A 60-year-old man with chronic open-angle glaucoma uses timolol eye drops. He is a known asthmatic and develops wheezing after a week of therapy. The intraocular pressure-lowering mechanism of timolol, and the basis of his wheeze respectively, are:

• A Increased trabecular outflow; alpha-1 blockade in nasal vessels
• B Increased uveoscleral outflow; beta-2 blockade in bronchial smooth muscle
• C Decreased aqueous humour secretion; beta-1 blockade in cardiac muscle
• D Decreased aqueous humour secretion; beta-2 blockade in bronchial smooth muscle ✓

Explanation

Beta-2 receptors on the ciliary epithelium drive aqueous humour formation; timolol blocks them and reduces secretion. Systemically absorbed drug blocks airway beta-2 receptors, precipitating bronchospasm in asthmatics, which is why topical beta-blockers are avoided in reactive airway disease. Option B describes latanoprost, which increases uveoscleral outflow and has no bronchial adverse effect profile.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

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Written and medically reviewed by the StethoPrep medical team.