Maximal response ERG waveform from a dark adapted eye.
Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors (rods and cones), inner retinal cells (bipolar and amacrine cells), and the ganglion cells. Electrodes (DTL silver/nylon fiber string) are usually placed on the surface of the cornea for Full Field/Global/Multifocal ERG's and brass/copper electrodes are placed on the skin near the eye for EOG type testing. During a recording, the patient's eyes are exposed to standardized stimuli and the resulting signal is displayed showing the time course of the signal's amplitude (voltage). Signals are very small, and typically are measured in microvolts or nanovolts. The ERG is composed of electrical potentials contributed by different cell types within the retina, and the stimulus conditions (flash or pattern stimulus, whether a background light is present, and the colors of the stimulus and background) can elicit stronger response from certain components.
If a dim flash ERG is performed on a dark-adapted eye, the response is primarily from the rod system. Flash ERGs performed on a light adapted eye will reflect the activity of the cone system. Sufficiently bright flashes will elicit ERGs containing an a-wave (initial negative deflection) followed by a b-wave (positive deflection). The leading edge of the a-wave is produced by the photoreceptors, while the remainder of the wave is produced by a mixture of cells including photoreceptors, bipolar, amacrine, and Muller cells or Muller glia. The pattern ERG (PERG), evoked by an alternating checkerboard stimulus, primarily reflects activity of retinal ganglion cells.
Inherited retinal degenerations in which the ERG can be useful include:
- Retinitis pigmentosa and related hereditary degenerations
- Retinitis punctata albescens
- Leber's congenital amaurosis
- Gyrate atrophy of the retina and choroid
- Goldman-Favre syndrome
- Congenital stationary night blindness - normal a-wave indicates normal photoreceptors; absent b-wave indicates abnormality in the bipolar cell region.
- X-linked juvenile retinoschisis
- Cone dystrophy
- Disorders mimicking retinitis pigmentosa
- Usher Syndrome
Other ocular disorders in which the standard ERG provides useful information include:
- Diabetic retinopathy
- Other ischemic retinopathies including central retinal vein occlusion (CRVO), branch vein occlusion (BVO), and sickle cell retinopathy
- Toxic retinopathies, including those caused by Plaquenil and Vigabatrin. The ERG is also used to monitor retinal toxicity in many drug trials.
- Autoimmune retinopathies such as Cancer Associated Retinopathy (CAR), Melanoma Associated Retinopathy (MAR), and Acute Zonal Occult Outer Retinopathy (AZOOR)
- Retinal detachment
- Assessment of retinal function after trauma, especially in vitreous hemorrhage, dense cataracts, and other conditions where the fundus cannot be visualized.
The ERG is also used extensively in eye research, as it provides information about the function of the retina that is not otherwise available.
The multifocal ERG is used to record separate responses for different retinal locations.
The international body concerned with the clinical use and standardization of the ERG, EOG, and VEP is the International Society for the Clinical Electrophysiology of Vision (ISCEV).
In addition to its clinical diagnostic purpose, the ERG can be used during the course of drug development and in clinical trails for testing ocular safety and efficacy of new or existing drugs and treatment modalities.
A 2013 study by Nasser et al. found that the retinal dopaminergic response to eating a brownie is equivalent in magnitude to the response to a 20 mg dose of methylphenidate, which implies that the activity of dopamine neurons in the retina reflects brain dopaminergic activity. The study concludes that, if verified by further research, "ERG could provide the neurotransmitter specificity of PET at a much lower cost".
- Webvision at University of Utah
- Electroretinography, U.S. National Library of Medicine, 11 April 2005 (accessed 19 January 2007)
- Maa; et al. (2015). "A novel device for accurate and efficient testing for vision-threatening diabetic retinopathy". Journal of Diabetes and its Complications. 30: 524–32. doi:10.1016/j.jdiacomp.2015.12.005. PMID 26803474. Retrieved 3 February 2015.
- ISCEV Website
- Brigell; et al. (2005). "An overview of drug development with special emphasis on the role of visual electrophysiological testing". Doc Ophthalmol. 110 (1): 3–13. doi:10.1007/s10633-005-7338-9. PMID 16249953.
- Nasser, J.a.; Parigi, A. Del; Merhige, K.; Wolper, C.; Geliebter, A.; Hashim, S.a. (2013-05-01). "Electroretinographic detection of human brain dopamine response to oral food stimulation". Obesity. 21 (5): 976–980. doi:10.1002/oby.20101. ISSN 1930-739X. PMC 4964968.