Evaluation Guidelines Table82

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Neuroimaging. Most patients with suspected optic neuropathies should undergo neuroimaging to exclude a compressive

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TABLE 8-2 -- USEFUL STUDIES IN T

HE EVALUATION OF DISO

RDERS OF CRANIAL NERVE II AND AFFERENT VISUAL PATHWAYS

Syndrome

Neuroimaging

Electrophysiology

Fluid and Tissue Analysis

Neuropsychological Tests

Other Tests

Retina

MRA: carotid occlusion, stenosis, or dissection

Abnormal electroretinogram if photoreceptors are affected

Elevated ESR in giant cell arteritis

N/A

Vascular occlusion or leakage on fluorescein angiography

Thromboembolic source on cardiac echography or carotid ultrasound

Optic nerve

MRI with gadolinium and fat saturation, coronal views: optic nerve enlargement or enhancement, or compressive mass lesion

Visual evoked potentials: decreased amplitude or increased latency

Elevated ESR in giant cell arteritis. CSF pleocytosis if inflammatory

N/A

N/A

Optic chiasm

MRI with gadolinium, thin coronal and sagittal cuts through sella: chiasmal enlarglement or enhancement, or sellar compressive mass

N/A

CSF pleocytosis if inflammatory, endocrine studies showing evidence of hypopituitarism or hormone hypersecretion

N/A

N/A

Optic tract

MRI: compressive mass

N/A

N/A

N/A

N/A

Lateral geniculate body

MRI: infarction

N/A

N/A

N/A

N/A

Optic radiations

MRI: infarction or tumor MRA: carotid or MCA occlusion

N/A

N/A

Deficits in spatial ability and attention

Thromboembolic source on cardiac echography or carotid ultrasound

Occipital lobe

MRI: infarction or tumor

MRA: posterior circulation occlusion

N/A

N/A

N/A

Thromboembolic source on cardiac echography

Higher cortical areas

MRI: infarction or tumor

N/A

N/A

Deficits in color perception or object or face recognition

Thromboembolic source on cardiac echography

Visual hallucinations

MRI: infarction or tumor

Epileptiform discharges on EEG if hallucinations are due to seizures

N/A

N/A

N/A

Other positive visual phenomena

MRI: infarction or tumor

Epileptiform discharges if positive phenomena are due to seizures

N/A

N/A

N/A

MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; ESR, erythrocyte sedimentation rate; EEG, electroencephalogram; CSF, cerebrospinal fluid; N/A, not applicable.

MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; ESR, erythrocyte sedimentation rate; EEG, electroencephalogram; CSF, cerebrospinal fluid; N/A, not applicable.

or inflammatory process. The major exceptions are typical optic neuritis (although magnetic resonance imaging [MRI] scan may be appropriate to determine the prognosis for developing subsequent multiple sclerosis) or classic anterior ischemic optic neuropathy. Transient visual loss in the setting of migraine does not require neuroimaging when the historical features are characteristic and results of the neurological examination are normal. All patients with chiasmal and retrochiasmal patterns of visual loss should undergo imaging examination.

In general, MRI, with and without gadolinium, is the preferred technique for the evaluation of patients with suspected lesions of the afferent visual pathway. [d , y In addition to the usual brain studies, some special instances require further neuroimaging. For instance, when an optic nerve process is suspected, MRI of the orbits with fat

saturation and coronal views should be obtained. The chiasm is best imaged with additional thin cuts, coronal and sagittal, through the sellar area. Vascular disturbances, whether occlusive or aneurysmal, require MRI-angiography, and then in some instances, formal angiography. Computerized tomography (CT) is helpful when fractures, bony erosion, or calcification is suspected, as in meningiomas or craniopharyngiomas.

Some experimental functional neuroimaging techniques may be helpful in cases when MRI or CT evaluation is inconclusive. Using positron emission tomography (PET) or single-proton emission computed tomography (SPECT) techniques, hypoperfusion in visual association areas may be demonstrated in patients with visual agnosias, achromatopsia, and deficits in motion perception, for example. y Functional MRI can highlight areas of cortical activation by detecting small changes in local blood flow. Functional MRI technique may be more helpful for localization of normally functioning areas of brain.

Amaurosis fugax, central retinal artery occlusion, and ischemia in the middle and anterior cerebral artery distributions necessitate Doppler examination or MRI-angiography of the carotids. Echocardiography should be performed when a cardiac embolus or patent foramen ovale is suspected.

Electrophysiology. Other diagnostic tests used in combination with the clinical examination include electrophysiologic testing such as an electroretinogram (ERG) or visual evoked potential (VEP). ERGs measure rod and cone photoreceptor function and help to distinguish the retinal degenerations and dystrophies. VEPs measure the cortical activity in response to flashes of light or checkerboard stimuli and if abnormal suggest a lesion of the afferent visual pathway. VEPs, when the result is normal, are particularly helpful in excluding organic lesions when functional visual loss is suspected. In addition, they can detect clinically silent lesions in the afferent visual pathway of patients with multiple sclerosis. VEPs should never take the place of the clinical examination and field testing, however, in the diagnosis or follow-up of patients with optic neuropathies or chiasmal disturbances. When cardiac source emboli are suspected, arrhythmias should be excluded by telemetry or Holter monitoring.

Fluid and Tissue Analyses. Blood studies are indicated when specific pathological processes are suspected. Vascular events may require evaluation of the erythrocyte sedimentation rate (ESR), rapid plasmin reagin (RPR), antinuclear antibody (ANA), and coagulation indices such as prothrombin time, partial thromboplastin time (PT and PTT), and platelet count. The ESR is especially important in transient or permanent visual loss in the elderly to rule out giant cell arteritis. In young individuals without an obvious risk factor for stroke, protein C, protein S, antithrombin III, antiphospholipid antibody, and anticardiolipin antibody levels should be obtained. A toxicology screen is necessary if drug use is suspected. Most intracranial mass lesions require either at least brain biopsy if a primary neoplasm is suspected, or a metastatic evaluation if multiple metastases are more likely. In many cases of demyelinating disease, other autoimmune, inflammatory, and infectious disorders should be excluded by evaluating the ANA titer result, angiotensin converting enzyme (ACE) level, Lyme disease titer, and ESR, for instance.

When visual loss due to a sellar process is suspected, pituitary and hypothalamic function should be evaluated. The basic outpatient endocrinological panel should include serum prolactin, growth hormone (GH) (after a 75-g oral glucose load), insulin-like growth factor-I, adrenocorticotropic hormone (ACTH), cortisol (serum morning sample or 24-hour urine free cortisol), thyroid-stimulating hormone (TSH), T3, T4, luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol, and testosterone level. Low values, and even normal ones in some instances, are consistent with hypopituitarism. High values imply pituitary hypersecretion by a pituitary adenoma.

Cerebrospinal Fluid. In some instances, CSF examination may aid in establishing the cause of visual loss. Inflammatory and infectious processes require CSF analysis (cell count, protein, glucose, Gram's stain, and culture [bacterial, fungal, and viral when indicated]) when suspected. Carcinomatous or lymphomatous meningitis may require CSF cytology testing, including flow cytometry if available.

When pseudotumor cerebri is suspected in an individual with papilledema and normal neuroimaging results, a lumbar puncture (LP) is necessary to rule out meningitis, for example, and to document the CSF opening pressure. The lumbar puncture should be performed with the patient relaxed in a lateral decubitus position, with the head and spine at the same level and the neck and knees slightly flexed. To establish the diagnosis of pseudotumor cerebri, the CSF opening pressure should exceed 250 mm H2 O, which is the upper limit of normal for most obese and nonobese people. y Approximately 20 to 30 ml of CSF can be removed, although the optimal amount has not been studied. In suspected cases with a normal CSF opening pressure, monitoring for 1 hour with an epidural transducer or subarachnoid bolt may be considered,y but such a procedure is rarely done in clinical practice. The cell count and glucose levels should be normal, and the protein level normal or low.y

LPs are not necessary in the evaluation of patients with typical isolated optic neuritis, because they rarely change the diagnosis and do not add to information garnered from an MRI in predicting the future development of multiple sclerosis. y The protein and glucose levels are usually normal, and only one third of patients have a pleocytosis that is between 6 to 27 white blood cells (WBC)/mm y and rarely ever higher. Myelin basic protein can be detected in about one fifth of patients and IgG synthesis in about two fifths. Oligoclonal banding occurs in approximately half of these patients and is associated with the future development of clinically definite multiple sclerosis. Oligoclonal banding usually occurs in the presence of white matter lesions on MRI, however, which has been found to be highly correlated with the future evolution of multiple sclerosis. Therefore, oligoclonal banding adds little to the prognosis when neuroimaging, which is less invasive, has already been performed.y

Neuropsychological Testing. Neuropsychological assessment of higher cognitive functions can be used as an adjunct to the neuro-ophthalmic examination. By combining the disciplines of neurology and cognitive psychology, it is possible to analyze systematically the subcomponents

of complex cognitive abilities or "information processing." The general functions that may be analyzed in detail include attention, speech and language, memory (short- and long-term, verbal and visual), visuospatial and perceptual functions (neglect, construction, praxia, and agnosia), and specific thought processes that include calculations, abstractions, judgment, and problem solving. y

Other Tests. Color stereo disc photographs aid frequently in the management of patients with optic disc swelling and pseudopapilledema due to disc drusen, for instance. Fluorescein angiography highlights the choroidal and retinal vasculature and involves intravenous injection of an aqueous fluorescein solution. Funduscopic pictures are taken before injection and at specific intervals after injection. This method detects vascular occlusion, abnormal retinal pigmentation or hemorrhages, and disturbances of the retinal pigmented epithelium. Truly swollen discs leak fluorescein, whereas, in general, discs with pseudopapilledema do not.

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