Electrooculography (EOG) is a specialized electrophysiological test used in ophthalmology to assess the health and function of the eye’s outer layers. The test measures the steady electrical voltage that naturally exists across the eyeball. Its primary purpose is to evaluate the functional status of the retinal pigment epithelium (RPE), the layer of cells that supports the retina’s light-sensing cells.
How Electrooculography Measures Eye Function
The biological mechanism relies on the fact that the eye acts like a naturally occurring battery, establishing the cornea-retinal potential (CRP). This constant electrical voltage is maintained between the front (cornea) and the back (retina). The cornea is electrically positive relative to the retina, creating an electrical dipole.
When the eye moves, this standing potential rotates, and electrodes placed near the eye detect the shift in voltage. The magnitude of this potential is largely generated and maintained by ion transport activity within the RPE cells. This cellular activity creates a transepithelial potential (TEP) that contributes significantly to the overall CRP. The RPE’s metabolic rate is highly sensitive to changes in light and darkness, which influences the strength of the electrical dipole.
The EOG measures how this RPE-generated potential changes in response to prolonged periods of darkness followed by light. By tracking these voltage changes, the test indirectly assesses the health and responsiveness of the RPE. Changes in the RPE’s ability to maintain ion transport are reflected as distinct peaks and troughs in the recorded electrical signal.
The EOG Testing Experience
Small electrodes are affixed to the skin near the eyes, typically at the inner and outer corners (nasal and temporal canthi), with a reference electrode placed on the forehead. Patients must refrain from wearing makeup or contact lenses to ensure proper electrode adhesion.
The procedure begins with the patient sitting in a darkened room for an initial period of dark adaptation, usually lasting about 15 minutes. During this phase, the patient is instructed to look back and forth between two fixed light markers to generate a consistent eye movement signal.
After the dark phase, the room lights are switched on, and the patient enters the light adaptation phase, which continues for another 15 minutes or more. The entire test typically takes around 30 to 45 minutes. The electrical signals generated by the eye movements are recorded continuously throughout both the dark and light phases, providing a functional curve for analysis by an ophthalmologist.
Specific Conditions Diagnosed
The EOG is primarily used to investigate inherited retinal dystrophies affecting the retinal pigment epithelium. It is definitive for diagnosing Best’s vitelliform macular dystrophy (Best disease), an inherited condition causing a yellow, egg yolk-like lesion under the macula.
An abnormal EOG result is a hallmark of Best disease and can be present even in individuals who have not yet developed visible symptoms. This makes the test a powerful diagnostic tool for identifying the condition in its earliest stages or in asymptomatic carriers. The abnormal EOG is caused by the underlying dysfunction of the RPE cells due to a mutation in the BEST1 gene.
The test helps differentiate Best disease from other macular conditions that may appear similar clinically but have different underlying causes. For example, some adult-onset vitelliform macular dystrophies often show normal EOG results, separating them from the generalized RPE dysfunction seen in Best disease. The EOG is also applied to assess RPE health in other inherited retinal disorders, though it may provide supplementary information rather than a definitive diagnosis.
Analyzing the Test Results
The data collected during the EOG test is quantified and interpreted using the Arden ratio. This ratio is calculated to objectively compare the eye’s electrical response during the dark and light phases.
The Arden ratio is derived by dividing the maximum voltage recorded during the light adaptation phase (the “light peak”) by the minimum voltage recorded during the preceding dark adaptation phase (the “dark trough”). A normal Arden ratio is generally considered to be 1.8 or higher (180%). A ratio significantly lower than this range, such as below 1.5 (150%), indicates an abnormality in RPE function. A severely reduced ratio is a strong indicator of Best’s vitelliform macular dystrophy.