ELECTRO-OCULOGRAPH

Typical Number in Hospital: 2

Cost Bands: 5

References: 9

There is a steady d.c. potential between the cornea and retina of the eye. electrodes placed on the skin each side of the eye record a zero voltage when the gaze is straight ahead but a negative or positive voltage as the eye moves to the left and right of this position. There is an almost linear relationship between the horizontal angle of the gaze and the voltage produced. The recording of this voltage is called the electro- oculogram (EOG). electrodes may also be placed above and below the eye to record vertical movements.

The electro-oculograph is commonly used for recording eye movements in sleep and dream movements and also for recording nystagmus, which is the involuntary eye movement which occurs during dizziness or vertigo. The EOG is also used in the eye clinic or electrophysiology laboratory for assessing the strength of the corneo-retinal potential during changes in ambient light conditions. Some disorders of the retina exhibit abnormal or absent changes in the corneo-retinal potential during dark and light adaptation processes. For these tests the patient is asked to move his eyes to focus on two fixation lights 15 degrees either side of centre to produce alternating voltages which are dependent on the corneo-retinal potential. As the ambient light conditions are changed the voltage will change producing a graph of this change on the electro-oculograph.

The instrument for this employs silver-silver chloride electrodes each side of the eyes and two recording amplifiers with frequency response from d.c. to 50 Hz. The potentials detected may vary from 0.05 to 3.5 mV. The recording system may be integrated with a generalized electrophysiology set used for recording the ERG, VEP/VER, the EMG, and possibly the EEG. The patient may be given verbal instructions to look at the fixation lights or the lights may flash alternately. Red fixation lights are normally used since these do not affect the dark adaptation process.