Evoked Potential Test

Overview

An evoked potentials test measures the time it takes for nerves to respond to sight, sound, or touch. The size of the response is also measured. Nerves from different areas of the body may be tested. Types of responses are:

• Visual evoked response or potential (VER or VEP). This response occurs when the eyes are checked by looking at a test pattern.
• Auditory brain stem evoked response or potential (ABER or ABEP). This occurs when hearing is checked by listening to a test tone.
• Somatosensory evoked response or potential (SSER or SSEP). This occurs when the nerves of the arms and legs are checked using an electrical pulse.

Each type of response is recorded using sensors taped to the head. The VER is the test used most often to diagnose multiple sclerosis (MS).

Special gel and sensors are put on the scalp. The location depends on the type of response being recorded. For example, when VERs are recorded, the sensors are taped to the back of the scalp over the brain areas that sense input from the eyes.

Stimulation is given:

• For VER by a strobe light or a screen with a special pattern.
• For ABER by clicking noises or a tone sent through earphones.
• For SSER by an electrical pulse at the wrist, knee, or another part of the body. This pulse is a mild electrical shock.

Responses from the sensors are recorded.

Why It Is Done

Evoked potentials tests may be done to check problems with nerves. These tests can also help confirm the diagnosis of certain nerve conditions, such as multiple sclerosis. They are also used to monitor nerves during certain surgeries.

Results

Normal

The time between the stimulation and the brain's response is within the normal range.

Abnormal

The time between the stimulation and the brain's response is longer than expected. This can be due to problems with nerves that were tested. Some people will have an abnormal result but not have any symptoms in the areas tested.