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Evoked Potential Responses

Definition: 

Evoked Potential Responses are minute electrical signals generated by the brain and spinal cord when transmitting and processing responses to sensory stimuli. These sensory stimuli may be something seen (visual), heard (auditory) or felt (somatosensory). Since these electrical signals are very small they are normally obscured by random electrical activity in the environment. In order to measure them dozens of responses are measured sequentially and then averaged together. The random electrical signals tend to cancel each other out, leaving the evoked response to be seen and measured. Delays or reductions in these responses help define or locate any problem in the system of nerves that transmit and process the responses. Visual evoked responses are usually elicited by having the subject look at an alternating checkerboard pattern on a TV screen and measuring a response from an electrode on the scalp applied with a water soluble paste. Typical testing time is 1 hour. Auditory evoked responses are measured with repeated clicking noises in the ears from headphones, also measured from an electrode on the scalp. Typical testing time is 30-45 minutes. Somatosensory evoked responses are usually measured from a series of brief electrical shocks in the arms and legs and may be measured from the scalp, at points along the spine, or both. Testing time is usually about 2 hours.

What’s it used for? 

Depending on the sensory system being tested (visual, auditory or somatosensory) this test can be used to eliminate or help to confirm many different problems. These include small tumors on the nerve that transmits hearing, injuries to the spinal cord or brain, or problems with the nerves that transmit sight. They are also sometimes used together as a screening test for multiple sclerosis.

Preparation

If somatosensory evoked potentials are to be measured the arms and legs should be clean and easily accessible. For all three test types it is best if the scalp is clean so that the electrodes adhere. Make sure to bring your glasses or contacts if you wear them. Apart from this there is no particular preparation needed. After the test you should wash out the electrode paste that is used to apply the electrodes.

Are their side effects? 

These tests are usually well tolerated. The somatosensory evoked potentials can be somewhat uncomfortable due to the repetitive tiny shocks given which usually cause the arms and legs to twitch. Apart from this transient discomfort there are no side effects.

Where is it done? 

These tests are usually done in electrophysiology laboratories in hospitals of physician’s offices often in conjunction with electromyography or electroencephalography testing.

R. O'Brien MD