Evidence of aqueous outflow regulation within the episcleral venous system, visualised by Haemoglobin Video Imaging

Abstract

Background: Glaucoma is a chronic progressive optic neuropathy responsible for more irreversible blindness worldwide than any other eye condition. Intraocular pressure (IOP) reduction is the foundation of glaucoma treatment. Despite the prevalence of glaucoma, the role of the episcleral vascular system (EVS) in modulation of aqueous outflow (AO) and regulation of IOP remains incompletely understood.

Hypothesis: Regulatory mechanisms within the EVS contribute to IOP control and become dysfunctional in open-angle glaucoma.

Aims: To identify evidence of regulatory mechanisms within the conventional AO system that may contribute to IOP homeostasis. The secondary aim is to detect differences in AO in health and glaucoma.

Methods: Haemoglobin Video Imaging (HVI), a novel AO imaging technique, was used to compare responses to the water-drinking test (WDT) and trabecular bypass surgery (TBS). Aqueous vein distribution, AO drainage patterns and quantification of AO, estimated by aqueous column cross-sectional area (AqCA), were studied.

Results: All clinical studies indicated aqueous drainage was most prominent in the nasal quadrant. An increase in AqCA was recorded in all eyes in response to IOP elevation induced by ingestion of a water bolus. Unlike control eyes, glaucomatous eyes were unable to sustain the AqCA increase and IOP did not recover to baseline. Characteristic flow patterns were demonstrated in some glaucomatous eyes. TBS increased AqCA in most eyes, indicating improved AO. A novel finding of IOP elevation was identified in some eyes within one week of TBS. Variations in AO recovery were seen within the early post-operative period.

Conclusions: Visualisation of AO within the EVS under physiological conditions has been achieved with HVI. Responses following the WDT and TBS demonstrate the dynamic nature of the AO system and confirm trabecular meshwork is not solely responsible for IOP control. Differences between normal and glaucomatous eyes have been identified.