An evoked potentials test measures the speed of the messages along your sensory nerves to the brain. Evoked potentials tests are sometimes used in the diagnosis of MS, because they are painless, non-invasive and faster than MRI scans. Although MRI scans are more commonly used, there is still a role for evoked potentials tests, particularly at the earlier stages 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 that you listen to 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.