Visual evoked potentials

Visually evoked potentials (VEP) are changes in the electrical activity of the brain and other parts of the nervous system, induced by a brief visual stimulus. The examination determines the speed of nerve signals along the nerve pathways.

By default, the brain's reactions are almost immediate, in the event of a malfunction of the nerve pathways, the transmission slows down. Visual evoked potentials belong to auxiliary diagnostic methods in neurology, they are typically used to diagnose demyelinating diseases (e.g. multiple sclerosis ) and nerve damage (especially optic nerve).

History
Visual evoked potentials induced by a light pulse were first observed in the 1930s during the first encephalographic measurements.

VEPs can be observed in the background of the encephalographic recording and are extracted from it using a simple program. This process is called signal averaging. A computer program capable of this function was first demonstrated by George Dawson in 1951. The program is set up to respond only to stimuli that arrived at a certain, precisely defined interval after the signal (this way we filter out the "noise").

Investigation
Visual evoked potentials are an examination method that does not restrict the patient dietary or medically. The patient should wash his hair at least a day before the examination and not wear any metal objects or clips in his hair. The patient is seated in a chair, the assistant places electrodes on the patient's head (it is important to degrease the patient first).

The reference electrode is placed on the ear lobe or on the forehead, three standard electrodes are placed on the mastoid process or on the nape of the neck. The examinee observes visual stimuli, commonly used stimuli include a stroboscopic lamp, LEDs, or a so-called structured stimulus (pattern reversal, this is a black-and-white checkerboard that flashes in a certain period of time).

Each eye is always examined separately. Each eye is usually examined twice. The most frequently used frequencies range from 1 to 100 Hz. The time scale is 200–250 ms, higher in children under 5 years. The examination of children under three years of age is performed during mild anesthesia, as children of this age have a problem with maintaining concentration).

Usage
Any abnormality discovered using visual evoked potentials may be symptomatic of a disorder, but it is never appropriate to make a diagnosis based on this examination alone. The result must be put into context with the conclusion of other examinations, and only then it is appropriate to determine which disorder of the nervous system is involved.

Abnormality can be caused by:


 * meningitis, encephalitis (can cause cortical blindness),
 * optic atrophy,
 * heart attack
 * multiple sclerosis (VEPs of a patient affected by this diagnosis have a lower frequency),
 * compression of neural pathways caused by tumor, neurofibroma, hematoma (these VEPs have smaller amplitudes)

Youtube demos
Course of VEP examination - video demonstration

Resources
Visually Evoke Potentials by Donnell J. Creel

The Johns Hopkins Consumer Guide to Medical Tests