Sarmad Akkach1,2, Nicholas H. Andrew3, Robert J. Casson2

Purpose: The regulation of aqueous humour out- flow has clinical implications for the medical and surgical management of glaucoma. This is especially relevant in the era of micro-invasive glaucoma sur- geries (MIGS) that are highly targeted to bypassing specific sites of outflow resistance. We re-examined the evidence regarding the anatomy and physiology of aqueous outflow resistance to provide a compre- hensive summary with relevance to current and future MIGS.
Method Literature Review: Results Elevated IOP occurs due to impaired aqueous humour outflow. Both a passive model and a dynamic model have been used to explain outflow resistance. The passive model posits that the trabecular meshwork acts as a filter that exerts stable and passive resistance to out- flow. Several pieces of evidence argue against the passive model as being solely responsible for trabec- ular outflow resistance. Whereas, the dynamic model proposes that trabecular outflow requires a biomechanical pump’ in which the flexible trabecu- lar meshwork distends and recoils in sync with the cardiac cycle, actively moving aqueous into Schlemm’s canal. Unlike the passive model, the dynamic model links trabecular structure and func- tion. A unifying model to explain all of the observa- tions on trabecular resistance must incorporate both the passive and the dynamic model.
Conclusion: Technological advances are enabling increasingly sophisticated surgical management; however, optimal outcomes will require an accu- rate understanding of aqueous outflow resistance and a targeted approach. The potential for post-operative titration and avoidance of complications, particularly hypotony, are additional considerations.