Electroretinogram

The Electroretinogram is the result of an electroretinography, a special functional examination of the retina of the eye. The aim of the measurement is to check the functionality of the light sensory cells of the retina (cones and rods). The electrical impulses that are generated by the rods and cones on the basis of predetermined light stimuli and recorded in the electroretinogram are measured.

What is the electroretinogram?

In the retina of the human eye there are two different types of photoreceptors, sensory cells, which convert light stimuli into electrical nerve impulses and pass them on via the optic nerve for image creation and "image processing". Mainly in the area of ​​the yellow spot (macula / fovea), the zone of sharpest vision and color vision, there are three different types of photoreceptors (cones).

They are responsible for color vision in daylight and are accordingly less sensitive to light. In contrast, the highly light-sensitive rods are responsible for seeing in weak 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 that the photoreceptors are working properly and to diagnose certain diseases. The result of the electroretinography is recorded in the electroretinogram (ERG).

Because of the very different light sensitivity between cones and rods, a distinction is made between light-adapted (photopic) conditions for checking the cones and dark-adapted (scotopic) conditions for checking the rods. The light stimuli converted into electrical nerve impulses by the photoreceptors are picked up by electrodes and recorded in the ERG. The most common procedure is to insert 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 a complex process or using contact lenses with built-in electrodes are rarely used today. A method of sticking the electrodes to the skin has proven inefficient due to inaccurate results.

Function, effect & goals

The 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 method originally had the shortcoming that diseases and functional disorders that are limited to certain regions of the retina, such as the yellow spot, were not always recognizable. Therefore, depending on the diagnostic goal, three ERG variants are used. It is the classic ERG, which is used to examine the functionality of the entire retina.

The entire field of view is exposed to white flashes of light with variable brightness and variable frequency. Special macular diseases cannot be detected. In the second method, the pattern ERG, distinctive, high-contrast, mostly checkerboard-like patterns in black and white are played and the colors reversed three times within a minute. Functional anomalies in the area of ​​the macula can be detected with the method. The third variant is the multifocal ERG, in which small hexagonal areas of the retina are exposed. This procedure also reveals possible malfunctions in the area of ​​the yellow spot. The various ERG procedures are an effective diagnostic method for detecting inherited or acquired diseases of the retina and choroid.

In addition, ERG procedures are also used to monitor the progression of incurable retinal diseases and, in positive cases, to monitor therapy progress. The most important and most common hereditary disease that causes gradual degeneration of the retina is rethinopathia pigmentosa, which affects all types of photoreceptors, i.e. cones and rods alike, and results in a gradual deterioration in vision leading to complete blindness. Other, sometimes very rare, degenerative diseases of the retina or choroid, which are caused by genetic defects, can also be diagnosed with an ERG.

Examples of this are juvenile macular degeneration, which only affects the yellow spot, or cone dystrophy, a hereditary disease in the course of which the rods that are important for night vision degenerate and become inoperable. Some acquired dysfunctions and diseases of the retina and choroid can also be diagnosed by ERG. For example, retinal inflammation (retinitis), retinal detachments (ablatio retinae) and age-related macular degeneration (AMD) can be diagnosed.

The ERG can also make an important contribution to a correct diagnosis if there is suspicion of damage to the retina from chronic high blood pressure (hypertensive retinopathy) or if diabetic retinopathy is suspected. Another important area of ​​application of the ERG is the detection of damage to the optic nerve due to chronically increased intraocular pressure, as is symptomatic of glaucoma. Retinal damage caused by a lack of vitamin A or the undesirable side effects of certain drugs or toxins can be localized and diagnosed using an ERG.

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Risks, side effects & dangers

The particular advantage of retinography is that it is an objective procedure, the results of which are independent of the subjective condition of the patient. Especially with certain, slow degenerations of the retina, there are hardly any symptoms at first. In such cases, the ERG can recognize changes at an early stage so that appropriate therapies can be initiated at an early stage and the person affected can adjust their behavior accordingly.

All ERG procedures are painless and can also be classified as non-invasive, apart from the fine thread electrodes that are inserted into the conjunctival sac. In rare cases it can happen that the measurement results are falsified because the electrodes applied to the cornea have slipped and this fact was not noticed. In a few cases there is slight irritation, redness or burning sensation in the eyes, symptoms that usually resolve on their own after a short time. No further risks are discernible.