The Clinical Electro-Oculogram

The clinical electro-oculogram (EOG) measures the electrical activity of the retina and retinal pigment epithelium (RPE) in response to light stimulation. The test is especially useful in conditions affecting the structural integrity of the rods and the RPE. At rest, there is a positive “standing potential” of the eye that is maintained by differences in the ionic conductance of the apical and basal membranes of the RPE. Changes in the flow of ions across these membranes alter the standing potential of the eye; these can be recorded from horizontal saccadic eye movements under dark—and light-adapted intervals. Clinically, the standing potential falls during the dark adaptation phase and increases during the light-adapting phase. The ratio of the recorded saccadic amplitudes during these phases is used as a clinical index for the functional integrity of the rods and the RPE. The EOG may be abnormal in conditions affecting bestrophin, a calcium and chloride regulator localized in the RPE. Mutations in the gene encoding bestrophin (BEST1) result in conditions such as Best’s vitelliform macular dystrophy.