An electrooculogram (EOG) is a diagnostic test in ophthalmology. This non-invasive procedure measures the electrical potential between the front (cornea) and back (retina) of the eye. The EOG provides valuable information about the function of the retinal pigment epithelium (RPE), a layer of cells behind the retina, helping professionals understand their health.
The Science of Eye Movement Potentials
The EOG test relies on a stable biological principle known as the corneoretinal potential. The eye functions as a small electrical dipole, with two oppositely charged poles. The cornea, located at the front of the eye, consistently maintains a positive electrical charge, while the retina at the back holds a negative charge. This difference in electrical potential creates a constant electrical field around the eye.
When the eye moves, this stable electrical field also shifts its orientation within the head. The EOG captures these changes in electrical potential. Electrodes placed on the skin near the eyes detect these shifts, translating the eye’s movements into measurable electrical signals. These recorded signals provide a direct representation of the underlying electrical activity generated by the eye’s specialized cells.
The Electrooculogram Procedure
The EOG procedure begins with minimal preparation for accurate readings. A technician cleans skin around the eyes to improve conductivity for the adhesive electrodes.
Once the electrodes are in place, the test proceeds through two distinct phases. The first phase involves dark adaptation, where the patient sits in a dimly lit or completely dark room for approximately 15 to 20 minutes. This period allows the eye’s light-sensitive cells to adjust to the low-light conditions, reaching a baseline state. During this time, the patient is often instructed to maintain a relaxed gaze.
Following dark adaptation, the test moves into the light adaptation phase. The room is then illuminated with a controlled light source, and the patient remains in this illuminated environment for another 15 to 20 minutes. Throughout both phases, the patient is asked to perform a specific task: repeatedly moving their eyes horizontally back and forth between two fixed points. These points are typically small lights positioned about 30 degrees apart, and the patient will move their eyes between them every 10 to 20 seconds as instructed.
Interpreting EOG Results
During the EOG, electrical signals from eye movements are continuously recorded. These signals reflect changes in the corneoretinal potential as the eye moves through the dark and light phases. The data is then processed to determine measurements that indicate retinal pigment epithelium health.
The primary metric used for interpreting EOG results is the Arden ratio. This ratio is calculated by comparing the highest electrical potential recorded during the light adaptation phase to the lowest electrical potential measured during the dark adaptation phase. A normal Arden ratio is typically 1.8 to 2.0 or higher. A ratio below this range suggests retinal pigment epithelium cell dysfunction.
Clinical Applications and Diagnoses
An abnormal EOG indicates certain ocular conditions affecting the retinal pigment epithelium. The EOG is significant for diagnosing Best’s vitelliform macular dystrophy, a genetic disorder impacting central vision. For this condition, the EOG often confirms RPE dysfunction, even in early stages.
The EOG also assists in diagnosing or monitoring other retinal disorders. These include certain forms of retinitis pigmentosa, a group of inherited diseases that cause progressive vision loss. The test can also help identify choroideremia, another rare genetic condition leading to progressive vision impairment. An EOG can also detect retinal toxicity from long-term medication use, such as chloroquine, by assessing its impact on RPE function.