Aqueous humor outflow system

The aqueous humor outflow system is a specialized physiological complex in the anterior segment of the eye responsible for the drainage of aqueous humor to maintain stable intraocular pressure (IOP) [1, 5]. It consists of two primary routes: the conventional (trabecular) pathway, which includes the trabecular meshwork (TM) and Schlemm's canal, and the unconventional (uveoscleral) pathway, where fluid drains through the ciliary muscle interstices [3, 11, 18]. In diseases such as primary open-angle glaucoma, increased resistance to drainage within this system—primarily at the juxtacanalicular tissue of the TM—leads to pathological elevation of IOP, which can cause progressive damage to the optic nerve [1, 4, 7]. Many glaucoma medications specifically target this system to enhance fluid egress: prostaglandin analogs increase uveoscleral outflow, while Rho kinase (ROCK) inhibitors and nitric oxide-donating compounds reduce resistance in the trabecular meshwork [2, 9, 17]. Surgical interventions like micro-invasive glaucoma surgery (MIGS) also aim to bypass or enhance the function of these natural drainage structures [18, 20].

Other names

Aqueous drainage systemAqueous outflow pathwayConventional and unconventional outflow pathwaysTrabecular and uveoscleral outflow system

Drugs targeting this system lower intraocular pressure by increasing fluid drainage through several distinct mechanisms: Prostaglandin analogs (e.g., latanoprost) primarily enhance unconventional (uveoscleral) outflow by remodeling the extracellular matrix of the ciliary muscle [7, 11]. Rho kinase (ROCK) inhibitors (e.g., netarsudil) act directly on the trabecular meshwork to increase its permeability by relaxing TM cells and widening Schlemm's canal [2, 17]. Cholinergic agonists (e.g., pilocarpine) induce contraction of the ciliary muscle, which mechanically pulls the scleral spur to open the trabecular meshwork spaces [8, 9]. Nitric oxide-donating compounds (e.g., latanoprostene bunod) provide a dual effect by targeting both the uveoscleral pathway and the trabecular meshwork via cell relaxation [4, 9].