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Evoked potentials can give important information about the brain and spinal cord, and about the nerve pathways leading to it from the eyes and ears.

If light is flashed in the eye or a small electrical pulse given to the skin over a nerve in an arm or leg, a characteristic response - the evoked potential - can be
recorded from the brain using electrodes placed on the scalp. There will be a very short delay - measured in fractions of a second - between the delivery of the stimulus and the appearance of the electrical response in the brain. This delay corresponds to the time that it takes for the signal to pass from the eye or skin to the brain, along the nerve pathways. If there is a delay in the appearance of the evoked potential in the brain, this may mean that something is wrong somewhere in the nerve pathways.

For example, if there is a delay in the appearance of the response over the scalp after a light is flashed in one eye, this may be due to disease affecting the optic nerve - the large nerve connecting the retina at the back of the eye with the brain. Similarly, if there is a delay in the appearance of the response in the brain after a small electrical pulse is applied over a nerve in a leg, there may be problem a with the spinal cord.

Delays of this kind may be produced by a wide variety of different problems - disease within the optic nerve or spinal cord itself, or tumours pressing on these structures from outside them and so on. In the past, evoked potentials were most commonly used in the diagnosis of multiple sclerosis. This is a disease of the central nervous system in which there is loss of the fatty insulation (“myelin sheath”) around the nerves, causing them to malfunction.

Loss of this fatty insulating sheath (demyelination) causes a delay in the conduction of signals along the nerve pathway, and this will be seen as a delay in the appearance of the evoked potentials at the scalp if the affected nerve pathway is stimulated. Evoked potential testing will also reveal whether the optic nerve, the brainstem and the spinal cord have been affected by the disease.

Magnetic resonance imaging (MRI) has to a large extent taken over the role that evoked potentials used to have in the diagnosis of multiple sclerosis. Nevertheless, evoked potential testing may still be used if there is any doubt about the interpretation of the MRI scans.

My doctor has sent me for evoked potential testing. Does this mean that he thinks I have multiple sclerosis?
If you are having evoked potential testing it doesn’t necessarily follow that your doctor thinks you have multiple sclerosis. The technique can be used to investigate many other types of disorder affecting the central nervous system. Also, it is common practice in medicine to carry out tests to try and rule out conditions that could possibly cause a person’s symptoms on the way to making an accurate final diagnosis.

If you are concerned about why you are being sent for a particular test, it is always worth simply asking your doctor.

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Last updated 26 June 2009