Retina: Electrophysiologic Examination Methods

Electrophysiologic Examination Methods

(electroretinogram, electro-oculogram, and visual evoked potentials; see Fig. 12.2a)

Electroretinogram (ERG): 

This examination method uses electrodes to rec-ord the electrical response of the retina to flashes of light (Fig. 12.12a). Photopic (light-adapted) and scotopic (dark-adapted) electroretinograms are obtain-ed. The electroretinogram (ERG) consists of a negative A wave indicating the response of the photoreceptors and a positive B wave primarily indicating the response of the bipolar cells and the supporting cells of Müller (Fig. 12.12b). A flicker ERG (repeated flashes) isolates pure cone response; a pattern ERG (such as a checkerboard) and oscillating potentials can be used to evaluate the inner layers of the retina. The ERG represents a summation response of the ret-ina. A focal ERG can record the response of isolated areas of the retina.

The classic indication for an electroretinogram is retinitis pigmentosa with early loss of scotopic and photopic potentials.

Electro-oculogram (EOG): 

The electro-oculogram detectsabnormal changesin the retinal pigment epithelium such as macular vitelliform dystrophy. Thisexamination method utilizes the dipole of the eye in which the cornea forms the positive pole and the retinal pigment epithelium the negative pole. The standing potential across cornea and retina in comparison to the cornea is measured indirectly with two temporal electrodes (Fig. 12.13). During the measuring process, the patient performs regular eye movements by alter-nately focusing on two lights. The standing potential is normally higher in the light-adapted eye than in the dark-adapted eye.  

The ratio of light-adapted potential to dark-adapted potential (Arden ratio) is obtained to evaluate the eye; this ratio is normally greater than 1.8. The ratio will be decreased in the presence of abnormal changes. The typical indication for an electro-oculogram is macular vitelliform dystrophy (Best’s vitelliform dystrophy) with a significantly decreased Arden ratio.

Visual evoked potential (VEP): 

This examination is used to diagnose dam-age along the visual pathway. The VEP is not a specific examination of the ret-ina such as an electroretinogram or electro-oculogram.