Using the portable ERG to measure visual function in glaucoma
Reviewed by Yvonne Ou, MD
The retina contains many subtypes of ganglion cells that respond to different stimuli. For example, some types are more sensitive to gradations or contrast, some respond to movement or specific directions, or local outlines.
“When all of this information is put together,” explained Yvonne Ou, MD, “the retina can process all of the visual information presented.”
In addition to these subtypes, retinal ganglion cells (RGCs) can be classified into centered or off-center types. Off RGCs respond to decreases in light and RGCs to increases in light, 1 according to Ou, academic director of the Glaucoma division and vice president of postgraduate training in the Department of Ophthalmology at the University of California at San Francisco .
Experimental studies on glaucoma have demonstrated that preferential damage occurs in off RGCs in terms of synapse loss, dendritic retraction and RGC responses. “We believe that there is a selective vulnerability of alpha-off-transient RGCs in glaucoma, which has been identified in mice,” she said.
The next step was to find out what is going on in humans. It seems that the off cells are providing clues to the continued damage of glaucoma in the retina, the so-called canary in the coal mine.
“Are the active cells charcoal miners, that is, very hardy and resistant, and less susceptible to glaucomatous damage?” She asked, which then took her to the next step, assessing visual function.
Visual field tests are subjective, uncomfortable, and difficult for patients. In addition, they require expensive equipment. Ou suggested that using an objective measurement of visual fields with a portable electroretinography (ERG) machine may be a better mousetrap to identify any changes in the RGC on and off pathways.
“The activation and deactivation pathways have been explored with other ERG modalities in glaucoma and there is some evidence that a flicker stimulus can separate the activation and deactivation pathways in mice and possibly be in nonhuman primates and humans, ”she said.
We reported on a study in which a mouse model of genetic glaucoma had a reduced ERG response to a flicker stimulus that was most noticeable at the higher frequencies.
“We found this particularly interesting as it suggested that the off-trail is more vulnerable,” she said.
A new look at ERG
This technology is not currently the standard of practice for assessing visual function in patients with glaucoma due to the need for corneal electrodes, the equipment is cumbersome, and the exam takes time.
When examining the practical application of the portable device in the clinic, Ou and his colleagues questioned whether the portable ERG can objectively detect the relative changes in the on and off pathways.
In their study, the investigators tested the photopic flash, the flicker stimulus, the negative photopic response, and the custom sine wave stimulus, the latter of which is a sine wave at varying frequencies ranging from 50 to 0.3 Hz. The study included 59 eyes in the glaucoma group and 19 control eyes; the average age of the participants was 69 years. The demographic characteristics of the two groups were similar. The ERG response was measured using the RETeval device (LKC Technologies. Participants received upper and lower sinusoidal rate stimuli and the data was analyzed.
Or demonstrated the sinusoidal response obtained from controls and patients using both 25 and 5 Hz frequency stimuli. She reported that at 25 Hz there was a marked decrease in ERG amplitudes in patients with glaucoma in relation to witnesses. However, at 5 Hz, no difference was observed between the 2 groups.
“Quantification of the ERG response as a function of the frequency of the sinusoidal stimulus showed separation between controls and patients with glaucoma, especially at the higher frequencies,” Ou said. “When we evaluated the percentage difference from the controls, we saw about a 20% to 25% reduction in the highest frequencies in the amplitudes of the ERG. “
When asked about their opinion of the portable ERG test compared to the standard visual field test, 55% of patients reported no discomfort during the test, compared to 27% of patients who reported no discomfort with the test. visual field test.
“When evaluating the activation and deactivation pathways using a novel stimulus and a portable ERG device in glaucoma patients compared to controls, the portable ERG using a sinusoidal stimulus can provide an objective assessment that demonstrates greater off-pathway sensitivity in glaucoma, ”concluded Ou.
Yvonne Ou, MD
This article has been adapted from Ou’s presentation at the 2021 Virtual Meeting of the 25th Annual Glaucoma Symposium. She has no financial interest in this matter.
1. Ou Y, Ullian EM, Wong ROL, et al. Selective vulnerability of specific types of retinal ganglion cells and synapses after transient ocular hypertension. J Neurosci 2016; 36: 9240-9252.
2. Harazny K, Scholz M, Buder T, et al. Electrophysiological Deficits in the DBA / 2J Mouse Retina. Doc Ophthalmol Adv Ophthalmol 2009; 119: 181-197.