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Editors Selection IGR 16-4

Clinical Examination Methods: Retinal Vessel Density in POAG

Comment by David Greenfield on:

75362 Peripapillary and Macular Vessel Density in Patients with Primary Open-Angle Glaucoma and Unilateral Visual Field Loss, Yarmohammadi A; Zangwill LM; Manalastas PIC et al., Ophthalmology, 2018; 125: 578-587

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Optical coherence tomography-angiography (OCT-A) is a non-invasive imaging technology that has facilitated visualization and measurement of the capillary networks in different layers of the retina. A variety of spectral domain and swept source OCT technologies are commercially available that enable calculation of superficial and deep vascular plexus measurements. Vessel density calculations within the optic disc, peripapillary retina, and macula have been shown to be associated with glaucomatous structural damage and visual field loss.

In the present cross-sectional study, Yarmohammadi and colleagues used OCT-A to compare the retinal microvasculature of 33 patients with primary open-angle glaucoma (POAG) with unilateral visual field (VF) loss to 33 healthy eyes. Study participants underwent VF testing, spectral- domain OCT measurements of the circumpapillary RNFL (cpRNFL) and macular ganglion cell complex (mGCC), and OCT-A imaging (Avanti AngioVue; Optovue, Inc, Fremont, CA) of the circumpapillary vessel density (cpVD), parafoveal vessel density (pfVD), and the vessel density of the whole image scanned (wiVD). Eyes were stratified into three categories: (1) healthy eyes; (2) POAG eyes with perimetric loss; and (3) perimetrically unaffected eyes of POAG patients. Unaffected eyes of POAG patients had significantly thinner structural parameters (macula GCC and RNFL) and capillary density (cpVD and pfVD) compared to healthy eyes, and the wiVD parameter had the highest discriminating power.

The fundamental question remains whether reductions in vascular density represent the so-called chicken or the egg

Key questions remain unanswered: (1) Will longitudinal OCT-A studies provide useful information regarding the underlying pathogenesis of glaucoma; (2) Do changes in capillary density precede or follow structural damage to the RGCs and their axons; (3) Is OCT-A useful for predicting or identifying progression; and (4) which vascular networks are most relevant in glaucomatous eyes, and is there a 'floor effect' in eyes with severe damage. Additionally, logistical hurdles must be overcome to reduce projection artifacts caused by large vessels and motion artifact caused by lengthy image acquisition times, reduce segmentation error, and improve the signalto- noise ratio.

This well designed study provides important information. Changes in capillary density represent a surrogate for blood flow and these results suggest a potentially useful role for OCT-A in glaucoma monitoring. The fundamental question remains whether reductions in vascular density represent the so-called chicken or the egg. Longitudinal studies are currently underway and data will be forthcoming.

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