Electroretinography measures the electrical responses of various cell types in the retina, including the photoreceptors, inner retinal cells, and the ganglion cells. Electrodes are placed on the surface of the cornea or on the skin beneath the eye to measure retinal responses. Retinal pigment epithelium responses are measured with an EOG test with skin-contact electrodes placed near the canthi. 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. 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 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 followed by a b-wave. 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, evoked by an alternating checkerboard stimulus, primarily reflects activity of retinal ganglion cells.
Diagnostics
Clinically used mainly by ophthalmologists and optometrists, the electroretinogram is used for the diagnosis of various retinal diseases. 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.
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. Other ERG tests, such as the photopic negative response and pattern ERG may be useful in assessing retinal ganglion cell function in diseases like glaucoma. 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.
Other uses
In addition to its clinical diagnostic purpose, the ERG can be used during the course of drug development and in clinical trials 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". The ERG has been shown to differ in people with schizophrenia and may be useful in helping to differentiate schizophrenia and bipolar disorder.
