Asthma bronchial challenge test

Overview
Asthmatics may remain asymptomatic for a long period unless provoked by a stimuli such as a chemical irritant, an environmental allergen, cold or dry air, or rigorous exercise that may precipitate an acute attack. The bronchial challenge test is a procedure performed to provoke airway obstruction using a stimuli that is known to trigger bronchospasm, sudden contraction of the bronchioles. This test helps to identify the specific environmental stimuli that triggers an acute attack and also helps to determine the extent of the reaction.

Bronchial hyperresponsiveness
The rationale for bronchoprovocation testing is to assess the degree of underlying bronchial hyper-responsiveness that occurs because of recurrent bronchial inflammation. Bronchial hyper-responsiveness is defined as a state of hyperactive airways that may be easily triggered by an external stimulus to precipitate an episode of bronchospasm.

Mechanisms of Benefit

 * Absence of bronchial hyper-responsiveness on bronchoprovocation test does rule-out the diagnosis of asthma.


 * Asymptomatic airway hyper-responsiveness has shown to be associated with airway inflammation and remodeling and the appearance of asthmatic symptoms is because of an increase in the airway inflammation.


 * The severity of disease has shown to be proportional to the degree of airway responsiveness.


 * The degree of bronchial hyper-responsiveness has shown to be beneficial in discriminating the risk of near-fatal attacks and hence predict outcomes in symptomatic patients.

Test Specificity
Bronchoprovocation test is not specific for the diagnosis of asthma; however, a negative test indicated by the absence bronchial hyper-responsiveness following allergen inhalation excludes asthma.

Indication

 * To identify specific environmental triggers
 * Evaluation of atypical symptoms such as unexplained cough
 * Evaluation of exercise induced asthma and occupational asthma
 * Assessment of response to therapy
 * Symptomatic patients with a normal lung function test and no reversal of symptoms with bronchodilator

Procedure

 * The patient's medical history is taken to evaluate for possible triggers and a baseline spirometry is conducted to assess initial lung function. Following which, under controlled circumstances, the patient is exposed to specific triggers to assess the extent of bronchial hyper-responsiveness. Spirometry tests are repeated again after inhalation of the allergen and compared with the baseline results.


 * Pharmaceutical agents such as methacholine or histamine may be used as a provocative stimuli to confirm the diagnosis.


 * Reversibility test or a post bronchodilator test helps to assess the reversibility of airway disease and differentiate between asthma and COPD; wherein, a bronchodilator is administered before performing another round of test for comparison.

Indication
In cases of inconclusive baseline spirometry results, the methacholine challenge test may be used to diagnosis asthma.

Dose
Lower doses of inhaled methacholine have shown to be sufficient to provoke bronchial hyperresponsiveness in asthmatics.

Sensitivity and Specificity
The sensitivity of a positive bronchial hyperactivity with methacholine challenge test is approximately 86%. Patients with allergic rhinitis, COPD, bronchitis may test falsely positive. However, irrespective of the false negatives, and false positive results associated with the test, a negative test demonstrating no airway reactivity, generally excludes the diagnosis of asthma.

Procedure
Based on the American Thoracic Society guidelines, individuals must be specifically trained to perform this test. In addition, tests must be conducted in an appropriate facility. After a baseline spirometry, the patient breathes in incremental doses of nebulized methacholine that provokes narrowing of the airways resulting in bronchoconstriction and subsequently the FEV1 is measured a minute after the inhalation. The procedure is repeated until a 20% FEV1 reduction is observed.

Interpretation
A reduction in FEV1 greater than 20% after provocation with methacholine is indicative of bronchial hyper-responsiveness.

Limitations

 * The test is physically demanding and the results may be affected if the patient is exhausted or has muscular fatigue or weakness.
 * Methacholine at sometimes can stimulate the upper airway responses sufficiently enough to cause violent coughing which can make spirometry difficult or impossible.