Electroretinogram represents the result of a performed electroretinography, a special functional examination of the retina of the eye. The purpose of the measurement is to check the functionality of the light sensory cells of the retina (cones and rods). The electrical impulses generated by the rods and cones in response to given light stimuli are measured and recorded in the electroretinogram.
What is the electroretinogram?
The electroretinogram represents the result of a performed electroretinography, a special functional examination of the retina of the eye. The retina of the human eye contains two different types of photoreceptors, sensory cells that convert incident light stimuli into electrical nerve impulses and transmit them via the optic nerve (optic nerve) for image formation and “image processing.” Three different types of photoreceptors (cones) are located mainly in the area of the yellow spot (macula/fovea), the zone of sharpest vision and color vision. They are responsible for color vision in daylight and are correspondingly not very sensitive to light. In contrast, the highly light-sensitive rods are responsible for vision in low light conditions. The rods are mainly concentrated outside the yellow spot and are responsible for monochromatic, blurred, vision at night. Electroretinography can be used to check the proper function of the photoreceptors and diagnose certain diseases. The result of electroretinography is recorded in the electroretinogram (ERG). Due to the very different sensitivity to light between cones and rods, a distinction is made between light-adapted (photopic) conditions for checking cones and dark-adapted (scotopic) conditions for checking rods. The light stimuli converted into electrical nerve impulses by the photoreceptors are recorded by electrodes and recorded in the ERG. The most common method involves inserting tiny gold or platinum electrodes into the conjunctival sac without direct contact with the cornea. The previously used methods of placing electrodes directly on the retina in an elaborate procedure or using contact lenses with embedded electrodes are rarely used today. A method of gluing the electrodes to the skin has not proven efficient because of inaccurate results.
Function, effect, and goals
ERG provides information about the functionality of the conversion chain from the incident light stimulus to the generation of the resulting electrical nerve impulse in the retina. The procedure originally had the shortcoming that diseases and dysfunctions confined to specific regions of the retina, such as the yellow spot, were not always detectable. Therefore, three ERG variants are used depending on the diagnostic objective. These are the classic ERG, which is used to examine the functionality of the entire retina. The entire visual field is exposed to white light flashes of variable brightness and frequency. Specific diseases of the macula cannot be detected. In the second procedure, pattern ERG, striking high-contrast, usually checkerboard, patterns are played in black and white and the colors are reversed three times within one minute. The procedure can be used to detect functional abnormalities in the macular region. The third variation is multifocal ERG, in which small hexagonal areas of the retina are exposed at a time. This procedure also allows the detection of possible functional abnormalities in the area of the yellow spot. The various ERG procedures are an effective diagnostic method to detect inherited or acquired diseases of the retina and choroid. In addition, ERG procedures are also used to monitor the progress of non-curable retinal diseases and, in the positive case, to monitor the progress of therapy. The most important and most common hereditary disease causing gradual degeneration of the retina is rethinopathy pigmentosa, which affects all types of photoreceptors, i.e. cones and rods alike, and results in gradual deterioration of vision until complete blindness. Other degenerative diseases of the retina or choroid, some of which are very rare and caused by genetic defects, can also be diagnosed by an ERG.Examples are juvenile macular degeneration, which exclusively affects the yellow spot, or cone dystrophy, a hereditary disease in the course of which especially the rods, which are important for night vision, degenerate and become non-functional. Some acquired functional disorders and diseases of the retina and choroid can also be diagnosed by ERG. For example, retinal inflammation (retinitis), retinal detachments (retinal detachment) and age-related macular degeneration (AMD) can be diagnosed. The ERG can also make an important contribution to a correct diagnosis if damage to the retina is suspected due to chronic high blood pressure (hypertensive retinopathy) or if diabetic retinopathy is suspected. Another important use of the ERG is in the detection of optic nerve damage caused by chronically elevated intraocular pressure, as is symptomatic in glaucoma. Retinal damage caused by vitamin A deficiency or adverse side effects of certain medications or toxins can be narrowed down and diagnosed using an ERG.
Risks, side effects, and hazards
The particular advantage of retinography is that it is an objective procedure whose results are independent of the patient’s subjective state of mind. Particularly in certain, slow-moving, degenerations of the retina, hardly any symptoms are initially present. In such cases, the ERG can detect changes at an early stage, so that appropriate therapies can be initiated at an early stage and the affected person can adjust his behavior accordingly. All ERG procedures are painless and can also be classified as non-invasive, except for the fine thread electrodes that are inserted into the conjunctival sac. In rare cases, the measurement results may be falsified because the electrodes applied to the cornea have slipped and this circumstance has not been noticed. In a few cases, slight irritation, redness or burning of the eyes may occur, symptoms that usually disappear by themselves after a short time. No other risks are apparent.
