A 58-year-old man is diagnosed with primary open-angle glaucoma (POAG) during routine eye screening. Gonioscopy reveals an open angle with normal-appearing trabecular meshwork. Which is the most common site of resistance to aqueous outflow in POAG?

Explanation

## Aqueous Outflow Resistance in POAG **Key Point:** In primary open-angle glaucoma, the primary site of increased resistance to aqueous outflow is the juxtacanalicular (cribriform) region of the trabecular meshwork, not the conventional trabecular beams. ### Pathophysiology of Outflow Resistance The conventional (trabecular) pathway accounts for ~85–90% of aqueous drainage in healthy eyes. Resistance to outflow in POAG occurs at multiple sites, but the **juxtacanalicular region** (also called the cribriform layer or inner wall endothelium of Schlemm's canal) is the primary site of increased resistance. | Site | Role in POAG | Mechanism | |------|--------------|----------| | **Juxtacanalicular region** | **Primary resistance site** | Endothelial cell loss, increased extracellular matrix deposition, loss of giant vacuoles | | Trabecular beams | Secondary resistance | Gradual narrowing and loss of cellularity | | Schlemm's canal | Tertiary | Reduced lumen, endothelial dysfunction | | Collector channels | Tertiary | Obstruction and reduced patency | ### Histopathological Changes in POAG 1. **Juxtacanalicular region changes:** - Loss of giant vacuoles (critical for transcellular flow) - Increased extracellular matrix deposition - Endothelial cell apoptosis and denudation - Accumulation of glycosaminoglycans 2. **Trabecular meshwork changes:** - Increased cross-linking of collagen - Loss of trabecular cells - Narrowing of trabecular spaces **High-Yield:** The juxtacanalicular region accounts for ~50% of total outflow resistance in normal eyes; in POAG, this resistance increases disproportionately due to endothelial dysfunction and loss of transcellular pathways. **Clinical Pearl:** Gonioscopy in POAG typically shows a normal-appearing angle because the pathology is at the microscopic level (inner wall endothelium and juxtacanalicular tissue), not visible on clinical examination. ### Why Other Sites Are Not Primary - **Schlemm's canal:** Secondary changes occur here, but it is not the primary site of resistance. - **Ciliary body:** Not involved in aqueous outflow resistance; it is the site of aqueous production. - **Iris root:** Not a site of aqueous outflow; anatomically unrelated to drainage pathways.