Evoked potentials

What are evoked potentials?

Evoked potential (EP) tests or evoked response tests are a diagnostic tool used by neurophysiologists to measure the time it takes for the brain to respond to various stimuli to the different senses, including sight, hearing and touch. They can be used to diagnose problems in the nervous system, such as multiple sclerosis.

The nervous system is a complex web of nerve cells threaded throughout our bodies, which send information to and from the brain as electrical impulses. Visual information is sent from the eyes after converting the light that enters it into an electrical signal; auditory information is sent from the ears; and somatosensory (touch) information is sent from the skin.

What does an EP involve?

There are three main types of evoked potential, which test responses to sight, sound and touch. In all three, electrodes are placed on the scalp to read the brain’s response.

1. Visual evoked response (VER) test – the eye is presented with stimuli such as flashes of light or chequered patterns.
2. Brainstem auditory evoked response (BAER) test – clicking sounds or tones are used to see if there is a problem with the auditory pathway in the brain or the acoustic nerve, which connects the ear to the brain.
3. Somatosensory evoked response (SSER) test – tests response times to low-level electric shocks administered to a part of the body where the nervous system could be compromised, for example if the doctor suspects a spinal cord lesion.

Why are evoked potentials done?

EPs are generally used to confirm a diagnosis or monitor a patient’s nervous system. It is often used to support or confirm a diagnosis of multiple sclerosis, as slow response times to stimuli can indicate that the myelin sheath that insulates nerve cells is damaged, which is a key symptom of MS.

EPs can also be used to:

• evaluate brain activity in coma patients
• test certain nerves, such as the optic nerve, for damage
• detect lesions and tumours
• test sight and hearing
• diagnose and/or monitor conditions that cause nerve damage.

Advances/developments

With the arrival of more advanced diagnostic tools, such as MRI scans, the use of EPs has declined somewhat, but they can still be a useful tool in providing extra evidence to make a diagnosis.