A 62-year-old man with open-angle glaucoma using timolol eye drops presents with acute wheeze and breathlessness. He has a history of bronchial asthma. Which property of timolol best explains this adverse event?
- A It blocks beta-2 receptors in bronchial smooth muscle even after ocular administration ✓
- B It stimulates muscarinic receptors causing bronchoconstriction
- C It causes histamine release from mast cells
- D It inhibits phosphodiesterase in airway tissue
Correct answer: A. It blocks beta-2 receptors in bronchial smooth muscle even after ocular administration
Explanation
Timolol is a non-selective beta-blocker. Even when given as eye drops, systemic absorption through nasal and conjunctival mucosa is significant, so drug reaches the airways and blocks bronchial beta-2 receptors, producing bronchospasm in asthmatics. The safest alternatives are topical carbonic anhydrase inhibitors or prostaglandin analogues. The tempting distractor is option D because timolol shares its name with drugs like theophylline that affect airways, but timolol has no phosphodiesterase activity; the event is purely beta-2 blockade.
Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.
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Written and medically reviewed by the StethoPrep medical team.