A 62-year-old man with a history of hyperopia and shallow anterior chamber is found to have intermittent episodes of subacute angle closure with IOP spikes to 35–40 mmHg. He is awaiting laser peripheral iridotomy. Which drug is the preferred choice for chronic prophylaxis to prevent acute attacks until surgery?

Explanation

## Chronic Prophylaxis in Subacute Angle-Closure Glaucoma ### Clinical Context: Intermittent vs. Acute ACG Subacute (intermittent) angle closure occurs when the angle is anatomically narrow but not completely blocked. Patients experience recurrent episodes of elevated IOP (35–45 mmHg) with mild symptoms (blurred vision, halos) but NOT the severe pain and corneal edema of acute ACG. The goal of prophylaxis is to prevent progression to acute attack until definitive laser treatment. ### Drug of Choice: Pilocarpine **Key Point:** Pilocarpine is the preferred prophylactic agent in subacute angle-closure because it is the ONLY drug that directly widens the angle by: 1. **Muscarinic agonism** → ciliary muscle contraction (accommodation) 2. **Pulls the ciliary body posteriorly**, which: - Relaxes the iris root - Deepens the anterior chamber - Opens the drainage angle mechanically **High-Yield:** Pilocarpine is a **miotic** (pupil-constricting) agent—the iris sphincter contraction pulls the iris away from the angle, further opening it. This is the ONLY mechanism that directly addresses the anatomical narrowness. ### Mechanism Diagram: How Pilocarpine Widens the Angle ```mermaid flowchart TD A[Pilocarpine: Muscarinic agonist]:::action --> B[Ciliary muscle contraction<br/>Iris sphincter contraction]:::action B --> C[Ciliary body pulled posteriorly]:::action C --> D[Iris root relaxed<br/>Anterior chamber deepened]:::action D --> E[Drainage angle widened]:::outcome E --> F[IOP reduced<br/>Angle closure prevented]:::outcome ``` ### Dosing - **Pilocarpine 1–2% eye drops**, 3–4 times daily - Onset: 20–30 minutes - Duration: 4–8 hours - Oral pilocarpine 5 mg TDS is alternative (slower onset, longer duration) ### Why Pilocarpine Beats Other Options in Subacute ACG | Drug | Mechanism | Why NOT First-Line in Subacute ACG | |------|-----------|------------------------------------| | **Pilocarpine** | Widens angle mechanically (miosis + ciliary contraction) | None—gold standard for prophylaxis | | Dorzolamide (CAI) | ↓ Aqueous production + shrinks vitreous | Reduces IOP but does NOT widen angle; cannot prevent progression to acute attack | | Apraclonidine (alpha-2) | ↓ Aqueous production + ↑ outflow | Reduces IOP but does NOT address anatomical narrowness; risk of tachyphylaxis | | Betaxolol (beta-blocker) | ↓ Aqueous production | Reduces IOP but does NOT mechanically open angle; insufficient for prophylaxis | **Clinical Pearl:** In subacute ACG, IOP control alone is NOT enough—you must mechanically widen the angle to prevent acute attacks. Only pilocarpine does this. ### Pilocarpine: Advantages & Limitations **Advantages:** - Only drug that directly widens the angle - Prevents progression to acute attack - Inexpensive - Rapid onset (20–30 min) **Limitations:** - Frequent dosing (3–4 times daily) → poor compliance - Accommodative spasm → myopic shift, blurred vision - Headache, brow ache (from ciliary muscle contraction) - Contraindicated in acute ACG (miosis can worsen corneal edema) - Tachyphylaxis may develop with prolonged use **Warning:** Pilocarpine is CONTRAINDICATED in acute angle-closure glaucoma (corneal edema prevents visualization, miosis worsens inflammation). It is used ONLY for subacute/intermittent ACG or chronic angle-closure, and ALWAYS as a bridge to laser iridotomy. ### Complete Management of Subacute ACG 1. **Pilocarpine 1–2% TID–QID** (prophylaxis while awaiting laser) 2. **Topical beta-blocker ± CAI** (adjunctive IOP control if needed) 3. **Avoid mydriatics** (phenylephrine, tropicamide) — they can precipitate acute attack 4. **Laser peripheral iridotomy** (definitive treatment; pilocarpine discontinued post-laser) **High-Yield:** After successful laser iridotomy, pilocarpine is discontinued because the angle is now open and miosis is no longer needed. [cite:Parson's Diseases of the Eye 22e Ch 10; Harrison 21e Ch 423]