cycloplegia

The ciliary muscle is innervated by parasympathetic fibers (CN III via the ciliary ganglion); muscarinic blockade relaxes it, flattening the lens and paralyzing accommodation. The same drops also block the sphincter pupillae, producing mydriasis. Clinical uses include cycloplegic refraction (especially in children, where accommodative tone otherwise masks hyperopia), reduction of painful ciliary spasm in uveitis/iritis (homatropine, cyclopentolate), and pupil dilation for fundus exam (tropicamide is short-acting). Adverse effects include photophobia, blurred near vision, increased intraocular pressure (risk of acute angle-closure glaucoma in susceptible eyes), and systemic anticholinergic toxicity, especially with atropine in children. Atropine has the longest duration (up to ~1-2 weeks); tropicamide the shortest (hours).