## Anticholinergics and Glaucoma Risk **Key Point:** Atropine and other anticholinergics cause mydriasis (pupil dilation) by blocking M3 receptors on the iris sphincter muscle. In narrow-angle glaucoma, mydriasis pushes the peripheral iris forward, obstructing aqueous humor drainage and precipitating acute angle closure. ### Mechanism of Glaucoma Risk 1. **Narrow-angle anatomy:** Iris-lens diaphragm is already close to cornea 2. **Anticholinergic effect:** Blocks M3 → iris sphincter relaxes → pupil dilates 3. **Mechanical consequence:** Dilated iris bunches forward → blocks trabecular meshwork → IOP spike → acute glaucoma attack ### Glaucoma Type Comparison | Glaucoma Type | Anatomy | Anticholinergic Risk | Mechanism | | --- | --- | --- | --- | | Narrow-angle | Shallow anterior chamber, narrow angle | **CONTRAINDICATED** | Mydriasis → angle closure → IOP ↑↑ | | Open-angle | Wide angle, normal drainage | Safe | Angle remains open despite mydriasis | **High-Yield:** Atropine is absolutely contraindicated in narrow-angle glaucoma but safe in open-angle glaucoma. This is a high-yield exam trap — students must know the anatomical reason. **Mnemonic:** **NARROW = NO anticholinergics** (because mydriasis narrows the angle further and blocks drainage) **Clinical Pearl:** Patients with undiagnosed narrow-angle glaucoma may present with acute angle closure after receiving anticholinergics for other indications (e.g., atropine for organophosphate poisoning or benztropine for Parkinson's). **Warning:** ~~Anticholinergics are universally contraindicated in all glaucoma~~ — they are safe in open-angle glaucoma; the risk is specific to narrow-angle anatomy. [cite:Harrison 21e Ch 397]
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