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Evoked potentials

An evoked potentials test measures the speed of nerve messages along sensory nerves to the brain and is sometimes used in the diagnosis of MS.

Your brain produces electrical current in response to information that comes in through your senses. This current can be detected on your scalp, using electrodes attached with sticky pads. As damage to the nerves in MS can slow down the transmission of nerve signals, evoked potential tests can indicate nerve pathways that are damaged, even before you notice any clinical symptoms. Delays of as little as 10 milliseconds can indicate that there is damage to the nerve pathway.

The most commonly used test is called visual evoked potentials (VEP), which measures how long it takes the brain to respond to messages sent by the eyes. Your neurologist may also suggest running an auditory evoked potential test which tests your hearing using clicks delivered through headphones. Another alternative is a somatosensory evoked potential, which tests your skin sensation using tiny electrical shocks. 

If you have a VEP test, you will be shown a flashing chessboard pattern on a computer screen. The electrodes on your scalp will detect brain activity, and be synchronised with the changing visual pattern on the screen to detect any delay in responding. You don't have to do anything except stay alert and look at the screen. The test will be repeated for each eye in turn, and the whole process should take around 30-45 minutes. 

Evoked potentials tests are painless, non-invasive, cheaper and faster than MRI scans, and in some cases can be more sensitive, such as investigating early damage to the brainstem. MRI scans are much more commonly used at present, although there is still a role for evoked potential tests in some circumstances. Recent research suggests that a set of evoked potential tests, analysed in a particular way, might help to predict disability levels over the longer term.

What do I need to do for an evoked potentials test?

Performance on an evoked potential test is affected by fatigue, so try to get plenty of rest before you go in for your test.

  • Wash your hair the night before, and don't use any hair products on it on the day of the test.
  • You will need to remove all jewellry, braids and hair clips for the test.
  • You can normally eat and take your regular medication before an evoked potentials test, but you may be asked to delay taking medications that make you drowsy or sleepy until after the test.
  • If you wear glasses, don't forget to take them along.

Last updated: July 2018
Last reviewed: June 2018
This page will be reviewed within three years

More references

  • Walsh P, et al. The clinical role of evoked potentials. Journal of Neurology, Neurosurgery and Psychiatry 2005;76 Suppl 2:ii16-22. Full article (PDF, 257KB)
  • Kraft, GH Evoked Potentials in Multiple Sclerosis Physical Medicine and Rehabilitation Clinics of North America Volume 24, Issue 4, November 2013, Pages 717-720 Summary
  • Pokryszko-Dragan, A et al. Assessment of visual and auditory evoked potentials in multiple sclerosis patients with and without fatigue Neurol Sci. 2015; 36(2): 235–242. Full article
  • Schlaeger, R et al. Combined visual and motor evoked potentials predict multiple sclerosis disability after 20 years Multiple Sclerosis Journal 2014, Vol. 20(10) 1348 –1354 Summary

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