During a community screening programme in rural Maharashtra, a 62-year-old farmer is found to have elevated intraocular pressure (IOP 26 mmHg bilaterally) with open angles on gonioscopy and early optic disc changes suggestive of glaucoma. What is the most common cause of primary open-angle glaucoma in this population?

Explanation

## Pathophysiology of Primary Open-Angle Glaucoma **Key Point:** The most common underlying cause of POAG is **decreased trabecular outflow facility** due to structural and functional changes in the trabecular meshwork, NOT elevated aqueous production. ### Mechanism of IOP Elevation in POAG ```mermaid flowchart TD A[Aqueous Humor Dynamics]:::outcome --> B[Production vs Outflow]:::decision B -->|Normal production| C[Decreased outflow facility]:::action B -->|Elevated production| D[Rare in POAG]:::outcome C --> E[Trabecular meshwork dysfunction]:::action E --> F[Increased resistance to flow]:::action F --> G[IOP elevation]:::outcome G --> H[Optic nerve ischemia & damage]:::urgent ``` ### Structural Changes in POAG Trabecular Meshwork | Component | Change | Effect | |-----------|--------|--------| | Trabecular beams | Thickening, fusion, loss of cellularity | ↑ Resistance | | Endothelial cells | Degeneration, reduced phagocytosis | ↓ Clearance | | Extracellular matrix | Accumulation of glycosaminoglycans | ↑ Obstruction | | Juxtacanalicular tissue | Increased stiffness, reduced compliance | ↑ Resistance | | Schlemm's canal | Endothelial dropout, collapse | ↓ Drainage | **High-Yield:** Aqueous humor production remains **normal** in POAG — the problem is **outflow**, not inflow. ### Quantitative Basis Using the Goldmann equation: $$IOP = (F_{in} - F_{out}) / C + P_v$$ Where: - $F_{in}$ = aqueous inflow (normal in POAG: ~2.5 μL/min) - $F_{out}$ = trabecular outflow (reduced in POAG) - $C$ = outflow facility (decreased in POAG) - $P_v$ = episcleral venous pressure (normal) **Clinical Pearl:** Tonography (measurement of outflow facility) shows reduced C values (< 0.21 μL/min/mmHg) in POAG patients, confirming the outflow defect. ### Why Trabecular Outflow Fails 1. **Age-related degeneration** of trabecular endothelium 2. **Oxidative stress** and mitochondrial dysfunction 3. **Impaired phagocytosis** → debris accumulation 4. **Extracellular matrix remodeling** → tissue stiffness 5. **Genetic predisposition** (mutations in MYOC, OPTN, WDR36 genes) **Mnemonic: POEM** — **P**roduction normal, **O**utflow decreased, **E**levated IOP, **M**eshwork dysfunction (the core defect in POAG).