Overview

An evoked potential test measures the time it takes for nerves to respond to stimulation. 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 stimulated by looking at a test pattern.
• Auditory brain stem evoked response or potential (ABER or ABEP). This occurs when hearing is stimulated 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 stimulated by an electrical pulse.

Each type of response is recorded from brain waves by using electrodes taped to the head. The VER is the most commonly used evoked potential test in the diagnosis of multiple sclerosis (MS).

Conducting gel and electrodes are applied to the scalp. The location will depend on the type of response being recorded. For example, when VERs are recorded, the electrodes are applied to the rear (occipital region) of the scalp over the brain areas that register visual stimuli.

Stimuli are delivered:

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

Responses from the electrodes are recorded. The time between the stimulation and the response is called the latency, which is the speed at which the nerves pass a signal.

Why It Is Done

Evoked potential tests may be done to assess neurological problems. These tests can also help confirm the diagnosis of certain neurological conditions, such as multiple sclerosis.

Results

Normal

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

Abnormal

Some people who are free from symptoms in the nerve area tested will still have abnormal responses in that area.

Abnormal response times can also be linked with other neurological diseases or with damaged optic nerves and eyes.