An electroretinogram (ERG) tests the electrical response of the light-sensitive cells in the retina. The retina is a tissue layer in the eye responsible for receiving light, converting it into neural signals, and sending the information to the brain for color & visual recognition.
The retina is made up of ganglion cells that are responsible for transmitting images to the brain, photoreceptors (rods and cones), responsible for detecting light and specialized cells. The ERG, more specifically, measures electrical signals from the photoreceptors, Muller and bipolar cells. There are about 7 million cones and 120 million rods in the eye. The rods are sensitive to light and the cones are responsible for color sensitivity.
ERG can be conducted together with other eye tests, such as dark adaptometry testing and electrooculography (EOG).
Also Known As
Full-field ERG (ffERG)- It is also known as standard or flash ERG. This is the most common form of ERG testing. ffERG assesses the general function but can’t provide specific information about various sectors of the retina.
Multifocal ERG (mfERG)- Unlike full-field ERG, the stimuli used in testing are densely arranged black & white hexagonal boxes. These elements change from light to dark independently.
Pattern ERG (PERG)- Provides information about ganglion cells and generalized macular function.
Focal ERG- This type of ERG is used for detecting tiny lesions or pathologies that could go unnoticed by a full-focal ERG. Due to errors such as light scattering, focal ERG is used in research rather than clinical testing.
This is a procedure that is conducted at the eye specialist’s office by:
- An ophthalmologist
- An optometrist
The electroretinogram is used to diagnose various retinal diseases. It is an important test as it helps the doctor conclusively determine whether the patient has an acquired or inherited retinal disorder. The eye specialist will then be able to recommend treatment or surgery if needed.
There are various purposes of conducting an ERG such as:
- To determine the state of retinal function.
- To confirm the progression of retinal degeneration.
- To check for visual function in infants and children.
- For early detection of damage to the retina.
This test can also be carried out as laboratory experiments e.g., in drug development and clinical trials to determine the efficacy of drugs and treatments (both existing & new).
Preparation & Expectation
ERG is usually painless and without complication. In exceptional cases, such as when conducting the test on young children, sedation may be necessary. The test takes an average of one hour, depending on whether or not anesthesia will be administered. Patients are asked not to wear any eye makeup on the day of testing.
The patient will be instructed to either lay down or be seated. The specialist will then put dilating eyes drops into the eyes. The patient will sit in total darkness for 20 minutes to allow the eyes to adapt to darkness. The rods will be stimulated and the cones suppressed.
After the 20 minutes, the specialist will put anesthetic/numbing drops into the patient’s eyes. The eye specialist will proceed to gently put in the electrodes in the eye. Electrodes come in many forms. Some are contact lenses, while others resemble fine threads that are laid on each eye. A third electrode is put on the forehead.
The specialist starts to introduce flashes of light that stimulate the retina. The first flash of light is very dull to stimulate the rods. The flashing lights’ intensity is progressively increased. This stage is called the dark-adapted phase and lasts about 15 minutes. The electrode senses the eyes' response to the light. It is then amplified and displayed on a monitor.
The patient is left alone in medium bright light for about ten minutes to suppress the rods and stimulate the cones. Another series of light flashes is introduced and the electrode records the response. Here, the flashes appear in much faster frequencies than in the dark-adapted phase. This is the light-adapted phase, and it also takes about 15 minutes.
Results will show standard wave patterns that respond accordingly to each flash of light. Normal results are supposed to show a-wave patterns (photoreceptor alertness) and b-wave patterns (Muller. bipolar cells alertness) in both adapted phases.
Abnormal ERG readings can detect the specific abnormality as well as the cells that are affected by the disease. Abnormal results may indicate any of the following:
- Giant cell arteritis- Causes quick and painless permanent vision loss in one or both eyes.
- Retinal detachment
- Vitamin A deficiency- Which leads to choroideremia (night blindness).
- Retinal abnormalities
- Retinoblastoma- The cancer of the retina.
- Cone rod dystrophy- Vision loss caused by deteriorating photoreceptors.
- Achromatopsia- Which is total color blindness.
- Retinoschisis- Possible farsightedness due to the abnormal splitting of the retina's neurosensory layers
- Retinitis pigmentosa- The loss of peripheral and night vision.
- The eye specialist may start a course of treatment to treat diagnosed conditions. S/he may also decide to conduct further tests. You may need to undergo surgery for issues such as retinal separation.
Risks & Complications
There are no serious complications associated with the procedure. The procedure gives the patient a feeling similar to an eyelash in the eye because of the wires that rest on the eyes during the procedure. This may persist for several hours.
The patient is advised to refrain from scratching their eyes for at least an hour after completion of the ERG. This helps in reducing the chances of corneal abrasion, which is a scratch on the eye’s surface.
Sometimes, patients get mild discomfort long after the procedure. If a patient reports this discomfort, the eye specialist should check for corneal abrasion.