Which of the following is the primary site of aqueous humor resistance in primary open-angle glaucoma?

Explanation

## Primary Site of Resistance in POAG **Key Point:** In primary open-angle glaucoma, the elevated intraocular pressure results from increased resistance to aqueous humor outflow at the level of the trabecular meshwork and Schlemm's canal, despite a clinically open drainage angle. ### Mechanism of Outflow Resistance The trabecular meshwork consists of three layers: 1. Corneal meshwork (outer) 2. Uveal meshwork (middle) 3. Juxtacanalicular meshwork (inner) — **the primary site of resistance** In POAG, pathological changes occur in the juxtacanalicular tissue and inner wall of Schlemm's canal, leading to: - Increased extracellular material deposition - Loss of endothelial cells lining Schlemm's canal - Reduced trabecular meshwork cellularity - Increased stiffness of trabecular beams **High-Yield:** The angle remains gonioscopically open (>20°) in POAG, distinguishing it from angle-closure glaucoma. The problem is not anatomical obstruction but rather functional/structural changes in the outflow pathway. ### Aqueous Humor Dynamics | Component | Role | Status in POAG | |-----------|------|----------------| | Production (ciliary body) | Normal | Normal (6–7 μL/min) | | Outflow resistance (TM/SC) | Primary determinant of IOP | **Increased** | | Uveoscleral pathway | Secondary drainage route | May be relatively preserved | **Clinical Pearl:** Tonography measures outflow facility (C-value). In POAG, facility of outflow is reduced (<0.22 μL/min/mmHg), confirming trabecular resistance as the pathogenic mechanism. ### Why the Angle Appears Open Despite elevated IOP, gonioscopy reveals: - Wide angle (Shaffer grade III–IV) - Normal angle structures - No peripheral anterior synechiae - No iris bombe or pupillary block This distinguishes POAG from primary angle-closure glaucoma, where the angle is physically narrow or occluded.