Ocular Pharmacology and Diagnostic Agents
This domain tests the agents you use to examine the eye: what they do, how long they last, and which patients need caution before instillation.
Classes and behaviour
- Cycloplegic mydriatics block muscarinic receptors on the sphincter and ciliary muscle. Atropine 1% is longest acting (up to 1–2 weeks of cycloplegia), cyclopentolate 1% intermediate (roughly a day), tropicamide short (hours).
- Phenylephrine 2.5% dilates without cycloplegia; the 10% concentration is avoided because of systemic hypertensive risk.
- Topical anaesthetics (proparacaine, benoxinate, tetracaine) are for procedures only; chronic use causes epithelial toxicity and masks disease.
- Dyes: sodium fluorescein for corneal and tear assessment, lissamine green and rose bengal for devitalised epithelium.
- IOP agents: prostaglandin analogues raise uveoscleral outflow; beta-blockers and carbonic anhydrase inhibitors reduce production; pilocarpine raises trabecular outflow; alpha-2 agonists do both.
Worked example
A 64-year-old hyperope with a 2.5 mm pupil in dim light needs funduscopy. Shallow peripheral anterior chamber on van Herick grading suggests angle-closure risk. Reasoning through: gonioscopy or referral before dilation; if dilation is still required, use tropicamide with phenylephrine 2.5% rather than atropine, and warn about the signs of acute angle closure. The short-acting combination limits the duration of any risk.
Traps
Believing phenylephrine produces cycloplegia; dilating a shallow anterior chamber without grading or gonioscopy; prescribing topical steroids without monitoring IOP and corneal status; ignoring benzalkonium chloride toxicity with frequent dosing; and using fluorescein on a soft lens.