Glaucoma and Alzheimer’s disease patients appear to lose retinal ganglion cells (RGC) at about the same rate, according to a recent study in the Canadian Journal of Ophthalmology.
The investigation enrolled a small patient sampling: nine patients divided equally into three groups: a glaucoma group, an Alzheimer’s group and a group of healthy controls.
The researchers obtained postmortem paraffin-embedded retinal sections from the Human Eye Biobank for Research in Toronto. Patients were about 74 years old, and each group included two females and one male.
The team analyzed the macular, peripapillary, mid-peripheral and far-peripheral retinal regions to derive regional estimates of RGC density. Independent of the group, the researchers observed a progressive decrease in RGC density from the macula, peripapillary, midperipheral and far-peripheral retina.
In normal subjects, the mean RGC density was 189, 60, 24 and 7cells/mm for the macular, peripapillary, mid-peripheral and far-peripheral regions, respectively. In the glaucoma group, the mean RGC density was 184, 43, 18 and 5cells/mm for the same regions. In Alzheimer’s patients, the measurements were 117, 35, 14 and 5cells/mm.
In addition to quantifying multiple retinal sections per eye, the study implemented additional strategies to mitigate the limitations of the small sample sizes. The first was to express RGC loss as a percentage in the four different retinal regions. Using this method, the found that the glaucoma group showed little to no loss in the macular region, compared to an approximate 70% loss in the peripapillary, mid-peripheral and far-peripheral regions. In the Alzheimer’s group, researchers observed about a 60% to 70% loss of RGCs across the three regions compared with the healthy eyes. With the exception of the macula in glaucoma eyes, the study noted a consistent loss of RGCs in all retinal regions of glaucoma and Alzheimer’s eyes compared with the healthy group.
The second strategy normalized each subject’s RGC density to his/her own peripapillary region to determine whether there was a regional preferential loss of RGCs in the two peripheral areas. In all glaucoma and Alzheimer’s subjects, the investigators reported a relatively greater preservation of the RGCs in the far-peripheral compared with the mid-peripheral region. The macula was excluded from this analysis due to high variability relative to the peripapillary region.
Notably, the greatest difference between glaucoma and Alzheimer’s RGC density appeared in the macular region, where there was relatively little loss in glaucoma subjects, while losses in the peripapillary, midperipheral and far-peripheral regions were more comparable.
“Regardless of the limitations of this study, the implementation of RGC-specific immunohistochemical analysis, which has not been published elsewhere, provides the groundwork for future research with increased sample size and correlation with disease severity,” the researchers wrote in their paper.
Rawlyk B, Chauhan BC. Retinal ganglion cell loss in postmortem tissue of Alzheimer disease, glaucoma, and healthy normal subjects. Canadian Journal of Ophthalmology. October 17, 2020. [Epub ahead of print].