Electroretinography

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 are usually placed on the cornea and the skin near the eye, although it is possible to record the ERG from skin electrodes. 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. Stimuli include brief flashes of light (flash ERG) and reversing checkerboard patterns (pattern ERG). The flash ERG is usually recorded using an integrating sphere (referred to as a 'ganzfeld') to allow light to evenly enter the eye from all directions.

Applications are predominantly in ophthalmology, where the electroretinogram (ERG) is used for the diagnosis of various retinal diseases [ [http://www.nlm.nih.gov/medlineplus/ency/article/003388.htm Electroretinography] , U.S. National Library of Medicine, 11 april 2005 (accessed 19 January,2007)] . Inherited retinal degenerations in which the ERG can be useful include:
*Retinitis pigmentosa and related hereditary degenerations
*Retinitis punctata albescens
*Leber's congenital amaurosis
*Choroideremia
*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
*Achromatopsia
*Cone dystrophies
*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 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 and other conditions where the fundus cannot be visualized.

Other ERG tests, such as the Photopic Negative Response (PhNR) and pattern ERG (PERG) are useful in assessing retinal ganglion cell function in diseases like glaucoma.

The multifocal ERG is used to record separate responses for different retinal locations.

Electroretinograms can be broken down into three components: an initial a-wave, caused by extracellular ionic currents generated by photoreceptors during phototransduction, the b-wave, which corresponds to bipolar cell activity, and the later c-wave, which is generated by the retinal pigment epithelium and Müller cells. Depending on the species the ERG is taken from, the c-wave may be positive, negative, or absent in part or in whole. [ [http://webvision.med.utah.edu/ClinicalERG.html Clinical Electrophysiology] ,Donnell Creel, 2003 (accessed 19 January,2007)]

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 [ [http://iscev.org ISCEV Website] ]

References

See also

*Electrooculography
*Visual evoked potential