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Editors Selection IGR 9-4

Clinical Examination Methods: Real-Time Aqueous Outflow Imaging

Comment by Arthur Sit on:

66251 Aqueous Angiography: Real-Time and Physiologic Aqueous Humor Outflow Imaging, Saraswathy S; Tan JC; Yu F et al., PLoS ONE, 2016; 11: e0147176

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Aqueous humor outflow through the conventional outflow system has long been known to be segmental in nature, with higher flow occurring across some regions of the trabecular meshwork.¹ This has significant implications for angle surgeries, including minimally invasive glaucoma surgery (MIGS), since location of the surgery may affect the outcome. Intraoperatively, a fluid wave in the episcleral vessels near the surgery site is a positive prognostic factor, possibly indicating that flow resistance has been reduced in an area of well-functioning distal outflow system.² However, success from MIGS is currently unpredictable with highly variable results,^3,4 and there are currently no techniques that can pre-operatively assess the regional variations in the aqueous humor outflow system.

Selecting a single quadrant (e.g., nasal) as the site for angle surgery may not be an ideal strategy, and individualization is required

Saraswathy et al.⁵ reported on a novel technique (aqueous angiography) for assessing the segmental distribution of aqueous humor outflow using fluorescein angiography. In this study, cadaveric pig and human eyes were perfused with fluorescein solution at constant pressure and the distribution in the aqueous veins was assessed using a Spectralis HRA+OCT (Heidelberg Engineering, Heidelberg, Germany).

The authors noted that there was significant variability in the flow distribution of the distal outflow system based on the fluorescein signal. In particular, no single quadrant was consistently identified as having the highest angiographic signal. This suggests that consistently selecting a single quadrant (e.g., nasal) as the site for angle surgery may not be an ideal strategy, and individualization is required. To validate the results from angiography, concurrent OCT was performed, demonstrating open intrascleral lumens where angiography demonstrated flow, and an absence of intrascleral lumens where angiographic signal was absent.

There are a number of potential limitations to this study, which have been acknowledged by the authors. Most importantly, the experiments in this study used cadaver eyes in which the conjunctiva has been removed, and the episcleral veins have been severed. The authors have tried to minimize the potential artifacts by focusing on early fluorescein patterns, which limits the effect of fluorescein accumulation. Other potential confounders are post-mortem cellular changes, clotted blood in the episcleral veins, and tissue swelling. Nevertheless, aqueous angiography appears to be a very promising technique for the assessment of regional variations in the aqueous humor outflow system. If it can be successfully applied in vivo, it may fundamentally alter our approach to performing angle surgeries, and unlock the full potential of MIGS devices.

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