Section 7-C-5: Cholinergic Agents

The cholinergic agonists act at a receptor called the muscarinic receptor. Once the receptor is activated, the outflow of aqueous through trabecular meshwork is increased. The pupil also constricts (miosis). Agents within this class can act directly at the receptor or indirectly by inhibiting the breakdown of the neurotransmitter acetylcholine by the enzyme acetylcholinersterase. The topical medications within this class include pilocarpine, echothiophate iodide, and carbachol. With the number of better tolerated and more easily administered glaucoma drops on the market, these agents are no longer as popular in 2006.

Direct-Acting Cholinergic - Pilocarpine

Pilocarpine is a direct-acting cholinergic agonist. It opens the drainage angle and increases the trabecular meshwork aqueous outflow. There are different percentages of pilocarpine, and it has been noted that darker eye colors may require higher dosages of the medication. Dosing is frequently four times daily which can be difficult for a patient to comply. Pilocarpine was used frequently in the past due to its efficacy with rare systemic side effects (Table 7-7).

Table 7-7 Side effects of pilocarpine:
Ocular	Systemic (uncommon)
Ciliary muscle spasm (eye ache)	Headache
Pupillary constriction	Perspiration
Retinal detachment	Stimulation of salivary, lacrimal, gastrointestinal glands
Corneal toxicity	Increased bronchial secretions
Inflammation	Worsening of Alzheimer’s disease

Indirect-Acting Cholinergics – Carbachol, Echothiophate Iodide

Carbachol is both a direct and indirect-acting agent which is used as a topical medication. Echothiophate iodide (Phospholine Iodide) is an indirect-acting agent which inhibits acetylcholinesterase. The side effects of carbachol are similar to pilocarpine. Both carbachol and echothiophate iodide are not used commonly in 2006 due to newer and more easily tolerated agents on the market. Although they are effective in lowering IOP, they can have unwanted side effects which make newer agents more attractive. Occasionally patients will continue on these medications if they were initially started on them with good success and tolerance. A potentially serious complication of echothiophate iodide is pseudocholinesterase depletion. Echothiophate iodide may also inhibit pseudocholinesterase which breaks down succinylcholine, a paralytic medication sometimes used for general anesthesia. There can be prolonged respiratory paralysis if used prior to general anesthesia. Other side effects include nausea, diarrhea, cataracts, iris cysts, and ocular inflammation (Table 7-8).

Table 7-8: Side effects of echothiophate iodide:
Ocular	Systemic
Ciliary muscle constriction	Headache
Pupillary constriction	Respiratory depression
Retinal detachment	Nausea
Cataract formation	Diarrhea
Iris cysts	Abdominal cramping
Corneal toxicity	Malaise
Inflammation	Prolonged paralysis by succinylcholine during general anesthesia

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