Commentary: Can the vessels help solve the puzzle? Distinguishing high myopes from primary open-angle glaucomas

Ocular blood flow has been an area of great interest and research in the field of glaucoma in the recent past. The paradigms have shifted from focusing exclusively on the intraocular pressures to maintaining adequate optic nerve head (ONH) perfusion.[1] The configuration of vessels around the optic nerve has been studied in detail, and many objective methods are being devised to analyze the same.

The mind-bending conundrum

Differentiating between a myopic and a glaucomatous disc can be a mind-bending job, especially in the background of pathological myopia. There have been constant efforts to objectively distinguish between the two, as management of these conditions are poles apart. Fundus imaging is an objective and comparative tool that can aid in the detailed evaluation of the pathological disc.[2] It can highlight prominent and visible pathologies, but an element of subjectivity persists, which can cause inter-observer variations.[3] Moreover, a decrease in the blood flow at the vascular level, an early pathology in the spectrum of glaucoma, cannot be captured by a fundus photo. Hence, higher-end methods are necessary to facilitate the same.

This iteration of the Indian Journal of Ophthalmology has published an article on the diagnostic ability of superficial vascular density (SVD) measured by optical coherence tomography angiography (OCTA) to differentiate between high myopia and primary open-angle glaucoma (POAG).[4] The radically different management of POAG in contrast to high myopia treatment, and follow-up patterns make this non-invasive modality of assessment a worthy addition to an ophthalmologist’s armamentarium.

The vessel density and different vascular plexuses

Vessel density is usually defined as the percentage of surface area occupied by the vessels inside the ONH. It can be evaluated using OCTA in the peripapillary retina, peripapillary choroid, and macula.[5] Two different vascular plexuses are curently being studied, namely the superficial and deep vascular plexus. Different OCTA devices employ different methods of vessel density measurement. OCTA enhances our understanding of the role of ocular blood flow in glaucoma.

What we know from the past

It is noted that the peripapillary SVD and macular SVD have been variably studied in the literature. However, a new parameter for diagnosis and possible classification, the correlation of vessel density in glaucomatous eyes has come under scrutiny. In POAG eyes, peripapillary SVD strongly correlates with the visual field index. Comparison between retinal nerve fiber layer thickness and vascular density have yielded good relatable results.[5] The role of peripapillary SVD in primary angle-closure glaucoma has also been studied. Vessel densities have shown a significant drop after six weeks following an angle closure attack.[6] Decreased perfusion has also been demonstrated in patients with normal tension glaucoma.[7] We believe that owing to the asymmetrical presentation of POAG, vessel density as a parameter, once well-established, may prove to be pivotal in the management.

Chihara et al.[8] revealed results similar to this study, where SVD differed significantly between normal and glaucomatous eyes or eyes with ocular hypertension. The results have established significant deviation from normalcy between vascularity in cases of glaucoma and ocular hypertension. Another recent study by Lee et al.[9] has tried to correlate the degree of the optic nerve and visual field damage with the amount of vascularity in both superficial and deep vascular plexuses. The study showed that the affected deep plexus had a more reliable correlation with glaucomatous damage than the superficial plexus. A similar approach has been adopted in this study. A stratified comparison at three different levels of severity in glaucoma has been made based on the extent of field defects.

A progressive decrease in the peripapillary perfused capillary density has been found in open-angle glaucoma when compared with myopia.[10] In the accompanying article, it is stated that there was a greater reduction of peripapillary SVD in glaucomatous eyes compared to myopic eyes. The authors serve us with a hypothesis that highlights the fact that in glaucoma, the death of ganglion cells from the three inner layers of the retina leads to a consequent reduction in oxygen demand and a reduction in vascular density, whereas with high myopes, there is thinning and stretching of all layers of the retina as a result of an increasing axial length, but no cell death or tissue loss.[1]

Understanding the outcomes

The outcomes of this study have been gratifying, but with certain limitations. Though it has established apparent deviations in comparison with normal controls, its utility in recognizing the fox in the sheep’s disguise is still not achieved, i.e., in the early stages of glaucoma and its differences from a myopic disc.

Hurdles and the way forward

With further studies which dive deeper into newer and more reliable parameters being needed at this juncture, this study aids in understanding the significant role of OCTA, the upcoming investigation under the radar, in helping us decipher the code to isolate the myopic discs from the perilous glaucomatous ones.

The role of OCTA as a tool to non-invasively distinguish high myopia and glaucoma are demonstrated well, based on their peripapillary SVD in this study. The inclusion of high myopes with glaucomatous changes and their corresponding vessel density architecture was excluded and warrants further study to better recognize, classify, and treat this entity.