Electroretinography (ERG) is a test that measures the electrical response of the light-sensitive cells in your eyes.
These cells are called rods and cones, and form part of the back of the eye known as the retina. There are around 120 million rods in the human eye and six to seven million cones. The cones are responsible for providing the eye’s sensitivity to color and are more concentrated in the eye’s macula. The rods are more sensitive to light than the cones but not to color.
Your doctor may perform an ERG to determine if you have an inherited or acquired disorder of the retina such as:
- retinitis pigmentosa: a genetic disease causing loss of peripheral and night vision
- macular degeneration: loss of vision caused by the death of cells in the macula
- retinoblastoma: cancer of the retina
- retinal separation: detachment of the retina from the back of the eyeball
- cone-rod dystrophy (CORD): vision loss due to impaired cone and rod cells
An ERG may also be used to assess your need for retinal surgery or other types of eye surgery, such as the removal of cataracts.
Your doctor will ask you to lie down or sit in a comfortable position. Your eyes will have been dilated with eye drops in preparation for the test. The doctor will place anesthetic drops into your eyes, which will make them numb.
Your eyelids will then be held open with a device called a retractor. This will enable the doctor to carefully place a small electrode on each eye. The electrodes are about the size of a contact lens. Another electrode is attached to your skin so that it functions as a ground for the faint electrical signals made by the retina.
While the ERG is being recorded, you will be asked to watch a flashing light. The doctor will conduct the test in normal light and in a darkened room. The electrode enables the doctor to measure your retina’s electrical response to light. The responses recorded in a light room will mainly be from your retina’s cones. The responses recorded in a darkened room will mainly be from your retina’s rods.
The information from the electrodes is transferred to a monitor, where it is displayed and can be recorded. The information from the electrodes is displayed as two types of waves that are called a waves and b waves.
The a wave is a positive wave that originates mainly from your eye’s cornea. It represents the initial negative deflection of a flash of light. The b wave (positive deflection) follows. The plot of the b wave’s amplitude reveals how well your eye reacts to light.
If your results are normal, they will show the wave patterns of a normal eye in response to each flash of light.
Abnormal results may be linked to the following conditions:
- arteriosclerosis damage to the retina
- congenital retinoschisis (splitting of layers in the retina)
- congenital night blindness
- giant cell arteritis
- retinal detachment
- CORD (cone-rod dystrophy)
- certain medications
- vitamin A deficiency
There are no risks linked to the ERG test. You may feel a slight discomfort during the procedure. The placement of the electrode feels something like having an eyelash lodged in your eye. Your eyes may feel slightly sore for a short time after the test.
In very rare cases, some people suffer from a corneal abrasion from the test. If this happens, it will be detected early and is easily treated.
If you have continued discomfort following an ERG, you should contact the doctor who performed the test.
You should avoid rubbing your eyes for up to an hour after the test. This may cause corneal damage because they will still be numb from the anesthetic.
Your doctor will discuss your results with you. He or she may order further tests to assess your eye. If you have a disorder such as retinal separation or trauma, you made need eye surgery.
You may be prescribed medication to treat other retinal conditions.