Methacholine challenge testing for Asthma

  • Asthma is a chronic disorder of conducting airways caused by immunological reaction to varieties of stimuli.
  • An asthma attack is triggered mostly by environmental allergens, such as dusts, pollen, cockroach or animal dander and foods. This type of asthma is called Atopic asthma.
  • Most asthma attacks are not triggered by inhalation of allergens, but instead by viral respiratory infections. This type of asthma is called Non-Atopic Asthma.
  • In some adults bronchospasm is triggered by a completely non allergic stimuli such as cold air, exercise, distilled water etc.
  • Now suppose we make the latter category of people inhale aerosols of cholinergic agonist “Methacholine” in serially increasing concentrations and measure their maximal expiratory flow rate, we could access that there is a fall in maximal expiratory flow rate with each dose increase (this happens because of bronchoconstriction induced by Methacholine). The concentration at which methacholine reduces FEV1 by 20% is noted. This will tell us about –
          • The severity of bronchial hyper-reactivity
          • The responsiveness of bronchial airway to drugs.
  • Now this test is rarely used clinically  but can be used to get the differentials of chronic cough in a patient with normal pulmonary function tests.
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