Evoked Potentials

General Description

Evoked potential tests are a set of neurophysiological diagnostic techniques aimed at examining the function and integrity of nerve pathways by recording the brain's electrical activity in response to various sensory stimuli. In other words, they study the central nervous system's response to sensory stimuli by exploring the peripheral nerve pathways that transmit information to the brain.

Depending on the nerve pathways studied, different types of evoked potentials are classified:

• Visual evoked potentials: These evaluate the integrity of the visual pathway by recording changes in brain bioelectrical activity after a light stimulus.
• Brainstem auditory evoked potentials: These assess the function of the auditory pathway, from the auditory nerve to the brainstem, by measuring brain activity in response to auditory stimuli.
• Somatosensory evoked potentials: These examine the somatosensory pathway, responsible for transmitting tactile, vibratory, and proprioceptive information. Electrical current stimulation is applied to the skin of the limbs.
• Motor evoked potentials: These monitor the function of the corticospinal and bulbar pathways, from the motor cortex in the brain to peripheral muscles, using transcutaneous electromagnetic stimulation.
• Vestibular myogenic evoked potentials: These evaluate the function of the inner ear region responsible for balance (vestibular function) by applying high-intensity auditory stimuli.

When is it indicated?

The purpose of evoked potentials is to provide information on nerve conduction and detect lesions in sensory pathways. Therefore, they are used in the diagnosis and follow-up of various neurophysiological conditions, including:

• Multiple sclerosis.
• Hearing loss.
• Optic neuritis.
• Spinal cord or brainstem injuries.
• Neuropathies.
• Head trauma.

Evoked potential tests are especially useful for the early diagnosis of nerve lesions that do not present visible symptoms. Additionally, they are used to evaluate brain function in patients in a coma to assess prognosis.

How is it performed?

The test is based on placing electrodes on the patient's body, which are attached using a conductive gel, and presenting specific stimuli according to the type of evoked potential being studied:

• Visual evoked potentials: Surface or subdermal needle electrodes are placed on the scalp. Generally, two types of light stimuli are used: a light source emitting repetitive flashes at a specific frequency or a screen displaying a checkerboard pattern alternating colors while the patient focuses on an image in the center.
• Brainstem auditory evoked potentials: In addition to scalp electrodes, surface electrodes are placed on the earlobes or mastoids. The patient receives auditory stimuli, such as tones or clicks, of varying intensities through headphones. Each auditory nerve is studied individually.
• Somatosensory evoked potentials: Electrodes are placed on the scalp, cervical region, lumbar region, and proximal limb areas. Low-intensity electrical currents are applied to the limbs, usually on the wrists and ankles, and sometimes on the thighs.
• Motor evoked potentials: Electrodes are placed on the upper or lower limbs, and electromagnetic stimulation is applied using a coil positioned over the patient's head.
• Vestibular evoked potentials: Electrodes are placed on the sternocleidomastoid muscle and the lower edge of the eyes. The patient receives high-intensity auditory stimuli through headphones. Each auditory pathway is evaluated separately.

Risks

The evoked potential test is a harmless procedure for the patient, although the applied stimuli may be uncomfortable. Additionally, there is a possibility of mild skin irritation due to the electrodes or an allergic reaction to the conductive gel. If subcutaneous needle electrodes are used, there is a minimal risk of infection or bleeding.

Evoked potential tests may also be contraindicated in patients with epilepsy, metallic prostheses, or electrical or electronic devices such as pacemakers or stimulators.

What to expect from an evoked potential test

The test is performed with the patient sitting or lying on an examination table, depending on the specific study. It is important for the patient to remain relaxed and calm. Discomfort may be felt during electrode placement, especially with subcutaneous electrodes. If electrical stimuli are applied, mild cramps or tingling sensations may occur, but they are generally well tolerated. Additionally, dizziness or vertigo may be experienced during vestibular evoked potential tests, but these symptoms are mild and short-lived. Once the study is completed, the electrodes are removed, and any remaining conductive gel is cleaned off.

Evoked potential studies are lengthy procedures, as hundreds of stimuli need to be applied to obtain accurate results. Visual and auditory evoked potentials last about 30 to 40 minutes, while somatosensory and motor evoked potentials can take up to 60 minutes. These are outpatient procedures, and patients can resume their normal routine immediately afterward.

Medical specialties that request evoked potential tests

Evoked potentials are requested in the fields of neurology and clinical neurophysiology.