| An evoked potential test measures electrical activity in the brain that is produced (evoked) in response to an external sensory stimulus. Types of stimuli that are commonly used for the test include visual (a flashing light), auditory (a clicking noise delivered to the ear via headphones), or tactile (a light touch or mild electrical shock). The resulting stimulation of a sensory nerve cell (neuron) results in an electrical discharge (depolarization) of the cell, which generates an electrical impulse called an action potential. Action potentials can also be elicited from a cognitive stimulus (recognition of a specified target or pattern) or by the omission of a stimulus (an increased time gap between stimuli). Brain and/or spinal cord electrical activity is recorded by a computer which then analyzes the speed, duration, and intensity of the neural response. These tests are used to detect problems in the sensory functions of the nervous system, including vision, hearing, and touch.
The most commonly measured responses are brain stem/auditory, visual, and somatosensory. A brainstem auditory evoked potential (BAEP) checks the pathway from the ear to the brain and may help uncover the cause of hearing and balance problems or other symptoms. A visual evoked potential (VEP) checks the pathway from the eyes to the brain and is used to help find the cause of certain vision problems and other conditions. It is often utilized to diagnose multiple sclerosis. A somatosensory evoked potential (SEP) checks the pathway from the nerves in the arms or legs to the brain and is used to study the function of these nerves, the spinal cord, and brain. A dermatomal evoked potential (DEP) is similar to an SEP, except that stimulation is done over areas thought to be more or less specific to a single nerve root, making it theoretically of greater use in disorders of spinal root function (radiculopathies). A cognitive evoked potential (P300) is a specialized auditory potential that detects response to a novel or unusual sound, making it a general measure of awareness and higher auditory processing. |
Source: Medical Disability Advisor
| Evoked potential (EP) tests are used to evaluate nervous system functioning, detect nervous system abnormalities or lesions, monitor coma patients, monitor patients during surgical procedures, and test hearing or vision in infants and others whose hearing cannot be tested in standard ways. These studies can be used to determine a patient's baseline (usual) evoked response, and then be compared with information from EP tests conducted during or after surgery that might compromise function of a particular part of the nervous system. For example, dermatomal evoked potentials can monitor spinal nerve root function during anterior cervical fusion, posterior cervical fusion, spinal fusion, tumor removals, or surgeries to correct narrowing of the spinal column opening that contains the spinal cord (spinal stenosis) or curvature of the spine (scoliosis). Somatosensory EPs can monitor peripheral nerves that might be damaged by traction or manipulation during surgical procedures, such as carpal tunnel surgery, or orthopedic procedures involving traction that might damage the pudendal nerve supplying sensation to the genital area. While most EP studies track sensory pathways, motor EPs can be monitored during abdominal aneurysm surgery to prevent loss of blood flow (ischemia) to the spinal cord, which could result in paralysis of both legs (paraplegia). EP tests can also help diagnose multiple sclerosis or detect brain damage in head injury patients. EP tests can help predict outcome in coma, with greater accuracy than that of emergency room physicians looking only at clinical data. They can help localize an abnormality in function to a specific region of the nervous system, such as peripheral nerves, spinal cord, brain stem, or higher brain centers. Because EP tests are objective, and not subject to voluntary manipulation by the individual, they can be used to help distinguish organic disease of the nervous system from malingering. Cognitive EPs generally reflect degree of cognitive deterioration in Alzheimer's disease and other dementias. |
Source: Medical Disability Advisor
| The nerve responses evoked by stimuli during a test are detected by means of electrodes (thin metal discs) placed on the skin over the scalp, neck, arm, or leg. Signals from the electrodes during the testing process are detected by a computer, which amplifies, averages, and analyzes the responses, creating a print-out of the results, which are then interpreted by a physician. There are no pre-test preparations required, although individuals being tested should refrain from taking sedatives or other medications that could interfere with test results, and should not use hair spray or skin creams that could interfere with electrode function. Each evoked potential (EP) test usually lasts from 1 to 2 hours and is conducted with the individual lying comfortably on a couch. The tests may be performed singly or in various combinations, depending upon the symptoms being evaluated. During the brain stem auditory evoked potential (BAEP) test, electrodes are placed in several locations on the scalp and earlobes. Earphones are placed over the ears to deliver the stimulus (clicks or tones), usually into one ear at a time. The clicks or tones will sound many times per second during the test process. After one ear is tested, the other is usually tested as well. For the cognitive evoked potential, the individual is asked to count how many times he or she hears a tone that is different from the background tone. During the visual evoked potential (VEP) test, electrodes are attached to the individual's scalp in various locations. One eye is covered, and the individual is instructed to have a fixed gaze on a spot in the center of a video screen. A checkerboard pattern is displayed and then quickly reversed numerous times. The stimulus responses are then recorded and the procedure repeated for the other eye. During the somatosensory evoked potential (SEP) test, electrodes are attached to the skin over somatosensory pathways such as the wrist, knee, and ankle. Additional electrodes are placed above the clavicle (collarbone) and at the second cervical vertebra (spinal column in the neck) for upper limb stimulation, and over the lower lumbar vertebrae in the back for lower limb stimulation. A painless electric shock is sent to the peripheral nerve through the stimulating electrode. The intensity of the shock is adjusted to produce a slight muscle response. Analysis is based on the measurement and averaging of the time it takes the current to reach the brain. The electrodes are repositioned, and the procedure is repeated on the other side of the body. |
Source: Medical Disability Advisor
| After the evoked potential test is performed, a neurologist or other physician will interpret the test results and give the individual being tested information about diagnosis, treatment, or the need for further tests. The outcome depends upon any underlying conditions, not on the evoked potential test itself. |
Source: Medical Disability Advisor
| No rehabilitation is required as a result of these tests, although it may be needed for any underlying neurological disorders. |
Source: Medical Disability Advisor
| There are no side effects to evoked potential test procedures, other than possible skin sensitivity to the paste used to attach electrodes (thin metal discs) to the skin. |
Source: Medical Disability Advisor
| Accommodations are based on any underlying disease or condition, not on the evoked potential test itself. |
Source: Medical Disability Advisor
| Emerson, Ronald G., Thaddeus S. Walczak, and T. A. Pedley. "Electroencephalography and Evoked Potentials." Merritt's Neurology. Ed. Lewis P. Rowland. 10th ed. Philadelphia: Lippincott, Williams & Wilkins, 2000. 64-73.Maurice, Victor, and Allan H. Ropper. Adam's and Victor's Principles of Neurology. 7th ed. New York: McGraw-Hill, 2001. |
Source: Medical Disability Advisor
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