What are evoked potentials?
Evoked potentials (also called evoked response) measure the electrical signals to the brain generated by hearing, touch, or sight. These tests are used to assess sensory nerve problems and confirm neurological conditions including multiple sclerosis, brain tumor, acoustic neuroma (small tumors of the inner ear), and spinal cord injury. Evoked potentials are also used to test sight and hearing (especially in infants and young children), monitor brain activity among coma patients, and confirm brain death.
Testing may take place in a doctor’s office or hospital setting. It is painless and risk-free. Two sets of needle electrodes are used to test for nerve damage. One set of electrodes, which will be used to measure the electrophysiological response to stimuli, is attached to the patient’s scalp using conducting paste. The second set of electrodes is attached to the part of the body to be tested. The physician then records the amount of time it takes for the impulse generated by stimuli to reach the brain. Under normal circumstances, the process of signal transmission is instantaneous.
Auditory evoked potentials (also called brain stem auditory evoked response) are used to assess high-frequency hearing loss, diagnose any damage to the acoustic nerve and auditory pathways in the brainstem, and detect acoustic neuromas. The patient sits in a soundproof room and wears headphones. Clicking sounds are delivered one at a time to one ear while a masking sound is sent to the other ear. Each ear is usually tested twice, and the entire procedure takes about 45 minutes.
Visual evoked potentials detect loss of vision from optic nerve damage (in particular, damage caused by multiple sclerosis). The patient sits close to a screen and is asked to focus on the center of a shifting checkerboard pattern. Only one eye is tested at a time; the other eye is either kept closed or covered with a patch. Each eye is usually tested twice. Testing takes 30-45 minutes.
Somatosensory evoked potentials measure response from stimuli to the peripheral nerves and can detect nerve or spinal cord damage or nerve degeneration from multiple sclerosis and other degenerating diseases. Tiny electrical shocks are delivered by electrode to a nerve in an arm or leg. Responses to the shocks, which may be delivered for more than a minute at a time, are recorded. This test usually lasts less than an hour.
Reference: The National Institute of Neurological Disorders and Stroke
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