Leave a comment

Chapter 196 – Retrochiasmal Pathways, Higher Cortical Function, and Nonorganic Visual Loss

Chapter 196 – Retrochiasmal Pathways, Higher Cortical Function, and Nonorganic Visual Loss

 

ANDREW W. LAWTON

 

 

 

 

 

DEFINITION

• The retrochiasmal pathways consist of parallel streams of information diverted to appropriate areas of the brain for identification, storage, and retrieval.

 

KEY FEATURES

• Lesions of numerous areas of the brain will produce characteristic interruptions of pathway functions and visual processing.

• Careful analysis of visual fields and function frequently can localize white matter and cortical lesions accurately.

• Because patients with conversion reactions and malingerers may have complaints that mimic those caused by retrochiasmal pathway and cortical injuries, careful evaluation is necessary to avoid unnecessary testing and patient discomfort.

 

 

 

RETROCHIASMAL PATHWAYS AND HIGHER CORTICAL FUNCTION

OPTIC TRACTS

The optic tracts connect the optic chiasm to the lateral geniculate visual nuclei. Although the chiasm sorts information from the right field of each eye to the left visual cortex and vice versa, lesions of the optic tracts tend to produce highly incongruous visual fields.

This disparity manifests in other ways, as well. An injury to an optic tract may yield a relative afferent pupillary defect in the contralateral eye. The primary cause of this phenomenon is the temporal crescent. The temporal visual field is 50% larger than the nasal field of the contralateral eye. Hence the nasal retina produces axons that constitute approximately 55% of the contralateral optic tract.[1]

The optic tracts do not maintain a strict retinotopic architecture as might be expected ( Fig. 196-1 ). Fibers from corresponding parts of the retinas do not pair in the optic tracts. Larger diameter, faster-conducting axons predominate superficially, under the pia. Tracer studies indicate that these fibers correspond to the magnicellular layers in the lateral geniculate nuclei. The parvicellular axons dominate the center of the optic tract, with fibers from the opposite eye running in the deepest, dorsal regions of the tract. The ipsilateral parvicellular fibers sit slightly ventrally. Optic tract axons have achieved this orientation by the time of their arrival during axonogenesis. This disparity in neuronal migration explains the incongruity of optic tract visual field defects.

 

 

Figure 196-1 Optic tract cross-section. Note that the parvicellular fibers run centrally and the magnicellular fibers peripherally.

 

 

Figure 196-2 Lateral geniculate body section. The layers are numbered from ventral to dorsal in this posterior view. K fibers travel between the lamellae.

LATERAL GENICULATE BODIES

The lateral geniculate bodies represent the first site at which information from corresponding axons arising from the retinal ganglion cell layers pairs together. Evidence suggests, however, that early embryos do not have this orientation of fibers.[2] The axons must rearrange themselves into regular layers. The retina directs the rearrangement process via generation of electrical impulses even before the system is visually active. These impulses arise from ganglion and amacrine cells prior to the appearance of photoreceptors.

Myelinated nerve fibers divide each lateral geniculate body into six neuronal layers ( Fig. 196-2 ). [3] Traditionally, the layers are numbered ventral to dorsal. Axons from the contralateral eye synapse in layers 1, 4, and 6; axons from the ipsilateral eye synapse in layers 2, 3, and 5.

 

1296

The layers of the lateral geniculate body may be categorized by neuronal size. Large, magnicellular neurons (M cells) predominate in layers 1 and 2; small, parvicellular neurons (P cells) constitute layers 3–6. At this level, visual processing is divided into at least two parallel pathways. Primate studies indicate, but have not proved, that the parvicellular pathway carries information related to color perception and visual resolution (high spatial frequency contrast sensitivity).[3] The magnicellular pathway apparently contains information that deals with motion detection and lower contrast, lower spatial frequency.

Many authors have challenged the notion of only two parallel pathways. [4] Primate research shows that numerous small neurons (koniocellular or K cells) sit in the interlaminar zones and superficial layers of the lateral geniculate. These cells receive input from both the retina and the region of the superior colliculus. Authors speculate that the koniocellular pathway’s role is to modulate information derived from the other two pathways.

The lateral geniculate nuclei are organized by retinotopic visual field loci, as well. The macula tends to project to the caudal 75% of the nucleus. These fibers straddle the midline of the nucleus and form a rhombus. The unpaired sections of the visual fields appear to project peripherally within the nucleus. Fibers from the superior retina tend to migrate medially in the lateral geniculate nuclei; those from the lower retina tend to move laterally.

OPTIC RADIATIONS

Axons arising in the lateral geniculate nuclei form the optic radiations and project to the calcarine cortex. Superior retinal fibers course inferiorly in the radiations and the inferior retinal fibers migrate superiorly ( Fig. 196-3 ). Axons corresponding to central vision travel between the two other bundles.[5]

The fibers from the inferior retina travel deep within the parietal lobe relatively close to the internal capsule and to a tract that carries pursuit information from both occipital lobes to the ipsilateral paramedian pontine reticular formation. The superior retinal fibers course ventrally into the temporal lobe in an arc (Meyer’s loop) around the temporal horn of the lateral ventricle.[6]

 

 

Figure 196-3 Optic tract paths. Fibers that correspond to the inferior retina course rostrally and laterally into the temporal lobes to form Meyer’s loops. The superior retinal fibers take a much more direct course through the parietal lobes.

These geographical relationships gain considerable importance in localizing a lesion of the visual pathways.

HIGHER CORTICAL FUNCTION

Information from the optic tracts projects to the calcarine cortices of the medial occipital lobes. This visual cortex performs multiple processing functions to prepare information for detailed analysis elsewhere in the brain.

The occipital cortex involved in primary visual processing straddles the calcarine sulci. Brodmann[7] designated this region as area 17 in his topographical nomenclature; area 17 also is referred to as area V1. The actual surface area of the visual cortex varies in range from 20–25?cm2 and occupies approximately 3.5% of the surface area of the brain. The neurons of area 17 receive information via the myelinated stripe of Gennari, which gives the area its distinctive histology. Information from central vision projects to the caudal half of the visual cortex while that from peripheral vision projects rostrally.

The visual cortex contains multiple identifiable layers of cells. Layer I, the most superficial, contains small granule cells and a few horizontal cells; layer II consists of pyramidal neurons and numerous interneurons; and layer III contains pyramidal and granule cells. Layer IV contains three subdivisions—IVa consists of stellate neurons, IVb contains predominantly granule cells, and the deepest stratum, IVc , contains granule, pyramidal, and stellate cells.[8] Input from the optic tracts tends to terminate in layer IVc .[9] Layer V contains numerous pyramidal cells. Layer VI demonstrates fewer neurons but stains darkly because of star pyramids.

The calcarine cortex plays less of a role in visual image processing than previously theorized. It appears to be a coordination center, where information from both hemifields is paired into parallel, vertically oriented, ocular dominance columns ( Fig. 196-4 ).[10] Small regions of the visual field are analyzed in the primary visual cortex by an array of complex cellular units called hypercolumns. A single hypercolumn represents the neural machinery necessary to analyze a discrete region of the visual field. Each hypercolumn contains a complete set of the orientation columns, which represents

 

 

Figure 196-4 Ocular dominance columns. Each ocular dominance column receives input from either the contralateral or ipsilateral eye via projections from cells in individual layers of the lateral geniculate nucleus that serve one or the other eye. (Adapted with permission from Kandel ER, Schwartz JH, Jessell TM. Principles of neural science, ed 3. New York: 1991, McGraw-Hill.)

 

1297

360°, a set of left and right ocular dominance columns, and several blobs (regions of the cortex in which the cells are specific for color).[11] These hypercolumns merge data from corresponding points in each retina. Because the temporal field (nasal retina) from the contralateral eye is considerably larger than the nasal field (temporal retina) from the ipsilateral eye, each calcarine cortex receives unpaired information from the contralateral eye; this forms the “temporal crescent.”[12] This information is processed most anteriorly and represents an important clinical feature critical for the diagnosis and localization of many occipital lobe lesions. Recent evidence suggests that connections between the two hemispheres via the corpus callosum allow synchronization of information generated by both fields; this enables information that arises from both fields to be combined.[13] [14]

The visual cortex contains four basic types of cells that respond in specific and characteristic ways to retinal stimuli.[15] Circularly symmetrical cells react to small lights independent of movement or orientation. Simple cells respond to a moving light or a dark line or pattern with a specific orientation and direction of motion that projects on the center of their field. Simple cells may turn either “on” or “off” in response to the stimulus. Complex cells respond to linear stimuli almost anywhere in their field but are less specific as to orientation. They also may be “on” or “off” cells. Hypercomplex cells are similar to complex cells but require a linear stimulus of a specific length. The information generated among all these cells is synchronized through the extensive interconnection between visual cortical areas ( Fig. 196-5 ). [16] As a result of this exchange of information, certain stimuli in patterns “pop out” and catch an individual’s attention, while other details (small gaps in a large pattern especially outside the central 15° or the defect associated with the blind spot) may “fill in” and disappear into the background. [17]

TOPOGRAPHICAL DIAGNOSIS OF RETROCHIASMAL DISEASE

Unfortunately, the anatomy of the visual pathways means that any lesion of these tracts tends to produce some form of homonymous hemianopia. A total homonymous hemianopia involving the temporal crescent is nonlocalizing. In most cases, however, careful examination of the visual field and associated clinical findings yields clues to the location of a lesion.

 

 

Figure 196-5 Parallel visual pathways. Their suggested functions in the macaque monkey. (Adapted with permission from DeYoe EA, Van Essen DC. Concurrent processing streams in monkey visual cortex. Trends Neurosci. 11:219–26, 1988.)

 

1298

Lesions isolated to the optic tracts account for less than 5% of patients with a homonymous hemianopia.[18] Injury to the optic tracts tends to produce exceedingly incongruous field defects. Because the optic tracts include fibers of the afferent pupillary pathway, patients with optic tract lesions tend to demonstrate a relative afferent pupillary defect in the contralateral eye and, eventually, optic atrophy on one or both sides.[19] [20] Patients may demonstrate a larger pupil on the side of the hemianopia (Behr’s pupil) or pupillary hemiakinesia (Wernicke’s pupil).

Lesions to the lateral geniculate nuclei also tend to produce an incongruous homonymous hemianopia.[21] The vascular supply of the lateral geniculate nucleus may include the adjacent thalamus and corticospinal tracts, which provides additional neurological data to localize a lesion clinically. Because pupillary fibers leave the optic tracts rostral to the lateral geniculate nuclei, lesions here do not produce afferent papillary defects.

Lesions of the deep parietal lobe may involve the superior (superior and peripheral retinal) fibers of the optic radiations. This damage results in a wedge-shaped, inferior, contralateral homonymous hemianopia ( Fig. 196-6 ).[22] Because the optic radiation fibers are still orientating themselves for cortical innervation, the hemianopia is incongruous. Because the macular fibers pass between the parietal and temporal fibers, the defect generally spares central vision. The lesion may involve the posterior limb of the internal capsule and produce a contralateral hemiplegia and hemianesthesia. Involvement of the pursuit pathways, headed for the ipsilateral paramedian pontine reticular formation,

 

 

Figure 196-6 Inferior, incongruous, homonymous visual field defect. Injuries to the parietal lobe tend to spare central fixation, as is characteristic of temporal lobe lesions.

 

 

Figure 196-7 Visual field defect, temporal lobe injury. Interruption of this segment of the optic radiations yields an incongruous, superior, homonymous visual field defect, more dense above than below—the “pie-in-the-sky” pattern.

tends to result in an alteration of optokinetic nystagmus—the patient cannot pursue stimuli moving toward the side of the lesion and does not generate optokinetic nystagmus in that direction.

Damage to the temporal optic radiations interrupts the inferior (inferior and peripheral retinal) fibers of Meyer’s loop.[5] The typical visual field defect is an incongruous, wedge-shaped, superior homonymous hemianopia sparing central vision ( Fig. 196-7 ). Injury to adjacent structures may yield memory loss, hearing loss, and auditory hallucinations.

Lesions of the calcarine cortex tend to be silent other than for visual field defects. These defects tend to be highly congruous ( Fig. 196-8 ).[23] Preservation of the temporal crescent identifies a defect as cortical. Patients may show sparing of central vision (macular sparing); this phenomenon generally results from separate arterial supply between the occipital pole and the rostral calcarine cortex.

Horton and Hoyt[24] mapped the visual cortex in depth by correlating magnetic resonance imaging (MRI) findings and visual dysfunction in patients with occipital lobe lesions. Information from the fovea occupies an extremely large segment of the calcarine cortex; input from the central 10° involves more than 50% of the caudal striate cortex. Hence, lesions that spare only the temporal crescent are unusual; the central 1° of vision and the entire temporal crescent engage an equivalent cortical volume.

Improved neuroimaging techniques now simplify the evaluation of patients with homonymous hemianopia.[25] MRI is the method of choice for watershed lesions, small-vessel disease, thrombotic infarction, leukodystrophy, primary or secondary neoplasia, demyelinating white matter disease, shear injuries, or contusion. The MRI should include contrast and noncontrast multiplanar T1- and T2-weighted images. Lesions dominated by blood (acute subarachnoid or intraparenchymal hemorrhage, for example) show poorly on MRI. Noncontrast CT studies are more productive in these cases. A lumbar puncture, at times, may be the only conclusive tool for finding blood.

CORTICAL REPRESENTATION OF VISION

INTRODUCTION

The visual association areas of the brain create a recognizable image of the world through complex combinations of information from several parallel pathways. The brain separates information by position and category and correlates this information

 

 

Figure 196-8 Visual field defect, inferior left calcarine cortex lesion. Note the high congruity and involvement of fixation.

 

1299

with surrounding objects and associated sounds ( Fig. 196-9 ). The brain must maintain a reference library of previously viewed images ready for instantaneous recall. Consider the identification of the human face. An individual views a jogger for the first time wearing a blue shirt and shorts and has a conversation. On the second encounter, the jogger is now sitting, reading a book, and wearing a blue dress. Despite the differences, recognition occurs and creates the expectation of a specific voice and speech pattern. An understanding of the relationship of cortical visual processing pathways enables the recognition of characteristic syndromes by clinical features.

OBJECT IDENTIFICATION AND MEMORY

Identification of an object requires the ability to retain an image in memory and use this image for future comparison. Milner et al.[26] and Goodale[27] describe a patient with visual agnosia. She could identify objects by touch but not by sight. If asked to take an object, however, she could turn her wrist appropriately, open her hand, and grasp the object so that it did not drop. The primary damage appeared to be in her ventrolateral occipital lobes. In contrast, damage to the superior posterior parietal cortex results in difficulty in making the movements needed to manipulate objects despite a preserved ability to describe the objects and their orientation. [28]

One theoretical basis for this separation of function targets visual association areas in the medial occipital lobes.[29] Injury to the right occipital lobe results in failure to identify complex objects (including faces, i.e., prosopagnosia) in general. Apparently, many animals sort out various types of visual stimuli in this area.[30] A lesion in the left occipital lobe yields impairment of recognition of objects with numerous parts, including words.

 

 

Figure 196-9 Distribution of higher order visual processing among different cortical areas. The magnicellular system (inferior stream) is considered generally with the location and motion of objects, while the parvicellular system (superior stream) is concerned with the fine resolution (acuity), form, and color of objects.

A second theory concentrates on the connections among lobes of the brain[31] and postulates that primate brains separate visual information into two streams, dorsal and ventral. The ventral stream originates in the primary visual cortex, projects to the inferotemporal region, and carries information needed to identify objects and their positions in space. The dorsal pathway transmits the information on size, shape, and orientation needed to grasp the object. One stream may be damaged without injuring the other.

The positron emission tomography (PET) scanner has provided assistance in resolution of these two theories. Sargent et al. [32] tested subjects with three separate stimuli—sine wave gratings, male and female faces, and simple objects. Sine wave gratings yielded activity restricted to the striate and extrastriate cortices. Faces stimulated the right parahippocampal region and both fusiform and anterior temporal cortices. Simple objects activated the left occipitotemporal cortex alone. This study tends to support both theories as critical steps in visual processing. Farah[33] provides an excellent synopsis of current theories of image generation.

Lack of image recognition, however, does not mean an individual cannot see an image clearly.[34] Patients with prosopagnosia fail in selecting two matching faces from a picture set. Invert the faces, however, and the patients fare much better. Inverting the face apparently allows the brain to treat faces as simple objects.

When an image is repeated in a series, necessitating the recurrent identification of the same object, the brain adds another region to the loop. The prefrontal cortex becomes active on PET scan during this task.[35] Although long-term memory for vision is a temporal lobe function, short-term visual memory is a frontal lobe function.

The inability to identify objects visually, however, does not necessarily affect a person’s generation of a mental image. Despite a deficit in naming seen objects, patients may copy visual objects, generate accurate pictures from the memory of an object, or draw a picture based upon tactile examination of an object.[36] [37] [38] Ironically, when shown these drawings, patients do not recognize them as their own.

Stimulation of storage areas may produce accurate visual hallucinations that may occur as a release phenomenon resulting from visual loss [39] [40] [41] or abnormal electrical stimulation.[42] Visual hallucinations may result in isolated midbrain injury,[43] although the exact mechanism remains undetermined.

READING AND DYSLEXIA

Reading represents a very specialized form of visual processing. The unimpaired reader can recognize even sloppy and garbled writing. Multiple regions of the brain are involved, and injury to any of these areas produces recognizable syndromes.

The primary center for reading and writing language appears to be in the dominant angular gyrus in the parietal lobe.[44] Alexia (the inability to read) with agraphia (the inability to write) results from destruction of this area; exceptions to this rule do exist. Darius and Boller[45] reported a right-handed patient who suffered a form of alexia with agraphia following a right temporo-occipital hematoma. The patient could copy words but not read them and could not write from dictation or spontaneously. The authors postulated a double disconnection syndrome—alexia from disconnection of the right angular gyrus and occipital association areas by a subcapsular lesion and agraphia from disconnection of semantic stores in the right angular gyrus.

Alexia without agraphia results from disconnection of the dominant angular gyrus from input from both occipital lobes. [46] [47] [48] This syndrome most commonly results from an infarct in the distribution of the left posterior cerebral artery.[49]

The association of alexia with damage to the angular gyrus may represent a Western bias. Sakurai et al.[50] reported a patient who had alexia with agraphia for Japanese characters (kanji).

 

1300

The patient also suffered problems with names. Scans indicated an infarct involving the dominant inferotemporal and fusiform gyri from the temporo-occipital junction to the anterior one third of the temporal lobe. The authors postulated a disconnection of fibers to the parahippocampal region. They suggested that pictographs are processed differently from words composed of letters. They questioned whether a similar lesion might have an impact on reading irregular words in English.

Dyslexia represents a very special form of alexia.[51] By definition, patients with developmental dyslexia have a discrepancy between the acquisition of reading skills and other intellectual abilities, and this disability is not related to environmental conditions, sensory deficits, or acquired neurological disorders.

Investigators have failed to determine a specific site for the origin of developmental dyslexia. Livingstone et al. [52] proposed that developmental dyslexia results from deficiencies and depletion in magnicellular pathways. They reported decreased numbers of magnicellular neurons in the lateral geniculate nuclei from postmortem studies in dyslexic patients. They also identified decreased responses to low spatial–frequency patterns in patients who have developmental dyslexia.

Sadun[53] has challenged this hypothesis. He emphasized the small number of autopsies and amount of clinical material and questioned the statistical significance of any findings of the Livingstone study. The changes seen in the lateral geniculate nuclei may represent a physical abnormality that does not directly cause reading defects but is a secondary finding in patients who have central nervous system anomalies elsewhere. The definitive answer to an anatomical cause of developmental dyslexia awaits further clinical research.

Phonological dyslexia represents a disorder of association between sounds (phonemes) and letters (graphemes). Some evidence now exists that phonological dyslexia may result from abnormal development of the dominant inferior frontal lobe in areas responsible for the development of control for tongue and lip articulator movements.

COLOR PERCEPTION

The pathway for the interpretation of color remains separate from those responsible for object identification.[54] PET scanning findings indicate that the lingual and fusiform gyri become stimulated when a normal individual scans for a colored target. [55] [56] [57] Patients who have lesions that cause visual agnosia may maintain the ability to identify the color of objects.[58]

Patients who have acquired, central cerebral achromatopsia (inability to identify colors) may have complete loss or miss only one primary color.[59] The isolation of single color defects links with research performed in macaque monkeys, which showed that an area of prestriate cortex, identified as area V4, contains neurons that respond to specific color stimuli.[60] [61]

Patients with cerebral achromatopsia generally describe objects as “washed out” or “faded.” Patients still may be able to use contrast clues to separate the edge of one intense color from another. If two colors or a color and a shade of gray match pseudoisochromatically, however, patients demonstrate a distinct inability to isolate colored targets. Despite the achromatopsia, other parts of the parvicellular system may remain intact. Patients may have normal visual acuity and contrast sensitivity. Postmortem and radiological studies of these patients reveal bilateral lesions of the inferior occipital cortex.

INTEGRATION OF VISUAL–AUDITORY SPACE

The brain frequently receives contradictory information from the visual and auditory systems. For example, when an individual watches a movie, an image is seen directly ahead, but sounds are heard from numerous speakers throughout the theater. The brain integrates this information to provide a meaningful and logical integrated experience.

The ability to reconcile auditory and visual cues appears to be learned.[62] This reconciliation is a complicated task, because visual information is received by direct stimulation of an individual retina, while auditory localization requires a binaural triangulation of sound. The complexity heightens when the individual or the target moves.

The brain prioritizes visual input.[63] If a sound seems to originate from a seen object, the brain transfers the perception of that sound to the visible source. The process of correlation occurs in the midbrain tectum.

MOTION DETECTION

Once the brain identifies an object, it must localize that object in relation to the perceiver and the environment, and determine the relative rate of motion of the object to the perceiver. The observer also may be moving, and multiple environmental targets may be moving in different directions. The brain has developed efficient mechanisms to resolve these factors.

The primary step in motion detection involves neurons in area V1 of the calcarine cortex supplied by the magnicellular pathway. Motion-sensitive neurons react to movement in a specific direction.[64] The information from these individual neurons then travels to an area (referred to as MT or V5) in the medial temporal lobe. In primates, MT sits in the posterior segment of cortex, bordering the superior temporal sulcus. Almost 100% of the neurons in MT demonstrate directional sensitivity. Evidence suggests that MT represents the first area in which the information related to motion becomes attached to a texture, color, or pattern. [65]

Unfortunately, for simplistic views of motion detection, a moving object in the environment generates multiple bits of information in the MT region. Some bits may appear contradictory. The brain must integrate these signals to form one coherent, 3-dimensional representation of relative motion. Approximately 25% of neurons in MT do not react just to linear motion in a single direction but react to motion in multiple vectors. These cells may be responsible for motion integration.

Evaluation of clinical data supports this concept of a parallel pathway for motion detection. As far back as World War I, Riddoch[66] found patients who could detect motion of “invisible” targets placed within a supposedly absolute scotoma. Zihl et al. [67] may have reported the only patient known to suffer a complete, isolated loss of motion perception; the onset of symptoms corresponded to the development of a lesion in the temporal lobe that involved the area where MT would be located.

NONORGANIC VISUAL LOSS

Nonorganic (or functional) visual loss represents one of the most difficult challenges faced by ophthalmologists. Patients affected by organic disease demonstrate true concern about their condition, but patients with nonorganic complaints may go out of their way to confuse or mislead the examiner or, alternatively, show no concern for the problem.

Nonorganic visual loss represents a visual complaint that is not explained by physical examination or ancillary testing. Purported visual disturbances may vary from mild visual blurring or focal visual field defects to a complete loss of light perception. Nonorganic visual loss falls into one of two categories—conversion reaction or malingering.

Patients with a conversion reaction, previously called hysterical blindness, react to environmental stress. Adolescents seem particularly prone to this kind of response. By becoming “blind,” individuals may justify perceived or real failure as no fault of their own; if one cannot see, one cannot perform. Such individuals gain an apparent resolution of psychological conflict. Because a conversion reaction alleviates tension, patients generally show a flat, relaxed affect despite severe visual complaints. Patients with a conversion reaction appear to honestly believe they are disabled, even when initially confronted with

 

1301

evidence to the contrary. They tend to be cooperative with testing. By cooperating, these patients readily display behavior that contradicts their complaints.

Malingering patients mimic visual loss consciously to obtain an external secondary gain. Their visual complaints appear out of proportion to the underlying original injury. Such patients may seek medical advice at the behest of an attorney and have received coaching in advance. Malingerers pay close attention to the actions of an examiner and try to circumvent tests. Physicians must take great care, because any conclusions may require documentation for a judge and jury.

An accurate visual acuity assessment may not be obtainable for a patient who has nonorganic visual loss. Fortunately, all the examiner must do is to demonstrate that the patient’s vision is significantly better than stated.

To evaluate patients who have nonorganic visual loss, the examiner starts with the smallest letters possible, in most cases the 20/10 (6/3) line. The physician should pause at each letter and demonstrate concern and confusion that the patient cannot identify these letters. After making the point that the next letters are much larger, the examiner shifts to the 20/15 (6/4.5) line and repeats the process. By the time the patient looks at the 20/20 (6/6) or 20/25 (6/7.5) line, the power of suggestion has set in and the patient generally is convinced the letters are now large enough to read. This technique works well with complaints of either monocular or binocular visual loss.

A 4-diopter prism is an indispensable tool in the evaluation of the visual acuity of patients who have nonorganic monocular visual complaints. The examiner occludes the patient’s “bad” eye, then places the prism over the patient’s “good” eye such that the base is up and the apex splits the pupil. If the prism is positioned in just the right spot, the patient experiences monocular vertical diplopia. The examiner asks the patient if both of the perceived lines appear equally clear; the answer will be “yes.” Once the patient is certain that this test measures the function of the “good” eye, the tester simultaneously uncovers the “bad” eye and slides the prism downward to cover the “good” eye completely. The patient now experiences binocular diplopia but intellectually remains convinced of a monocular phenomenon. At this point, the patient often reads well down the eye chart without hesitation, even when asked to attend to the upper line that corresponds to the “bad” eye. A very accurate measure of visual acuity may be obtained for otherwise uncooperative patients.

The red–green eyeglasses provided with the Worth four-dot test may be useful. The examiner asks the patient to put on the glasses and then inserts the red–green filter installed in the vision chart projector. The patient sees the letters on the red half of the eye chart with the eye covered by the red lens and the letters on the green half with the eye covered by the green lens. The patient often progresses well down the eye chart before realizing over-achievement has occurred.

The red–green eyeglasses may be used with the Ishihara color plate series, as well. If a patient complains of poor vision in one eye, have the patient put on the red–green glasses with the green lens over the “good” eye and the red lens over the “bad” eye. Under normal circumstances, an individual can read the Ishihara numbers through the red lens but not the green lens. If the subject who has nonorganic complaints reads the numbers under the above circumstances, this discrepancy confirms better-than-stated ocular function.

Ophthalmologists often use phoropters and trial frames to confuse patients and obtain a measure of visual acuity. These methods generally fail, however, when patients are malingerers. In both circumstances, patients may close an eye surreptitiously and determine that these are tests of deception. Optokinetic nystagmus only helps ascertain that vision is grossly intact in each eye.

Perimetry remains an excellent tool for the evaluation of nonorganic complaints. Both confrontational and tangent screen techniques yield the best information, because they allow variable test distances. Patients who are determined to produce

 

 

Figure 196-10 Characteristic “tunnel” field of nonorganic visual field loss. Paradoxically, the visual field appears to expand as the patient approaches the screen. Such a pattern does not correspond to any known ocular or central nervous system lesion.

a factitious visual field defect may confound Goldmann and automated perimetry techniques easily, however.

The most common defect discovered during perimetry is a tunnel field. If a visual field is constricted because of organic disease, the absolute size of an isopter for a given test object increases as the distance from the screen increases. Patients who have tunnel fields, however, tend to have field constriction, but they always generate the same absolute size of an isopter on the tangent screen no matter what the test distance ( Fig. 196-10 ). The examiner may enhance this tendency by using large, easily discriminated pins to mark the edge of an isopter. Testing at two distances is necessary, however, because several medical conditions (end-stage glaucoma, end-stage papilledema, tapetoretinal degeneration, chiasmal compression, or bilateral occipital lobe infarcts) may produce authentic generalized constriction of the visual field.

The tangent screen may prove useful in another way for the evaluation of patients who have unilateral visual complaints. In one method the visual field is tested for the “good” eye and the location of the blind spot determined. The examiner then tests the “bad” eye and elicits the characteristically small tunnel field inside the blind spot. Finally, the physician evaluates the patient with both eyes open. Patients who have nonorganic complaints frequently lose the blind spot from the “good” eye, even though the claimed field for the “bad” eye was smaller than 10°. Occasionally, patients tested in this manner may yield totally inexplicable and impossible visual field changes under binocular conditions. For example, a patient who has a full field in one eye and a tunnel field in the other may report a hemifield loss on the side of the “bad” eye with both eyes open.

Should a patient claim severe bilateral vision loss and be noncompliant on perimetry testing, the examiner must take every opportunity to observe the patient’s behavior. If a call to the patient from a distance results in an accurate fix on the examiner’s location by the patient, a peripheral field much larger than stated is indicated. Patients who have small, bilateral tunnel fields may be able to maneuver easily without bumping into objects. Patients may pick up or take objects held well away to one side, which indicates they can see the objects. Finally, patients who feel no one is watching may perform tasks inconsistent with their level of claimed disability.

Tests for stereoscopic vision may assist the evaluation of patients who have nonorganic complaints. Most methods for stereopsis evaluation require good peripheral fields and good visual acuity in both eyes. Patients often become intrigued with the challenge of stereoacuity testing and perform at a level well beyond that claimed under other conditions.

The examiner may be able to use motility testing to advantage. If a patient complains of a tunnel field, the tester should

 

1302

evaluate saccades initially with two targets very close together. The examiner gradually increases the distance between the two targets and asks the patient to continue to make saccades back and forth. Because a saccade requires voluntary generation to a visible target, patients who have organic visual loss do poorly, but those with nonorganic complaints may perform well.

Appropriate evaluation of the pupils constitutes a critical part of the examination of patients who have nonorganic disorders. Asymmetrical visual acuity or field loss between the two eyes must result from intraocular disease or lesions of the optic nerves. Although intraocular disease may not cause an afferent pupillary defect, the examiner usually is able to identify the lesion visually. Unilateral optic nerve disease must produce a relative afferent pupillary defect on the side of the lesion. A patient who complains of markedly poor vision in one eye only, has normal ocular examination findings, and a normal response of the pupils to a light in the “bad” eye, is likely to have a nonorganic syndrome.

Ancillary testing, in certain circumstances, may prove helpful. If a patient generates complaints or findings that suggest a retrochiasmal lesion, imaging studies may be used to identify or eliminate such a lesion as a cause. Should the examiner need further documentation of optic nerve function, visual evoked response testing that gives a normal latency and amplitude essentially rules out organic disease as a cause of serious injury to the optic nerve. Malingering patients, however, may prove uncooperative and thwart the efforts of the electrophysiology technician.

Once the examiner has established a patient’s complaints as nonorganic, all appropriate findings must be documented carefully in the patient record. The physician must perform all critical tests in the presence of a reliable witness who can corroborate the results in a courtroom.

Patients who have a conversion reaction, in most instances, respond very favorably to a report of a healthy visual system. The physician must remember that these patients’ complaints stem from anxiety; assuaging that anxiety allows the patient to “recover” without stigma. Only in unusual circumstances does the patient require the assistance of a psychiatrist. The patient and family must understand that a conversion reaction represents an adaptation to stress; they must work to alleviate the cause of this stress to prevent the development of other somatic complaints.

Malingering patients react poorly to confrontation. Because they seek secondary gain, they immediately challenge anyone who claims they are lying. The physician approaches these patients supportively. A good approach is to state that, because the physician has not examined the patient previously, an organic lesion may have existed at one time. The examiner can then express concern and relief that the patient’s problem has resolved so well. By using this approach, the physician may avoid significant unpleasantness.

 

 

REFERENCES

 

1. Reese B. Clinical implications of the fibre order in the optic pathway of primates. Neurol Res. 1993;15:83–6.

 

2. Shatz CJ. Emergence of order in visual system development. Proc Natl Acad Sci U S A. 1996;93:602–8.

 

3. von Noorden GK, Middleditch PR. Histological observations in the normal monkey lateral geniculate nucleus. Invest Ophthalmol Vis Sci. 1975;14:55–8.

 

4. Casagrande VA. A third parallel visual pathway to primate area V1. Trends Neurosci. 1994;7:305–10.

 

5. van Buren JM, Baldwin M. The architecture of the optic radiation in the temporal lobe of man. Brain. 1958;81:15–40.

 

6. Meyer A. The connections of the occipital lobes and the present status of the cerebral visual affections. Trans Assoc Am Physicians. 1907;22:7–15.

 

7. Brodmann K. Vergleichende Localisationslehre der Grosshirnrinde in ihren Prinzipien Dargestelltg auf Grund des Zellenbaues. Leipzig: Barth; 1909.

 

8. Lund JS. Organization of neurons in the visual cortex, area 17, of the monkey. J Comp Neurol. 1973;147:455–96.

 

9. Hubel DH, Wiesel TN. Laminar and columnar distribution of geniculocortical fibers in the macaque monkey. J Comp Neurol. 1972;146:421–50.

 

10. Hubel DH, Wiesel TN. Sequence regularity and geometry of orientation columns in the monkey striate cortex. J Comp Neurol. 1974;158:267–94.

 

11. Kandel ER, Schwartz JH, Jessell TM. Principles of neural science. New York: Elsevier; 1991:431–4.

 

12. Benton S, Levy I, Swash M. Vision in the temporal crescent in occipital infarction. Brain. 1980;103:83–97.

 

13. Innocenti GM, Aggoun-Zouaoui D, Lehmann P. Cellular aspects of callosal connections and their development. Neuropsychologia. 1995;33:961–87.

 

14. Salin PA, Bullier J. Corticocortical connections in the visual system: structure and function. Physiol Rev. 1995;75:107–54.

 

15. Hubel DH, Wiesel TN. Functional architecture of macaque monkey visual cortex. Proc R Soc London Ser B. 1977;198:1–59.

 

16. Bressler SL. Interareal synchronization in the visual cortex. Behav Brain Res. 1996;76:37–49.

 

17. Derrington A. Vision: filling in and popping out. Curr Biol. 1996;6:141–3.

 

18. Smith JL. Homonymous hemianopia: a review of one hundred cases. Am J Ophthalmol. 1962;54:616–22.

 

19. Savino PJ, Paris M, Schatz NJ, et al. Optic tract syndrome: a review of 21 patients. Arch Ophthalmol. 1978;96:656–63.

 

20. O’Connor PS, Kaston D, Tredici TJ, et al. The Marcus Gunn pupil in experimental optic tract lesions. Ophthalmology. 1982;89:160–4.

 

21. Gunderson CH, Hoyt WF. Geniculate hemianopia: incongruous homonymous field defects in two patients with partial lesions of the lateral geniculate nucleus. J Neurol Neurosurg Psychiatry. 1971;24:1–6.

 

22. Pahwa JM. Homonymous hemianopias from lesions of parietotemporal lobes. Mediscope. 1963;5:543–7.

 

23. McAuley DL, Ross Russell RW. Correlation of CAT scan and visual field defects in vascular lesions of the posterior visual pathways. J Neurol Neurosurg Psychiatry. 1979;42:298–311.

 

24. Horton JC, Hoyt WF. The representation of the visual field in human striate cortex. A revision of the classic Holmes map. Arch Ophthalmol. 1991;109:816–24.

 

25. Davis PC, Newman NJ. Advances in neuroimaging of the visual pathways. Am J Ophthalmol. 1996;121:690–705.

 

26. Milner AD, Perrett DI, Johnson RS, et al. Perception and action in “visual form agnosia.” Brain. 1991;114:405–28.

 

27. Goodale MA. Perceiving the world and grasping it: is there a difference? Lancet. 1994;343:930–1.

 

28. Jakobson LS, Archibald YM, Carey DP, et al. A kinematic analysis of reaching and grasping movements in a patient recovering from optic ataxia. Neuropsychologia. 1991;29:803–9.

 

29. Ogden JA. Visual object agnosia, prosopagnosia, achromatopsia, loss of visual imagery, and autobiographical amnesia following recovery from cortical blindness: case MH. Neuropsychologia. 1993;6:571–89.

 

30. Mehta Z, Newcombe F, De Haan E. Selective loss of imagery in a case of visual agnosia. Neuropsychologia. 1992;30:645–55.

 

31. Goodale MA, Milner AD. Separate visual pathways for perception and action. Trends Neurosci. 1992;15:20–5.

 

32. Sargent J, Ohta S, MacDonald B. Functional neuroanatomy of face and object processing. Brain. 1992;115:15–36.

 

33. Farah MJ. Current issues in the neuropsychology of image generation. Neuropsychologia. 1995;11:1455–71.

 

34. Farah MJ, Wilson KD, Drain HM, et al. The inverted face inversion effect in prosopagnosia: evidence for mandatory, face-specific perceptual mechanisms. Vision Res. 1995;35:2089–93.

 

35. Ungerleider LG. Functional brain imaging studies of cortical mechanisms for memory. Science. 1995;270:769–75.

 

36. Gurd JM, Marshall JC. Drawing upon the mind’s eye. Nature. 1992;359:590–1.

 

37. Servos P, Goodale MA, Humphrey GK. The drawing of objects by a visual form of agnosia: contribution of surface properties and memorial representations. Neuropsychologia. 1993;31:251–9.

 

38. Behrmann M, Winocur G, Moscovitch M. Dissociation between mental imagery and object recognition in a brain-damaged patient. Nature. 1992;359:636–7.

 

39. Cogan DG. Visual hallucinations as release phenomena. Graefes Arch Klin Exp Ophthalmol. 1973;188:138–50.

 

40. Lance JW. Simple formed hallucinations confined to the area of a specific visual field defect. Brain. 1977;99:719–32.

 

41. Lepare FE. Spontaneous visual phenomena with visual loss: 104 patients with lesions of the retinal and neural afferent pathways. Neurology. 1990;40:444–7.

 

42. Brugger P, Agosti R, Regard M, et al. Heautoscopy, epilepsy, and suicide. J Neurol Neurosurg Psychiatry. 1994;57:838–9.

 

43. Howlett CC, Downie AL, Banerjee AK, et al. MRI of an unusual case of peduncular hallucinosis (Lhermitte’s syndrome). Neuroradiology. 1994;36:121–2.

 

44. Peterson SE, Fox PT, Posner MI, et al. Positron emission tomographic studies of the cortical anatomy of single-word processing. Nature. 1988;331:585–9.

 

45. Darius P, Boller F. Transcortical alexia with agraphia following a right temporo-occipital hematoma in a right-handed patient. Neuropsychologia. 1994;32: 1263–72.

 

46. Quint DJ, Gilmore JL. Alexia without agraphia. Neuroradiology. 1992;24:210–4.

 

47. Lichter C, Horber F. Transitory alexia without agraphia in an HIV-positive patient suffering from toxoplasma encephalitis: a case report. Eur Neurol. 1992;32:26–7.

 

48. Rentschler I, Treutwein B, Landis T. Dissociation of local and global processing in visual agnosia. Vision Res. 1994;34:963–71.

 

49. Geschwind N. Disconnection syndromes in animals and man. I and II. Brain. 1965;88:237–94, 585–644.

 

50. Sakurai Y, Sakai K, Sukuta M, et al. Naming difficulties in alexia with agraphia for kanji after a left posterior inferior temporal lesion. J Neurol Neurosurg Psychiatry. 1994;57:609–13.

 

51. Heilman KM, Voeller K, Alexander AW. Developmental dyslexia: a motor–articulatory feedback hypothesis. Ann Neurol. 1996;39:407–12.

 

52. Livingstone MS, Rosen GD, Drislane FW, et al. Physiological and anatomical evidence for a magnocellular defect in developmental dyslexia. Proc Natl Acad Sci U S A. 1991;88:7943–7.

 

53. Sadun AA. Dyslexia at The New York Times: (mis)understanding of parallel visual processing. Arch Ophthalmol. 1992;110:933–4.

 

54. Humphrey GK, Goodale MA, Jakobson LS. The role of surface information in object recognition: studies of a visual form agnosic and normal subjects. Perception. 1994;23:1457–81.

 

55. Corbetta M, Miezin FM, Dobmeyer S, et al. Selective and divided attention during visual discriminations of shape, color and speed: functional anatomy by positron emission tomography. J Neurosci. 1991;11:2383–402.

 

56. Zeki S, Watson JDG, Lueck CJ, et al. A direct demonstration of functional specialization in human visual cortex. J Neurosci. 1991;11:641–9.

 

1303

 

 

57. Gulyas B, Heywood CA, Popplewell DA, et al. Visual form discrimination from color or motion cues. Functional anatomy by positron emission tomography. Proc Natl Acad Sci U S A. 1994;91:9965–9.

 

58. Schnider A, Landis T, Regard M, et al. Dissociation of color from object in amnesia. Arch Neurol. 1992;49:982–5.

 

59. Heywood C, Cowey A, Newcombe F. On the role of parvocellular (P) and magnocellular (M) pathways in cerebral achromatopsia. Brain. 1994;117:245–54.

 

60. Kennard C, Lawden M, Morland AB, et al. Colour identification and colour constancy are impaired in a patient with incomplete achromatopsia associated with prestriate cortical lesions. Proc R Soc London. 1995;260:169–75.

 

61. Rizzo R, Nawrot M, Blake R, et al. A human visual disorder resembling area V4 dysfunction in the monkey. Neurology. 1992;42:1175–80.

 

62. Miyashita Y. Neuronal correlate of visual associative long-term memory in the primate temporal cortex. Nature. 1988;335:817–20.

 

63. Knudsen EI, Brainard MS. Creating a unified representation of visual and auditory space in the brain. Annu Rev Neurosci. 1995;18:19–43.

 

64. Albright TD, Stoner GR. Visual motion perception. Proc Natl Acad Sci U S A. 1995;92:2433–40.

 

65. Braddick O. Seeing motion signals in noise. Curr Biol. 1995;5:7–9.

 

66. Riddoch G. Dissociation of visual perception due to occipital injuries, with especial reference to appreciation of movement. Brain. 1917;40:15–57.

 

67. Zihl J, von Cramon D, Mai N. Selective disturbance of movement vision after bilateral brain damage. Brain. 1983;106:313–40.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: