Electroretinogram

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Definition

The electroretinogram (ERG) is a diagnostic test that measures the electrical activity of the retina in response to a light stimulus. Electrodes embedded in a corneal contact lens measure a summation of retinal electrical activity at the corneal surface.

Components

a-wave: derived from the photoreceptor layers
b-wave: derived from the inner retina, predominantly Mueller and ON-bipolar cells

c-wave: derived from the retinal pigment epithelium and photoreceptors

Types

The full-field ERG measures the stimulation of the entire retina with a flashlight source under various types of retinal adaptation, primary dark (scotopic conditions) and bright (photopic conditions).This useful in detecting disease with widespread generalized retinal dysfunction i.e. cancer associated retinopathy, toxic retinopathies, and cone-rod dysfunction.


The multifocal ERG (mfERG) measures local retinal responses from up to 250 retinal locations within the central 30 degrees mapped topographically. This technology is especially helpful in distinguishing isolated maculopathies, which may have a normal full-field ERG.


The pattern ERG (PERG) uses pattern-reversal stimuli similar to VEP testing and captures retinal ganglion cell activity in the N95 waveform component. The PERG is used to detect subtle optic neuropathies. In demyelinating optic neuropathy, the PERG is relatively normal, while it may be abnormal is ischemic optic neuropathies

Abnormalities in various disease states

Disease entity
full-field ERG findings
multifocal ERG findings
Multiple evanescent white dot syndrome (MEWDS)
depressed a-wave responses
focal areas of retinal dysfunction
Vitamin A deficiency
marked rod dysfunction and elevated threshold of rods and cones on dark adaptation

Cone dystrophy markedly depressed photopic response and less affected scotopic response
Cancer associated retinopathy (CAR) significantly reduced a-wave and b-wave amplitudes
Melanoma associated retinopathy (MAR) extinguished rod responses, normal a-wave, reduced b-wave (electronegative ERG)


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Additional Resources

Electrophysiologic Testing in Disorders of the Retina, Optic Nerve, and Visual Pathway (Pearls Series) by Gerald Allen Fishman M.D. Publication Date: January 2, 2001 | ISBN-10: 1560551984 | ISBN-13: 978-1560551980 | Edition: 2

References

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Original article contributed by: Hema L. Ramkumar, MD
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