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Muscarinic antagonists

Last updated: December 4, 2020

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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

Effects of muscarinic antagonists

Overview of the effects of muscarinic receptors blockage
Muscarinic receptors Organ/Tissue Effects
M1, M4, M5

Central nervous system

  • Influences neurologic function (e.g., cognitive impairment)
M2

Heart

M3

Smooth muscle

Exocrine glands

  • ↓ Secretions (sweat)

Important muscarinic antagonists

List of antimuscarinic agents
Chemical characteristics Drug Effect Indication

Tertiary amines

Lipophilic (good oral bioavailability and CNS penetration)

  • Atropine
  • ↓ Vestibular disturbances (antiemetic)
  • Homatropine
  • Tropicamide
  • Ophthalmology
    • Therapeutic use: in patients with uveitis
    • Diagnostic use: pupillary dilation to allow ocular fundus examination and cycloplegia to allow refractory testing
  • ↓ Cholinergic overactivity in CNS
  • Oxybutynin
  • Tolterodine
  • Solifenacin
  • Dicyclomine
  • Hyoscyamine
  • Darifenacin
  • ↑ Sphincter tone

Quarternary amines

Hydrophilic (poor oral bioavailability and CNS penetration)

  • Butylscopolamine (hyoscine butylbromide)
  • Glycopyrrolate
  • ↓ GI and respiratory secretions
  • Ipratropium bromide
  • Tiotropium bromide

References:[1][2][3][4][5][6][7]

Side effects of antimuscarinic agents

Antimuscarinic side effects
System Side effect Contraindications
Impaired secretion by exocrine glands
  • Respiratory distress
Cardiovascular system
Decreased smooth muscle tone
Eye
CNS

"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)

References:[1][8][9][10]

We list the most important adverse effects. The selection is not exhaustive.

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  3. American Heart Association. American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care - Part 7.3: Management of symptomatic bradycardia and tachycardia 2005. Circulation. 2005; 112 (24, suppl): p.IV-67-IV-77. doi: 10.1161/circulationaha.105.166558 . | Open in Read by QxMD
  4. Brocks DR. Anticholinergic drugs used in Parkinson's disease: An overlooked class of drugs from a pharmacokinetic perspective.. J Pharm Pharm Sci. 1999; 2 (2): p.39-46.
  5. Posey EL. Management of peptic ulcer with glycopyrrolate. Am J Dig Dis. 1962; 7 (10): p.863-872. doi: 10.1007/bf02231863 . | Open in Read by QxMD
  6. Schlueter DP. Ipratropium bromide in asthma. A review of the literature.. Am J Med. 1986; 81 (5A): p.55-60.
  7. Acetylcholine Receptors: Muscarinic and Nicotinic. http://pharmacologycorner.com/acetylcholine-receptors-muscarinic-and-nicotinic/. Updated: January 1, 2018. Accessed: January 8, 2018.
  8. Lieberman JA. Managing anticholinergic side effects. Prim Care Companion J Clin Psychiatry. 2004; 6 (Suppl 2): p.20-10.
  9. Atropine. https://www.drugs.com/pro/atropine.html. Updated: March 3, 2017. Accessed: May 10, 2017.
  10. Berdai MA, Labib S, Chetouani K, Harandou M. Atropa belladonna intoxication: a case report.. Pan Afr Med J. 2012; 11 : p.72.