## Primary Mechanism of PACG **Key Point:** Pupillary block is the fundamental mechanism in primary angle-closure glaucoma, accounting for the majority of cases. ### Pathophysiology In PACG, the iris-lens contact creates a pressure gradient between the posterior and anterior chambers: 1. **Pupillary block** — the iris bulges forward, physically obstructing the drainage angle 2. **Angle closure** — the peripheral iris apposition to the trabecular meshwork prevents aqueous outflow 3. **IOP elevation** — impaired aqueous drainage through the trabecular meshwork causes acute pressure rise ### Anatomical Predisposition - **Shallow anterior chamber** (short axial length, hyperopia) - **Thick lens** (age-related) - **Narrow angle** (gonioscopic finding) - **Crowded anterior segment** **Clinical Pearl:** The acute angle closure occurs when the pupil is mid-dilated (45–60° dilation), as this position maximizes iris-lens contact and pupillary block effect. **High-Yield:** Pupillary block is reversible with peripheral iridotomy, which allows aqueous flow from posterior to anterior chamber, pushing the iris back and reopening the angle. ### Why Not Other Mechanisms? - ~~Trabecular meshwork degeneration~~ occurs in open-angle glaucoma, not PACG - ~~Excessive aqueous production~~ is not the primary defect (production is normal) - ~~Posterior synechiae~~ are a *consequence* of repeated angle closure, not the initiating mechanism 
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