131 PNEUMONIA AND LUNG ABSCESS
Harrison’s Manual of Medicine
PNEUMONIA AND LUNG ABSCESS
ETIOLOGY Pneumonia is an infection of the pulmonary parenchyma caused by various bacterial species (including mycoplasmas, chlamydiae, and rickettsiae), viruses, fungi, and parasites. Compromised hosts are particularly vulnerable to pulmonary infections caused by a variety of pathogens.
EPIDEMIOLOGY Factors such as travel history, exposure to pets, exposure to other people who are ill, occupation, age, presence or absence of teeth, season of the year, geographic location, setting (community vs. hospital acquisition), smoking status, and HIV status all influence the types of pathogens to consider in the etiology of pneumonia.
PATHOGENESIS The most common mechanism for acquiring pneumonia is aspiration of organisms from the oropharynx. The usual organisms are aerobic gram-positive cocci and anaerobes that colonize the oropharynx. Normally, 50% of adults aspirate during sleep. Aspiration increases with impaired consciousness—e.g., in alcoholics and drug users; in pts with stroke or seizure, other neurologic or swallowing disorders, or nasogastric or endotracheal tubes; and during or after anesthesia. Aerobic gram-negative bacilli colonize the oropharynx or stomach more frequently in hospitalized or institutionalized pts than in other individuals. Other routes of transmission for pneumonia include inhalation of infected particles (diameter <5 µm), hematogenous spread, contiguous spread from another infected site, and direct inoculation from open trauma to the chest.
CLINICAL MANIFESTATIONS The “typical” pneumonia syndrome is characterized by the sudden onset of fever, cough productive of purulent sputum, and pleuritic chest pain; signs of pulmonary consolidation; and a lobar infiltrate on CXR. This syndrome is most commonly caused by Streptococcus pneumoniae and other bacterial pathogens. The “atypical” pneumonia syndrome is characterized by a more gradual onset, a dry cough, a prominence of extrapulmonary symptoms (e.g., headache, malaise, myalgias, sore throat, GI distress), and minimal signs on physical exam (other than rales) despite an abnormal, often patchy or diffuse pattern on CXR. Atypical pneumonia is classically caused by Mycoplasma pneumoniae but may also be due to Legionella pneumophila, Chlamydia pneumoniae, oral anaerobes, Pneumocystis carinii, and S. pneumoniae.
Other, rarer pathogens causing atypical pneumonia include Chlamydia psittaci, Coxiella burnetii, Francisella tularensis, Histoplasma capsulatum, and Coccidioides immitis. While recent data suggest that the distinction between typical and atypical pneumonia syndromes may be less reliable than was once thought, the differences are of some diagnostic value. Certain viruses, including influenza virus, respiratory syncytial virus, measles virus, varicella-zoster virus, and cytomegalovirus, may produce an atypical pneumonia. Hantavirus causes an initial febrile prodrome followed by rapidly progressive respiratory failure and diffuse pulmonary infiltrates.
DIAGNOSIS Radiography Findings on CXR range from lobar consolidation with air bronchograms to diffuse patchy interstitial infiltrates. Other findings include multiple nodules suspicious for septic emboli, cavitation, pleural effusion, and hilar adenopathy.
Sputum Examination Whenever possible, sputum should be obtained and evaluated grossly for purulence and blood and by Gram’s stain. The presence of >25 PMNs and <10 epithelial cells per high-power field suggests that the specimen is adequate. The finding of mixed flora on Gram’s stain suggests anaerobic infection. The presence of a single, predominant type of organism suggests the etiology of the pneumonia. Sputum may also be examined directly for acid-fast organisms and by special stain or immunofluorescence for Legionella or Pneumocystis. Sputum culture may yield the causative agent but is usually less sensitive than Gram’s stain.
Blood Cultures Since sputum examination and culture do not always reveal a pathogen, blood for cultures should be obtained before therapy begins.
Other Diagnostic Maneuvers Sputum induction (with ultrasonic nebulization of 3% saline), bronchoscopy with bronchoalveolar lavage or protected brush specimens, transtracheal aspiration, thoracentesis, percutaneous lung puncture, open-lung biopsy, acute and convalescent serologies, and chest CT are all useful in selected cases.
Initial antibiotic treatment for pneumonia is often empirical. However, establishing a specific microbial etiology is important, for it allows institution of specific pathogen-directed antimicrobial therapy, exposes the pt to fewer potential adverse drug effects, and reduces the pressure for selection of antimicrobial resistance. See Table 131-1 for clinical syndromes, likely organisms, and antibiotic choices. See Table 131-2 for doses of antibiotics used for inpatient treatment of pneumonia.
Table 131-1 Treatment of Pneumonia
Table 131-2 Dosage of Antimicrobial Agents for the Treatment of Pneumonia in Hospitalized Patientsa
Outpatient and Home Care Considerations Some persons with pneumonia may be treated as outpatients with oral antibiotics. Criteria for hospitalization of pts with pneumonia are listed in Fig. 131-1. These criteria are from the Pneumonia Patient Outcomes Research Team (PORT) and attempt to stratify pts into five risk classes for death and other adverse outcomes on the basis of a cumulative point score. Outpatient management is appropriate for many pts in the low-risk groups (I and II).
FIGURE 131-1. Criteria for hospitalization of pts with pneumonia: the PORT score. *A risk score (total point score) for a given pt is obtained by summing the pt’s age in years (age minus 10 for females) and the points for each applicable pt characteristic. (From: ME Levison: HPIM- 15, p. 1480.)
CLINICAL MANIFESTATIONS Lung abscess is usually a complication of the aspiration of oral anaerobes. In most pts it is a subacute disease with an indolent presentation. The pt usually has a cough that may or may not be productive of large quantities of purulent, foul-smelling sputum and often has fevers, night sweats, and weight loss. Pleuritic chest pain and blood-streaked sputum also may be noted.
DIAGNOSIS CXR usually shows a cavitary lesion with an air-fluid level, often in dependent, poorly ventilated portions of the lung. The sputum is not necessarily foul-smelling but usually contains a mixed flora revealed by Gram’s stain.
The treatment of choice is clindamycin (600 mg q8h IV, then 300–450 mg PO); ampicillin (2 g IV q6h) or amoxicillin (500 mg PO tid) with metronidazole (500 mg PO q6h); ampicillin/sulbactam; or amoxicillin/clavulanate. Antibiotics rarely need to be directed at each organism isolated if the abscess is aspirated. Treatment should be continued until CXR findings have resolved, which may require months.
For a more detailed discussion, see Levison ME: Pneumonia, Including Necrotizing Pulmonary Infections (Lung Abscess), Chap. 255, p. 1475, in HPIM-15.