Electrophysiologic Examination Methods

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

Electroretinogram (ERG): This examination method uses electrodes to record the electrical response of the retina to flashes of light (Fig. 12.12 a). Photopic (light-adapted) and scotopic (dark-adapted) electroretinograms are obtained. 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 retina. A focal ERG can record the response of isolated areas of the retina.

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

Electro-oculogram (EOG): The electro-oculogram detects abnormal changes in the retinal pigment epithelium such as macular vitelliform dystrophy. This examination 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 alternately 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

Electroretinogram (ERG).

Electroretinogram (ERG).

Fig. 12.12 a Retinal potentials are recorded with a corneal contact lens electrode and skin electrode.

b Normal electroretinogram.

Fig. 12.12 a Retinal potentials are recorded with a corneal contact lens electrode and skin electrode.

b Normal electroretinogram.

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.

H 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 damage along the visual pathway. The VEP is not a specific examination of the retina such as an electroretinogram or electro-oculogram. This method is briefly discussed in Chapter 13, Optic Nerve.

314 12 Retina Eiectro-ocuiogram (EOG).

314 12 Retina Eiectro-ocuiogram (EOG).

left eye

potential left eye

Fig. 12.13 The eye forms a dipole in which the anterior pole is positive and the posterior pole is negative. The EOG records the change in position of the standing potential of the retina with two temporal electrodes.

potential

Fig. 12.13 The eye forms a dipole in which the anterior pole is positive and the posterior pole is negative. The EOG records the change in position of the standing potential of the retina with two temporal electrodes.

Left gaze Straight ahead Right gaze

EOG

potential

Dark

Subnormal

Abnormal

Normal

Subnormal

Abnormal

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