Multifocal Electro Retinogram (MFERG)
The multifocal ERG (mfERG) is a test to determine local retinal function by recording from the corneal surface of the eye. The test is similar to the more commonly recorded full field ERG that tests the function of the whole retina. The mfERG is recorded with dilated pupils, and thin fiber electrodes called DTL electrodes are lightly placed underneath the lower lid (both eyes) of the patient. The patient is asked to look at the center of a display consisting of an array of black and white hexagons on a video monitor. The number of hexagons can vary from 37 to 241; an array of 103 hexagons is most commonly used. The hexagon sizes are scaled to obtain responses from same number of retinal photoreceptors (the cone photoreceptors) and to produce equal ERG responses at all locations. During recording, the display alternates between black and white in a predetermined sequence for each hexagon. One complete recording from each eye takes about 8 minutes and for the comfort of the patient the recording is broken into 15 second segments to allow the patient to blink. At the end of the recording, ERG responses are calculated by the recording system for all hexagon locations. By comparing the responses from each retinal location with those from age-matched normal eyes, it can be determined if there are any retinal areas with abnormal function.