Electroretinography: Purpose, Procedure & Results

If you’ve ever wished your eye doctor could just “plug in” your eyes to see how well they work,
electroretinography (ERG) is about as close as it gets. This painless test measures the tiny
electrical signals your retina produces in response to light. In other words, it records how
your eye’s wiring behaves when the lights go on and off.

ERG sounds technical (and, okay, it is), but understanding the basics can make the whole experience
far less intimidating. In this guide, we’ll walk through what electroretinography is, why your
doctor might order it, what actually happens during the test, and what those mysterious “waves”
in your results mean. By the end, you’ll know exactly what to expect and how ERG fits into
protecting your vision.

What Is Electroretinography?

Electroretinography is a diagnostic test that measures the electrical responses of the retina,
the light-sensitive layer lining the back of your eye. When light hits the retina, specialized
cells called rods and cones generate small electrical signals. ERG captures those signals and
turns them into a waveform your eye specialist can analyze.

A popular analogy is that an ERG is to the retina what an ECG (electrocardiogram) is to the heart.
An ECG shows how your heart’s electrical system is working; an ERG does the same for your retina,
giving your doctor an objective look at how well different retinal cells are functioning.

During the test, light flashes or patterns stimulate the retina. Electrodes placed on the eye
or nearby skin record the electrical activity and produce a series of peaks and valleys on a
graph. Those peaks and valleys correspond to different layers and cell types in the retina,
including photoreceptors (rods and cones), bipolar cells, and supporting cells.

Why Your Doctor Might Order an ERG

ERG is not a routine part of a basic eye exam. It’s usually ordered when your doctor needs more
detailed information about retinal function, especially when symptoms or imaging don’t fully
explain what’s going on. Think of ERG as the “deep dive” test for your retina.

Common reasons for electroretinography

• Diagnosing inherited retinal diseases. Conditions such as retinitis pigmentosa,
  cone-rod dystrophies, achromatopsia, and some genetic macular disorders can be detected and
  monitored with ERG. The test helps determine which retinal cells are affected and how severely.
• Evaluating unexplained vision loss. When vision is reduced but the front of
  the eye and optical media (cornea, lens, vitreous) look relatively normal, ERG can reveal
  hidden retinal dysfunction.
• Distinguishing retinal disease from optic nerve problems. If ERG is normal
  but vision is still poor, your doctor may look more closely at the optic nerve or brain. If ERG
  is abnormal, it points the finger back at the retina.
• Monitoring diabetic retinopathy and other acquired retinal conditions. ERG can
  help assess functional damage in conditions like diabetic retinopathy, retinal vein occlusion,
  and some forms of macular degeneration, sometimes showing dysfunction before changes are obvious
  on imaging.
• Assessing retinal toxicity from medications. Certain drugs (for example, some
  antimalarials or chemotherapies) can affect the retina. ERG helps detect early functional changes
  so treatment can be adjusted if needed.
• Pre-surgical and research evaluations. ERG may be used to evaluate candidacy for
  retinal surgery, gene therapy, or clinical trials by documenting baseline retinal function.

In short, if your doctor wants to know how your retina is functioning at a cellular levelbeyond
what can be seen with a regular exam or imagingthey may send you for an ERG.

Types of Electroretinography Tests

Not all ERGs are the same. Your doctor may choose a specific type depending on what part of the
retina they are most concerned about.

Full-field (Ganzfeld) ERG

A full-field ERG exposes the entire retina to flashes of light inside a dome or bowl-shaped device
(called a Ganzfeld). This test measures the combined response of rod and cone cells across the whole
retina. It’s especially useful for inherited retinal diseases that affect large areas of the retina,
such as retinitis pigmentosa.

During full-field ERG, recordings are taken under dark-adapted (scotopic) conditions to emphasize
rod function and under light-adapted (photopic) conditions to highlight cone function. Comparing
these responses helps pinpoint which cell types are affected.

Multifocal ERG (mfERG)

Multifocal ERG zeroes in on specific regions of the retina, particularly the maculathe area
responsible for sharp, central vision. Instead of uniform flashes, the test uses a pattern of
many small hexagons that flicker in a controlled sequence. The computer then maps the electrical
responses from different zones.

mfERG is especially helpful for conditions that primarily affect the central retina, such as some
forms of macular degeneration, macular edema, or drug toxicity that targets the macula.

Pattern ERG (PERG)

Pattern ERG uses alternating black-and-white checkerboard or grating patterns rather than simple
flashes. This technique stresses the central retina and retinal ganglion cellsthe cells whose
fibers form the optic nerve.

PERG can provide information about early glaucoma and other conditions affecting ganglion cells,
especially when structural changes on imaging are subtle but functional loss is suspected.

Other specialized ERG techniques

In some specialty centers, additional ERG variants may be used, such as focal macular ERG or
flicker ERG protocols tailored to specific diseases. The exact setup may vary, but the core idea
is the same: measure how the retina responds to light under controlled conditions.

How to Prepare for an ERG Test

The good news: ERG preparation is usually simple and low-stress. Your eye care team will give you
specific instructions, but here are common steps:

• Medication review: Tell your doctor about all medications and supplements you
  take. Most medicines do not need to be stopped, but your provider may review anything that could
  affect your eyes or your ability to sit comfortably for the test.
• Avoiding eye makeup: Mascara, eyeliner, or heavy creams around the eyes can
  interfere with electrodes or contact lenses, so you’ll typically be asked to come in with a
  clean face.
• Arranging a ride if needed: Your pupils may be dilated, and your eyes may feel
  a little light-sensitive afterward. Many people can still get themselves home, but having a
  driver is more comfortable, especially if your appointment is later in the day.
• Comfortable clothing: You’ll be seated in a darkened room for a while. Wear
  something comfortable and bring a light jacket if you tend to get cold.

You usually won’t need to fast, and in most cases you can take your usual medications as
prescribed, unless your doctor says otherwise.

Step-by-Step: What Happens During the Procedure

ERG may look a little futuristic, but it follows a predictable sequence. Here’s what most people
experience:

1. Check-in and consent. You’ll review your history, sign consent forms, and have
   an opportunity to ask questions. If your pupils need to be dilated, drops are placed first.
2. Dark adaptation (for some tests). For full-field ERG, you may sit in a dim or
   dark room for about 20–30 minutes so your rod cells are fully sensitized to low light. Think of
   it as a mini movie-theater experience, just without the popcorn.
3. Electrode placement. Depending on the lab, the recording electrode may be:
   • a special contact lens with an embedded electrode placed on the eye after numbing drops, or
   • a thin fiber, thread, or strip resting gently on the surface of the eye, or
   • adhesive skin electrodes placed just below the lid (commonly used in some modern handheld devices).

   Additional reference and ground electrodes are placed on the skin near the eye or on the forehead.

4. Light stimulation. You’ll look toward a light sourceoften inside a dome or
   screenwhile brief flashes or patterns are presented. For some parts of the test you’ll be in
   darkness; for others, in a gently lit environment. You’re usually asked to keep your eyes open
   and as still as possible.
5. Recording responses. Each flash or pattern generates a tiny electrical signal.
   The computer averages many responses to create a clean waveform. This is repeated under different
   conditions (dark vs light adapted, varying flash strengths, or pattern changes).
6. Wrapping up. The electrodes and any contact lenses are carefully removed, your
   eyes may be rinsed with lubricating drops if needed, and you’re free to go. The entire session
   generally takes 30–60 minutes.

Most people describe ERG as strange but not painful. The brightest flashes can be a little startling,
and the contact lens electrode may feel odd at first, but serious discomfort is uncommon.

Understanding Electroretinography Results

ERG outputs look like a series of waves on a graph, not a simple “pass/fail” readout. Two of the
main components your doctor may mention are:

• a-wave: An initial negative dip that mainly reflects photoreceptor activity
  (rods and cones).
• b-wave: A larger positive peak that primarily reflects inner retinal cells,
  especially bipolar cells and Müller cells.

Your eye specialist evaluates several features:

• Amplitude: How tall the waves are, which relates to the strength of the retinal
  response. Reduced amplitudes may indicate fewer functioning cells or decreased cell activity.
• Timing (implicit time): How long it takes for peaks to occur after a flash of
  light. Delayed responses can suggest slower retinal processing or disease affecting specific layers.
• Pattern of responses: Comparing rod-dominant and cone-dominant responses,
  as well as full-field vs multifocal or pattern ERG, helps pinpoint which regions and cell types
  are affected.

Importantly, ERG results are interpreted in context. Your doctor will consider your symptoms,
visual acuity, visual field tests, imaging (like optical coherence tomography), genetic testing
if relevant, and other lab results. An abnormal ERG doesn’t automatically mean you’re losing vision
quickly, and a normal ERG doesn’t rule out every eye conditionso avoid trying to self-interpret
the printout at home.

Risks, Side Effects, and Recovery

ERG is considered very safe and noninvasive. The most common issues are mild and short-lived:

• Temporary irritation or dryness. The contact lens or electrode can leave the eye
  feeling a bit scratchy for a short time. Lubricating drops usually help.
• Light sensitivity. After bright flashes and pupil dilation, your eyes may be
  more sensitive to light for a few hours. Sunglasses on the way home are your best friend.
• Mild redness. Some people notice slight redness from contact with the electrode or
  from rubbing their eyes afterward. This typically resolves quickly.

Serious complicationslike corneal injury or infectionare rare. Your eye care team takes precautions
with sterile equipment, numbing drops, and close monitoring. If your eye feels very painful, stays
red, or your vision worsens after the test, you should contact your doctor promptly.

Electroretinography vs Other Eye Tests

ERG is one piece of the diagnostic puzzle. You may also hear about other tests, such as:

• Optical coherence tomography (OCT): This imaging technique creates high-resolution
  cross-sectional images of the retina, showing its structure. OCT is fantastic for spotting swelling,
  thinning, or scarring, while ERG shows how well the retina actually works.
• Visual field testing: Maps out your peripheral and central vision, revealing
  blind spots or areas of reduced sensitivity. ERG helps explain why those visual field defects
  might be happening at the cellular level.
• Standard eye exam: Checks visual acuity, eye pressure, and the health of the front
  and back of the eye. ERG goes deeper by providing objective functional data that is not visible
  with a slit lamp or ophthalmoscope alone.

Together, these tests give a fuller picture: structure (what the retina looks like) plus function
(how it behaves). That combination helps your doctor diagnose conditions earlier and tailor your
treatment plan more precisely.

Living With a Condition That Requires ERG Monitoring

For many people, ERG isn’t a one-time event. If you have an inherited retinal disease, diabetic
retinopathy, or another chronic retinal condition, your specialist may repeat ERG testing
periodically to track changes over time.

While it can be unsettling to think about your retinal function changing, ERG offers a crucial
benefit: it can reveal subtle shifts before you notice major symptoms. That opens the door to
earlier interventions, clinical trial discussions, and more personalized planning for your visual
future.

If you feel anxious about repeated testing, don’t hesitate to talk to your care team. Many clinics
are happy to walk you through the process step-by-step, schedule longer appointments so you don’t
feel rushed, and discuss how the results will be used to guide your carenot to surprise you with
scary news.

Real-World Experiences with Electroretinography

Reading about ERG in theory is one thing; living through it is another. While everyone’s experience
is unique, a few themes come up again and again when patients describe what ERG is actually like.

“It looked scarier than it felt.”

The setup can look intimidating at first: wires, electrodes, a large dome, and lights that flash
in a dark room. Many people walk in thinking they’re about to experience something painful or
overwhelming. Then the test begins, and they realize it’s mostly about sitting still while lights
flickernot exactly a day at the spa, but far from torture.

The oddest part for many patients is the contact lens electrode. With numbing drops, it usually
doesn’t hurt, but you do feel that something slightly “plasticky” is sitting on your eye. After a
minute or two, most people adapt and focus instead on the instructions: “Look here, keep your eyes
open, try not to blink during the flashes.”

Small annoyances, big peace of mind

The biggest complaints are usually minor: watery eyes, a brief scratchy feeling afterward, or
temporary light sensitivity from dilated pupils and bright flashes. Some people describe the
light bursts as surprisinglike camera flashes going off in a dark room. Others find the dim
adaptation period oddly relaxing, a chance to sit quietly with no screens or notifications.

On the plus side, many patients say the test gives them a sense of control. Instead of just
wondering whether their retina is getting better or worse, they know that their doctor has an
objective, measurable way to track change. That can be especially reassuring in conditions that
progress slowly over time.

How clinicians see ERG

From the clinician’s side, ERG is a powerful tool for answering tough questions: Is this problem
truly retinal? Are rods or cones more affected? Is treatment helping? For inherited retinal
diseases, ERG helps build a clear functional profile; for acquired diseases like diabetic
retinopathy, it can highlight functional stress even when structural damage is still subtle.

As newer treatmentssuch as gene therapy, retinal implants, and advanced drug delivery systems
become more widely available, ERG will likely play an even bigger role in selecting candidates
and tracking outcomes. In that sense, saying yes to an ERG today is not just about understanding
your current vision, but also about keeping doors open for future options.

Tips to make your ERG visit easier

• Ask for a walkthrough. Before the test starts, ask the technician to show you
  the equipment and describe each step. Knowing what’s coming reduces anxiety.
• Practice steady breathing. Slow, even breaths can help you stay relaxed and
  keep your body still, which improves the quality of the recordings.
• Bring sunglasses. After bright flashes and dilation, stepping into daylight
  can feel intense. A good pair of sunglasses makes the ride home much more pleasant.
• Plan a low-key schedule after. If your eyes feel tired or light-sensitive,
  give yourself permission to rest and avoid heavy reading or screen time for a few hours.

Overall, most people find that electroretinography is more “weird” than painful, and that the
information it provides is worth the temporary inconvenience. When you understand why it’s being
done and what your doctor hopes to learn, ERG becomes less of a mysterious machine test and more
of a smart, high-tech ally in protecting your sight.

Bottom Line

Electroretinography is a sophisticated yet patient-friendly way to measure how your retina
responds to light. It helps eye specialists diagnose a wide range of retinal conditions,
distinguish retinal disease from other causes of vision loss, and monitor how your eyes respond
to treatment over time.

While the equipment may look high-tech and the waveforms may seem cryptic, you don’t need to be an
engineer to appreciate what ERG does for you. By providing an objective snapshot of retinal
function, it gives your doctor clearer answersand gives you more confidence that your eye care
plan is based on solid data, not guesswork.