A patient using topical timolol maleate 0.5% eye drops develops bradycardia and worsening of asthma. Timolol causes these systemic side effects primarily because:

• A It is absorbed through the conjunctival vessels and metabolized in the liver
• B Nasolacrimal drainage carries the drug to nasal and pharyngeal mucosa where systemic absorption occurs, bypassing hepatic first-pass metabolism ✓
• C It crosses the cornea and enters the anterior chamber where it diffuses into systemic circulation
• D Conjunctival melanin binds and slowly releases the drug systemically

Explanation

A significant proportion of topically applied eye drops drain via the nasolacrimal duct into the nasal cavity and nasopharynx, where absorption occurs through the richly vascularized nasal and pharyngeal mucosa. This route delivers the drug directly into the systemic circulation, bypassing hepatic first-pass metabolism — resulting in relatively high systemic bioavailability. This explains why topical beta-blockers can cause bronchospasm in asthmatics and bradycardia/AV block in susceptible patients. Punctal occlusion (nasolacrimal occlusion technique) after instillation reduces systemic absorption.

Reference: Khurana Comprehensive Ophthalmology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.