USMLE

Methacholine

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Cholinomimetics
  1. Bethanechol
  2. Carbachol
  3. Methacholine
  4. Pilocarpine
  5. Donepezil Rivastigmine and Galantamine
  6. Edrophonium
  7. Neostigmine
  8. Physostigmine
  9. Pyridostigmine

Summary

Methacholine is a muscarinic agonist primarily used clinically in asthma challenge tests due to its ability to induce bronchospasm and dyspnea. A strong bronchospastic response at a low dose of methacholine indicates high airway reactivity, confirming a diagnosis of asthma.

Key Points

  • Methacholine
    • Mechanism
      • Muscarinic agonist (cholinomimetic)
        • Typically administered in inhaled formulation
        • Targets M3 receptors in the bronchioles
    • Clinical Use
      • Asthma challenge (diagnostic test)
        • Stimulation of M3 receptors causes bronchoconstriction and mucus production
        • Overreactivity of airways to very small doses of methacholine suggests diagnosis of asthma (called “methacholine challenge test” or “bronchoprovocation test”)
    • Adverse Effects
      • Bronchospasm, dyspnea (obvious)