An EOG (electrooculography) test measures the electrical potential between the front and back of your eye to assess the health of a specific layer of the retina. It’s a non-invasive eye test that involves looking back and forth between two fixed points while small electrodes placed on the skin near your eyes record changes in voltage. The entire test takes about 30 to 40 minutes, and its primary use today is diagnosing a group of inherited eye conditions, most notably Best disease.
How the EOG Works
Your eye naturally generates a small electrical charge. The front of the eye (the cornea) carries a positive charge relative to the back, creating what’s called a standing potential. This voltage isn’t random. It’s produced by the retinal pigment epithelium, a thin layer of cells that sits behind the light-sensing photoreceptors and supports their function. When this layer is healthy and interacting normally with the middle layers of the retina, the standing potential responds in predictable ways to changes in lighting.
When you move your eyes, the electrodes on your skin detect shifts in this electrical field. By measuring how the voltage changes as the eye adapts to darkness and then to bright light, the test reveals whether the pigment epithelium is functioning properly. This is information that other eye tests, including standard imaging, can miss in early stages of disease.
What Happens During the Test
Small adhesive electrodes are placed on the skin near the inner and outer corners of each eye. You’ll be asked to look back and forth between two small fixation lights set at a fixed distance apart. The test has two main phases, each lasting 15 minutes.
During the first phase, you sit in total darkness (except for dim fixation lights). Every minute, you perform a brief 10-second recording by moving your eyes between the two lights. This dark adaptation phase allows the standing potential to settle to its lowest point, called the dark trough.
Then the room lights come on gradually over about 20 seconds to avoid discomfort, and a bright, evenly lit background stays on for the next 15 minutes. You continue the same eye movements every minute. During this light phase, the standing potential rises and eventually hits a peak called the light peak. In most people under 25, this peak arrives between 6 and 10 minutes into the light phase. For people over 55, it tends to take a couple of minutes longer.
How Results Are Interpreted
The key number from an EOG is the Arden ratio: the light peak voltage divided by the dark trough voltage. A healthy eye produces a significantly higher voltage in the light than in the dark, so the ratio is well above 1.0.
The average Arden ratio in the general population is about 2.5, meaning the voltage roughly doubles and a half from dark to light. Normal values fall between roughly 1.7 and 3.4 (the 5th and 95th percentiles). These thresholds shift with age. A 10-year-old’s lower limit of normal sits around 2.0, while a 60-year-old’s drops to about 1.7.
An Arden ratio above 1.85 is generally considered normal. Ratios between 1.55 and 1.85 fall into a borderline or equivocal zone. A ratio below 1.2 is severely reduced and strongly suggests dysfunction of the retinal pigment epithelium, which is the hallmark finding in Best disease and related conditions.
What Conditions the EOG Detects
The most common reason doctors order an EOG is to confirm or rule out Best disease (also called Best vitelliform macular dystrophy). This inherited condition causes a distinctive egg yolk-shaped lesion on the retina, typically appearing in childhood or adolescence. While the lesion itself can be seen on imaging, a severely reduced Arden ratio remains the most sensitive clinical test for confirming the diagnosis. It can also distinguish Best disease from a similar-looking condition called adult-onset vitelliform macular dystrophy, which has a different genetic basis, a later age of onset, and often produces a less abnormal EOG.
The EOG also plays a valuable role in monitoring for retinal toxicity in people taking antimalarial medications like chloroquine and hydroxychloroquine over long periods. These drugs can damage the retinal pigment epithelium, and the EOG can detect changes before other tests like the ERG (electroretinogram) show abnormalities. This makes it useful as an early warning system during long-term treatment.
EOG vs. ERG
Both the EOG and the ERG (electroretinogram) are electrophysiology tests for the eye, but they measure different things. The ERG records the electrical activity of the photoreceptors and other retinal neurons in response to flashes of light. It’s broadly useful for diagnosing conditions that affect the retina’s ability to detect and process light, such as retinitis pigmentosa and cone-rod dystrophies.
The EOG, by contrast, specifically evaluates the retinal pigment epithelium and its interaction with the middle retina. It doesn’t measure how well your photoreceptors respond to light directly. Instead, it measures how well the support layer behind those photoreceptors is doing its job. This is why the EOG is the go-to test for conditions rooted in pigment epithelium dysfunction, where the ERG may still look normal.
In some clinical situations, both tests are performed together to build a more complete picture of retinal health. A normal ERG paired with a severely abnormal EOG, for example, points strongly toward a pigment epithelium problem rather than a photoreceptor disease.
What the Test Feels Like
The EOG is painless and entirely non-invasive. No eye drops, no contact lenses on the eye, and no bright flashes directed into your pupils (unlike the ERG, which does use flashes). The electrodes sit on the skin beside your eyes, not on the eyes themselves. The main challenge is staying alert and consistently moving your eyes on cue during 30 minutes of alternating darkness and light. Some people find the dark phase slightly tedious, and the transition to bright light can feel momentarily uncomfortable, but the gradual ramp-up minimizes this.
Pupil-dilating drops are not typically required for the EOG, though your clinic may dilate your pupils if other tests are being performed during the same visit. The results are available immediately after the recording is complete, and your doctor can usually discuss them the same day.