Evoked Brain Potentials, Evoked Responses, Evoked Potential Studies
Evoked potential tests measure the brain's response to stimuli that are delivered through sight, hearing, or touch. These sensory stimuli evoke minute electrical potentials that travel along nerves to the brain, and can be recorded with patch-like sensors (electrodes) that are attached to the scalp and skin over various peripheral sensory nerves.
These signals are transmitted to a computer, where they are amplified, averaged, and displayed. The 3 major types of evoked potential tests are described here.
Visual evoked potentials, which are produced by exposing the eye to a reversible checkerboard pattern or strobe light flash, help to detect vision impairment caused by optic nerve damage, particularly from multiple sclerosis.
Brainstem auditory evoked potentials, generated by delivering clicks to the ear, are used to identify the source of hearing loss. They help to differentiate between damage to the acoustic nerve and damage to auditory pathways within the brainstem.
Somatosensory evoked potentials, produced by electrically stimulating a peripheral sensory nervethat is, a nerve responsible for sensation in an area of the bodycan be used to diagnose peripheral nerve damage and locate brain and spinal cord lesions.
Purpose of the Evoked Potential Tests
- To assess the function of the nervous system
- To aid in the diagnosis of nervous system lesions and abnormalities
- To monitor the progression or treatment of degenerative nerve diseases such as multiple sclerosis
- To monitor brain activity and nerve signals during brain or spine surgery, or in patients who are under general anesthesia
- To assess brain function in a patient who is in a coma
Who Performs Evoked Potential Tests
- A neurologist, a nurse, or a lab technician
Special Concerns about Evoked Potential Tests
- It may be difficult to determine visual evoked potentials accurately in people with extremely impaired vision.
- Earwax, severe inflammation of the middle ear, or severe hearing impairment may interfere with the results of brainstem auditory evoked potential tests.
- Severe sensory loss (neuropathy) may interfere with the results of somatosensory evoked potential tests.
- Muscle spasms in the head or neck may also interfere with test results.
Before the Evoked Potential Tests
- Wash your hair the night before the test, and do not use hair spray, gel, or other hair care products after shampooing.
- Your hair should be free of any braids, pins, or jewelry.
- Avoid taking sedative drugs, such as benzodiazepines and barbiturates, before the test.
- Remove all jewelry and metal objects before the procedure.
What You Experience during Evoked Potential Tests
Visual evoked potentials:
- You sit in a chair, about 3 feet away from a TV screen.
- A special adhesive paste is used to attach electrodes to your scalp.
- The eye that is not being tested is usually covered with a patch.
- You are asked to focus your gaze on a dot at the center of the TV screen as it displays a visual stimulus, usually a rapidly moving checkerboard pattern.
- Electrical activity in the optic nerve and brain is recorded by the electrodes.
- The procedure is repeated for the other eye.
- Each eye is usually tested twice. The entire procedure takes about 30 to 45 minutes.
Brainstem auditory evoked potentials:
- You will sit in a soundproof room and put on headphones.
- Electrodes are attached to the top of your head and to the earlobe of the ear being tested.
- A series of clicking sounds is delivered through the headphones to one ear, then the other. Signals produced by the brain in response to the clicks are recorded.
- Each ear is usually tested twice. The entire procedure takes about 30 to 45 minutes.
Somatosensory evoked potentials:
- Recording electrodes are attached to your scalp and neck. Stimulating electrodes may be placed over your wrist, lower back, back of the knee, or ankle.
- Mild, painless electrical shocks are delivered to the stimulating electrodes. The stimulus lasts for about 2 minutes at a time and may cause some twitching and tingling in the target area.
- The brain’s response to the shocks is measured by the recording electrodes.
- The procedure takes about 30 minutes.
Risks and Complications of Evoked Potential Tests
- Evoked potential tests are painless and carry no significant risk. The mild electrical shocks delivered in the somatosensory tests are usually perceived as a tingling sensation.
After the Evoked Potential Tests
- The electrodes are removed and the adhesive paste is washed away.
- You may resume taking any medications that were withheld before the test, according to your doctor’s instructions.
Results of Evoked Potential Tests
- A neurologist will evaluate the electrical tracings, or wave forms, for any abnormalities that indicate damage to the nerve pathways leading to the brain from your eyes, ears, or limbs.
- In most cases, further testing will be needed to provide more specific information or further evaluate abnormal findings. Additional tests may include a brain CT scan, magnetic resonance angiography, or electroneurography.
The Johns Hopkins Consumer Guide to Medical Tests
Simeon Margolis, M.D., Ph.D., Medical Editor
Updated by Remedy Health Media