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Editors Selection IGR 21-2

Anatomical Structures: Imaging the Aqueous Veins

Comment by Alex Huang & Seung Hyen Lee on:

113216 , Chen ZQ; Chen W; Deng CH et al., International Journal of Ophthalmology, 2023; 16: 1482-1488

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This cross-sectional study by Chen et al. used enhanced depth of imaging anterior segment optical coherence tomography (OCT) and OCT-angiography (OCTA) to attempt study of aqueous veins (AVs) before and after water drinking tests. The authors reported that AVs have a lower blood flow density than conjunctival veins. After drinking water, the authors reported that the area and blood flow density of AVs decreased before returning to baseline.

There are a number of important limitations in this study. Firstly, the authors specifically state in the Discussion that they are the first to study AVs using OCT and OCTA. This is not true, and too numerous to count papers have imaged AVs and episcleral veins (EVs) using OCT and OCTA. Secondly, the authors are unlikely studying AVs. The authors do not distinguish AVs from EVs. Looking at Asher's original description,¹ there are three basic veins. (1) AVs carry aqueous. Hence, they are invisible and near impossible to visualize. Then there are two types of EVs: (2) EVs carrying blood; and (3) EVs carrying both blood and aqueous. Based on the first figure, the authors are clearly studying a blood carrying vessel which means that these are not AVs. The authors are studying one of the two EV types. Further, as the authors reported no difference in depth between the conjunctival vessel and the blood-carrying study vessel in question, it is again more likely that the authors are studying EVs as they are more superficial. Lastly, measuring vessel parameters in cross-section is challenging. Acquiring OCTs scans at different angles yield different vessel shape on the B-scan. It is difficult to guarantee that images are obtained exactly at the same location and angle before and after the water drinking test. Thus, 3D reconstruction of imaged vascular pathways with subsequent quantitative assessment would be the solution to this problem.

Ultimately, this paper was interesting as it combined the water drinking test with anterior segment imaging. However, the findings are more likely related to episcleral veins, and numerous important limitations need to be considered. It cannot be emphasized enough the major unmet need for the field to use consistent words and terminology. For the future, work by the Advised Protocol for OCT Study Terminology and Elements Anterior Segment (APOSTEL-AS) working group is underway to create consensus definitions for these types of anterior segment structures.

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