Antimuscarinic Anticholinergic Agents

The term antimuscarinic is derived from the action of acetylcholine at the postganglionic synapse which is closely imitated by the alkaloid, muscarine. 

The antimuscarinic effects mainly include an elevation of heart-rate. Besides, a diminution in the production of bronchial, lachrymal, gastric, nasal, intestinal, sweat and saliva secretions are observed together with a reduction in intestinal motility. 

Antimuscarinic agents may be classified on the basis of their chemical structures under the following heads  

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Physostigmine Suggested for antimuscarinic anticholinergic agents not for tricyclic antidepressants Adult dose is 0.5-1 mg IV slowly. The effects are transient (30-60 minutes), and the lowest effective dose may be repeated when symptoms return. May cause bradycardia, increased bronchial secretions, seizures. Have atropine ready to reverse excess effects. Do not use for tricyclic antidepressant overdose. 

The prototypical antimuscarinic anticholinergic drug is atropine (Fig. 19-2). Atropine is a naturally occurring substance that can be obtained from the extract of plants such as belladonna and jimsonweed. Other natural, semisynthetic, and synthetic antimuscarinic anticholinergic agents have been developed that are similar in structure or function to atropine. 

The antimuscarinic action of clozapine and thioridazine can cause tachycardia and enhance the peripheral and central effects (confusion, delirium) of other anticholinergic agents, such as the tricyclic antidepressants and antiparkinson agents. 

The case in context Clozapine is a newer antipsychotic that can, like other agents in its class, produce antimuscarinic side effects. Although Ms. Doe had not complained of anticholinergic effects prior to beginning treatment with a moderate dose of diphenhydramine, it is likely that the additive anticholinergic effects of clozapine and diphenhydramine resulted in urinary retention. 

Some systemic agents may possess sufficient anticholinergic activity to produce mydriasis and a weak cycloplegic effect.These medications include antimuscarinic drugs, antihistamines, phenothiazines, and tricyclic antidepressants (Table 35-9). 

Of the systemic antihistamines, the ethanolamines, including diphenhydramine, have significant antimuscarinic activity. In addition, the antipsychotic agents, particularly the phenothiazines such as thioridazine (Mellaril), have well-dociunented anticholinergic properties. Therapeutic doses of tricyclic antidepressants, like amitriptyline hydrochloride (Elavil) and imipramine (Tofranil), produce significant anticholinergic actions and thus have the potential for ocular side effects. 

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