Electroretinography, is used to measure the electrical responses of various cell types in the retina, including the light-sensitive cells (rods and cones) and the ganglion cells. Electrodes are placed on the cornea and the skin near the eye. During a recording, the patient is watching a standardized stimulus and the resulting signal is interpreted in terms of its amplitude (voltage) and time course. Stimuli include flashes (flash ERG) and reversing checkerboard patterns (pattern ERG). Applications are predominantly in ophthalmology, where the electroretinogram (ERG) is used for the diagnosis of various retinal diseases:
- Retinitis pigmentosa and related hereditary degenerations
- Retinitis pigmentosa sine pigmento
- 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 dystrophies
- Disorders mimicking retinitis pigmentosa
- Usher Syndrome
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.