A 62-year-old woman with primary open-angle glaucoma is on latanoprost monotherapy. Her IOP remains 24 mmHg despite good compliance. She has no history of asthma or cardiac arrhythmias. What is the preferred second agent to add for combination therapy?

Explanation

## Combination Therapy in POAG: Second Agent Selection **Key Point:** When prostaglandin analog monotherapy fails to achieve target IOP, a beta-blocker (timolol) is the preferred second agent due to additive efficacy, complementary mechanism, and established safety profile in combination. ### Rationale for Beta-Blocker as Second Agent Prostaglandin analogs increase uveoscleral outflow, while beta-blockers decrease aqueous humor production—these **complementary mechanisms** provide additive IOP reduction when combined. ### Comparison of Second-Line Agents | Agent | Mechanism | Additional IOP Reduction | Advantages | Disadvantages | |-------|-----------|-------------------------|------------|---------------| | **Timolol (Beta-blocker)** | ↓ Aqueous production | 15–20% | Additive with PGA; well-studied combinations; good efficacy | Systemic effects (bradycardia, bronchospasm); BID dosing | | **Brimonidine (Alpha-2)** | ↓ Production + ↑ uveoscleral outflow | 10–15% | Neuroprotection (theoretical); minimal systemic effects | Tachyphylaxis; weaker efficacy; TID dosing | | **Acetazolamide (CAI)** | ↓ Aqueous production | 10–15% | Oral option; useful in acute cases | Metabolic acidosis; electrolyte abnormalities; TID dosing | | **Pilocarpine (Cholinergic)** | ↑ Trabecular outflow | 10–15% | Rarely used now | Miosis, accommodative spasm, poor efficacy; obsolete | **High-Yield:** Timolol + latanoprost is a **gold-standard fixed-dose combination** (Xalacom) widely used in POAG because the two drugs have complementary mechanisms and no significant pharmacokinetic interactions. **Clinical Pearl:** When adding a second agent, ensure the patient has no contraindications to the chosen drug class. In this case, the absence of asthma and cardiac arrhythmias makes timolol a safe choice. **Mnemonic:** **PACE** — **P**rostaglandin first, then **A**dd **C**omplementary **E**ffector (beta-blocker or CAI). ### Why Not Other Options? - **Brimonidine:** While safe, it is weaker than timolol and carries risk of tachyphylaxis; reserved for patients with contraindications to beta-blockers. - **Acetazolamide:** Systemic CAI is reserved for adjunctive therapy or acute angle-closure; topical dorzolamide is preferred if a CAI is needed as second agent. - **Pilocarpine:** Obsolete in POAG management; causes miosis and accommodative spasm; very weak efficacy. [cite:Khurana 7e Ch 10]