Visual Evoked Potential
The VEP tests the function of the visual pathway from the retina to the occipital cortex. It measures the conduction of the visual pathways from the optic nerve, optic chiasm, and optic radiations to the occipital cortex. (The author has assumed that the reader is familiar with the anatomy of the optic system.) The most important fact to consider is that, although the axons from the nasal half of the retina decussate at the optic chiasm, the temporal axons do not. Therefore, retrochiasmatic lesions may not be detected by full-field checkerboard stimulation. VEPs are most useful in testing optic nerve function and less useful in postchiasmatic disorders. In retrochiasmatic lesions, the MRI is a more useful test. Partial-field studies may be useful in retrochiasmatic lesions; however, they are not performed routinely in clinical settings. Also note that the macula projects to the occipital pole, while the rest of the retina projects to the mesial calcarine cortex. The VEP is very useful in detecting an anterior visual conduction disturbance. However, it is not specific with regard to etiology. A tumor compressing the optic nerve, an ischemic disturbance, or a demyelinating disease may cause delay in the P100; only additional clinical history and, often, MRI are needed to uncover the etiology. The usual waveform is the initial negative peak (N1 or N75), followed by a large positive peak (P1 or P100), followed by another negative peak (N2 or N145). Maximum value for P100 is 115 milliseconds (ms) in patients younger than 60 years; it rises to 120 ms thereafter in females and 125 ms in males. Even though published norms are available in the medical literature, each individual laboratory should have its own norms to control for lab-to-lab variability in technique.
The W morphology, in the author's experience, is most often an individual variation, although decreasing the stimulation frequency from the ubiquitous 2 Hz to 1 Hz usually converts the W shape into a conventional P100 peak. Check size and alternation rate are factors in this; the responses can be manipulated to a W or a conventional P100 response by changing these parameters. Large checks tend to produce VEPs similar to those produced by flash stimulation.
Factors influencing VEP
The usual VEPs are evoked by checkerboard stimulation and, because cells of the visual cortex are maximally sensitive to movement at the edges, a pattern-shift method is used with a frequency of 1-2 Hz. The size of the checks affects the amplitude of the waveform and the latency of the P100. In addition, pupillary size, gender, and age all affect the VEP. Visual acuity deterioration up to 20/200 does not alter the response significantly; large checks may be required. In some studies, women have slightly shorter P100 latencies. Sedation and anesthesia abolish the VEP. Some subjects, by "fixating" beyond the plane of stimulation, may alter or suppress P100 altogether.Certain drugs, such as carbamazepine, prolong VEPs. The effects of carbamazepine and sodium valproate monotherapy on VEPs were studied in 18 epileptic children by Yuksel et al. Pattern-reversal VEPs were determined before administration of the antiepileptic drugs and after 1 year of therapy. The VEP amplitude showed no consistent changes after 1 year of therapy, but VEP P100 latencies were significantly prolonged after 1 year of carbamazepine therapy. The conclusion was that carbamazepine slows down central impulse conduction.
According to Trip et al, atrophy of the optic nerve was correlated with decreased VEP amplitude (Trip, 2006).