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Visually Evoked Response

The Visually Evoked Response test (VER), also known as the Visually Evoked Potential test (VEP), is a test for Optic Neuritis or other demyelinating events along the Optic Nerve or further back along the optic pathways.

The test involves watching a black and white checkered pattern on a TV screen in a darkened room. The black and white squares alternate on a regular cycle which generates electrical potentials along the optic nerve and into the brain. These can be detected with electroencephalographical (EEG) sensors placed at specific sites on the top of the head (the occipital scalp). Each eye is tested independently while an eye patch is worn on the other eye.

VEPs are very sensitive at measuring slowed responses to visual events and can often detect dysfunction which is undetectable through clinical evaluation and the person is unaware of any visual defects.

Because of their ability to detect silent lesions and historic demyelinating episodes, they are very useful diagnostic tools. A definite diagnosis of multiple sclerosis requires at least two distinct demyelinating episodes, in two different central nervous system sites which are separated by at least one month (the Schumacher criteria). VEPs can often provide evidence of such episodes when other tests, even MRI, cannot.

Each pattern reversal stimulates a nerve transmission along the optic nerve, the optic chiasm and the optic tract to the lateral geniculate body. From there a second axonal signal moves along the optic radiations and the posterior periventricular white matter to the occipital cortex. This stimulates the striate cortex to generate a large electrical potential which is detectable on the EEG sensors.

The whole trip (the latency) normally takes about 100 milliseconds. This latency is called the P100. White matter lesions anywhere along this pathway will slow or even stop the signal.

Almost everyone with optic neuritis will have an abnormal VEP latency. Studies show that 85-90% of people with definite MS and 58% of people with probable MS have abnormal VEPs.

Other conditions including Friedreich's ataxia, vitamin B12 deficiency and neurosyphilis can also slow the P100 latency.

Visually Evoked Response Refs:

Nuwer, MR "Evoked Potentials in Multiple Sclerosis" (1997)
published in Multiple Sclerosis: Clinical and pathogenic basis ISBN 0 412 30890 8

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