What's the next step for a patient with a dry cough and exertional shortness of breath (SOB) after exercise, who had normal spirometry results?

Methacholine Challenge Test is the Next Step for a Patient with Dry Cough and Exertional SOB After Normal Spirometry

The methacholine challenge test (option B) is the most appropriate next step for a patient presenting with dry cough and exertional shortness of breath after exercise who has normal spirometry results. 1

Rationale for Methacholine Challenge Test

When a patient presents with symptoms suggestive of exercise-induced bronchoconstriction (EIB) but has normal spirometry, further diagnostic testing is required to confirm or rule out underlying airway hyperresponsiveness. The American Thoracic Society guidelines specifically recommend:

• In a subject with no history of current clinical asthma, normal pulmonary function test results, and no response to bronchodilator, an exercise challenge or surrogate challenge such as methacholine is indicated 1
• Self-reported symptoms and therapeutic trials without objective diagnosis are insufficient for diagnosing EIB 1

The methacholine challenge test has:

• High sensitivity (96.5%) for detecting airway hyperresponsiveness 2
• Strong negative predictive value - a negative test effectively rules out asthma 2

Why Other Options Are Less Appropriate

1. Repeat spirometry (Option A):

   • Not indicated when initial spirometry is already normal
   • Would likely yield the same normal results without providing additional diagnostic information 1

2. Arterial blood gases (Option C):

   • Not recommended for diagnosing exercise-induced symptoms
   • More appropriate for evaluating gas exchange abnormalities or respiratory failure
   • Will not detect underlying airway hyperresponsiveness 1

3. Peak expiratory flow (Option D):

   • Less sensitive than FEV1 for detecting airway obstruction
   • Has poorer repeatability compared to spirometry 1
   • Not recommended as the primary diagnostic tool when spirometry is normal 1

Methacholine Challenge Test Procedure

The methacholine challenge test involves:

1. Baseline spirometry measurement
2. Inhalation of diluent as control
3. Inhalation of increasing concentrations of methacholine
4. Serial measurements of FEV1 after each concentration
5. Administration of bronchodilator at test completion 1, 3

A positive test is defined as:

• A fall in FEV1 of ≥20% from baseline at any methacholine concentration 1, 4
• This indicates the presence of airway hyperresponsiveness, supporting a diagnosis of asthma or EIB

Clinical Pearls and Caveats

• Exercise-induced symptoms with normal spirometry are common in asthma, particularly in early or mild disease 5
• Up to 64% of patients with symptoms suggestive of asthma but normal spirometry will have a positive methacholine challenge test 5
• Important to rule out vocal cord dysfunction and other causes of exercise-induced laryngeal dysfunction, which can mimic asthma symptoms 1, 6
• Methacholine challenge should be performed under supervision with appropriate safety measures and bronchodilators readily available 3
• The test should not be performed in patients with severe airflow obstruction (FEV1 <60% predicted) or recent respiratory infection 3

In conclusion, the methacholine challenge test is the most appropriate next diagnostic step for this patient with exercise-induced respiratory symptoms and normal spirometry, as it will help establish or exclude the diagnosis of airway hyperresponsiveness.