Definition and Overview
Evoked potential testing is a diagnostic test used to measure the brain’s electrical activity in response to sensory stimulation, in order to detect any slowed or delayed transmission of nerve signals caused by nerve damage. This test is performed on individuals who are suspected of having multiple sclerosis (MS) and in cases wherein a neurological exam is not able to provide sufficient information. The procedure is done by evoking neural response through visual, auditory, and somatosensory stimulation. The test is considered safe, with the patient experiencing very little discomfort, and risks are limited to those that may arise due to medical conditions that already exist.
Who Should Undergo and Expected Results
The most common use of evoked potential testing is for the diagnosis of multiple sclerosis, but it can generally be used to detect damage or demyelination, especially when the problem is too small to be detected in a neurological exam. The type of damage that the test detects is also associated with brain tumors, spinal cord problems, and specific nerve issues. Thus, it is not considered as a test specific or exclusive only for MS diagnosis.
The test will either have a normal or abnormal result:
Normal – A normal result is when the time between the delivery of stimulation and the response of the nerve is within normal values.
Abnormal - An abnormal result is when the response time is slower than the normal range. If so, this means there is an existing abnormality in the brain or the nerve pathways.
However, the test cannot determine the root cause of the delayed brain response or damage. Thus, it can only be used either as a preliminary test or, in the case of MS, to confirm a suspected diagnosis. If two demyelinations or damages are detected, there is a high risk of a positive diagnosis of MS.
How is the Procedure Performed?
An evoked potential test is performed and interpreted by a neurologist or a neurophysiologist; however, special training in administering the different types of the test is required. The test is conducted in a laboratory, as there is specific equipment required. It normally takes around 30 minutes to an hour.
There are three ways in which an evoked potential test is conducted; each one uses a different type of stimulation:
1. Visual evoked response (VER) – This is the most common type used especially in diagnosing MS, due to the high occurrence rate of optic nerve damage observed as an early symptom of the disease. In this test, the patient is asked to look at a test pattern that changes or reverses color or a strobe light that flashes for several minutes to stimulate the nerves of the eyes. The patient, seated at a distance of three feet away from the display, will be instructed to focus on the center of the pattern and to close one eye at a time.
2. Auditory brain stem evoked response (ABER) – This test stimulates the nerves of the ears by making the patient listen to a test tone, usually a clicking noise that is delivered to one ear at a time. While also used for MS diagnosis, an ABER can also detect tumors in the brain stem.
3. Somatosensory evoked response (SSER) – The nerves of the legs and the arms are stimulated using mild electrical shock administered at either the wrist or knee. Aside from MS, this test can also detect spinal issues that cause numbing and weakening of the limbs.
In all these tests, the rate at which the nerves respond is recorded by placing electrodes attached to the patient’s head and other parts of his body. The specific type of testing dictates the exact location of these electrodes. For VER tests, the electrodes are placed behind the head, which is the occipital region, because this is where the brain registers visual stimulus. For ABER tests, electrodes are also placed on the earlobes aside from the scalp. For SSER tests, electrodes are attached to the scalp, the wrist, and the back of the knee.
Possible Risks and Complications
Evoked potential testing is a simple and safe diagnostic procedure that causes little to no discomfort, and has a high efficacy rate in detecting damage or demyelination.
However, in people with existing medical conditions, there may be some risks involved. Thus, it is important to disclose all pertinent health information to the doctor before undergoing the procedure.
There are also some factors that may impair the accuracy of the test. These include:
- Severe nearsightedness, which will keep the patient from seeing the test pattern properly
- Middle ear infection or an earwax blocking the ear, inhibiting proper delivery of auditory stimuli
Severe hearing impairment, which will prevent the patient from hearing the stimulus tone
Shy ME. Peripheral neuropathies. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier. 2007: chap 446.