What is electroretinography (ERG)?
An electroretinography test also called an electrocardiogram measures the electrical response of light-sensitive cells in the eyes.
These cells are called rods and cones. These form in the back of the eye called the retina. The human eye has approximately 120 million rods and six to seven million cones.
The cones are responsible for the sensitivity to the color of the eyes. They are found mainly in the macula of the eye. Rods are more sensitive to light than cones, but they are not more sensitive to color.
Purpose of electroretinography test
Your doctor can perform electroretinography to find out if you have an inherited or acquired retinal disorder:
- Retinitis pigmentosa is a genetic disease that causes loss of peripheral and night vision
- Macular degeneration, which is the loss of vision due to cell death in the macula
- Retinoblastoma, which is a cancer of the retina
- Segmentation of the retina, which is the detachment of the retina from the back of the eyeball
- Rod-cone dystrophy (CRD), which is the loss of vision caused by weak rod-cone cells
- Electroretinography can help your doctor assess the need for other types of eye surgery, such as retinal surgery or cataract removal
Electroretinography tests which should be taken
The following tests are performed routinely. Each test takes 30 minutes, but not all tests are done on all patients. It depends on the clinical question that is asked. We also spend up to half an hour on basic tests that measure things like visual tests and color vision.
Visual evoked potentials (VEP)
This is a way of analyzing the brain waves that occur when the patient sees the waves on a screen or in a flashing light. By linking brain responses to stimuli, we can check the function of the optic nerve and the part of the brain that sees it. The sensors are attached to the scalp using a harmless and easily removable adhesive paste.
Flash electrocardiograms (ERG)
The electrical signals generated by the eye in response to flashing lights can be analyzed by recording sensors placed near the eye. Part of this test is done in the light and part of it follows in the dark for a period of 15-20 minutes. Therefore, we can analyze the global function of the photosensitive membrane (photoreceptors or rods and cones) of the retina.
Two sensors are glued to the temples. We place a fine thread along the eyelid so that it is against the eye or (with small patients) adhere a small sensor to the skin of the lower eyelid. This test is usually done with dilated pupils, for which eye drops are placed. Since these drops can cause temporary clouding and dilation of students, we recommend that patients do not drive themselves to their appointment.
Pattern electroretinograms (PERG)
Using the same sensors as Flash ERG, but with a sample stimulus, we can further evaluate the function of the optic nerve and the macula (the most sensitive central part of the retina).
It is another test of retinal function and is used to assess the integrity of the retinal pigment epithelium (RPE). It is the membrane of the retina that nourishes the light-sensitive cells. The sensors are placed on the skin on both sides of each eye and the patient is asked to perform a series of guided eye movements at half-hour intervals. Half the time is spent in the dark and the other half in bright light.
What happens in the electroretinography test?
The following occurs during electroretinography:
- Your doctor will ask you to lie down or sit in a comfortable position.
- They usually separate your eyes with eye drops for analysis.
- If your doctor places the electrode directly in your eye, they will put numbing drops in your eyes, making them numb.
They use a device called a retractor to hold the eyelids open. This allows a small electrode to be carefully placed in each eye. About the size of some kind of electrode contact lens. Another type is the fine thread that is placed on the cornea.
Your doctor will place another electrode on your skin to act as a ground for the faint electrical signals emitted by the retina. Depending on what your doctor is looking for, they may only place electrodes on the skin around the eye instead of the eye.
You will then see a flashing light. Your doctor will perform the test in normal light and a dark room. The electrode allows the doctor to measure the electrical response of your retina to light. The responses recorded in the light chamber come mainly from the retinal cones. The responses recorded in the darkroom come mainly from the retinal rods.
The information from the electrodes is transferred to the monitor. The monitor displays and records information. It appears as waves and b waves. Wave A is a positive wave emanating from the corner of the eye. This indicates the initial negative deviation of the light gauge rods and the cones blink. It follows a B wave or positive deflection. The graph of the amplitude of wave B reveals how well your eye responds to light.
How does electroretinography work?
When you sit down, your doctor will roll your eyes with the drops. After the drops have taken effect, your eyes are held open with a device called a speculum. Next, the doctor will place an electrical sensor in the eye.
This electrode measures electrical activity in your retina when the light is on. These measurements are recorded on a screen similar to that of a television. The electroretinography is done in normal light and the dark. Some report that the electrodes are scratched
If your results are normal, they will show normal eye wave patterns in response to each light.
Abnormal results may indicate any of the following conditions:
- Arteriosclerosis damage to the retina
- Congenital retinal sclerosis, which is the division of the membranes in the retina.
- Congenital night blindness
- Giant cell arteritis
- Retinal detachment
- Tapered rod dystrophy (CRD)
- Some drugs
- Vitamin a deficiency
- Diabetic retinopathy
- Open-angle glaucoma
Typical test results include a pattern of A, B, C, and D waves that appear after each flash of light.
Abnormal results appear as short or late waves and give clues as to the types of cells damaged. Doctors know that starangard disease and age-related macular degeneration, especially the wet form of the disease, can lead to abnormal electroretinography results.
Other conditions that can cause abnormal electroretinography results are :
- Hardening of the arteries that damage the retina (arteriosclerosis)
- Retinal detachment
- Eye injuries
- Congenital night blindness and congenital retinitis
- Retinitis pigmentosa
- Vitamin a deficiency
- Some drugs