## Primary Site of Outflow Resistance in POAG **Key Point:** In primary open-angle glaucoma, the trabecular meshwork (TM) and Schlemm's canal represent the primary site of increased outflow resistance, leading to elevated intraocular pressure (IOP). ### Pathophysiology of POAG The disease is characterized by: - Progressive degeneration and dysfunction of trabecular meshwork cells - Accumulation of extracellular material and glycosaminoglycans in the TM - Loss of endothelial cell integrity within Schlemm's canal - Decreased aqueous humor drainage despite an anatomically open angle ### Why the Angle Remains Open Unlike angle-closure glaucoma, POAG maintains a structurally normal, open drainage angle on gonioscopy. The problem is **functional obstruction** at the level of the TM-Schlemm's canal complex, not anatomic closure. **High-Yield:** The trabecular meshwork dysfunction in POAG is progressive and irreversible, making early detection and treatment critical to prevent optic nerve damage. ### Aqueous Humor Drainage Routes | Route | Percentage | Status in POAG | | --- | --- | --- | | Conventional (TM → Schlemm's canal → episcleral veins) | 85–90% | **Primary site of resistance** | | Uveoscleral (unconventional) | 10–15% | Relatively preserved | **Clinical Pearl:** Gonioscopy in POAG shows a wide, open angle with normal-appearing angle structures, but the TM may appear darker or have reduced translucency due to pigmentation and fibrosis. [cite:Khurana Comprehensive Ophthalmology Ch 11] 
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