## Mechanism of Acute Angle-Closure Glaucoma **Key Point:** Pupillary block is the PRIMARY and most common mechanism in primary angle-closure glaucoma (PACG). The iris is pushed forward against the trabecular meshwork, mechanically obstructing aqueous outflow. ### Pathophysiology of Pupillary Block In predisposed eyes (short axial length, shallow anterior chamber, thick lens, narrow angles): 1. **Relative pupillary block** occurs when the lens-iris contact is excessive 2. Aqueous pressure builds up behind the iris (posterior chamber pressure > anterior chamber pressure) 3. The iris bows forward (iris bombé) 4. The peripheral iris contacts the trabecular meshwork 5. Aqueous outflow is mechanically obstructed → acute IOP spike ### Why Pupillary Block is Primary - Present in **>90% of acute PACG cases** - Occurs in anatomically predisposed eyes even with normal aqueous production - Resolves with peripheral iridotomy (YAG laser) — the definitive treatment - Aqueous production remains normal; the problem is mechanical obstruction **High-Yield:** Peripheral iridotomy (YAG or surgical) is curative because it **equalizes pressure** between posterior and anterior chambers, relieving iris bowing and reopening the angle. **Clinical Pearl:** The angle closes because the iris physically blocks the trabecular meshwork — NOT because the meshwork itself is damaged or has reduced facility (that is secondary inflammation). 
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