Electrooculography

The electro-oculogram (EOG) refers to measurement of the standing potential of the eye. This potential difference is generated across the retinal pigment epithelium and manifests as a dipole between the back of the eye and the electropositive cornea (for a recent review see [25]). The ERG is a global response and allows assessment of the photoreceptor/RPE interface. A normal EOG depends on the integrity of the pho-toreceptors and a functioning RPE. A reduced EOG is generally accompanied by a reduction in the full-field ERG unless dysfunction is confined to the RPE (see Sect.9.6.1.2). The ISCEV-standard EOG is measured by recording the potentials generated by fixed excursion eye movements during a standard period of dark adaptation, and then during restoration to full photopic conditions [56]. The eye movement excursions are prompted by alternately flashing lights and accurate testing depends on the child's cooperation and ability to follow these fixation lights; it is rarely possible to test children younger than about 6 years of age. The EOG is usually expressed as a light peak:dark trough ratio, the Arden index.

Accurate diagnosis and phenotyping of retinal dystrophies often relies on the pattern of electrophysiological abnormality. The pattern ERG is used to assess central retinal function and the full-field ERG generalised retinal function. Patients with generalised retinal dysfunction and severe ERG abnormalities can have normal PERGs if the macula is spared. Conversely, patients with disease confined to the macula have normal ERGs, but the PERG P50 may be profoundly abnormal. Thus, optimal phenotyping requires the use of both techniques [40, 25]. The examples that follow are discussed in relation to common presenting symptoms. Such a classification inevitably results in a degree of overlap; selected examples are used here to illustrate this integrated approach to visual electrodiagnosis.

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