Evoked potential (EP) tests are one way of helping clinicians diagnose multiple sclerosis (MS). In MS, the immune system attacks the myelin, the fatty substance that surrounds and protects the nerve cells, causing lesions (called demyelination).
EP tests measure the electrical activity of the brain in response to hearing, sound or sight stimulus. These tests can detect the speed of the impulse that passes through the nerves and are sensitive enough to detect lesions even before they show up on neurological exams or symptoms become obvious.
Besides MS, EP tests can be used to confirm other conditions such as brain tumors, acoustic neuroma (small tumors in the inner ear), and spinal cord injuries.
How evoked potential tests are done
EP tests are painless, risk-free, and usually performed in a doctor’s office or in a hospital.
Two sets of electrodes are used in the test. One is placed on your scalp to measure the electrophysiological response to stimuli and the other is placed on the part of the body to be tested. The doctor records how long it takes the electrical impulse generated by different stimuli to reach the brain. In normal conditions, this signal transmission is instantaneous.
The entire test takes about 45 minutes and results are interpreted by a neurologist or a neurophysiologist who specializes in these tests.
Evoked potential tests and multiple sclerosis
There are three different types of EP tests that have been used to help to diagnose MS.
- Visual evoked potentials (VEP). This test detects loss of vision from optic nerve damage. The patient is seated in front of a screen and focuses on the center, where a checkboard pattern is shifting. One eye is tested at a time and each eye is tested twice.
- Brain stem auditory evoked potentials (BAEP). This test assesses high-frequency hearing loss, diagnoses hearing nerve damage, and can detect small tumors in the inner ear (acoustic neuroma). The patient is seated in a room with headphones on. A series of clicks will be sent through the headphones, one at a time to one ear. Each ear is tested twice.
- Somatosensory evoked potentials (SEP). This measures how long it takes the peripheral nerves (nerves outside the brain and spinal cord) to respond to stimuli. It can detect nerve damage or spinal cord damage due to MS or other degenerative diseases. In this test, the patient receives electrical impulses to the arm or leg.
The current criteria for a diagnosis of MS considers only visual evoked potential testing because this test has been shown to be more useful. As mentioned above, the VEP test detects loss of vision from optic nerve damage, which is a common early symptom in MS.
Although EP tests are used for a diagnosis of MS, they are not specific to this condition. Other tests need to be considered before a diagnosis of MS can be made.
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