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Editors Selection IGR 7-2

Clinical Examination Methods: Macular Vessels and Ganglion Cells

Comment by Kouros Nouri-Mahdavi on:

85134 Ganglion Cell Complex Thickness and Macular Vessel Density Loss in Primary Open-Angle Glaucoma, Hou H; Hou H; Hou H; Moghimi S; Proudfoot JA et al., Ophthalmology, 2020; 127: 1043-1052

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Hou and co-investigators compared the rates of change for GCC thickness measures and OCTA-derived vessel density (VD) measures in the macular region in 23 healthy eyes, 36 eyes with pre-perimetric glaucoma, and 80 POAG eyes (total of 94 subjects) who had at least three measurements available during follow-up. The average follow-up was 2.0-2.6 years with an average visit number of 3.4-3.8.

The rates of change for both thickness and OCTA parameters was higher on average in the preperimetric glaucoma group than normal subjects and in the group with established glaucoma compared with pre-perimetric glaucoma eyes. Of note, the glaucoma group consisted mostly of early glaucoma eyes (average MD: -4.2; 95% CI: -5.0 to -3.4 dB). Based on percent rates of change, they found overall higher VD rate of change in all three groups as compared to GCC thickness, which points to the possible utility of OCTA for earlier detection of structural change in the macular region.

By evaluating change rates in POAG eyes with different glaucoma severity, the investigators found that the slowest rate of GCC thinning was observed in advanced glaucoma eyes whereas the slowest rate of macular vessel density decrease was observed in mild glaucoma eyes; the results were not provided, but were based on only five advanced eyes defined as visual field MD less than -12 dB; if confirmed they would suggest that OCTA may be a viable measure for detecting change in more advanced stages of glaucoma.

Approximately 35% of the macular scans were excluded because of poor quality. It is not clear how many OCTA scans vs. thickness scans were excluded. This is important as an unbalanced exclusion of the images (GCC vs OCTA) could lead to biased results.

The rates of change are notoriously unstable early during follow-up when a small number of datapoints are available

One has to keep in mind that the rates of change are notoriously unstable early during follow-up when a small number of datapoints are available (< 4 on average for all subgroups of this sample). For example, the loss rate of -2.5% per year for vessel density in normal subjects is evidently unsustainable during a lifetime, given the average age of 52 years in this group. The authors reported a stronger relationship between baseline visual field loss (based on MD) with VD rates of change as compared to GCC rates of change. However, the results were likely influenced by an outlier eye for both parameters and given the smaller number of eyes with advanced glaucoma, this hypothesis will have to be verified in future studies.

While the authors provide some evidence for the hypothesis that OCTA parameters could provide additional information with regard to glaucoma progression compared with GCC thickness measures, larger samples of patients with a wider range of glaucoma severity and longer follow-up are needed to definitively answer this important question.

The investigators are to be commended for uncovering another piece of the glaucoma puzzle and contributing to our understanding of the pathophysiology of change in this disease.

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