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Editors Selection IGR 21-4

Anatomical Structures: Posterior pole Vascularization and Structural Progression

Comment by Andrew Tatham on:

77939 Macular and Optic Nerve Head Vessel Density and Progressive Retinal Nerve Fiber Layer Loss in Glaucoma, Moghimi S; Zangwill LM; Penteado RC et al., Ophthalmology, 2018; 125: 1720-1728

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OCT angiography (OCTA) provides a method for non-invasive imaging of the microvasculature of the optic nerve head and retina. Several studies using OCTA have shown patients with glaucoma have reduced peripapillary and macular vessel density,^1-3 with vessel density frequently reported to have similar ability to differentiate healthy and glaucomatous eyes compared to retinal nerve fibre layer (RNFL) thickness.^1,2 These and other studies support the theory that impaired ocular blood flow is associated with the development and progression of glaucoma.

Moghimi and colleagues take a step beyond these cross-sectional analyses and report the results of the first study to examine the relationship between baseline OCTA measurements and subsequent rates of change in RNFL thickness over time. This is an important study as it is the first to show low vessel density is a risk factor for glaucoma progression. Eighty-three patients with mild to moderate glaucoma were followed for an average of over two years, with OCT circumpapillary RNFL and macular ganglion cell inner plexiform layer (mGCIPL) measurements obtained semi-annually. Baseline OCTA measurements included macular whole image vessel density (m-wiVD) and optic nerve head whole image vessel density (onh-wiVD).

The main finding of the study was that eyes with lower baseline macular and optic nerve head vessel density had significantly faster rates of loss of RNFL during follow-up compared to eyes with higher vessel density. The overall average rate of RNFL loss was -1.07 µm/ year, however, rates of loss were 0.11 µm/year and 0.06 µm/year faster for each 1% lower baseline m-wiVD and onh-wiVD respectively.

Eyes with lower baseline macular and optic nerve head vessel density had significantly faster rates of loss of RNFL during follow-up compared to eyes with higher vessel density

The association between lower vessel density and faster rates of RNFL loss was present even when accounting for potential cofounders known to affect rates of progression, including age, intraocular pressure, central corneal thickness, and disease severity at baseline. Vessel density also provided additional predictive value compared to conventional structural measurements such as baseline RNFL and mGCIPL thickness. This suggests that impaired optic nerve head and retinal perfusion may lead to faster rates of retinal ganglion cell loss in glaucoma, however, it is possible that vascular dropout may occur due to reduced metabolic demands of already dysfunctional retinal ganglion cells.

Determining a patient's risk of progression is essential to inform decisions regarding monitoring intervals and appropriate treatment, however risk assessment remains challenging. This study shows measurements of vessel density using OCTA may improve our ability to assess to risk. OCTA also has the potential to improve our understanding of glaucoma pathogenesis and better explore the role of ocular blood flow in glaucoma.

Further work is needed to elucidate the relationship between vessel density and structural measurements across different severities of disease and to determine the dynamic range of vessel density.

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