What is the difference between allergic cough and cough variant asthma in patients presenting with a persistent cough?

Distinguishing Allergic Cough from Cough Variant Asthma

Cough variant asthma (CVA) is a distinct clinical entity characterized by bronchial hyperresponsiveness and eosinophilic airway inflammation that responds to bronchodilators and corticosteroids, while "allergic cough" is a poorly defined term that likely represents an overlap syndrome with allergic rhinitis, adenoid hypertrophy, or early CVA rather than a separate diagnostic category. 1, 2

Key Diagnostic Features of Cough Variant Asthma

CVA presents with the following defining characteristics:

• Persistent nonproductive cough lasting >2-3 weeks as the sole or predominant symptom, without wheezing, chest tightness, or dyspnea 1, 2, 3
• Bronchial hyperresponsiveness demonstrated by positive methacholine challenge test (though CVA patients show lesser degree of hyperresponsiveness compared to typical asthma) 1, 2
• Physical examination and spirometry are typically completely normal, making diagnosis challenging 2
• Cough characteristically worsens at night, after exercise, or with cold air exposure 1
• Definitive diagnosis requires documented improvement with bronchodilator therapy or inhaled corticosteroids 1, 2, 4

Pathophysiology That Distinguishes CVA

CVA shares identical pathologic features with classic asthma:

• Eosinophilic airway inflammation present in sputum, bronchial mucosa, and bronchoalveolar lavage fluid 2, 5
• Airway smooth muscle infiltration by mast cells 2
• Subepithelial layer thickening indicating airway remodeling 2, 5
• Significantly more sensitive cough reflex compared to typical asthma or healthy individuals 2

The Problem with "Allergic Cough" as a Diagnosis

"Allergic cough" lacks clear diagnostic criteria and clinical validation:

• In children, "allergic cough" is poorly defined even in adults and likely represents overlap with asthma, allergic rhinitis, and adenoid tonsillar hypertrophy 1
• Studies examining atopy in children with isolated chronic cough show inconsistent findings, with some reporting increased atopy and others showing no influence of atopic status 1
• Markers of atopy (skin prick tests, specific IgE) do not predict which children with cough will respond to asthma therapies 1
• In children with atopy, cough sensitivity is not elevated 1

Atopic Cough: A Separate Entity from CVA

Some research describes "atopic cough" as distinct from CVA:

• Atopic cough presents with isolated chronic nonproductive cough, atopic constitution, eosinophilic tracheobronchitis, and airway cough receptor hypersensitivity 6
• Critical difference: atopic cough lacks bronchial hyperresponsiveness and does NOT respond to bronchodilators 6
• Atopic cough is NOT a precursor to typical asthma (only 1/82 patients developed asthma over median 4.8 years), whereas CVA frequently progresses to typical asthma 6
• Patients with CVA show higher rates of sensitization to house dust mite, dog dander, and molds compared to atopic cough 7

Clinical Algorithm for Differentiation

For patients with persistent cough >2-3 weeks:

1. Obtain detailed history focusing on nocturnal worsening, exercise triggers, cold air triggers, and absence of wheezing 1, 2

2. Perform spirometry (usually normal in CVA) and methacholine challenge test 1, 2

   • Positive methacholine test supports CVA but is not diagnostic alone
   • Negative methacholine test essentially excludes CVA due to very high negative predictive value 2

3. Consider induced sputum analysis for eosinophil count (>3% is diagnostic of eosinophilic inflammation) 2

4. Measure fractional exhaled nitric oxide (FeNO) as additional evidence of eosinophilic inflammation 2

5. Initiate therapeutic trial with inhaled corticosteroids (≤200 mcg/day beclomethasone equivalent) 2, 4

   • Expect response within 6 hours to 2 weeks 2
   • If no response after 2 weeks, escalate to oral prednisolone 30 mg/day for 2 weeks 1, 2
   • CVA diagnosis confirmed by documented cough resolution with treatment 2, 4

Critical Pitfalls to Avoid

Do not diagnose asthma or CVA based on cough alone in children:

• Most children with isolated chronic cough do NOT have asthma 1
• Cough sensitivity and specificity for wheeze is poor (34% and 35% respectively) in children 1
• Chronic cough in children without wheeze should not be considered a variant of asthma 1
• "Cough variant asthma is probably a misnomer for most children in the community who have persistent cough" 1

Recognize that CVA can progress to typical asthma:

• Approximately 30% of CVA patients develop typical asthma within several years 2
• Treatment with inhaled corticosteroids may prevent transformation to typical asthma 6
• Patients sensitized to multiple allergens, especially house dust mite and dog dander, have higher risk of progression 7

Treatment Response as Diagnostic Confirmation

CVA responds specifically to asthma medications:

• Bronchodilators provide symptomatic relief 1, 4, 5
• Inhaled corticosteroids are first-line maintenance therapy 2, 4
• Nine of ten CVA patients report significant improvement within 3 days of prednisone trial 4
• Complete resolution may require up to 8 weeks of treatment 2

In contrast, if "allergic cough" represents upper airway disease:

• Treatment should target allergic rhinitis or post-nasal drip rather than bronchodilators 1
• Lack of response to bronchodilators argues against CVA diagnosis 6