What are the diagnostic criteria and functional loss classification for asthma hypersensitivity airway syndrome and cough‑variant asthma across mild, moderate, and severe cases?

Asthma Hypersensitivity Airway Syndrome and Cough-Variant Asthma: Diagnostic Criteria and Functional Loss

Diagnostic Criteria

Cough-variant asthma (CVA) is diagnosed when patients present with chronic cough (>8 weeks) as the sole or predominant symptom, demonstrate bronchial hyperresponsiveness (BHR) on challenge testing, and show clinical response to bronchodilator therapy. 1

Core Diagnostic Elements for CVA:

Three essential criteria must be met 1:

1. Clinical presentation: Spasmodic cough, with or without wheezing and dyspnea
2. Objective evidence (at least one of the following):
   • Positive airway hyperresponsiveness on bronchial challenge test
   • Sputum eosinophils ≥3%
   • %V50 <70% predicted
   • %V25 <50% predicted
   • Positive bronchodilator response (required when cough is the only symptom)
3. Exclusion: Rule out other lung and heart diseases

Key Diagnostic Distinctions from Classic Asthma:

CVA patients differ significantly from classic asthma (CA) in several measurable parameters 2, 3:

• Better baseline pulmonary function: Higher FEV1/FVC ratio, FEV1% predicted, and flow rates at 50% and 25% of vital capacity
• Lower airway inflammation: Reduced fractional exhaled nitric oxide (FeNO) values (24.0 ppb vs 36.0 ppb in CA) 3
• Milder BHR: Higher PC20 values and lower proportion of moderate-to-severe hyperresponsiveness (41.72% vs 64.70% in CA) 2
• Demographics: More common in younger patients and females 2

Important caveat: By definition, CVA excludes patients with normal methacholine sensitivity, though some chronic cough patients with normal airway sensitivity may still cough during challenge testing (termed "COUGH" phenotype) 4.

Functional Loss Classification

Mild CVA:

Pulmonary function characteristics 5, 6:

• FEV1% predicted: typically >80%
• Small airway dysfunction predominates:
  • MMEF% pred: 65-80% (74.50% median) 6
  • FEF50% pred: 70-85% (78.9% median)
  • FEF75% pred: 60-80% (69.70% median)
• FEV1/FVC: >80% (84.65% mean) 6
• Predominantly mild BHR on challenge testing 5

Clinical impact:

• Chronic cough without significant dyspnea or wheezing
• Normal daily activities generally preserved
• Minimal rescue medication use

Moderate CVA:

Pulmonary function characteristics 5, 7:

• Progressive small airway dysfunction:
  • MMEF% pred: 50-65%
  • FEF50% and FEF75% showing 15-30% reduction from predicted
• Increased airway resistance on impulse oscillometry 7
• Moderate BHR (PC20 values indicating moderate sensitivity) 5

Clinical impact:

• Persistent cough affecting sleep and quality of life
• Occasional limitation of activities
• Regular need for bronchodilator therapy

Severe CVA (Progression toward Classic Asthma):

Pulmonary function characteristics 7, 2:

• Significant small airway obstruction:
  • MMEF% pred: <50%
  • FEF50% pred: <65%
  • FEF75% pred: <50%
• May show FEV1% predicted <80% if progressing to classic asthma
• Severe BHR with low PC20 values 5
• Elevated airway resistance parameters (R5, R20, resonant frequency) 7

Clinical impact:

• Severe, unremitting cough significantly impairing quality of life
• Risk of progression to classic asthma with wheezing and dyspnea 7
• Daily symptoms requiring controller medications

Severity Grading Based on BHR:

The most objective classification uses histamine or methacholine challenge PC20 values 5:

• Very mild: PC20 approaching borderline positive
• Mild: PC20 indicating mild hyperresponsiveness (most common presentation, 58% of CVA patients)
• Moderate: PC20 indicating moderate hyperresponsiveness (22% of CVA patients)
• Severe: PC20 indicating severe hyperresponsiveness (rare in pure CVA)

Inflammatory Endotypes:

Recent molecular profiling identifies two distinct CVA endotypes with different functional implications 8:

Mixed-inflammatory CVA (56% of cases):

• Elevated sputum eosinophils, neutrophils, and multiple cytokines
• Better response to anti-asthma therapy (80% complete cough resolution at 6 months)
• More pronounced airway inflammation signatures

Pauci-inflammatory CVA (44% of cases):

• Minimal sputum inflammation despite BHR
• Lower treatment response (56.2% complete resolution)
• Higher cough persistence despite therapy
• May represent distinct pathophysiological mechanism not driven by eosinophilic inflammation 9

Predictive Markers for Treatment Response:

The most valuable predictors of positive anti-asthma therapy response are 3:

• ΔFEV1 ≥110 ml after bronchodilator in CVA patients
• ΔFEV1% ≥2.59% after bronchodilator in CVA patients

These thresholds help identify patients likely to benefit from controller therapy and distinguish CVA from other causes of chronic cough.

Clinical Progression:

CVA represents an early stage of asthma that can evolve 7:

1. CVA (earliest stage): Cough only, mild BHR, preserved lung function
2. Classic asthma with positive BPT (intermediate): Development of wheezing/dyspnea, moderate BHR
3. Classic asthma with positive BDT (advanced): Fixed airway obstruction, severe small airway dysfunction, poorest lung function

The interval from onset to diagnosis is shortest in CVA, emphasizing the importance of early recognition to prevent progression 7.