The retina, the inside back wall of the eye, is made of light-sensing cells called rods and cones. These rods and cones detect light and turn it into electrical signals that are transmitted to the brain through the optic nerve. This is how we “see.” Rods lie mostly in the peripheral retina and help us see in low light or darkness. Cones are found mostly in the center of the retina, an area called the macula, and allow us to see fine detail and color.
ERG and macular degeneration
If you have been diagnosed with age-related macular degeneration or Stargardt disease, your doctor may order a special test that measures the eye’s electrical response to light. This test is called electroretinography (ERG) or electroretinogram.
What is electroretinography (ERG)?
An ERG detects and measures any problems your retina may have in processing light. The results can help your doctor better understand the health of your retina. You may also hear ERG called electrophysiologic testing.1-4 James Dewar of Scotland performed the first ERG on a human in 1877, but the test did not gain widespread acceptance until 1941.5
Types of ERG
There are two types of electroretinography: simple and multifocal. Both tests are performed basically the same way, but the multifocal test searches for specific abnormalities and takes longer. The basic test searches the whole retina and takes about one hour to perform.3-5
How the test works
While you are sitting, your doctor will dilate and numb your eyes with drops. Once the drops have taken effect, your eyes will be held open with an instrument called a speculum. Then, the doctor will place an electrical sensor on your eye.
This electrode measures the electrical activity on your retina when a light flashes. These measurements are recorded on a TV-like screen. The test is performed in normal light and in the dark. Some people report that the electrodes feel scratchy.1,2
What do the test results mean?
Normal test results will have a pattern of A, B, C and D waves that appear after each flash of light.5
Abnormal results appear as smaller or delayed waves and provide clues about the types of damaged cells. Doctors know that Stargardt disease and age-related macular degeneration, particularly the wet form of the disease, create abnormal ERG results.
Other conditions
Other conditions that may cause abnormal ERG results include1,2:
- Hardening of the arteries (arteriosclerosis) that has damaged the retina
- Retinal detachment
- Eye injuries
- Congenital night blindness and congenital retinoschisis
- Retinitis pigmentosa
- Vitamin A deficiency
- Certain medicines