An electroretinogram, or ERG, measures the electrical activity of light-sensitive cells in your retina. It’s a diagnostic eye test that helps doctors determine whether the rods and cones at the back of your eye are responding to light the way they should. When these cells are damaged or deteriorating, the ERG picks up abnormal electrical signals before you might even notice vision changes.
How an ERG Works
Your retina contains two main types of light-sensitive cells: rods (which handle dim-light and peripheral vision) and cones (which handle color and detail). When light hits these cells, they generate tiny electrical signals that travel through layers of the retina and on to the brain. An ERG records these signals by placing an electrode on or near the surface of your eye while flashing calibrated lights.
The test produces a waveform with two key components. The first dip, called the a-wave, reflects how well your photoreceptors themselves are working. The second, larger peak, called the b-wave, reflects the health of deeper retinal layers, including the cells that relay signals from photoreceptors toward the optic nerve. By comparing the size and timing of these waves against normal values, your doctor can pinpoint which layer of the retina is affected and how severely.
What Conditions It Detects
ERGs are especially valuable for inherited retinal diseases like retinitis pigmentosa, a progressive condition that damages the photoreceptors and pigment layer of the retina. In these cases, the ERG often shows reduced signals well before standard imaging reveals structural damage, making it useful for early diagnosis and tracking how quickly the disease is progressing.
The test also helps evaluate acquired conditions. Central retinal vein occlusion, where blood flow to the inner retina is disrupted, produces a distinctly different ERG pattern than inherited diseases do. Doctors use ERGs to assess unexplained vision loss, monitor retinal toxicity from certain medications, evaluate how well the retina is functioning before surgery, and screen for conditions like cone-rod dystrophy and congenital stationary night blindness.
Types of ERG Tests
Not every ERG test works the same way. The version your doctor orders depends on what part of the retina they need to evaluate.
- Full-field ERG (ffERG): Flashes light across the entire retina and records a combined response from all the rods and cones at once. This is the standard version, best for widespread retinal diseases.
- Multifocal ERG (mfERG): Stimulates many small zones of the retina individually, producing a detailed map of function across the central retina. It’s particularly strong at detecting localized damage, such as early changes in glaucoma, where it outperforms other ERG types in correlating with structural nerve fiber loss.
- Pattern ERG (PERG): Uses a checkerboard pattern instead of flashes to specifically assess the ganglion cells, the retinal neurons closest to the optic nerve. It’s often used in glaucoma evaluation, though its diagnostic sensitivity tends to be lower than multifocal ERG for that purpose.
What Happens During the Test
Before a standard full-field ERG, your pupils are dilated and you sit in a dark room for at least 20 minutes. This dark adaptation period allows your rods to reach full sensitivity so the test can accurately measure their response. For certain specialized protocols, shorter adaptation times of around 5 minutes may be used when the doctor only needs to isolate cone-driven signals.
Once adapted, a small electrode is placed on or near your eye. The most common types are a thin fiber electrode that rests along your lower eyelid (generally well-tolerated) or a contact lens electrode that sits directly on the cornea. Contact lens versions give a stronger signal but can feel uncomfortable, so numbing eye drops are applied first along with a protective lubricating solution. A second reference electrode goes near your temple, and a ground electrode is placed on your forehead or earlobe.
With everything in position, a series of light flashes are presented. You simply look straight ahead while the equipment records your retina’s electrical responses. Each recording takes only seconds, but a full session with multiple stimulus conditions typically runs 45 minutes to over an hour when you include the dark adaptation period. You’ll need to keep your eyes still and avoid blinking during recordings, since eye movements can shift the electrode and distort results.
Side Effects and Recovery
An ERG is noninvasive and generally safe. The most common complaint is mild irritation from the electrode, especially with contact lens types. There is a small risk of a superficial corneal abrasion from the lens electrode, similar to what can happen with a standard contact lens. Using a thinner fiber electrode significantly reduces this risk and improves comfort.
After the test, your eyes may feel slightly scratchy or watery for a few hours. Your pupils will remain dilated from the drops, so bright light will be uncomfortable and your near vision will be blurry until the dilation wears off, typically within four to six hours. Most people return to normal activities the same day.
For young children and infants, the test can be more challenging. Short attention spans and difficulty holding still mean the recording environment needs to be calm, and skin electrodes (placed on the lower eyelid rather than on the eye) are sometimes used instead of contact types to reduce stress. A parent or caregiver typically stays in the room during the procedure.
How Results Are Used
Your doctor compares the amplitude (height) and timing of your a-wave and b-wave against established norms. A reduced a-wave suggests the photoreceptors are the problem. A reduced b-wave with a relatively normal a-wave points to trouble in the inner retinal layers instead. Both waves being diminished can indicate widespread retinal disease.
ERG results rarely stand alone. They’re typically combined with imaging, visual field testing, and clinical examination to build a complete picture. Where the ERG excels is in providing functional data: it tells your doctor not just what the retina looks like structurally but how well it’s actually working. That distinction matters for tracking disease progression over time, evaluating whether a treatment is preserving retinal function, and catching problems that imaging alone would miss.