy” have been divided into nonproliferative and proliferative groups. Nonproliferative changes include so-called “salmon patch” hemorrhages, iridescent spots, and black sunbursts. The latter term is used to describe lesions that occur in the peripheral retina; as the retina becomes ischemic, neovascular growth starts at abnormal arterial venous anastomoses resulting from vascular occlusions. These vascular growths extend toward the periphery. Because these abnormal vascular fronds resemble the marine invertebrate Gorgonia flabellum, the lesions are called “sea fans.”
Examination of the conjunctiva may reveal multiple short comma-shaped capillary segments that often appear isolated from the vascular network because the afferent and efferent lumens are empty. These transient sites of tightly clumped intravascular erythrocytes are found on the bulbar conjunctiva underneath the eyelids (Fig. 47-7). They occasionally disappear during the course of a lengthy examination because of the warmth of the light. Visual loss is most common in SC disease and is due principally to vitreous hemorrhage, secondary to bleeding from the neovascularized areas.
FIGURE 47-7 Lower bulbar conjunctiva in a patient with sickle cell anemia, showing many segmentations. (Paton,457 by permission.)
The orbital compression syndrome, consisting of fever, headache, orbital swelling, and optic nerve dysfunction, has been documented in a number of patients with sickle cell disease.181 The most common cause appears to be orbital marrow infarctions.
Central Nervous System Cerebrovascular accidents are one of the most devastating complications of sickle cell disease. Once thought to be due to obstruction of small blood vessels, it now appears to be due to lesions of major vessels, particularly the internal carotid and anterior and middle cerebral arteries.116,182 Even children with no history of stroke may show evidence of infarction on MRI.183 The prevalence of cerebrovascular accidents has been found to be 4.01 percent and the incidence 0.61 per 100 patient-years in sickle cell anemia (SS) patients, but cardiovascular accidents occur at somewhat lower frequencies in all common genotypes.184,185 and 186 Stroke has even been reported in more than a dozen children and adults with sickle trait, but the cause-and-effect relationship must be considered unproven.187,188 The incidence of infarctive cerebrovascular accidents is lowest in sickle cell anemia patients 20 to 29 years of age and higher in children and older patients. On the other hand, the incidence of hemorrhagic stroke in SS patients is highest among patients aged 20 to 29 years. The mortality rate was 26 percent in the 2 weeks after hemorrhagic stroke. No deaths occurred after infarctive stroke.184 The incidence of stroke among patients with hemoglobin SC disease is significantly lower, approximately 2 percent.184,185 and 186 Measurement of the velocity of cerebral blood flow by transcranial Doppler ultrasonography has some predictive value with respect to the probability of developing a stroke. 189 In most patients the stroke occurs without any warning, but in about one-quarter of the cases the stroke occurs in the context of some other complication, such as a painful crisis, priapism,186 or an aplastic crisis.190 Risk factors include low steady-state hemoglobin, previous transient ischemic attacks, occurrence of priapism,184,191 and increased plasma homocysteine levels.192 Preliminary studies suggest that the inheritance of the prothrombin Leiden mutation and the 677C®T mutation in the methylenetetrahydrofolate reductase (MTHFR) are not major factors in the development of strokes.193,194 Recurrence of strokes is a prominent feature of this complication; at least 67 percent of patients who have one stroke will suffer at least one more if untreated. Such episodes are particularly common within the first 36 months after a stroke.185
Many other neurologic symptoms have been described, including drowsiness, coma, convulsions, headache, temporary or permanent blindness, cranial nerve palsies, and paresthesias of the extremities.195 Multiple cerebral aneurisms appear to be more common in patients with sickle cell disease.196,197
Leg Ulcers Although encountered in patients with other types of hemolytic disease, ulcers around the ankles are a particularly common feature of sickle cell disease.198,199 They are unusual in the younger child, and stasis clearly plays some part in their formation. They usually start as a small break in the skin or a blisterlike area that breaks down and rapidly extends to form a painful, indolent ulcer. Usually the ulcers become infected, and the base is covered with a yellow, purulent layer. They may extend deeply enough to expose muscle. Once formed, leg ulcers do not heal spontaneously, and they become a major source of morbidity for affected patients.
Infections Patients with sickle cell disease are particularly prone to develop infections, and this may be the single most common reason for hospitalization.200 Because of functional asplenia, impaired phagocytic function,201 and a defect in activation of the alternate complement pathway, infections may be quite hazardous, particularly so in children. The risk varies significantly from patient to patient, with some patients having very few infections. Pneumonia seems to be the most common infection encountered and often is of pneumococcal origin, particularly in children. As noted above, osteomyelitis due to Staphylococcus and to Salmonella also is relatively common.139 Babesiosis has been reported to occur in one patient,202 possibly as a result of the impaired splenic function.
Pregnancy Pregnancy in women with sickle cell anemia is accompanied by an increased incidence of pyelonephritis, pulmonary infarction, pneumonia, acute chest syndrome, antepartum hemorrhage, prematurity, and fetal death.203 Megaloblastic anemia responsive to folic acid, especially in late pregnancy, also occurs with increased frequency. The birth weight of infants of mothers with sickle cell anemia is below average,204,205 and the fetal wastage is high.206,207 The cause of neonatal death is obscure, but it may sometimes result from vasoocclusion of the placenta204; the postmortem findings are those of intrapartum anoxia.208 The maternal mortality in sickle cell disease was formerly prohibitively high, with rates averaging 33 percent, but is now much lower, averaging about 1.5 percent in various series.206,207,209,210,211,212 and 213 Higher mortality rates are still observed in some parts of the world, however, with maternal mortality rates of up to 9.2 percent and a perinatal mortality of up to 19.5 percent.210,214,215
The steady-state hemoglobin level of patients with sickle cell anemia is usually between 5 and 11 g/dl. The anemia is normochromic and normocytic in spite of the elevated reticulocyte count.216 In comparison with patients with similarly increased reticulocyte counts, patients with SS disease may be considered to have a “microcytic” anemia, presumably because the sickle mutation impairs the efficiency of production of hemoglobin. The range of red cell densities is increased in sickle cell anemia,217 but the average cellular MCHC is normal. In SC disease, however, the average MCHC is increased.217 Erythropoietin levels may be reduced relative to the degree of anemia218 but have also been reported to be appropriate.219 The anemia is accompanied by laboratory signs of hemolysis, with increased indirect-reacting serum bilirubin and reticulocytosis and often circulating nucleated red cells. As in any hemolytic anemia, endogenous CO production is increased220,221 and haptoglobin absent. Sickled erythrocytes are often evident on inspection of the blood film. Target cells may be present, particularly in sickle cell–hemoglobin C disease and in sickle cell b-thalassemia. In sickle cell hemoglobin C disease, folded cells are sometimes seen (Fig. 47-8, Fig. 47-9). Examination of the red cells by inference phase-contrast microscopy reveals surface indentations, presumably resulting from splenic hypofunction, in approximately 20 percent of the cells.153 A modest polymorphonuclear leukocytosis with a left shift is common even in the steady state 222,223 and may be due in part to redistribution of leukocytes from the marginal to the circulating granulocyte pool.222 It does not necessarily signify an infection. Thrombocytosis is also common, but evidence of intravascular coagulation with thrombocytopenia has been noted rarely during crisis.224
FIGURE 47-8 Scanning electron microscopy of individual SC cells: (1) multifolded cells; (2) unifolded cell resembling pita bread and most likely the same as the “fat cell” shown in Fig. 47-9; (3) tridimpled cell, also called a triangular cell. (Lawrence et al,52 by permission.)
FIGURE 47-9 Bizarre-shaped erythrocytes in the blood film of patient with hemoglobin SC disease. (A) “Fat sickle cells.” (B) Crescent-shaped erythrocyte with three deep-hued crystals (center left). Two bizarre condensed hemoglobin masses in a red blood cell (lower right). (C) Elongated red corpuscle with concentration of hemoglobin at each end and hemoglobin-free central area (center). (D) Red cell with two parallel, dark, crystal-like structures of different lengths, terminating in a pyramid tip (center). (E) Erythrocyte with two parallel formations separated by a clear area (upper right). Red cell with one elongated mass (lower left). (F) Erythrocyte with densely stained hemoglobin masses (upper right). Red cell with one dark, elongated, rounded bulge and one small triangular hemoglobin mass, leaving two areas relatively free of hemoglobin (lower left). (Diggs and Bell,458 by permission.)
The marrow shows erythroid hyperplasia. Immunoglobulin levels are frequently increased. IgA levels are particularly elevated in all forms of sickle cell disease. Elevations of IgG levels are also sometimes seen, while IgM levels appear to be elevated particularly in patients with sickle cell thalassemia and in individuals with other combinations such as hemoglobin SC disease.225 A decreased number of T lymphocytes and increased B lymphocytes in the blood have been reported.226 Activation in the alternative complement pathway has been detected in some patients227 and is apparently a result of phosphatidylserine exposure by erythrocytes.228 This may be responsible, in part, for increased susceptibility to infection.
Plasma tocopherol229 and zinc230,231 levels are often low, the latter possibly due to zincuria.151,231 Serum ferritin levels are normal in the first two decades of life but tend to rise in older patients, and modest elevations in plasma iron content are also frequently encountered.232 High ferritin levels and increased iron burden occurs in patients who receive chronic transfusion therapy, and such patients are often treated with desferrioxamine.233,234 and 235 However, although the development of hemochromatosis has been reported,165 this seems to be a relatively uncommon complication, even in extensively transfused patients. Frank iron deficiency is not rare, and overt iron deficiency with microcytosis has sometimes been observed in patients with sickle cell anemia.236,237 Thus, the presence of microcytosis does not necessarily indicate the concurrent presence of thalassemia.
Diagnosis depends upon documentation of the presence of sickle hemoglobin, preferably by electrophoresis.233 Many different media and buffers are used to distinguish different mutant hemoglobins from one another, but several relatively simple systems suffice for the differentiation of most variants from one another.238 Rapid methods that are less reliable for the detection of sickle hemoglobin include the observation of sickling of red cells containing sickle hemoglobin microscopically under a coverslip by suspending the cells in a droplet of a 2% solution of sodium metabisulfite239 and solubility tests. The latter depend on the low solubility of reduced sickle hemoglobin which results in the development of turbidity under appropriate conditions.240 However, such tests do not detect hemoglobin C or b-thalassemia and do not reliably distinguish between sickle trait and sickle disease and are therefore of limited value. With the refinement and automation of techniques it has also been possible to detect sickle hemoglobin accurately and economically by high-pressure liquid chromatography and by isoelectric focusing.241 Use of the polymerase chain reaction to detect the sickle mutation is the method of choice for prenatal diagnosis.242
Because there are no normal b polypeptide chain genes, patients with sickle cell anemia or hemoglobin SC disease have no normal adult hemoglobin. In the heterozygote for the sickle cell gene and bo-thalassemia no hemoglobin A is found, but small amounts of normal hemoglobin are present in the compound heterozygote for the sickle cell and b+-thalassemia genes. The concentration of fetal hemoglobin usually is increased in sickle cell b-thalassemia and is heterogeneously distributed among the red cells. The quantitation of hemoglobin A2 is of value in differentiating sickle cell anemia from sickle cell bo-thalassemia; hemoglobin A2 levels tend to be increased in the latter condition. Family studies are particularly helpful if sickle cell bo-thalassemia is to be clearly differentiated from sickle cell anemia.
Sickle cell anemia can be diagnosed at birth by subjecting cord blood samples to electrophoresis.243 Ideally, all babies of ethnic groups with a high frequency of the sickle cell gene should be screened at birth, because of a demonstrated decrease in mortality of very young children when the diagnosis is made.244 The cost-effectiveness of screening depends on the composition of the target population; it has been estimated to be $206,000 per death averted in Alaska. Screening is particularly desirable if the mother has sickle cell trait.
Chorionic villus biopsy has been used extensively to obtain fetal DNA for diagnosis in the first trimester.245 The availability of techniques for the amplification of genomic DNA makes feasible DNA-based prenatal diagnosis of sickle cell disease (see Chap. 9 and Chap. 46). The mutant and normal sequences can be differentiated with an appropriate restriction endonuclease or by the use of synthetic oligonucleotide probes.242,246
THERAPY, COURSE, AND PROGNOSIS
An authoritative guide for the management of patients with sickle cell diseases has been published under the auspices of the Heart, Lung, and Blood Institute of the National Institutes of Health.233
General Measures Since no fully satisfactory, specific treatments for the sickle cell disorders have yet become available, physicians must concentrate their therapeutic efforts in the direction of continuous and effective general medical care and appropriate management of complications as they arise.247,248 Folic acid supplementation has been suggested, but there is little evidence that it is beneficial249 except in pregnancy and in patients with other disorders that increase the requirement for folate. Transfusions are not usually required, except in special circumstances such as stroke, abnormal transcranial Doppler findings, leg ulcers, or intractable or frequently recurring painful crises.250 Prophylactic transfusion does, as expected, decrease the frequency of crises,166 but it requires administration of desferrioxamine to prevent iron overload and subjects patients to the risk of the complications of transfusion such as alloimmunization251 and transmission of infection. A randomized double-blind study showed that conservative transfusion therapy (designed to keep the hemoglobin level over 10 g/dl) was as effective in preventing perioperative complications as more aggressive therapy (designed to maintain the hemoglobin S level under 30%) and was safer.252 Acute neurologic symptoms have been reported to occur after partial exchange transfusion, but a cause-and-effect relationship is not established.253 The use of neocytes (young erythrocytes; see Chap. 140) is probably not justified because of inconvenience and high cost.
Exposure to cold and high altitudes should be avoided. Special vocational training of patients with sickle cell anemia for suitable occupations is useful. It is important that patients live as normal a life as possible. Occupations that do not require heavy manual labor and in which occasional absences from work are practical may be excellent and can make these patients productive members of society.
Acute Chest Syndrome Rapid and correct diagnosis is of paramount importance. It has been recommended that if normal flora are seen on gram-stained sputum in a patient who is not seriously ill with the acute chest syndrome, no antibiotics should be used. However, more symptomatic patients with sputum production should receive antibiotics based on the organisms found in the gram-stained sputum. In adults, in contrast to children, such pulmonary events rarely appear to be due to infection with pneumococci.254 Because of the life-threatening nature of the acute chest syndrome, some clinicians prefer a more aggressive approach, immediately instituting empiric antibiotic therapy including erythromycin because of the frequent involvement of bacteria such as Chlamydia or Mycoplasma.176 Adequate hydration is important, but fluid overload resulting in pulmonary edema occurs not infrequently, and thus careful monitoring of fluid balance is required.176 Exchange transfusion has been advocated.255
Infections The administration of pneumococcal vaccine is recommended,256 but a number of failures of the vaccine to protect children with sickle cell disease against infection with the pneumococcus have been reported, and children with sickle disease should receive pneumococcal vaccine and penicillin prophylaxis at least until the age of five.233,257,258 and 259 Other infectious diseases against which patients with sickle diseases should be immunized include hepatitis B, diphtheria, tetanus, pertussis, poliomyelitis, and Haemophilus influenzae.233 Infections should be treated vigorously with antibiotics. Because patients with sickle cell anemia are unable to concentrate urine adequately, dehydration during the course of infection represents a special risk to be avoided by adequate fluid administration.
Crises Once a small blood vessel is totally obstructed by sickled cells in the development of a painful crisis, the obstruction is probably irreversible. Yet the function of neighboring blood vessels in the areas obstructed by rigid sickled cells may be preserved by a number of therapeutic measures. The patient should be kept warm, and adequate hydration should be maintained by the oral or intravenous route. The role of oxygen therapy in the treatment of vasoocclusive crises is poorly defined. Although the administration of oxygen was once considered to be contraindicated because of a putative negative effect on erythropoiesis, it seems doubtful that it does any harm aside from the minor discomfort incident to its administration, and it may be useful in patients with a decreased arterial oxygen saturation. Hyperbaric oxygen usually fails to benefit the patient,260 although occasional success using this treatment has been claimed.261
The anticoagulants dicumarol262 and the defibrinating enzyme Arvin263 have been tried without success. Intravenous administration of magnesium sulfate264 has been reported to be beneficial, although a therapeutic effect has not been confirmed.265 Promising results of the treatment of sickle crisis with pentoxifylline, a drug reported to increase erythrocyte deformability, were reported in a double-blind study266 but could not be confirmed.267 Oral sodium bicarbonate or sodium citrate therapy has been tried in the treatment of an established vasoocclusive crisis, as well as in its prevention,265 but the efficacy of this treatment could not be confirmed in a controlled study.268
Management with analgesics of the pain of infarctive crises represents a particularly difficult problem for the physician233,269 and is discussed in Chap. 21. In most instances the manifestations of vasoocclusive crisis may gradually disappear over a period of hours or days on symptomatic management.
Splenic sequestration crises are a life-threatening complication that must be treated vigorously. Transfusion with red cells (exchange transfusions if there is respiratory distress) and splenectomy (see below) have been recommended.122,125
Strokes Because of the high recurrence rates of strokes, special attention has been paid to this group of high-risk patients. Regular transfusion programs to maintain the sickle hemoglobin concentration at 30 percent of the total hemoglobin reduce recurrence rates.167 Allowing no more than 50 percent of the hemoglobin to be sickle hemoglobin may provide similar protection.270 A randomized study showed that transfusion greatly reduces the risk of a first stroke in children with sickle cell anemia who have abnormal results on transcranial Doppler ultrasonography.183,271
Hypersplenism and Splenectomy Because of “autosplenectomy,” hypersplenism is seldom a problem in sickle cell anemia. Hypersplenism may be suspected in other forms of sickle disease if a long-term transfusion program becomes necessary to maintain life or if leukopenia and thrombocytopenia are associated with a palpable spleen. Under these circumstances splenectomy may very occasionally be warranted. It has been recommended that splenectomy be performed in all children over the age of two in which one major or two minor splenic sequestration crises have occurred because of the danger of recurrent crises.125,233
Cholelithiasis It is useful to examine adolescent and adult sickle cell anemia patients for the presence of gallstones. It has been suggested that elective cholecystectomy be performed when stones are present,272 but since 50 to 70 percent of adult patients with sickle disease have been found to have gallstones,161 gallstones that do not cause symptoms should probably not be removed. Laparoscopic cholecystectomy has been found to be safe and cost-effective in children273 and adults.274
Contraception and Pregnancy Oral contraception may offer some additional hazard of thromboembolism in patients with sickle hemoglobin,275 but the risk is probably small compared to the risk of pregnancy itself. The contraceptives medroxyprogesterone acetate (Depo-Provera®) given parenterally monthly for 3 months and then every month or levonorgestrel + ethinyl estradiol (Microgynon 30®) given daily were associated with a decrease in the number of attacks of pain in one study.276
Although very high maternal mortality rates have been greatly reduced with good prenatal care, pregnancy and the postpartum period are still potentially hazardous for a mother with sickle cell disease.209 The patient should be closely supervised during pregnancy.277 Although prophylactic blood transfusions have been given to some patients with what appear to be satisfactory results,278,279,280 and 281 the effectiveness of this type of therapy is not proven.209,282 Studies demonstrating that exchange transfusions are not required have been presented283 and contested.280
Leg Ulcers Leg ulcers may respond to conservative treatment such as bed rest, elevation of the affected limb, zinc sulfate pressure dressings, or maintenance transfusion or may require surgical grafting.199 In a study of 172 patients,198 no difference in the rate of healing was associated with any of the different treatment modalities.
Bone and Joint Disease Joint replacement may be helpful to patients who have suffered osteonecrosis, but the number of complications and the number of revisions needed is extraordinary, so that the risk-to-benefit ratio is high.284 Core decompression has been found to be useful in the management of early avascular necrosis of the hip.285
Retinal Changes Vitreous hemorrhages and subsequent blindness may be the end result of the neovascularization that follows retinal infarction. Laser photocoagulation of new vessels may help to prevent this complication.180 When hemorrhages have occurred vitrectomy may be indicated. The administration of nifedipine seemed to improve conjunctival and retinal perfusion and color vision performance in patients with sickle cell disease.286
Priapism Surgical intervention is commonly practiced, particularly in postpubertal patients with priapism. However, there is no clear evidence of benefit from shunting procedures.148 Hydration and exchange transfusions have been associated with detumescence,150 and it has been suggested that the oral administration of the alpha-adrenergic agent etilefrine prevents recurrence of priapism.287 Penile prostheses have been found to be useful when impotence results from priapism.288,289
Anesthesia and Surgery The patient with sickle cell disease is at increased risk during anesthesia. If surgery is indicated, scrupulous care is needed to avoid factors known to precipitate crisis, including hypoxemia, dehydration, circulatory stasis, acidosis, cold, and infections.290,291,292 and 293
Preoperative transfusion with packed red cells may help to avoid complications in patients with sickle cell disease undergoing major surgery.294 Partial exchange transfusion has been advocated,166,295 and this has the advantage of immediate removal from the circulation of sickle cells that may obstruct the microcirculation. However, this more complex procedure probably has little if any advantage over simple transfusion if surgery is elective, as might be the case with patients requiring cholecystectomy or hip replacement.291 Exchange transfusion requires more blood to achieve an equivalent increment in the blood hemoglobin level, and therefore entails more risk than simple transfusion. Elevation of the hemoglobin level of the blood will markedly reduce the production of sickle cells by the marrow, and in view of the short life span of the patient’s own circulating erythrocytes, few sickle cells will remain in the circulation after a week or two. The complication rate of patients receiving exchange transfusions is, in point of fact, no lower than that observed in patients receiving simple transfusions.296 Exchange transfusion provides an advantage if iron overload is a concern or if removal of sickle cells is desired within a period of less than 5 to 7 days.
Transplantation Sickle cell disease is fundamentally a disease of the hematopoietic stem cell, and replacing the genetically defective cell with a normal one should cure the disease. One patient with sickle cell disease received a marrow transplant from a sib with sickle cell trait in the course of treatment of acute leukemia.297 As expected, the sickle cell disease was cured—converted into sickle cell trait.
Subsequently a considerable number of patients with sickle cell disease have undergone marrow transplantation. Groups in France and Belgium had transplanted 42 patients by 1992, with only 1 death; the other patients were alive with follow-ups of from 1 to 75 months.298,299 Over 90 percent of 50 patients transplanted in Belgium between April 1986 and January 1997 survived.300,301 In the United States an increasing number of patients are undergoing transplantation, and the overall results have been quite favorable.302 Twenty of 22 patients survived, with a median follow-up of 23.9 months (range, 10.1– 51.0), and 16 patients had stable engraftment of donor hematopoietic cells. In 1997 it was estimated that worldwide 140 patients had undergone transplantation.303
The decision of whether to transplant a patient with sickle cell disease is a difficult one, because the expected mortality rate for transplantation in young children with a good family donor match is still of the order of 10 percent, and the potential morbidity from chronic graft-versus-host disease needs also be taken into account. Thus, the initial focus must be upon those children with a poor prognosis, and apart from those who have already suffered a stroke, accurate prognostication is impossible.304,305 and 306
Agents with In Vitro Antisickling Activities For a number of years, attempts have been made to modify red cells containing hemoglobin S in a manner that will suppress the sickling process. Examples of this approach have included conversion of hemoglobin to carboxyhemoglobin307,308 and 309 or methemoglobin310; acetylation of the hemoglobin molecules with aspirin,311,312 methyl acetyl phosphate,313 or succinyldisalicylate314; cross-linking hemoglobin molecules with dimethyl adipimidate315,316; and use of carbonic anhydrase inhibitors to reduce the formation of H2CO3.317 Distilled water has been given intravenously to lower the MCHC.318 Glutamine has been given to change the oxidative state of the cell.319 Other antisickling agents that have been studied for a possible therapeutic effect include urea,320 cyanate,321 o-carbamoylsalicylates,322 methyl acetyl phosphate,323 lysyl-phenylalanine,324 procaine,325 zinc,326 pyridoxine327,328 and its derivatives,329,330 phenothiazines,331 steroids,315 nitrogen mustard,332 glyceraldehyde,333 hexamethylenetetramine,334 vitamin E,335 lawsone,336 substituted benzaldehydes,337 bepridil,338 and cetiedil.339 The usefulness of none of these has been confirmed.
Clinical Studies of Putative Antisickling Agents Most putative antisickling agents have been tested only with in vitro model systems, but a few have had clinical trials. The induction of methemoglobinemia310 by the administration of sodium nitrite or p-aminopropiophenone lengthened the life span of sickle cells, and the inhalation of carbon monoxide309 was found to have a similar effect. A patient with sickle cell anemia who was accidentally exposed to carbon monoxide levels presented with a hematocrit rising to 46 percent. A fatal outcome was attributed to extreme hyperviscosity occurring as the carboxyhemoglobin was converted to oxyhemoglobin and the cells again began to sickle.340 Pyridoxine,327 in contrast, did not influence red cell life span. The use of alkali to counteract the Bohr effect (the reduction of the oxygen affinity of hemoglobin at acid pH)341 has been thought to have some therapeutic value, but no beneficial effect could be demonstrated in controlled trials.342 The rationale for the use of urea was the ability of this chemical to dissociate hydrophobic molecular bonds and thus interfere with the sickling process. The concentration required to achieve such an effect cannot be reached in vivo, and clinical trials have proved disappointing.220 Carbamylation of the hemoglobin molecule by cyanate increases the affinity of the hemoglobin for oxygen.343 Because the sickling process requires the hemoglobin to be in the deoxy conformation, any agent capable of affecting the equilibrium between the oxy and deoxy conformations and thereby increasing the avidity of hemoglobin for oxygen must have an antisickling effect.86 Unfortunately, in clinical trials cyanate provoked polyneuropathy,344 retinal changes,344 and cataracts345 and therefore appears to be too toxic for systemic use. However, extracorporeal treatment with removal of excess cyanate by washing the red cells before returning them to the patient may overcome this problem.346,347 and 348 A number of substituted benzaldehyde compounds have been given experimentally to patients, producing a left shift in the oxygen dissociation curve and suggestive evidence of a decrease in hemolysis.337,349 It has been suggested that their effect may be due not only to stabilization of the oxy conformation of hemoglobin but also to decreasing potassium loss.337
Because sickling is highly concentration dependent, efforts to treat the disorder by swelling the red cells have been made. These have included the administration of distilled water intravenously318 and the lowering of serum sodium by the administration of a long-acting vasopressin derivative and vigorous hydration.350,351 The effectiveness and safety of the latter treatment has been questioned.352,353 Such treatment must still be regarded as experimental.
Increasing the Level of Fetal Hemoglobin Efforts have also been made to ameliorate the sickling process by stimulating the formation of fetal hemoglobin. Attempted originally by the administration of chorionic gonadotropin and estrogens,310 more recent efforts have focused on 5-azacytidine, a drug that inhibits the methylation of DNA and was shown to increase fetal hemoglobin concentrations of the red cells of baboons.354 The administration of 5-azacytidine to patients with sickle cell anemia resulted in an increased concentration of fetal hemoglobin355,356 and in a rise in the hemoglobin concentration of the blood.355,356 Other antineoplastic agents, including cytosine arabinoside357,358 and hydroxyurea357,359,360 and 361 or hydroxyurea in combination with erythropoietin,362 and erythropoietin alone363 also increase the fetal hemoglobin level. Butyric acid and related compounds364,365 and 366 increase fetal hemoglobin production in progenitor cells, experimental animals, and humans. However, isobutyramide given orally was not found to be useful.367 In vitro, interferon gamma has also been shown to increase fetal hemoglobin production.368
Of these agents, hydroxyurea is the one that has been tested most extensively and that has been introduced selectively into clinical practice. In a randomized study of 299 patients the median number of painful crises in patients given hydroxyurea was 2.5 per year, as compared with 4.5 per year in patients given placebo. Drug administration was started at 15 mg/kg body weight per day and was increased by 5 mg/kg/day every 12 weeks unless there were signs of marrow suppression.369 A number of open-label studies have been conducted and have in general shown a decrease in the incidence of painful crises370,371,372,373,374 and 375 without serious side effects. However, careful supervision is obviously required in the administration of a myelosuppressive agent. Compliance among children appeared to be very satisfactory in one study.376
Poloxamer 188, a nonionic surfactant with hemorheologic properties, has been tested in a double-blind randomized trial and found to decrease the severity of painful sickle crises.377
COURSE AND PROGNOSIS
For a number of years it was unclear why sickle cell anemia was relatively common in African Americans and yet appeared to be a rare disease in Central Africa. Subsequently it was recognized that the early mortality associated with sickle cell anemia in Central Africa378,379 was responsible for its apparent rarity: the surveys of the distribution of sickle hemoglobin in Africa did not include the afflicted who had died. With good medical care, patients with sickle cell anemia usually survive to middle age.380,381 and 382 Assessment of the overall mortality of sickle cell anemia must take into account the fact that cases first diagnosed in late childhood, adolescence, or adult life are likely to result in a preponderance of the clinically more benign patients. In the two and one-half decades after 1968, mortality rates of African American children with sickle cell disease decreased considerably.383 In the 1–4 age group the mortality had fallen from 37 per thousand persons in those born between 1967 and 1969 to 22 per thousand among those born between 1986 and 1988. Corresponding figures for the 5–9 age group were 19 and 10 per thousand, and for the 10–14 age group 17 and 8 per thousand.383 These improvements in survival may probably best be ascribed to newborn screening programs,384 penicillin prophylaxis of disease caused by Streptococcus pneumoniae, and perhaps the use of pneumococcal vaccines. There were considerable regional differences. The mortality was considerably higher in Florida than in Maryland and Pennsylvania, probably related to the health care facilities available in different regions.385 Astonishingly, in California and Illinois, mortality from all causes among African American children born during 1990–1994 with SC disease was slightly less than overall mortality for all African American children born in the same time period.386
The manifestations of sickle cell disease vary with age.111 Acute manifestations often are associated with severe infections in childhood, while in the adult, symptoms are characteristically chronic and organ-related, albeit still potentially life threatening. Until more data on the disease in infancy become available, it is not possible to predict whether the sudden death syndrome in infants with sickle cell anemia is a common or a rare event. In the meantime, the diagnosis must be considered in cases of acute general illness and unexplained death, especially in ethnic groups where the sickle cell gene is known to occur commonly.
Prevention of some of the sequelae of sickle cell diseases can be achieved by newborn screening (see “Diagnosis,” above). Another form of prevention is based on prenatal diagnosis. Parents can be screened for the carrier state, and if they are carriers they can be provided with genetic counseling and educated about the options of not having children or of having pregnancies monitored for the occurrence of a sickle cell disease in the fetus. Since approximately half of the children with sickle cell diseases have only one parent with sickle hemoglobin, effective screening programs must do more than merely detect the presence of this abnormal hemoglobin. They must also use means that will permit detection of hemoglobin C and of b-thalassemia trait. Because of the benign clinical nature of b-thalassemia, hemoglobin C, and sickle cell traits, no useful purpose other than that of genetic counseling seems to be served by screening populations for these carrier states. Indeed, misunderstanding concerning the significance of the carrier states has led to unwarranted harm to individuals who are detected as carriers in screening programs.387
Many screening programs have been implemented and the number and background of participants have been described.388,389 and 390 However, only scant data permitting assessment of the actual effect of screening programs on birth frequency of infants with sickle cell disorders are available. In Guadeloupe 62 percent of the group of mothers at risk for bearing children with sickle disease underwent prenatal diagnosis, which allowed identification of 27 SS fetuses, with an induced abortion rate of 70 percent. Such data are, of course, highly culture dependent, and very different results might be obtained elsewhere.
SICKLE CELL TRAIT
DEFINITION AND HISTORY
Sickle cell trait is the heterozygous state for the sickle cell diseases and is the most benign form of the sickling disorders.
ETIOLOGY AND PATHOGENESIS
The properties of sickle hemoglobin have been described above. In sickle cell trait less than one-half of the hemoglobin in each red cell is hemoglobin S. The abundance of normal hemoglobin A in the cell prevents sickling under most physiologic circumstances; sickle cell trait cells will sickle at an oxygen tension of about 15 torr.79
Sickle cell trait is inherited as an autosomal dominant disorder. It affects some 8 percent of African Americans and an even higher percentage of the population in Africa (see “Inheritance,” above). Interaction between a-thalassemia and sickle cell trait to modify the amount of sickle hemoglobin has been described above.
Sickle cell trait does not produce any abnormalities of the blood counts and is an exceedingly rare cause of morbidity. Red cell life span is normal in normoxic persons with sickle cell trait.391 Not only patients but even physicians392 often appear to believe that sickle cell trait represents a mild type of sickle cell disease. Cerebral thrombosis, mishaps during anesthesia, and sudden death attract little notice when occurring in a person who does not have a known genetic variant, but the same occurrence in the 1 of 12 African Americans who have this trait immediately raises the question of a cause-and-effect relationship. Thus, there is a legion of anecdotal reports suggesting that sickle cell trait contributed to a patient’s illness.187,188,393,394,395 and 396 There may, however, be certain situations in which a risk is plausible. Thus, in severe cyanotic congenital heart diseases, such as tetralogy of Fallot, patients with sickle cell trait may show signs of hemolysis.397 In reality, the morbidity and possible mortality associated with sickle cell trait is very low and therefore difficult to document accurately. It seems to be limited largely to renal lesions (see Fig. 47-6) leading to hematuria that is otherwise unexplained and possibly to thromboembolic episodes involving the lung. In a massive study encompassing over 65,000 consecutively admitted African American male patients in 13 U.S. Veterans Administration hospitals,73 slightly higher incidences only of hematuria of unspecified cause (2.5 percent versus 1.3 percent) and pulmonary embolism (2.2 percent versus 1.5 percent) were found. No age stratification was found, indicating that the life span of patients with sickle cell trait is normal. Surgical patients with sickle cell trait had no greater perioperative mortality, no longer postoperative stay, and no greater mortality than those with normal hemoglobin. Similar conclusions have been drawn in other studies.398 It has not been possible to document any differences from normal in cardiovascular function of sickle cell trait subjects even when they were subjected to maximum exercise399,400,401,402 and 403; indeed, persons with sickle trait were overrepresented among champion athletes in the Ivory Coast.404
Sudden death resulting from rhabdomyolysis has been reported anecdotally in numerous subjects with sickle cell trait following severe exercise.396,405,406,407,408 and 409 An extensive investigation of episodes of sudden death showed a statistically significant excess in the number of patients with sickle cell trait.410 It is believed that the hyposthenuria (see “Other Clinical Manifestations, Genitourinary System,” above) in combination with heat and extreme stress may trigger this catastrophic and usually fatal event.
Because of reports of splenic infarction in individuals thought to have sickle cell trait who were flying in unpressurized aircraft411,412 or who ascended to very high altitudes,413 there has been concern about the safety of permitting persons with sickle cell trait to fly. Since commercial aircraft maintain a cabin pressure equivalent to that encountered at 5000 to 7000 feet (1500 to 2100 m), this concern is unwarranted.81 It appears that when splenic infarction does occur at high altitudes, non-African persons with sickle trait are much more likely to be affected than are Africans.80
The diagnosis of sickle cell trait depends upon demonstration of the presence of hemoglobin S and hemoglobin A in the affected individual. The amount of hemoglobin S is always less than the concentration of hemoglobin A. In contrast, in sickle cell b+-thalassemia the amount of hemoglobin S exceeds that of hemoglobin A.
THERAPY, COURSE, AND PROGNOSIS
Because of its benign features, sickle cell trait does not require treatment and does not appear to affect life span.73
HEMOGLOBIN C DISEASE
DEFINITION AND HISTORY
Hemoglobin C was the second abnormal hemoglobin to be described, not long after the description of hemoglobin S.414 The homozygous state (CC disease) was described independently by Spaet et al415 and by Ranney et al416 in 1953. Hemoglobin C trait is the heterozygous state in which hemoglobin C is inherited together with normal hemoglobin. The combination with sickle cell hemoglobin, SC disease, has been described in the discussion of sickle cell anemia, under “The Sickle Cell Diseases,” above.
ETIOLOGY AND PATHOGENESIS
In hemoglobin C, glutamic acid in the sixth position from the N terminal of the b chain has been replaced by lysine.417 Red cells containing principally hemoglobin C are more rigid than normal,418 and their fragmentation in the circulation may result in the formation of microspherocytes. Intraerythrocytic crystals of oxygenated Hb C are found in the red cells, especially in splenectomized patients,418,419 and the formation of crystals is inhibited by hemoglobin F.420 The red cell life span is shortened to a mean of 30 to 35 days.421 The rate of hemoglobin production in hemoglobin C disease has been reported to be 2.5 to 3 times normal.422 Erythrocytes from patients with hemoglobin C disease have a low oxygen affinity, possibly due to a reduction for unknown reasons of the intracellular pH.423 This may contribute to the mild anemia that is usually present.
Hemoglobin C is found in 17 to 28 percent of West Africans, particularly east of the Niger River in the vicinity of North Ghana.424,425 The selective factors that account for this high prevalence are unknown at present. The prevalence among African Americans is 2 to 3 percent.73,426 Sporadic cases also have been reported in other populations, including Italians427 and Afrikaners.101
Splenomegaly is a fairly constant feature of hemoglobin C disease and may be associated with fleeting abdominal pain. However, there is little evidence for clinically significant hemodynamic disturbances.428 Women with hemoglobin C disease appear to tolerate pregnancy well.429 Children have mild anemia with few symptoms and normal growth.430
In hemoglobin C disease the hemoglobin level ranges from 8 to 12 g/dl. There is a marked increase in the number of target cells in the blood film (see Fig. 47-9). Some target cells are also present in the trait. Occasionally, intraerythrocytic hemoglobin crystals may be seen on the blood film, and these may appear in larger numbers if the red cells have been dehydrated either by drying or by suspension in a hypertonic solution (see Chap. 22). The osmotic fragility of the red cells may be decreased.
The diagnosis of homozygous hemoglobin C disease is achieved by electrophoresis, hemoglobin C moving to the same position as hemoglobin A2, hemoglobin E, and hemoglobin OArab at an alkaline pH. Hemoglobin C is readily distinguished from other hemoglobins by acid agar gel electrophoresis.
THERAPY, COURSE, AND PROGNOSIS
No specific therapy is available or required for patients with hemoglobin C disease. Anemia may become more severe following infections, but the overall prognosis is considered to be excellent.
HEMOGLOBIN D DISEASE
DEFINITION AND HISTORY
In his early studies of the hemoglobinopathies, Itano431 encountered a white family with an abnormal hemoglobin that migrated at the same rate as hemoglobin S but did not sickle. Its solubility in the reduced state resembled that of hemoglobin A, and this new abnormal hemoglobin was designated hemoglobin D. Subsequently, this name was given to any hemoglobin variant that manifested the same electrophoretic properties as hemoglobin S at an alkaline pH but had normal solubility properties.
ETIOLOGY AND PATHOGENESIS
With the exact chemical analysis of hemoglobin variants, it became apparent that hemoglobin DLos Angeles was identical to hemoglobin DPunjab, both manifesting a substitution of glutamate for lysine at the 121st position in the b chain. Another “D” hemoglobin, GPhiladelphia, is, on the other hand, an a chain variant, with a substitution of asparagine for lysine at the sixty-eighth position.
Like the other structural mutations of hemoglobin, hemoglobin D trait is the heterozygous state for hemoglobin D and hemoglobin A, while the homozygous state for hemoglobin D is designated hemoglobin D disease. Hemoglobin DPunjab is found in frequencies of approximately 3 percent in Northwest India.
The heterozygous state for hemoglobin D is entirely asymptomatic.432 The abnormal hemoglobin constitutes between 35 and 50 percent of the total hemoglobin. Homozygous hemoglobin D disease is very rare, and some patients originally believed to be homozygous for hemoglobin D433 subsequently were found to be heterozygous for hemoglobin D and b-thalassemia. A small number of true homozygotes have been described, however, and the clinical consequences are very mild.434
HEMOGLOBIN E DISEASE
DEFINITION AND HISTORY
Hemoglobin E is so prevalent that it may be the most common abnormal hemoglobin,435 or second in prevalence only to hemoglobin S. It was first described in 1954, independently by Itano et al436 and by Chernoff et al.437
ETIOLOGY AND PATHOGENESIS
Hemoglobin E is the result of a b chain mutation, a2b226Glu®Lys.438 The amino acid substitution not only produces a hemoglobin that is somewhat unstable when subjected to oxidative stress,439 perhaps because of weakening of the bonds between the monomers constituting the hemoglobin tetramer, but the nucleotide substitution also creates a new potential splicing sequence, so that some of the messenger may be spliced improperly.440 The formation of unstable messenger accounts for the thalassemia-like nature of hemoglobin E trait and disease.
The inheritance of hemoglobin E is the same as that of the other b chain mutants. Heterozygotes for hemoglobin E and hemoglobin A have hemoglobin E trait, while homozygotes for hemoglobin E are designated as having hemoglobin E disease. Hemoglobin E, like hemoglobin S and hemoglobin C, occurs with sufficient frequency to be considered a polymorphism. The distribution of the gene for this b chain mutation is illustrated in Fig. 47-10. Decreased falciparum malaria parasitemia has been documented in patients with hemoglobin E trait,441 and resistance to malaria may be the advantage that has led to high gene frequencies.
FIGURE 47-10 Distribution of hemoglobin E in Southeast Asia. Gene frequencies: cross-hatching indicates >0.2 percent; narrow hatching indicates 0.1 to 0.2 percent; wide hatching indicates 0.02 to 0.1 percent; dotted area indicates <0.02 percent and sporadic occurrence. (Flatz,442 by permission.)
Hemoglobin E is found principally in Burma, Thailand, Laos, Cambodia, Malaysia, and Indonesia, and in some areas it occurs with a carrier rate of 30 percent.442 On the other hand, it is not prevalent among the Chinese. Studies of restriction length polymorphisms in the b-globin cluster indicate that the hemoglobin E mutation has arisen several times independently.443
Although the prevalence of the gene for hemoglobin E is quite high in Southeast Asia (see Fig. 47-10), relatively few patients with homozygous E disease, as distinguished from hemoglobin E b-thalassemia, have been described.444,445 When homozygous E disease is encountered, it is associated with marked microcytosis and hypochromia but little or no anemia. Splenomegaly is unusual, and the red cell life span is normal. Clinically, the state closely resembles b-thalassemia minor.
In the hemoglobin E carrier state 30 to 45 percent of the hemoglobin is hemoglobin E,437 and such carriers are asymptomatic but do manifest microcytosis.446
The clinical manifestations of the heterozygous state between hemoglobin E and b-thalassemia are quite variable in severity447 and resemble those of homozygous hemoglobin E disease, with moderate anemia and splenomegaly representing the usual manifestation.
Hemoglobin E is electrophoretically slow in an alkaline medium, comigrating with hemoglobin C and A2. The characteristic blood change is microcytosis—mild in the trait and more severe in the homozygous state and in hemoglobin E b-thalassemia. There is a modest decrease in the a-/non-a-globin chain synthetic ratio445 and a minimal decrease in whole blood oxygen affinity.437,444
THERAPY, COURSE, AND PROGNOSIS
The prognosis seems to be good, although no thorough studies of the natural history of the disease have been carried out. Splenectomy increases the red cell life span and ameliorates anemia in hemoglobin E b-thalassemia,448,449 but its role in homozygous hemoglobin E disease has not been delineated. In one family manifesting both pyrimidine 5′-nucleotidase deficiency and homozygous hemoglobin E disease, those with both defects had more severe anemia than those inheriting one alone.450
In comparison with hemoglobins S, C, D, and E, other abnormal hemoglobins are rare. Some, such as the unstable hemoglobins (see Chap. 48), the hemoglobins producing erythrocytosis (see Chap. 61), and those producing cyanosis (see Chap. 49), are of clinical importance. Many of the other hemoglobins do not produce significant clinical alterations but have nonetheless been important in clarifying the role of individual amino acids in the structure and function of the hemoglobin molecule. Some of the more common hemoglobin variants are summarized in Table 47-1. Complete compendia of mutations affecting hemoglobin have been published,451 and further sources may be found at http://globin.cse.psu.edu/.
TABLE 47-1 SOME REPRESENTATIVE HEMOGLOBIN VARIANTS
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Ernest Beutler, Marshall A. Lichtman, Barry S. Coller, Thomas J. Kipps, and Uri Seligsohn