Evoked potentials

Definition

Stimuli from the environment are perceived through the senses. They are conducted as nerve impulses from the perceptual organs to the brain, where they will be interpreted and given meaning. . In this way, exposure to a flash, intense noise or a tactile stimulus will give rise to characteristic electrical brain waves. If the visual, auditory stimulus, or the electrical sensation given to the feet and hands does not produce the expected wave, at the right time and place, it means that there is a problem with the nerve pathway. The study of evoked potentials is, therefore, a neurophysiological procedure that assesses the function and integrity of the perceptual system and its pathways.

Depending on the sensory modality explored, we will distinguish:

• Auditory evoked potentials (PEA)
• Visual evoked potentials (VEP)
• Somatosensory evoked potentials (SSEP)

How is the study done?

It is an outpatient examination, hospitalization is not required. It is carried out in clinics, hospitals or consultations enabled for this purpose. The examination room should have a table and an EEG. The environment in which it is carried out must be quiet, pleasant and with little light. It is important that the patient is relaxed. Anxiety, nervousness, or lack of relaxation will increase muscle tension. The signal generated by a contracted muscle can interfere with the EEG recording. Lack of relaxation and collaboration will prevent the realization of an evoked potential. Exceptionally, in uncooperative children, the responsible physician may assess the use of some sedative substance.

The recording electrodes will be placed on the scalp and on the auditory or mastoid pavilions, sticking them with conductive gel that after the test can be easily removed with acetone. The registration area must first be cleaned to avoid interference.

In order to obtain the evoked responses, the path to be explored is stimulated. Depending on the sensory modality evaluated, different types of stimuli will be applied:

• PEV: the patient will have to look at a square on a screen. In uncooperative patients, a flash of light can be used.
• PEA: earphones emitting audible tones are worn.
• PESS: light electrical stimuli are applied to the feet and hands. Responses can be recorded at the level of the scalp and in other intermediate areas (shoulder, neck, spine), helping to examine the different portions of the sensory pathway.

The duration of the test is usually 30 minutes to 1 hour.

Preparation for the study

It does not require fasting or preparation. It is recommended not to apply hairspray or other hair products, as they can interfere with the signal captured by the electrodes.

The responsible doctor must be aware of the usual treatments carried out by the patient, and may indicate their withdrawal in the days prior to the test.

What does it feel like during and after the study?

It is not uncomfortable, although it is a long examination that can be exhausting for the patient. Sometimes the study of somatesthetic evoked potentials can cause a sensation of cramp of constant intensity on the skin.

Study risks

It is a safe technique. The risks are practically nil and if they occur they are usually related to the electrodes and the discomfort of the stimuli.

Study contraindications

There are no contraindications.

Reasons why the study is carried out

Evoked potentials are support techniques for the diagnosis of alterations of the sensory pathways. They help detect injuries and clarify their causes.

They are also useful for the evolutionary control of already known processes and to detect signs of a disease that has not yet caused any symptoms. The results obtained are objective, conclusive data on injury or normality, which is very important for the correlation between the patient's complaints and the actual injury.

They are important in diseases such as optic neuritis, multiple sclerosis, hearing loss, head trauma, spinal cord injuries, or neuropathies.

Likewise, it is also useful for the diagnosis or determination of the risk of sequelae in comatose or anesthetized patients in whom the simple neurological examination is not conclusive.