mydriatic
Summary
A mydriatic is an agent that dilates the pupil. Classic mydriatics include muscarinic antagonists (atropine, tropicamide) and alpha-1 agonists (phenylephrine), used for fundoscopic exams and to break posterior synechiae in uveitis.
Detail
Pupil size reflects the balance between parasympathetic (M3) constriction of the iris sphincter and sympathetic (alpha-1) contraction of the iris dilator. Mydriatics work by either blocking M3 (antimuscarinics: atropine, homatropine, tropicamide, cyclopentolate), which also produces cycloplegia (loss of accommodation), or by stimulating alpha-1 (phenylephrine), which dilates without cycloplegia. Tropicamide is preferred for routine fundus exams due to short duration. Major boards association: anticholinergic mydriatics can precipitate acute angle-closure glaucoma in predisposed patients with shallow anterior chambers, causing painful red eye, fixed mid-dilated pupil, and halos around lights. Anisocoria with a blown pupil after head trauma suggests CN III compression from uncal herniation.
Sources
- First Aid for USMLE Step 1 2024
- Katzung Basic & Clinical Pharmacology
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