Visual evoked potentials

The visual evoked potentials (VEP) result from the changes produced in the cerebral bioelectrical activity after light stimulation. The most frequently used stimulus to obtain PEV is a checkerboard image (on a chessboard), with a series of black and white squares, which alternate (PEV-pattern). It manages to evoke large and reproducible potentials. It requires the collaboration of the patient.

In non-cooperative patients or who cannot see the screen with the checkerboard, other stimuli such as light flashes are used. These produce evoked responses with great inter-individual variability, in morphology and latencies, so they are only useful to determine if the light stimulus reaches the cerebral cortex, and to compare the response of both eyes, in search of asymmetries.

It is the only clinically objective test to assess the functional status of the visual system. It registers the potential variations in the occipital cortex caused by a stimulus on the retina. For this reason it can evaluate retinocortical function in children, with different abilities and aphasic patients. It can also distinguish between patients with psychological blindness and those who suffer from an organic cause.

We can explore the VEP-pattern by hemifields, when there is suspicion of a chiasmatic lesion, which usually begins with the involvement of the fibers of the external visual hemifield.

Visual evoked potentials represent a very sensitive neurophysiological examination, since they are altered in a high proportion of patients with visual abnormalities, even in patients with subclinical involvement of the visual pathway, as occurs in multiple sclerosis. However, it is not very specific when it comes to determining the type of pathology, since any problem that stands between the stimulus and the recording in the occipital cortex can cause anomalies in visual potentials as long as it causes enough visual dysfunction (defect optical correction, dense cataract, retinopathy, glaucoma, optic neuropathy, stroke, etc.). They should therefore be evaluated with caution and within a clinical context. In addition, we must take into account that postchiasmatic lesions with visual dysfunction can occur without abnormalities in VEP.

Another extremely valuable feature of visual evoked potentials is that they provide quantifiable latency and amplitude data. This allows us to identify a dysfunction in the visual pathway, guiding whether demyelinating phenomena predominate, with delayed potentials (increased latency) or if an axonal activation defect predominates in the visual pathway (reduction in amplitude). On the other hand, it allows us to carry out evolutionary monitoring, being able to evaluate the possible efficacy of a treatment, or the progression of a disease.

When the visual evoked potentials show abnormalities in only one eye, we can deduce that there is prechiasmatic pathology on that side. If the anomalies are bilateral, we cannot define the location of the lesion, since the fibers corresponding to the nasal retina decussate to the contralateral side in the chiasm.

Realization

Visual evoked potentials are performed by placing the patient in front of a screen on which a chessboard of black and white squares appears with a guide point in the center of the board. Once the stimulation has started, these squares will alternate rhythmically according to a set frequency, the guide point remaining fixed in the center of the screen.

The patient will be prepared by placing a series of electrodes or sensors on the back of the head, in the occipital region. Said sensors are located in the occipital midline, inion, left occipital and right occipital taking as reference another sensor located in the anterior area of the head and common to all the others. These sensors will be in charge of listening to the bioelectric variations produced by each stimulus, transmitting them to a device that will average and process the signals collected.

Once the patient is prepared and placed in front of the screen, they will be asked to stare at the guide point, trying not to lose sight of it or be distracted by the alternating movement of the squares. The patient must remain attentive and concentrated avoiding falling asleep.

Once the test begins, the squares on the screen will begin to change color alternately, causing a nervous reaction that is transmitted by the retina to the optic nerve and through it to the occipital area of the brain where the changes will be collected. produced by the sensors and taken to the averager or apparatus for processing.

Generally, two series of one hundred stimuli are evoked for each eye, covering the eye that is not stimulated and using at least two spatial frequencies for the chessboard, that is, a low frequency with large squares and fewer and a high frequency with smaller squares and in greater number.

The examination should be carried out without corrective lenses, although they can be used to reveal possible refractive errors or when neuropathy of the optic nerve due to multiple sclerosis is suspected.

In PEV FLASH, a strobe or flash is used instead of the checkerboard screen. It is performed by placing the stroboscope in front of the eye to be stimulated and giving the established series of light flashes, leaving the other eye covered. The placement of the sensors or electrodes on the patient is similar to the pev pattern.

These tests are completely painless and innocuous and do not cause significant side effects in patients except, on occasion, headaches, dizziness and/or drowsiness.

Indications

• Demylinizing disease of the central nervous system. Multiple sclerosis
• Optical neuritis of any cause
• Quiasmatic injury
• Estimate of visual acuity by PEV
• Ambliopia
• Cortical blind
• Neonatal hypoxia
• Neurodegenerative diseases
• Coma
• Neurofibromatosis
• Optical neuropathy: ischemic, metabolic, toxic