Visual evoked potentials are useful in evaluating the function of the anterior visual pathways. They are not useful in evaluating lesions posterior to the optic chiasm.
In patients with an acute optic or retrobulbar neuritis, the P100 response is initially lost; with time, it recovers and is then found to have a markedly prolonged latency that generally persists indefinitely (see Fig 24-12 ),y , y even if apparently complete clinical recovery occurs. The VEP is therefore an important means of evaluating patients with suspected optic neuritis and may be delayed even when magnetic resonance imaging of the optic nerve is normal. y Abnormalities may relate to either recent or long-standing pathological processes. The VEP may be abnormal in patients without a past history of optic neuritis, thereby providing evidence of subclinical involvement of the optic nerve. For this reason, it is useful to record the VEP in such patients when multiple sclerosis is a diagnostic possibility. y
Other disorders may also lead to VEP abnormalities. In particular, ocular pathology (e.g., refractive error, inability to focus on the pattern stimulus, or glaucoma) may be responsible. Compressive, ischemic, toxic, or nutritional optic neuropathies may also produce VEP changes, and abnormalities occur in Leber's hereditary optic atrophy.y , y Compressive lesions of the optic nerve typically lead to VEPs that are markedly abnormal in shape as well as delayed in latency y ; in ischemic or toxic neuropathies, the response is usually markedly attenuated in amplitude without being delayed significantly. y The findings, however, have always to be interpreted in the clinical context in which they were obtained.
The VEP findings are of little help in the evaluation of lesions posterior to the optic chiasm. It has been suggested that VEPs can be used to evaluate the visual fields, but the approach is time consuming, requires the close cooperation and attention of patients, and is no better or even less sensitive than standard tests of perimetry.y , y In patients with cortical blindness, the VEP may be normal or abnormal--the presence of a normal response should therefore not lead, in itself, to a diagnosis of nonorganic visual loss. y
The pattern-elicited VEP can be used to measure refractive error or detect amblyopia in preverbal children who are unable to cooperate for behavioral testing. y For refractive purposes, the VEP is recorded while different lenses are placed in front of the eye to determine the lens with which the VEP to small (15 minute) checks is of largest amplitude and shortest latency; this lens will be the optimal one for the patient. By an analogous approach, the interocular difference in size of the VEP elicited by small checks is important as a means of detecting unilateral amblyopia at an early age, when recovery may still be possible.
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