Evoked Potential Studies Health Article

Definition

Sensory evoked potential (EP) studies are the measurement of the electrical response of nervous tissue to auditory, visual, and somatosensory stimulation.

Purpose

EP studies, also known as evoked responses, measure the very faint electrical response of the brain, brainstem, or peripheral nerves to a mild stimulus, displaying it as a wave on a paper strip or computer monitor. Because the response is of such low amplitude, the responses to many stimuli must be averaged to distinguish the potentials from the background brain activity. The general purpose of this group of tests is to diagnose nerve disorders and to distinguish nonorganic loss of sensation from nerve damage. The tests can locate the site of the lesion and serve to evaluate the condition of a patient's nervous response after treatment or during surgery.

Auditory EP studies are often used to assess the cause of a hearing loss or balance problems, especially in children. They are the most common way hearing loss is evaluated in premature infants. Most auditory EPs record activity from the brainstem, and are therefore called brainstem auditory evoked potentials. This test is also particularly useful in screening for acoustic neuroma (a benign tumor of the nerves of the auditory canal).

Visual EP studies are used to assess eyesight in infants and children, as well as to diagnose disorders of the optic nerve and muscles. They can also distinguish hysterical blindness, a psychiatric disorder, from blindness caused by nerve damage.

Somatosensory EP studies are commonly used in the diagnosis of multiple sclerosis and transverse myelitis, although magnetic resonance imaging (MRI) has made this test less useful for these purposes. After trauma, somatosensory EP can determine whether loss of sensation in an arm or leg is due to injury in the brain or spinal cord.

Auditory, visual, or somatosensory testing can detect tumors and other abnormalities affecting the brain and spinal cord, assess brain stem function in coma, and assist in the determination of brain death. Perhaps the most prominent future role of this group of tests is the monitoring of brain activity and signals from the nerves during surgery on the brain, spine, or carotid region, and during general anesthesia.