A 62-year-old man with primary open-angle glaucoma has been on timolol 0.5% twice daily for six months. Intraocular pressure remains elevated at 24 mmHg. The physician discontinues timolol and prescribes an alternative. Within two weeks, the patient reports worsening of exertional dyspnoea and bradycardia. This adverse effect was present while on timolol but masked. What is the mechanism by which topical timolol lowers intraocular pressure?
- A Increased uveoscleral outflow via matrix metalloproteinase activation
- B Decreased aqueous humour production by blockade of beta-2 receptors in the ciliary epithelium ✓
- C Contraction of the ciliary muscle opening the trabecular meshwork
- D Osmotic reduction of vitreous volume
Correct answer: B. Decreased aqueous humour production by blockade of beta-2 receptors in the ciliary epithelium
Explanation
Topical beta-blockers like timolol lower IOP by reducing aqueous humour production through blockade of beta-2 adrenergic receptors in the non-pigmented ciliary epithelium. Systemic absorption can cause bronchospasm in asthmatics and bradycardia, as seen here. Prostaglandin analogues increase uveoscleral outflow (A), pilocarpine acts on the ciliary muscle (C), and mannitol reduces vitreous volume osmotically (D).
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.