Evoked Potential Test

What are evoked potential tests?

Evoked potential tests measure the electrical activity in areas of your brain and spinal cord in response to certain stimuli. The tests involve electrodes placed on specific parts of your scalp and/or other parts of your body and delivery of a stimulus (such as images, sounds or electrical pulses). The electrodes “catch” your brain’s and nerves’ electrical signal responses to the stimulus.

Evoked potential tests record how quickly and completely nerve signals reach your brain. They can find damage along nerve and brain pathways that are too subtle to show up during a neurological examination. The damage also may not yet be noticeable to the person. Healthcare providers use evoked potentials in combination with other tests to help diagnose neurological conditions.

Types of evoked potential tests

The three main types of evoked potential tests include:

• Brainstem auditory evoked response (BAER): This test measures the electrical signals the auditory pathway in your brain generates in response to sounds. It helps diagnose suspected neurologic abnormalities of the 8th cranial nerve (auditory nerve), auditory pathway and brainstem.
• Visual evoked potential (VEP): This test measures the electrical signals your visual cortex (a region of your brain) generates in response to visual stimulation — usually a flashing checkerboard pattern. It helps diagnose issues with your visual pathway, especially your optic nerve. It can also help diagnose MS.
• Somatosensory evoked potential (SEP): This test can detect damage within your spinal cord and brain. It measures your brain’s response to mild electrical stimulation in various places on your body. This test determines how long it takes for the nerve signals to go from your peripheral nerves to your brain via your spinal cord.

What is an evoked potential test used for?

Healthcare providers use evoked potential tests to show abnormalities in the function of nerve and brain pathways that can result from neurological conditions. They also use SEPs during certain surgeries to monitor your neurologic function during the course of surgery.

Providers most commonly use these tests in an outpatient setting to help diagnose multiple sclerosis (MS). But they use them to help diagnose other conditions, as well.

For example, a visual evoked potential test can help diagnose optic nerve tumors or optic neuropathy. A brainstem auditory evoked response (BAER) test can assess hearing ability (especially in infants) and can point to possible brainstem tumors.

Is an evoked potential test the same as an EEG?

No, an evoked potential test and an electroencephalogram (EEG) are different tests. However, healthcare providers who perform evoked potential testing are often those who also perform EEGs.

The difference is that an EEG measures and records your brain’s electrical signals as they naturally happen. Evoked potential tests measure how quickly your nerves respond to certain stimulation.

How does an evoked potential test work?

An evoked potential test involves electrodes placed on specific parts of your scalp (and potentially other parts of your body) and delivery of a stimulus. This is so your healthcare team can record your brain’s electrical response to the stimulus.

To understand how this test works, it helps to know how your nervous system functions. Your nervous system consists of a vast network of nerves that send electrical signals to and from other cells, glands and muscles all over your body. These nerves receive information from the world around you. Your brain then interprets the information and controls your response. It’s like an enormous information highway running throughout your body.

In an evoked potential test, electrodes on your scalp measure electrical signals (impulses) as they travel between brain cells. As you experience the stimulus — either a visual pattern, series of sounds or very mild electric pulses or “shocks” — your nervous system responds to and interprets the stimulus. The electrodes on your scalp record the electrical activity in your brain that results. Healthcare providers run evoked potential tests by analyzing electroencephalogram (EEG) response (brainwaves) as it reacts to the stimulation.

The evoked potential machine averages the EEG signals following multiple stimuli so that it can assess the functioning of a specific neurological system. The evoked potential machine records the electrical response to the stimulation on several channels or traces. Your brain has specific waveforms that occur at very specific times in response to a various stimuli.

A specialist then interprets the wave patterns and looks for abnormal activity, which can point to various issues with your brain, spinal cord or nerves.

How do I prepare for an evoked potential test?

Your healthcare provider will go over what to expect during the test and what to do before it. Don’t hesitate to ask questions, and be sure to follow their preparation instructions.

Before the test, you’ll need to make sure your hair is clean and doesn’t have any hair sprays, oils or gels in it. These products can interfere with the function of the electrodes.

What happens during evoked potential tests?

There are several different types of evoked potential tests, so the process may vary.

You can generally expect the following for each type:

• Visual evoked potential (VEP): A healthcare provider will place electrodes (small metal disks) in specific places on your scalp. You’ll wear a patch over one eye and look at a flashing visual stimulus (usually a checkerboard pattern) projected on a monitor. Once the provider is finished testing one eye, you’ll repeat the process for your other eye.
• Brainstem auditory evoked response (BAER): A provider will place electrodes in specific places on your scalp and earlobes. You’ll listen to clicking noises generated in a set of headphones. The provider will test one ear at a time.
• Somatosensory evoked potentials (SEP): A provider will attach electrodes to your wrist, the back of your knee or other locations on your body in addition to your scalp. They’ll send a mild electrical stimulus through the electrodes on your body to see how long it takes the signal to reach your brain.

How long does an evoked potential test take?

The time it takes to complete an evoked potential test varies based on the type. In general, the tests last 60 to 90 minutes. Your healthcare provider will give you more specific details on what to expect.

What do the results of an evoked potential test mean?

Specially trained neurologists or neurophysiologists will interpret the results of your test and put together a report. They’ll send it to your healthcare provider. Your provider will then share the results with you. They’ll let you know if you need any other additional tests.

Your healthcare team may consider the results of the test in addition to findings from other tests, such as a neurological exam or MRI, before making a diagnosis.

A note from Cleveland Clinic

An evoked potential test is a noninvasive way to help diagnose certain conditions that affect parts of your nervous system. Your healthcare provider will walk you through the process before and after the test. Don’t be afraid to ask questions — they’re available to help you.