Electroretinography

Electroretinography is an eye test that can detect a number of retinal conditions.

Electroretinography (ERG) is an eye test that detects function of the retina (the light-detecting portion of the eye). The retina is comprised of layers of specialized cells, including photoreceptors (rods and cones), that detect light and ganglion cells that transmit images to the brain. Specifically, the ERG picks up electrical signals from the photoreceptors, as well as other cells (Muller cells and bipolar cells) that act as intermediaries between the photoreceptors and the ganglion cells. Abnormal ERG readings can detect certain abnormalities of these cell layers. During the test, a medical professional places an electrode on the cornea (at the front of the eye) to measure the electrical responses to light of these cells.

ERG is one type of ophthalmic electrophysiology test. Depending on which eye condition is being studied, ERG may be performed in conjunction with other tests, such as electrooculography (EOG) or dark adaptometry testing. ERG is usually well tolerated, painless, and medical professionals can perform ERG even in cooperative children and infants. Occasionally, sedation may be necessary.

The patient assumes a comfortable position either lying down or sitting up. An eye doctor dilates the patient's eyes with standard dilating eyedrops. Anesthetic drops are also given. The doctor then props the eyelids open with a speculum and gently places a contact lens electrode or an electrode resembling a fine thread on each eye. The physician places an additional electrode on the forehead skin.

During an ERG recording session, the patient looks into a bowl displaying different amounts of light. Retinal cells emit small electrical signals when stimulated by certain types of light. The ERG machine records the resulting electric signals' amplitude (voltage) and time course.

The visual stimuli vary; some are done with no light in the background (dark-adapted, or scotopic readings), and some are done with light in the background (light-adapted, or photopic readings). The light stimuli include flashes of light (flash ERG) and flickering lights.

The electrodes measure the electrical activity of the retina in response to light. The information that comes from each electrode transmits to a monitor where it displays as wave patterns.

Dark-adapted readings are taken when light flashes are presented after the patient has adapted to the dark for 20 minutes, and light-adapted readings are taken when light flashes are presented after the patient has adapted to light for 10 minutes. Generally, dark-adapted readings correlate with the health of the cells known as rods, while light-adapted readings correlate with the health of the cone cells.

An ERG is useful in evaluating both inherited (hereditary) and acquired disorders of the retina.

There are a number of retinal conditions in which the ERG may provide useful information, including

• retinitis pigmentosa and Usher syndrome,
• disorders that mimic retinitis pigmentosa (RP), as well as carriers of RP genes who otherwise show little sign of the disease,
• Leber's congenital amaurosis,
• choroideremia,
• congenital stationary night blindness,
• X-linked juvenile retinoschisis,
• achromatopsia,
• cone and rod dystrophies and degenerations,
• total retinal detachment, and
• retinal abnormalities due to metallic foreign bodies, autoimmune disease, inflammatory diseases, toxic drug damage, retinal vascular occlusion, malnutrition, and metabolic storage diseases.

While a standard ERG detects activity of the entire retina, the multifocal ERG tests different areas of the retina, looking for localized areas of abnormality. This test takes longer than a standard ERG.

A normal ERG shows an a-wave (photoreceptor activity) and b-wave (Muller and bipolar cells activity) patterns in dark-adapted (scotopic) and light-adapted (photopic) settings. Wave patterns that are diminished in size or delayed or prolonged in time provide clues about the types of damaged cells.

Abnormal ERG results provide clues as to which specific retinal cells are affected by disease. There are retinal diseases in which specific cells are missing or weak at birth, while other abnormalities are acquired over time.

Some patients experience mild ocular discomfort during or after the procedure, but if you have irritation following an ERG, you should let your eye doctor know, so that he or she can rule out a corneal abrasion (scratch of the surface of the eye).

The test does not hurt. However, the electrode that rests on the eye may produce a foreign body sensation like having an eyelash on the eye. This sensation may persist up to several hours following completion of the ERG.

The standard ERG typically takes about an hour. For the dark-adapted portion of the test, the patient sits in the dark for 20 minutes. Dim light flashes that gradually increase in brightness then appear and the retinal responses recorded. This part takes another 10 to 15 minutes. Next is the light-adapted portion of the test. The patient views a medium-bright light for 10 minutes. Once the eyes are light-adapted, another series of flashes and flickering lights are presented and the retinal responses recorded. This takes another 10 to 15 minutes.

A multifocal ERG may take an additional hour to complete.

One should not rub the eyes for an hour after an ERG (or any test in which the cornea has been anesthetized), so as not to injure the cornea.

An ophthalmologist or electrophysiologist who has special training in interpretation of ERGs will review the results of the test.