A 55-year-old man with glaucoma and a history of asthma presents for treatment. He requires a cholinergic drug that lowers intraocular pressure by increasing uveoscleral outflow and pupillary constriction. Which of the following is the drug of choice for chronic management of open-angle glaucoma in this patient?

Explanation

## Drug of Choice in Open-Angle Glaucoma **Key Point:** Pilocarpine is the preferred muscarinic agonist for chronic open-angle glaucoma because it is selective for M3 receptors on the ciliary muscle, has good corneal penetration, and minimal systemic absorption. ### Mechanism of Intraocular Pressure Reduction Pilocarpine acts as a **direct-acting muscarinic agonist** (M3 > M1): 1. **Ciliary muscle contraction** → pulls on scleral spur → increases trabecular meshwork outflow 2. **Pupillary constriction (miosis)** → opens angle in angle-closure glaucoma 3. **Net effect:** IOP reduction by 20–30% within 1 hour ### Comparison of Cholinergic Agents in Glaucoma | Feature | Pilocarpine | Carbachol | Acetylcholine | Bethanechol | | --- | --- | --- | --- | --- | | **Route** | Topical (eye drops) | Topical | Topical (rare) | Oral/SC | | **Selectivity** | M3 > M1 | Non-selective | Non-selective | M3 > M1 | | **Onset** | 10–30 min | 10–30 min | Immediate | 30–90 min | | **Duration** | 4–8 hours | 8 hours | Minutes | 1 hour | | **Systemic effects** | Mild | Marked | Severe | Moderate | | **Corneal penetration** | Good | Moderate | Poor | N/A | | **Use in glaucoma** | First-line | Alternative | Rarely used | Not used | **High-Yield:** Pilocarpine is a **tertiary amine** — it crosses the cornea well and achieves high local concentration with minimal systemic absorption, making it ideal for topical use. ### Why Pilocarpine is Preferred in This Patient 1. **Selective M3 agonism** — minimal M2-mediated bronchospasm (safer in asthma) 2. **Topical route** — avoids systemic cholinergic effects 3. **Good corneal penetration** — effective at low doses 4. **Established safety profile** — decades of use in glaucoma 5. **Rapid onset** — therapeutic effect within 10–30 minutes **Clinical Pearl:** Pilocarpine causes **accommodative spasm** (myopia) and **brow ache** due to ciliary muscle contraction — counsel patients about these side effects. **Warning:** In **angle-closure glaucoma**, pilocarpine is contraindicated acutely because miosis can paradoxically worsen angle closure in some anatomies. Use only after laser peripheral iridotomy. **Mnemonic:** **PILO** = **P**upil constriction, **I**ntraocular pressure ↓, **L**ong duration, **O**phthalmic use. [cite:KD Tripathi 8e Ch 7]