An EOG (electro-oculogram) is a painless eye test that measures the electrical activity of the retinal pigment epithelium, a layer of cells at the back of your eye that supports your photoreceptors. Small skin electrodes near your eyes record changes in electrical potential as you look back and forth in dark and light conditions. The whole process takes roughly 30 to 45 minutes, and it’s primarily used to help diagnose inherited retinal conditions like Best disease.
What the Test Actually Measures
Your eye naturally produces a small electrical charge, roughly 5 millivolts, between the front (cornea) and the back (a structure called Bruch’s membrane). This “standing potential” comes from the retinal pigment epithelium (RPE), a single layer of cells that sits behind your photoreceptors and keeps them healthy. The standing potential changes when your eye shifts between darkness and bright light, and those changes tell your doctor whether the RPE and the outer retina are functioning normally.
The key thing the EOG captures is how strongly the RPE responds to light. A healthy RPE generates a noticeable rise in electrical potential when exposed to light after a period of darkness. If the RPE or the photoreceptor layer is damaged or diseased, that light rise is reduced or absent.
How an EOG Differs From an ERG
An ERG (electroretinogram) is another common retinal test, but it measures something different. The ERG records the mass electrical response of the entire retina to flashes of light. Its signal reflects the health of both the photoreceptors (outer retina) and the inner retinal layers, including supporting cells. It typically requires a contact lens electrode placed directly on the eye’s surface.
The EOG focuses specifically on the RPE and its interaction with the outer retina. One practical advantage: electrodes sit on the skin near your eyes rather than on the eyeball itself. Your doctor may order both tests together to build a fuller picture, since each one evaluates different retinal layers.
What Happens During the Test
Before the test begins, a technologist will place dilating drops in your eyes to widen your pupils. Dilation can take up to 45 minutes, so plan for some waiting time. Small electrodes are then placed on the skin near the inner and outer corners of each eye.
The test has two main phases:
- Dark phase: You sit in total darkness for about 15 minutes. During this time, two small fixation lights alternate every second for 10-second intervals, once per minute. You follow the lights with your eyes while the electrodes record the standing potential as it drops to its lowest point (the “dark trough”).
- Light phase: A bright background light comes on inside a bowl-shaped stimulator. The light may ramp up gradually over about 20 seconds for comfort. You continue looking back and forth between fixation points for another period while the electrodes track the rising potential until it reaches its peak (the “light peak”).
Throughout both phases, you keep your head positioned inside the stimulator bowl with your eyes open. The eye movements you make between the fixation lights are what generate the recordable signal, since the electrical dipole shifts as your eye rotates.
How to Prepare
There’s very little you need to do beforehand. You can eat normally and take your regular medications. The two important things: skip eye makeup on the day of the test (it can interfere with electrode placement and raise infection risk), and arrange for someone to drive you home, since your pupils will still be dilated afterward and your vision will be blurry for a few hours.
Understanding the Arden Ratio
Your results come down to one main number: the ratio between the light peak and the dark trough, commonly called the Arden ratio. The doctor divides the highest potential recorded during the light phase by the lowest potential from the dark phase. A higher ratio means the RPE responded strongly to light, which is a good sign.
Normal values depend on age. For a 10-year-old, the lower limit of normal is around 2.0. For a 60-year-old, it drops to about 1.7. Across studies, the lower cutoff for normal ranges from 1.5 to 2.0, which is why each testing lab establishes its own reference range. A ratio well below normal suggests the RPE or photoreceptor layer isn’t functioning properly.
Conditions the EOG Helps Diagnose
The EOG is most clinically valuable for Best vitelliform macular dystrophy, an inherited condition where yellowish deposits accumulate under the retina. In classic Best disease, the Arden ratio is severely reduced or essentially flat, even in early stages before vision loss is obvious. This makes the EOG useful for identifying carriers in affected families who may not yet show visible retinal changes.
That said, not every form of the disease produces an abnormal EOG. Multifocal variants of Best disease can have a completely normal Arden ratio, which is one reason genetic testing has become increasingly important alongside the EOG.
Beyond Best disease, the EOG can flag other diffuse disorders affecting the RPE and outer retina, including certain acquired retinopathies and retinal dystrophies involving widespread photoreceptor damage or thinning of the layer between the retina and its blood supply.
Medication Monitoring
The EOG was once considered a useful screening tool for retinal toxicity caused by antimalarial drugs like chloroquine and hydroxychloroquine, which are widely prescribed for conditions like lupus and rheumatoid arthritis. In practice, the test turned out to be unreliable for this purpose. A British Journal of Ophthalmology study found that even the best cutoff values yielded only about 61% sensitivity with 54% specificity, meaning the EOG missed a significant number of cases and flagged many false positives. The underlying rheumatic disease itself can also reduce the Arden ratio, further muddying the results. Today, other screening methods have largely replaced the EOG for drug toxicity monitoring.
Limitations of the Test
The EOG measures overall RPE function across the entire eye. It cannot pinpoint damage in a small area of the retina, so localized problems may not show up. The test also has natural variation from person to person, which is why interpreting a single result without baseline data can be tricky. Results are most meaningful when compared to previous readings from the same patient or when the abnormality is dramatic, as in classic Best disease.
Because the light rise requires both healthy photoreceptors and a functioning RPE, the EOG alone can’t always distinguish which layer is the source of the problem. That’s why it’s typically paired with an ERG, visual imaging, and sometimes genetic testing to narrow down a diagnosis.