Although patients typically show a conspicuous degeneration of the retina, ERG changes can precede ophthalmoscopically visible fundus changes in all genetic patterns of the disease. In fact, the ERG is considered to be the definitive test for this disorder. Patients with the autosomal recessive or the X-linked recessive forms of the disease have undetectable or much reduced ERG responses from an early age.

The autosomal dominant forms of RP usually have a later onset of symptoms and a less rapid progression. ERGs may be recordable from these patients. In early stages of the disease, both rod and cone ERGs can be recorded. As the disease progresses, some cone function may still be recorded after the rod response is extinguished.

Even in RP patients with "extinguished" ERGs, it is usually possible to record some signal from the cones. Berson and colleagues used the ERG to establish that large doses of Vitamin A may retard the degenerative processes in RP. If you are planning to use Vitamin A to treat patients with RP, you should follow them using the ERG.

Leber's congenital amaurosis is an inherited retinal disorder characterized by diffuse photoreceptor cell disease. This condition is not rare, and accounts for between 10% and 20% of all blindness in children. Once poor vision is suspected, an ERG showing markedly reduced amplitudes will indicate this disorder. This may spare the patient and family costly neurologic and radiologic examinations looking for a central visual disorder.