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Eye. Muscarinic receptor antagonists block the cholinergic responses of the pupillary sphincter muscle of the iris and the ciliary muscle controlling lens curvature (see Chapter 63). Thus, they dilate the pupil (mydriasis) and paralyze accommodation (cycloplegia). The wide pupillary dilation results in photophobia; the lens is fixed for far vision, near objects are blurred, and objects may appear smaller than they are. The normal pupillary reflex constriction to light or upon convergence of the eyes is abolished. These effects can occur after either local or systemic administration of the alkaloids. However, con- ventional systemic doses of atropine (0.6 mg) have little ocular effect, in contrast to equal doses of scopolamine, which cause definite mydriasis and loss of accommodation. Locally applied atropine or scopolamine produces ocular effects of considerable duration; accom- modation and pupillary reflexes may not fully recover for 7 to 12 days. Other muscarinic receptor antagonists with shorter durations of action are therefore preferred as mydriatics in ophthalmological prac- tice (see Chapter 63). Sympathomimetic agents also cause pupillary dilation but without loss of accommodation. Pilocarpine, choline esters,physostigmine(ophthalmicsolutiondiscontinuedintheUnited States), and isoflurophate (DFP) in sufficient concentrations can par- tially or fully reverse the ocular effects of atropine. Muscarinic receptor antagonists administered systemically have lit- tle effect on intraocular pressure except in patients predisposed to nar- row-angle glaucoma, in whom the pressure may occasionally rise dan- gerously. The rise in pressure occurs when the anterior chamber is narrow and the iris obstructs outflow of aqueous humor into the trabec- ulae. Muscarinic antagonists may precipitate a first attack in unrecog- nized cases of this relatively rare condition. In patients with open-angle glaucoma, an acute rise in pressure is unusual. Atropinelike drugs gen- erally can be used safely in this latter condition, particularly if the patient also is adequately treated with an appropriate miotic agent.