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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.
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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.
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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), ...