The EOG measures the electrical difference that exists between the cornea and the retina, called the resting or standard potential of the eye. The cornea is approximately 6 m positive with respect to the retina, which changes with differing retinal illumination. The standing potential of the eye is generated mainly by the transepithelial potential across the pigmented epithelium of the retina. EOG's change under different states of retinal illumination.
ISCEV standards for electro-oculography recommend that EOG's be recorded under light-adapted and dark-adapted states Marmor (1999). The EOG is measured indirectly, by surface electrodes placed on either side of the eye. The patient is encouraged to look between two fixation lights. The eye acts as a dipole; as it moves towards one recording electrode or the other, the changing electrical signal is detected. Although the amplitudes of the saccadic eye movements remain the same, recorded EOG voltages decrease with dark adaptation, reaching a trough after a period of adaptation (the dark trough); then, after a period of light adaptation, the EOG increases to reach a peak (the light peak). One of the ways of evaluating the EOG is to record the ratio of the light peak to the dark trough; this is called the Arden ratio. The Arden ratio in young children (2-3 years) is comparable to that recorded in adults.
Clinical Use of Electro-Oculograms in Children
EOGs are most appropriate when diseases that affect the retinal pigment epithelium may be present. Fishman (1990) outlines those dystrophies of the pigment epithelium that may give rise to EOG abnormalities. The only disease that is consistently associated with abnormal EOGs, however, is Best (vitelliform) macular dystrophy. Best disease is an autosomal-dominant macular degeneration that may be congenital or may have an onset of up to 7 years of age.