Chapter 581

The electroretinogram (also known as full-field ERG) is a measure of retinal photoreceptor integrity and thus retina function. The basic technique consists of measuring the action potential produced by the retina when stimulated by light of variable intensity and color. ERG allows differentiation between the responses of the retinal rod and cone systems. A contact lens with an imbedded electrode is placed on the cornea, or a skin electrode is placed on the lower lid. A reference skin electrode is placed on the forehead. The ERG can be performed awake in infancy and later childhood but usually requires sedation or general anesthesia between the ages of 1 and 6 years. The first step of the ERG is dark adaptation, which is done by double patching both eyes for at least 20 minutes. Then, in a completely dark environment, the test is started. Sleeping or awake, quiet infants and children under sedation or anesthesia lie supine while the bowl-shaped machine is brought over their face. Older children can sit upright and their chin is placed on a rest within the bowl. As variable light stimuli are presented within the bowl, the electrical potential between electrodes is then measured and recorded as a waveform. In anxious children dim light adaptation (mesoptic vision) can be substituted.

The normal ocular ERG recording is biphasic, consisting of a negatively deflected a-wave directly generated by photoreceptors and a positively deflected b-wave produced by the cells that transmit the electrical potential generated by the rods and cones to the ganglion cells, which ultimately carry the message to the visual cortex. The amplitude and configuration of the wave is a measure of photoreceptor integrity and function. The ERG consists of a series of dark-adapted (scotopic) and light-adapted (photopic) recordings. The scotopic ERG measures the rod system function, and photopic stimulation assesses the cone system. The photopic ERG occurs after 10 minutes of relatively bright light adaptation, which causes the rods to be bleached out and nonresponsive. Likewise, the cone system is assessed by the flicker response using 30 Hz light stimulation, to which rods cannot cycle quickly enough to record.

The ERG is helpful in diagnosing several conditions, such as generalized (eg, retinitis pigmentosa) and localized genetic retinal degeneration (eg, macular dystrophies). It is also helpful in assessing retinal function following retinal vascular occlusions and in determining potential retinal function when the retina cannot be clearly viewed due to opaque media (eg, cataract). Although the ERG does not test visual acuity (which is a measure of foveal function, an area of retina too small for the ERG to assess, as it is a mass retinal cone or rod response), it is useful in assigning the anatomic location of visual compromise in children with poor vision and nystagmus. We recommend that all children with no other obvious cause for nystagmus receive an ERG. Specific pediatric considerations include Leber congenital amaurosis, rod monochromatism (achromatopsia), and congenital stationary night blindness (CSNB), ...

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