Full-field electroretinography (ERG) is a visual electrophysiology test that measures the pan-retinal response by capturing the sum of responses from the entire retina when exposed to flashes of light. This test is essential for diagnosing and monitoring various retinal dysfunctions, including retinopathies, vascular conditions, and unexplained vision loss.
Performing a full-field ERG test
The full-field ERG test can be performed in both seated and supine positions using a Ganzfeld ColorDome or ColorFlash. This test utilizes noninvasive corneal electrodes to provide superior signal quality. During this test, patients should keep their eyes open as they are exposed to a series of light flashes. The full-field ERG necessitates both dark and light adapted phases to assess both rod and cone systems.
Photoreceptor stimulation
A standard full-field ERG test includes both dark-adapted and light-adapted phases. The dark-adapted phase tests the rod system using flashes of 0.01, 3, and 10 cd.s/m² intensity. The light-adapted phase employs a 3 cd.s/m² intensity flash and flicker to elicit responses from the cone system.
Rod response
Isolate rods by presenting very weak 0.01 cd.s/m² flashes to a dark-adapted patient.
Combined rod and cone
Present higher intensity 3 and 10 cd.s/m² flashes to dark-adapted patients to elicit combined rod and cone responses.
Cone response
Use 3 cd.s/m² flash and 30 Hz flicker stimuli on a 30 cd/m² background to isolate a cone response.
Full-field ERG waveforms
Each step of the full-field ERG test creates a unique waveform representing the cellular functioning of the retina. By assessing the shape, size, and timing of these waveforms, you can determine the health of the retinal layers. Small amplitudes typically reflect a reduced number of functioning cells while delayed responses can indicate cell disease or dysfunction.
Dark-adapted ERG
The standard full-field ERG test involves three dark-adapted steps that elicit responses from the photoreceptors, bipolar cells, and amacrine cells. The a-waves progressively increase in size as the light intensity rises, reflecting a corresponding increase in rod response with a minor contribution from cones. A digital filter separates the oscillatory potentials of the amacrine cells from the rest of the ERG signal.
Light-adapted ERG
During light-adapted testing, a background light is turned on to suppress the rod response. The cone system is stimulated by a bright single 3.0 cd.s/m² flash, along with a 30 Hz flicker. Abnormal light-adapted responses indicate conditions affecting the cone system.
Pediatric ERG
Did you know about the pediatric version of the full-field ERG test? This test uses skin electrodes and does not require dilation, anesthesia or a long dark-adaptation period.
Extended ERG tests
The International Society for Clinical Electrophysiology of Vision (ISCEV) released extended ERG tests for more extensive diagnostic investigation.
Dark-adapted red flash ERG
The dark-adapted red flash ERG elicits an x-wave response that can be used in the diagnosis of achromatopsia (rod monochromacy), cone dystrophy, and other cone system dysfunctions.
On-Off ERG
The On-Off ERG elicits bipolar and inter-retinal cell responses to investigate post-phototransduction or post-receptor dysfunctions. It may reveal details about Congenital Stationary Night Blindness (CSNB), melanoma-associated retinopathy, some forms of autoimmune retinopathy, and more.
S-cone ERG
The S-cone ERG tests the blue-light receptor pathway and is useful for diagnosing inherited retinal disorders. It also helps differentiate between rod monochromacy and S-cone monochromacy.
References
Perlman, I., Kondo, M., Chelva, E. et al. ISCEV extended protocol for the S-cone ERG. Doc Ophthalmol 140, 95–101 (2020). https://doi.org/10.1007/s10633-019-09730-6
Robson, A.G., Frishman, L.J., Grigg, J. et al. ISCEV Standard for full-field clinical electroretinography (2022 update). Doc Ophthalmol 144, 165–177 (2022). https://doi.org/10.1007/s10633-022-09872-0
Sustar, M., Holder, G.E., Kremers, J. et al. ISCEV extended protocol for the photopic On–Off ERG. Doc Ophthalmol 136, 199–206 (2018). https://doi.org/10.1007/s10633-018-9645-y
Thompson, D.A., Fujinami, K., Perlman, I. et al. ISCEV extended protocol for the dark-adapted red flash ERG. Doc Ophthalmol 136, 191–197 (2018). https://doi.org/10.1007/s10633-018-9644-z