D8 Visual Electrophysiology: ERG and VEP

Topic

The electroretinogram involves placing a corneal electrode directly on the cornea—along with a reference electrode elsewhere on the face—and presenting a brief flash of light to the eye. Because the corneal electrode is positioned so close to the retina—separated from it only by the eye's transparent structures—it directly captures the electrical activity generated by the entire retina in response to the light flash, without the signal having to pass through the optic nerve or any other stage of the visual pathway first.

The electrical signal captured by the corneal electrode in response to a light flash originates from the entire retina simultaneously, encompassing both the photoreceptors that directly receive the light and the retinal layers that process the signal before transmitting it to the optic nerve. Consequently, the electroretinogram is considered a measure of global retinal function; this function is recorded in its entirety even if the optic nerve or subsequent stages of the visual pathway are damaged, because the electrode captures the signal before it is passed from the retina to the optic nerve.

In contrast, the visual evoked potential test does not involve placing an electrode on the eye itself; instead, scalp electrodes are used—one at the back of the head, directly over the primary visual cortex, and another at the top of the head as a reference—while a visual stimulus, such as a rapidly changing checkerboard pattern, is presented to the eye. Unlike the corneal electrode used in the electroretinogram, the scalp electrodes used for visual evoked potentials do not directly capture retinal electrical activity; rather, they record the electrical activity that ultimately reaches the primary visual cortex after traveling the entire length of the visual pathway from the retina. Since the signal picked up by scalp electrodes must travel through the retina, the optic nerve, and the rest of the visual pathway to the primary visual cortex in order to be recorded, the visual evoked potential assesses the integrity of the entire visual pathway—not just the retina. If any stage of the visual pathway—from the optic nerve to the primary visual cortex—is damaged, the visual evoked potential will be altered, even if the retina (which captures light at the start of that same pathway) is functioning normally. This stands in direct contrast to the electroretinogram, which can appear completely normal even if damage exists in the optic nerve or at any subsequent stage of the visual pathway; this is precisely because the corneal electrode captures the signal before the retina transmits it to those later stages.

Therefore, when a patient presents with unexplained vision loss, comparing their electroretinogram and visual evoked potential helps pinpoint the location of the problem along the visual pathway: an abnormal electroretinogram indicates that the issue lies within the retina itself, whereas a normal electroretinogram combined with an abnormal visual evoked potential indicates that the retina is functioning correctly and the problem lies further along the pathway—in the optic nerve, the rest of the visual pathway, or the primary visual cortex itself.

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