How is cough‑variant asthma diagnosed in a 35‑year‑old woman with a 30‑day dry cough?

Diagnosing Cough Variant Asthma

In a 35-year-old woman with 30 days of dry cough, perform bronchial challenge testing with methacholine to demonstrate airway hyperresponsiveness, which is the key diagnostic feature that distinguishes cough variant asthma from other causes of chronic cough. 1

Initial Clinical Assessment

• Obtain a focused history looking specifically for nocturnal or seasonal cough patterns, exercise-induced symptoms, and any history of atopy or allergic conditions 2, 3
• Exclude alternative diagnoses including ACE inhibitor use, smoking history, and gastroesophageal reflux disease, which commonly coexists with cough variant asthma 4
• Perform baseline spirometry even though it will likely be normal, as physical examination and spirometry findings are typically non-diagnostic in cough variant asthma 1, 2

Definitive Diagnostic Testing

Bronchial Challenge Testing (Primary Diagnostic Tool)

• Methacholine inhalation challenge testing should be performed when physical examination and spirometry are non-diagnostic to confirm airway hyperresponsiveness 1, 4, 3
• A negative methacholine test essentially excludes cough variant asthma due to its very high negative predictive power 2, 3
• A positive methacholine test is consistent with but NOT diagnostic of cough variant asthma alone—it requires documented resolution of cough with antiasthmatic therapy for definitive diagnosis 3
• The methacholine challenge involves sequential inhalation of increasing concentrations (0.0625 to 16 mg/mL) with FEV1 measurements after each dose, stopping when FEV1 falls ≥20% from baseline 5

Non-Invasive Inflammatory Markers (Supportive Testing)

• Measure fractional exhaled nitric oxide (FeNO) or sputum eosinophil counts to assess eosinophilic inflammation and predict corticosteroid responsiveness 1, 4, 2
• Sputum eosinophilia >3% is diagnostic of eosinophilic airway inflammation, and patients without sputum eosinophilia do not respond to corticosteroids 4, 2
• These markers are particularly useful when bronchial challenge testing is unavailable or impractical 4

Diagnostic Trial Approach (When Testing Unavailable)

• A 2-week trial of oral prednisolone 30 mg daily can establish the diagnosis when diagnostic testing is unavailable or impractical, with cough control expected within 1-2 weeks if due to eosinophilic airway inflammation 4
• Alternatively, initiate inhaled corticosteroids with an inhaled bronchodilator and assess for response over 1-8 weeks, as most patients show partial improvement after 1 week with complete resolution requiring up to 8 weeks 3
• Definitive diagnosis requires documented resolution of cough with antiasthmatic therapy, making the therapeutic trial both diagnostic and confirmatory 3

Key Diagnostic Features

• Chronic nonproductive cough as the sole manifestation lasting >8 weeks without wheezing or dyspnea 2, 6, 7
• Airway hyperresponsiveness to methacholine, though typically less severe than in classic asthma 2
• Eosinophilic airway inflammation demonstrated by sputum analysis, bronchial mucosa biopsy, or elevated FeNO 2
• Responsiveness to bronchodilators, which is a key diagnostic feature distinguishing cough variant asthma from non-asthmatic eosinophilic bronchitis 7

Common Diagnostic Pitfalls

• Do not rely solely on normal spirometry to exclude asthma—spirometry is often entirely normal in cough variant asthma 2, 3
• Do not confuse cough variant asthma with non-asthmatic eosinophilic bronchitis (NAEB)—both present with chronic cough and eosinophilic inflammation, but NAEB lacks airway hyperresponsiveness on methacholine testing 1
• Ensure chest radiograph is obtained to exclude other pathology before initiating corticosteroid therapy 4
• Recognize that approximately 30% of cough variant asthma patients develop classic asthma within several years, making early diagnosis and treatment crucial 2, 3, 8

Diagnostic Algorithm Summary

1. Baseline evaluation: History, physical exam, chest radiograph, spirometry 4
2. If spirometry shows reversible obstruction: Proceed directly to treatment 3
3. If spirometry is normal: Perform methacholine challenge testing 1, 3
4. If methacholine positive: Measure FeNO or sputum eosinophils for treatment guidance 4, 2
5. If testing unavailable: Initiate therapeutic trial with inhaled corticosteroids plus bronchodilator 4, 3
6. Confirm diagnosis: Document cough resolution with treatment 3