An electroretinography (ERG) test, also known as an electroretinogram, measures the electrical response of the light-sensitive cells in your eyes.
These cells are known as rods and cones. They 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 the eye’s color sensitivity. They reside mostly in your eye’s macula. The rods are more sensitive to light than the cones, but they’re not more sensitive 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, which is a genetic disease causing loss of peripheral and night vision
- macular degeneration, which is a loss of vision due to the death of cells in the macula
- retinoblastoma, which is a cancer of the retina
- retinal separation, which is a detachment of the retina from the back of the eyeball
- cone rod dystrophy (CRD), which is vision loss due to impaired cone and rod cells
An ERG may also help your doctor assess your need for retinal surgery or other types of eye surgery, such as the removal of cataracts.
The following occurs during an ERG:
- Your doctor will ask you to lie down or sit in a comfortable position.
- They’ll usually dilate your eyes with eye drops in preparation for the test.
- If your doctor is placing an electrode directly on the eye, they will place anesthetic drops in your eyes, which will make them numb.
- They’ll use a device known as a retractor to hold open your eyelids. This will enable them to carefully place a small electrode on each eye. One type of electrode is about the size of a contact lens. Another type is a fine thread placed on the cornea.
- Your doctor will attach another electrode to your skin so that it functions as a ground for the faint electrical signals made by the retina. Depending on what your doctor is looking for, they may only place electrodes on the skin around the eye instead of in the eye.
- You’ll then watch a flashing light. Your 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 transfers to a monitor. The monitor displays and records the information. It appears as 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 measuring rods and cones. The b-wave, or 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’ll show the wave patterns of a normal eye in response to each flash of light.
Abnormal results may indicate any of the following conditions:
There are no risks linked to the ERG. You may feel a slight discomfort during the procedure. If the electrode is put on the cornea, 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, your doctor can detect it early and treat it easily.
Monitor your condition after the procedure and follow all aftercare instructions your doctor gives you. If you have continued discomfort following an ERG, you should contact the doctor who performed the test.
Your eyes may feel sensitive after the test. You should avoid rubbing your eyes for up to an hour after the test. This may cause corneal damage because they’ll still be numb from the anesthetic.
Your doctor will discuss your results with you. They may order further tests to assess your eye. You may need surgery if you have a disorder such as retinal separation or trauma.
Your doctor may prescribe you medication to treat other retinal conditions.