ELECTRORETINOGRAPH

Typical Number in Hospital: 1

Cost Bands: 5

References: 9

When the retina of the eye is stimulated by a flash of light, there is a characteristic sequence of electrical potentials generated within the retina. The clinical electroretinogram (ERG) is a recording of these potentials as detected between an electrode on or close to the cornea and an indifferent electrode placed on the forehead, cheek or ear lobe. The corneal electrode may be in the form of a contact lens with a steel or silver wire embedded in the inner surface, or it may be a piece of gold leaf tucked under the lower lid close to the cornea. A skin electrode on the lower lid of the eye can be used instead of the corneal electrodes but the result is less satisfactory.

It is common to perform the test with the eyes 'dark adapted', that is after spending several minutes in complete darkness. Under these conditions the ERG response is relatively large (e.g. 200 [u]V) and produces a wave with several distinct components covering about two seconds, but the components used for clinical diagnosis (known as A and B-waves) occur in the first half second.

The flash of light is normally provided from a stroboscopic flash unit and it is repeated a number of times so that the resulting electrical response can be fed to a signal averager to improve the signal-to-noise ratio and to reduce artefacts such as the blink response. Thus the recording apparatus will include a preamplifier capable of dealing with input signals between 0 and 1 mV, with frequency response from 0 to 50 Hz (true d.c. response is not in fact required), a signal averager, and a display or recording device.

The ERG apparatus is normally found in the eye clinic or electrophysiological laboratory or it is part of a generalized instrument which may also be used for the electro-oculogram (EOG), electromyogram (EMG), possibly the visual evoked response (VER), and the electroencephalogram (EEG). The ERG is commonly used to assist in making a diagnosis of the various inherited disorders of the eye.