Figure 123

Diagram of eye depicting major pathway for outflow of aqueous humor (arrow) and ocular smooth muscles, which contract in response to parasympathomimetics or cholinesterase inhibitors (i.e., iris sphincter and ciliary muscle).

and permeability to the outflow of fluid. This beneficial effect, however, comes at the price of a spasm of accommodation and miosis, which seriously disturb vision. Cholinomimetics, therefore, have been replaced by p-blockers and carbonic anhydrase inhibitors, both of which decrease the formation of aqueous humor without affecting vision. However, some patients simply do not respond to these treatments or do not tolerate the cardiovascular side effects of the p-blockers, and choli-nomimetics (most notably pilocarpine) remain as important treatment alternatives.

Contraction of the iris sphincter (miosis) by choli-nomimetic stimulation is less important than contraction of the ciliary muscle for treating angle-closure glaucoma, but it may be essential as emergency therapy for acute-angle glaucoma to reduce intraocular pressure prior to surgery (iridectomy). Contraction of the iris sphincter by pilocarpine pulls the peripheral iris away from the trabecular meshwork, thereby opening the path for aqueous outflow.

Pilocarpine is the first choice among cholinomimet-ics for the treatment of glaucoma. Pilocarpine can be applied to the eye as a gel (Pilopine HS Gel) or timerelease system (Ocusert) for the chronic treatment of open-angle glaucoma, or as drops (Pilocar) for an acute reduction of intraocular pressure, as in the emergency management of angle-closure glaucoma. Carbachol is sometimes effective in treating cases of open-angle glaucoma that are resistant to pilocarpine.

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