Methacholin

Methacholine, commonly encountered as methacholine chloride, is a synthetic, quaternary ammonium choline ester that acts as a direct-acting muscarinic receptor agonist. The term methacholin appears in some sources as a misspelling; the standard name in medical literature is methacholine.

Pharmacology and mechanism of action: Methacholine stimulates muscarinic receptors, with activity primarily on M3 receptors in airway smooth muscle, leading to bronchoconstriction and increased airway resistance. It is poorly absorbed from the gut and is typically administered by inhalation for diagnostic purposes. In the body, methacholine is hydrolyzed by acetylcholinesterase, resulting in a short duration of action compared with acetylcholine.

Medical use: Methacholine is used mainly in bronchial challenge tests to evaluate airway hyperresponsiveness, aiding in the diagnosis of asthma or other reactive airway diseases. During the test, patients inhale increasing concentrations of methacholine; a drop in forced expiratory volume in one second (FEV1) of about 20% or more from baseline is considered a positive result. The test is often performed alongside bronchodilator reversibility assessment and other clinical information. It is not used as a therapy for symptoms.

Safety and contraindications: The procedure should be conducted in a controlled clinical setting with appropriate emergency equipment. It is contraindicated in individuals with significant baseline airway obstruction, uncontrolled asthma, recent acute coronary events, or hypersensitivity to methacholine. Common adverse effects include coughing, throat irritation, chest tightness, and bronchospasm; rare severe reactions may occur.