Do not forget the visual evoked potentials (PEV)


A flash of light, which is projected on the retina, produces an electrical response on the visual cortex of the occipital lobe, a phenomenon known since a century. A chessboard that reverses its white and black pattern also produces a cortical response that is measurable (visual evoked potentials or PEV). The PEV are performed with the patient sitting 70-100 cm in front of a screen with a visual angle of 30 seconds of arc; the chessboard changes the color pattern at a frequency of 1-2 Hz (every 500 milliseconds) and 200 responses are recorded for each eye by electrodes applied to the occipital region of the scalp. The examination consists of stimulating one eye at a time (after measuring the visual acuity) and can show if there is a delay of the nerve conduction (from the retina up to the occipital lobe, through the optic nerve , the optic chiasma, the lateral geniculate body and the optical radiations up to the area 17 of the visual cortex). This examination is particularly useful for identifying a pathology of the optic nerve even when the visual function is good, and there is no evidence of abnormalities of the ocular fundus or the visual fields. The examination is therefore needed for detecting demyelinating lesions in case of multiple sclerosis when the diagnosis is uncertain. In this case there is a delay of the P100 wave. The PEV would be useful for investigating all the optic neuritis (toxic causes, ischemic neuropathy, hereditary Leber neuropathy) and also for glaucoma in search of a concomitant impairment of the optic nerve. The normality of the PEV suggests, in the case of blindness, a psychogenic origin.
Although MRI examination can easily demonstrate the existence of lesions on the visual pathways, the PEVs are still a very useful test for measuring the optic nerve function and should be used to monitor the evolution of the optic neuritis. We perform the PEV in our neurology center (http://www.neurolacote.ch). The examen is refunded by the health insurance.

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