Clinical Considerations
The ERG provides objective retinal functional data and is therefore important in the diagnosis, management, and follow-up of retinal disease. Symptomatic indications include night blindness, in which the potentially blinding rod–cone dystrophies must be distinguished from the relatively benign congenital stationary night blindness (CSNB). The dystrophies are associated with markedly abnormal a-waves in the dark-adapted bright-flash ERGs; CSNB is usually associated with a normal a-wave and a “negative” ERG waveform (see Fig 3-2). Other symptomatic indications include photophobia, which indicates generalized cone dysfunction (as in cone dystrophy), and photopsia or shimmering, which can sometimes signal the development of autoimmune retinopathy, possibly paraneoplastic. The ERG is increasingly used in the assessment and monitoring of inflammatory disorders such as birdshot chorioretinopathy; objective functional data allow clinicians to make management decisions with more confidence. The ERG facilitates an objective assessment of disease severity, aiding in decisions on when and how to treat; following treatment, it provides a valuable measure of treatment efficacy that is more sensitive than conventional clinical parameters.
ERGs must always be taken in a clinical context, and to enable accurate ERG diagnosis, a careful clinical history should include previous drug and/or surgical treatment as well as a family history. Results are diagnostic (pathognomonic) only for 3 relatively rare inherited disorders: bradyopsia (mutation in RGS9 or R9AP), enhanced S-cone syndrome (NR2E3), and “cone dystrophy with supernormal rod ERG” (KCNV2).
The ERG can be useful in assessing patients with vascular disease. In patients with central retinal artery occlusion (CRAO), the ERG is characteristically negative, reflecting the dual blood supply to the retina; the photoreceptors are supplied via choroidal circulation, but the central retinal artery supplies the inner nuclear layer. Thus, the b-wave amplitude is reduced but the a-wave is relatively preserved. In eyes with central retinal vein occlusion (CRVO), a negative ERG or delay in the 30-Hz flicker response suggests significant ischemia.
The ERG can also be helpful in determining the carrier state of individuals with X-linked disease. For example, carriers of X-linked RP usually have abnormal ERG findings that reflect lyonization, even with a healthy-appearing fundus. However, in choroideremia, carriers usually exhibit a normal ERG response despite an abnormal fundus appearance (also resulting from lyonization).
Electroretinography is suitable for use with children of all ages, providing objective functional data for patients who may not be able to describe their symptoms. In addition, the ERG may reveal retinal abnormalities prior to the development of fundus abnormalities. For young subjects, sedation, general anesthesia, or eyelid electrodes may be used. The latter are usually well tolerated and require neither sedation nor anesthesia. Interpretation of pediatric ERGs involves special considerations. Adult ERG values are not reached until 6–9 months of age, and, if general anesthesia is used, the effect of the anesthetic on the ERG must be considered.
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Johnson MA, Marcus S, Elman MJ, McPhee TJ. Neovascularization in central retinal vein occlusion: electroretinographic findings. Arch Ophthalmol. 1988;106(3):348–352.
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Vincent A, Robson AG, Holder GE. Pathognomonic (diagnostic) ERGs. A review and update. Retina. 2013;33(1):5–12.
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.