ERG is an abbreviation of Electro-Retino-Gram. This is an image written on paper of electrical currents that are created in the retina and go through the retina under the influence of a light stimulus. Like an EEG, an Electro-Encephalo-Gram, can measure the electrical current running through the brain at the scalp and it is also possible to measure the electrical current running through the eye on the outside of the eye. This is done with the help electrodes. Such an electrode usually is a kind of large contact lens.
This contact lens will be put onto the eye after the cornea has been anaesthetised by means of eye drops. The contact lens is transparent and by means of a wire connected with the ERG device. Now light stimuli can be offered. This can be done by making a patient look at a lamp or a television screen. The light stimuli vary with respect to shape, light intensity and speed of offering successive light stimuli. Sometimes the colour of the light stimulus varies.
Before the start of the actual measurement, the patient first has to sit in a dark room for some time. The retina completely calms down in the dark. In a calm retina a light stimulus will cause a stronger current and this is necessary. This involves very small electrical signals sent by the eye that are really difficult to record. By offering the same light stimulus a few times in succession and adding up the results using a computer, the ERG device can accurately show the electrical currents.
Dropping out of mainly cones and dropping out of mainly rods each results in its own typically changing ERG registration. In this way the ERG makes an important contribution to the research into Usher Syndrome of retinitis pigmentosa. Because of its great sensitivity, the study makes it possible to detect changes at a stage of the course of the disease in which no deviations are noticeable in the eyes. Even at a stage that there are not even any complaints!