- Clinical science
Muscarinic antagonists (antimuscarinic agents) are a group of anticholinergic drugs that competitively inhibit postganglionic muscarinic receptors. As such, they have a variety of applications that involve the parasympathetic nervous system. Which organ systems are most affected by an antimuscarinic agent depends on the specific characteristics of the agent, particularly its lipophilicity. Lipophilic agents (i.e., atropine or benztropine) are able to cross the blood-brain barrier and therefore affect the central nervous system (CNS) in addition to other organ systems. Less lipophilic agents (i.e., ipratropium or butylscopolamine) are administered if the CNS does not need to be targeted, specifically for respiratory (e.g., asthma), gastrointestinal (e.g., irritable bowel syndrome), or genitourinary applications (e.g., urinary incontinence). Numerous adverse effects have been reported, the most common of which is dry mouth. An overdose can result in anticholinergic syndrome, which manifests in disorientation, hyperthermia, tachycardia, and/or coma. To avoid toxicity, it is especially important to consider the anticholinergic effects of other drug classes before administering muscarinic antagonists.
Mechanism of action
Anticholinergic agents block the neurotransmitter acetylcholine in the central and the peripheral nervous system. Muscarinic antagonists (the majority of anticholinergic drugs) inhibit the effect of acetylcholine on muscarinic receptors, and antinicotinic agents inhibit the effects of acetylcholine on nicotinic receptors (mostly skeletal muscle relaxants). Antinicotinic agents are discussed in the learning card on .
Effects of muscarinic antagonists
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Central nervous system
Important muscarinic antagonists
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|Impaired secretion by exocrine glands|| || |
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|Decreased smooth muscle tone|
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"Blind as a bat (mydriasis), mad as a hatter (delirium), red as a beet (flushing), hot as a hare (hyperthermia), dry as a bone (decreased secretions and dry skin), the bowel and bladder lose their tone (urinary retention and paralytic ileus), and the heart runs alone (tachycardia).”
Anticholinergic syndrome (overdose)
- Belladonna poisoning
- Jimson weed/Angel's trumpet (Datura stramonium) poisoning
- Dry mouth, warm, flushed skin, thirst, tachycardia, arrhythmias, mydriasis, confusion, and agitation
- Possibly anticholinergic delirium: Excessive use of tricyclic antidepressants (or other medications with significant anticholinergic effects) can cause life-threatening delirium, hallucinations, and psychomotor symptoms.
- Treatment: antidote for purely anticholinergic poisoning (e.g. atropine): physostigmine
Differential diagnosis: See .
- Anticholinergic toxidrome: dry skin and mucous membranes, absence of bowel sounds
- Adrenergic toxidrome: diaphoresis, normal bowel sounds
We list the most important adverse effects. The selection is not exhaustive.