Extraocular blood vessels of the conjunctiva and sclera are readily accessible sources of information regarding systemic and ocular disease processes. Superficial conjunctival vasculature is affected by systemic diseases, such as diabetes and hypertension. Deep episcleral and intrascleral vasculature comprise the distal portions of the conventional aqueous humor outflow (AHO) pathway, which may reflect anatomical abnormalities associated with ocular hypertension and glaucomatous optic neuropathy. Therefore, ophthalmologists could benefit from new non-invasive methods to assess and quantify the state of these extraocular blood vessels.
This study by Akagi and colleagues proposes an innovative application of anterior segment optical coherence tomography angiography (AS-OCTA) for visualizing superficial conjunctival and deep episcleral and intrascleral vasculature. The authors imaged ten young, healthy subjects on a commercially available OCTA device. Extraocular blood vessels were segregated into superficial (0-200 um depth) and deep (200-100 um depth) layers using manufacturer-provided segmentation algorithms. Qualitatively, the superficial vessels resembled vasculature delineated by fluorescein angiography and the deep vessels resembled vasculature delineated by aqueous angiography with indocyanine green (ICG). Quantitative AS-OCTA measurements of vessel density and other vessel properties demonstrated modest and significant inter-quadrant differences in the superficial and deep layers, respectively.
Non-invasive imaging of distal AHO pathways could provide valuable insight into the relationship between the dynamics of extraocular blood flow and effectiveness of IOP lowering treatments
Non-invasive imaging of distal AHO pathways could provide valuable insight into the relationship between the dynamics of extraocular blood flow and effectiveness of IOP lowering treatments, including eye drops, laser, and surgery. This information could also be valuable for operative planning prior to minimally invasive glaucoma surgery (MIGS). However, it is important to acknowledge limitations associated with the proposed method. AHO channels are only detectable on AS-OCTA when they contain red blood cells, which means that Schlemm's Canal and the collector channels normally cannot be visualized. However, the relationship between flow in different segments of the AHO pathway is incompletely understood. In addition, it is unclear what proportion of deep vessels are involved in venous outflow as opposed to aqueous outflow. Therefore, additional work is needed before the long-term clinical value of AS-OCTA can be determined.