Electroretinography (ERG)

Electroretinography is used in veterinary medicine to diagnose various retinal diseases such as generalised progressive retinal atrophy (GPRA) and distinguish sudden acquired retinal degeneration (SARD) from optic neuritis or central blindness. It is also an important tool in preparation for cataract surgery when we are unable to visualise the retina beforehand.

An ERG measures the electrical responses of various cell types in the retina, including the light sensitive cells (rod and cones) and the ganglion cells. Electrodes are placed on the cornea and the skin near the eye. During a recording, the patient is anaesthetised or sedated and facing a standardised light stimulus. The resulting signal is interpreted in terms of its amplitude (voltage) and time course.

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 Muller cells. Depending on the species the ERG is taken from, the c-wave may be positive, negative, or absent in part or in whole.

It can be tricky to perform this procedure and comparisions of waves should ideally be made only with an age, sex and breed matched normal animal that has had an ERG performed under the same cirmcumstances. As a result there are no absolute values to produce a normal range with and interpretation of the results should be made in light of this and other clinical findings.