Evoked Potential Tests: How Are They Used in MS?
Multiple sclerosis (MS) is a complex demyelinating disease of the central nervous system for which there is no single diagnostic test. Common tools used to diagnose MS include the neurological exam, complete medical history, magnetic resonance imaging (MRI), and evoked potential (EP) studies.
What are evoked potentials?
As the brain “talks” to various parts of the body, messages are sent as electrical signals that travel along nerves. Demyelination and neurodegeneration caused by MS can disrupt these electrical signals. Evoked potential studies, also called evoked potentials (EPs), measure signals that travel through specific sensory nerve pathways in response to external stimuli. Those electrical signals are measured through sensors placed on the skin in combination with sophisticated computer programs.
What can evoked potentials reveal?
EP testing can detect the slowing or reduction of nerve signals caused by damage (e.g., demyelination or lesions) along sensory pathways. A significant benefit of EP testing is that it is capable of detecting changes that are too subtle to be detected by neurological examination or noticed by the patient. EP testing can also provide evidence of demyelination that may not be visible on MRI scans, helping to support an MS diagnosis in patients who have a “negative” MRI.
Evoked potential tests used in MS
There are three types of sensory evoked potential studies used to detect changes in nerve conduction in MS: visual evoked potential (VEP), brainstem auditory evoked potential (BAEP), and somatosensory evoked potential (SSEP). Before testing, sensors are placed on the scalp over specific parts of the brain and other parts of the body as appropriate. In general, most EP testing is painless.
Visual evoked potentials (VEP) are used to evaluate nerve function from retina via the optic nerves to the visual cortex of the brain. VEP is more effective than MRI in testing the functional integrity of optic pathways and has the sensitivity to detect past or current damage from optic neuritis that may not be visible on MRI scans of the optic nerves. In preparation for the test, five electrodes are attached to the scalp over the occipital lobe of the brain, close to the primary visual cortex. During the test, which takes about 45 minutes, the patient focuses on a checkerboard pattern (the stimulus) that flashes on a computer monitor. The test is painless and patients who wear contacts or glasses need to wear them during the test. Hair should be clean and dry, free of hair products, weaves, braids, hair extensions, or toupees. Other names for VEP include visual evoked response (VER) and visual evoked cortical potential (VECP).
Brainstem auditory evoked potentials (BAEP), also known as brainstem auditory evoked responses (BAER), are used to evaluate electrical activity in the cochlea and auditory pathways in the brain to the lower brainstem. In preparation of the test, two electrodes are attached to the forehead, and one on each ear lobe. A series of clicking sounds (the stimulus) are delivered to each ear separately through earphones. For better testing results, the patient should be comfortable and relaxed to avoid interference of tense neck or jaw muscles. BAEP testing can take from 30 to 45 minutes.
Somatosensory evoked potentials (SSEP) test the nerve pathways from peripheral nerves through the spine to the somatosensory region of the brain. SSEP testing focuses on the arms (median or ulnar nerve) or the legs (posterior tibial nerve). For SSEP-arms evaluation, electrodes are placed on the head, back of the neck, and shoulders near the neck. For SSEP-legs evaluation, electrodes are attached to the legs, back, and head. Electrodes may also be placed near the wrists or ankles. Skin needs to be clean and patients must not use body lotion or powder prior to testing. During the test, which takes about 30 minutes, short electrical impulses (the stimulus) are administered to an arm or leg and the speed of nerve conduction from the stimulus to the brain is measured through the electrodes. Some patients may find the electrical impulses uncomfortable.
While MRI testing shows anatomical abnormalities, evoked potential testing can reveal functional problems. However, both forms of testing are useful in detecting damage to the nervous system. Results from EP testing are not immediately available as the data will need to be evaluated by a neurologist. Abnormal results — delayed nerve conduction — suggest damage to the nerve pathways. Of these three types of tests, VEP is the only evoked potential study that is considered in making the diagnosis of MS; the other tests are helpful but not diagnostic.
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