An electroretinogram is a diagnostic eye test that evaluates the functional ability of the light-sensitive photoreceptors of the back of the eye, the rods and cones, and their interconnecting nerve cells in the light-detecting membrane of the eye, the retina. During the examination, an ophthalmologist positions an electrode on the cornea, the clear window at the front of the eye, to gauge the electrical responses of the rods and cones to light. In the course of an electroretinogram, the patient watches a standardized light display, and the rods and cones generate electrical impulses that are measured by the electrode, with both the amplitude and duration charted. Two types of analysis are possible using two types of visual stimuli including stroboscopic flashes of light for flash electroretinogram and alternating checkerboard patterns for pattern electroretinogram. An abnormal electroretinogram suggests dysfunction of the retinal photoreceptors due to hereditary conditions such as retinitis pigmentosa, or diseases affecting the retina, such as arteriosclerosis, retinal detachment, or vitamin A deficiency.
During an electroretinogram, the patient lies down or sits in a comfortable position. The patient's pupils are dilated. Numbing drops are then placed into the eyes. With the eyelids held open with a speculum, an electrode is lightly situated on each eye with a device similar to a contact lens, and an additional grounding electrode is placed on the skin. Each electrode then transmits information regarding the electrical activity of the rods and cones to a monitor.
An electroretinogram contains two parts. There is an initial a-wave that deflects downward, representing the photoreceptors converting light to an electrical signal. The second wave, the b-wave, curves upward above the baseline. Bipolar and amacrine cells, other nerve cells in the retina, produce the b-waves. By examining these waves in relation to each other, specific types of disease can be pinpointed.
A flash electroretinogram is used to ascertain the health of the cone and rod cells. The patient’s eyes are first adapted to light by having the patient sit in a normally lit room. A flash electroretinogram performed on a light-adapted eye will display activity primarily arising in cone cells. The patient then sits in a dim room for 20 minutes, producing dark-adapted eyes. Performed after dark adaptation, the flash electroretinogram will then reflect the activity of the rod cells.
The interconnecting nerve cells, the ganglion cells, are assessed using pattern reversal electroretinography. Checkerboards of white and black squares are alternated so that each square changes from white to black in rapid succession. Electrical impulses recorded on the ERG indicate whether the ganglion cells are functioning normally. A pattern electroretinogram can be used in the evaluation of glaucoma, an optic nerve disease characterized by elevation of the intraocular pressure and slowly progressive death of the retinal ganglion cells.