Electroretinography (ERG) is an eye test used to detect abnormal function of the retina (the light-detecting portion of the eye). Specifically, in this test, the light-sensitive cells of the eye, the rods and cones, and their connecting ganglion cells in the retina are examined.
During the test, an electrode is placed on the cornea (at the front of the eye) to measure the electrical responses to light of the cells that sense light in the retina at the back of the eye.
How is an ERG done?
The patient assumes a comfortable position (lying down or sitting up). Usually the patient’s eyes are dilated beforehand with standard dilating eye drops.
Anesthetic drops are then placed in the eyes, causing them to become numb. The eyelids are then propped open with a speculum, and an electrode is gently placed on each eye with a device very similar to a contact lens.
An additional electrode is placed on the skin to provide a ground for the very faint electrical signals produced by the retina.
During an ERG recording session, the patient watches a standardized light stimulus, and the resulting signal is interpreted in terms of its amplitude (voltage) and time course.
This test can even be performed in cooperative children, as well as sedated or anesthetized infants. The visual stimuli include flashes, called a flash ERG, and reversing checkerboard patterns, known as a pattern ERG.
What do the electrodes do?
The electrodes measure the electrical activity of the retina in response to light. The information that comes from each electrode is transmitted to a monitor where it is displayed as two types of waves, labeled the A waves and B waves.
How are electroretinography readings made?
Readings during electroretinography are usually taken first in normal room light. The lights are then dimmed for 20 minutes, and readings are again taken while a white light is shined into the eyes.
The final readings are taken as a bright flash is directed toward the eyes.
Why is an ERG done?
An ERG is useful in evaluating both inherited (hereditary) and acquired disorders of the retina. An ERG can also be useful in determining if retinal surgery or other types of ocular surgery such as cataract extraction might be useful.
What is a normal outcome for an ERG?
A normal ERG shows a normal A- and B-wave pattern with appropriate increases in electrical activity with increased light intensities.
What does an abnormal ERG mean?
An abnormal ERG result suggests abnormal function of the retina due to diseases of the retina or abnormal function of the retina as occurs with other conditions such as arteriosclerosis (hardening of the arteries) involving the:
- eye vessels
- giant cell arteritis with eye involvement
- metabolic diseases called mucopolysaccharidoses
- detachment of the retina
- siderosis (excess iron deposits), and
- vitamin A deficiency.
How to Prepare for the Test
No special preparation is necessary for this test.
How the Test Will Feel
The probes that rest on your eye may feel a little scratchy. The test takes about 1 hour to perform.
The cornea may get a temporary scratch on the surface from the electrode. Otherwise, there are no risks with this procedure.
You should not rub your eyes for an hour after the test, as this could injure the cornea. Your doctor will discuss with you the results of the test and what it means for you.