Question

Janine was diagnosed with multiple sclerosis. Write, in your own words, why her visual evoked potential test was abnormal but her NCV test was normal.

Answer 1

Multiple sclerosis (MS) is a central nervous system disorder caused by destruction of myelin sheath due to an autoimmune attack. Although, it predominantly affects the myelination along the CNS axons, it can affect myelination in PNS axons. This peripheral demyelination, results in peripheral neuropathy in few patients (around 5% of MS patients)

Nerve conduction velocity (NCV) is a test that detects whether the conduction of nerve impulses along the neuron is normal or not. If the NCV test shows changes, then there are chances of peripheral neuropathy. This association is possible because of epitope spreading that occurs during MS longer course. If NCV test is normal, it indicates that MS has only affected the CNS axons.

The visual evoked potentials (VEP) are tests measuring the time taken by the brain to respond to signals sent by the eyes. This test detects the activity of sensory neurons as evoked potentials created by visual stimulation are recordings of CNS activity.

In the case of the patient, MS has been recently detected. Hence, the chances of peripheral neuropathy are reduced. The normal NCV test and abnormal VEP test indicates that demyelination has occurred only in CNS neurons. There is very less chance that the PNS involvement is present..