The electroretinogram (ERG) is recorded through an electrode placed on the surface of the eye, typically on the cornea. The ERG is produced by the action of the photoreceptors and by in the proximal retina such as bioplar and Müller cells. The ERG is normally stimulated by a Ganzfeld or mini-Ganzfeld, and reflects the state of the entire retina. Pattern stimulators can also be used, and can test specific locations of the retina. The response typically consists of a negative-going a-wave, followed by a positive-going b-wave. The leading edge of the a-wave provides a direct measure of photoreceptor activity, while the b-wave provides a reflection of the action of glial and other cells. By changing the stimulus conditions, it is possible to preferentially obtain responses from either the rod or cone systems.

Multi-Focal ERG
Multi-focal ERGs is a technique used to measure the function of localized portions of the retina. Multi-focal stimulation is from an alternating black/white hexagonal pattern from a ultra-high brightness video monitor. The result can be presented either as a 3-diemnsional ERG response “map” or waveforms from multiple regions of the retina recovered through the software’s complicated algorithm.

This test is useful in the differential diagnosis of unexplained acuity loss, ARMD, Stargardt’s, central serous retinopathy, glaucoma, and age-related maculopathy.
Learn more about Multi-Focal ERG

Pattern ERG
An ERG can also be performed using a alternating checkerboard stimulus on a pattern monitor to test the functionality of the entire retina, specifically the activity of ganglion cells or structures closely dependent upon ganglion cell integrity. The responses are much smaller in amplitude than those of the flash ERG. The pattern ERG is most commonly used as a testing method for glaucoma.

Retinal Ischemia Monitor (RIM) ERG
The Retinal Ischemia Monitor (RIM) is a specialized ERG protocol designed to assess the extent of retinal ischemia in an eye. To date, this test has proved to be highly effective in determining which eyes with central retinal vein occlusion (CRVO) will go on to develop neovascularization of the iris (NVI). This protocol also provides useful information in diabetic eyes.

ISCEV Standard ERG Protocol
In 1989, the International Society for Clinical Electrophysiology of Vision (ISCEV) developed a protocol to standardize ERG testing so test results could be compared worldwide. The protocol consists of five separate tests, each designed to evaluate different areas or functions of the eye.

1) Dim Scotopic Flash ERG
This is the first step in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG protocol. It is conducted with a -24 dB flash. In a dark-adapted eye, a dim flash tests a response arising from the rods, primarily the photoreceptors and associated glial cells.

2) Moderate Scotopic Flash ERG
This is the second step in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG protocol. It is conducted with a 0 dB flash. In a dark-adapted eye, a moderate flash tests a response from both the rods and cones.

3) Oscillatory Potentials (OPs)
This is the third step in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG protocol. The oscillatory potentials are high-frequency oscillations or wavelets seen on the leading-edge of the b-wave. The oscillatory potentials of the ERG are a sensitive and objective measure of retinal ischemia.

4) Moderate Photopic Flash ERG
This is the fourth step in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG protocol. It is conducted with a 0 dB flash. In a light-adapted eye, a moderate flash tests a response arising from the cones.

5) 30 Hz Flicker ERG
This is the fifth step in the International Society for Clinical Electrophysiology of Vision (ISCEV) standard ERG protocol. In a light-adapted eye, a flicker ERG tests a response arising from the cones. The flicker ERG has also been shown to be useful in patients with diabetic retinopathy.

User-Defined Protocols

The software in both the UTAS and the EPIC-4000 allows the user to personalize the testing procedure depending on the subject or parameter evaluated. For instance, the flash can be set to dim (-25 dB), moderate (0 dB), or bright (25 dB). The flicker setting can achieve frequencies up to 60 Hz.