Electro-oculography

The electro-oculogram (EOG) assesses the health of the RPE and its interaction with the photoreceptors by measuring the corneo-retinal standing potential during dark adaptation (DA) and light adaptation (LA). The standing potential, which reflects the voltage differential across the RPE, is positive at the cornea (Fig 3-5). Estimates of trans-RPE potential range from 1 to 10 mV.

For an EOG test, the patient makes fixed 30° lateral eye movements for approximately 10 seconds each minute during 15 minutes of DA, and again during a 12-minute period of LA (Fig 3-6). The amplitude of the signal recorded between electrodes positioned at medial and lateral canthi reaches a minimum after approximately 12 minutes of DA—the dark trough—and a maximum at approximately 8 minutes of LA—the light peak. The ratio between the amplitude of the light peak to the dark trough is expressed as a percentage (the Arden index or ratio). A normal light rise will be greater than 170% and requires normally functioning photoreceptors in contact with a normally functioning RPE. The light peak reflects progressive depolarization of the RPE basal membrane via mechanisms that are not fully understood; however, the protein bestrophin is implicated in the final opening of chloride channels.

Figure 3-5 Electrical circuit of the standing potential. The retinal pigment epithelium (RPE) cells generate a voltage from apex to base due to the different ionic permeability characteristics on each surface and the presence of impermeable tight junctions between the cells. Changes in the voltage across the apical or basal RPE membrane are reflected in the standing potential and are measurable clinically as the c-wave on ERG or with electro-oculography. M = Müller cells.

(Modified with permission from Steinberg RH. Monitoring communications between photoreceptors and pigment epithelial cells: effects of “mild” systemic hypoxia. Friedenwald Lecture. Invest Ophthalmol Vis Sci. 1987;28(12):1888–1904.)

Any disorder of rod photoreceptor function will affect an EOG, and the light rise is typically severely reduced in an EOG of any widespread photoreceptor degeneration, including RP. However, the EOG is principally used in clinical practice in the diagnosis of bestrophin mutations (see Chapter 13) and AZOOR (acute zonal occult outer retinopathy). In patients with Best disease, a dominantly inherited disorder caused by mutations in BEST1, a severely reduced or absent EOG light rise is accompanied by a normal ERG response. Severe loss of the EOG light rise is also seen in patients with autosomal recessive bestrophinopathy (ARB). ARB is a recessively inherited progressive retinal dystrophy and, unlike Best disease, requires biallelic mutation. Affected patients have abnormal ERG responses, but not sufficiently abnormal to explain the degree of EOG response abnormality. Also unlike Best disease, ARB carriers do not show an EOG response abnormality. The EOG findings in patients with adult vitelliform macular dystrophy may be mildly subnormal, but they are not as reduced as much as in Best disease.

• Arden GB, Constable PA. The electro-oculogram. Prog Retin Eye Res. 2006;25(2):207–248.

• Burgess R, Millar ID, Leroy BP, et al. Biallelic mutation of BEST1 causes a distinct retinopathy in humans. Am J Hum Genet. 2008;82(1):19–31.

Figure 3-6 The clinical electro-oculogram (EOG). A, After electrodes are fixed to an area near the medial and lateral canthi, the subject is asked to look back and forth between 2 fixation targets (dimly lit lights), which are separated by a distance that results in a 30° horizontal eye movement. As each eye moves, the voltage between skin electrodes varies in proportion to the size of the standing potential of the eye (the voltage differential across the RPE). B, Plot of the amplitude of the oscillations. As the eyes turn toward the positive electrodes, an increased potential is measured; thus, the slow back-and-forth motions result in the relatively square-looking voltage curve. During testing, the standing potential diminishes to a minimum in the dark (the dark trough) and then rises to a maximum after the light is turned on (the light peak). In clinical practice, the EOG result is usually reported as the ratio between the light peak and the dark trough expressed as a percentage, the Arden index or ratio.

(Illustrations by Mark Miller.)

Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.