Defining Eosinophilic Asthma
Eosinophilic asthma is defined by the presence of eosinophilic airway inflammation, typically characterized by elevated sputum eosinophil counts (>3%), blood eosinophil counts (>300 cells/μL), and/or elevated FeNO levels, which may occur with or without the typical features of airway hyperresponsiveness seen in classic asthma. 1
Key Diagnostic Criteria
Sputum eosinophilia: The gold standard for diagnosis is an induced sputum eosinophil count >3%, which indicates significant eosinophilic airway inflammation 2, 1
Blood biomarkers: Peripheral blood eosinophil counts typically >300 cells/μL serve as a more accessible surrogate marker 1, 3
FeNO levels: Elevated fractional exhaled nitric oxide measurements correlate with eosinophilic inflammation and can be used as a non-invasive biomarker 1, 3
Clinical presentation: Often associated with:
- Late-onset asthma
- More severe disease
- Potential steroid refractoriness
- May occur with or without atopy 4
Pathophysiological Features
Eosinophilic asthma differs from other asthma phenotypes in several key ways:
Inflammatory pattern: Characterized by eosinophil accumulation in airways that release inflammatory mediators and cytokines, causing tissue damage 5
Mast cell localization: In contrast to classic asthma where mast cells infiltrate airway smooth muscle, in eosinophilic bronchitis they primarily infiltrate the epithelium 2
Airway function: Unlike classic asthma, nonasthmatic eosinophilic bronchitis is not necessarily associated with variable airflow limitation or airway hyperresponsiveness 2
IL-5 pathway: Interleukin-5 serves as a key driver of eosinophil priming, activation, and survival in the airways 6
Clinical Significance and Treatment Implications
Corticosteroid response: Sputum eosinophil counts consistently decrease with corticosteroid therapy, with a halving or doubling considered clinically significant 2
Exacerbation prediction: Elevated sputum eosinophil counts strongly predict asthma exacerbations during corticosteroid reduction 2
Treatment guidance: Management strategies aimed at normalizing sputum eosinophil counts have been associated with up to 60% reduction in severe asthma exacerbations 2
Biologic therapy candidates: Patients with severe eosinophilic asthma often benefit from targeted biologics that affect the IL-5/IL-5Rα pathway 1, 7
Diagnostic Pitfalls
Dissociation between symptoms and inflammation: There can be significant discordance between eosinophilic airway inflammation and symptoms/disordered airway function in some asthma phenotypes 2
Biomarker limitations: While blood eosinophils and FeNO have good diagnostic accuracy, especially when used together, they are not perfect surrogates for sputum eosinophilia 3
Induced sputum challenges: Sputum induction and processing requires specialized facilities and trained technicians, limiting widespread availability 2
Phenotype overlap: Eosinophilic asthma can overlap with other asthma phenotypes, complicating diagnosis and treatment selection 5
By understanding these defining characteristics of eosinophilic asthma, clinicians can better identify this phenotype and implement appropriate targeted therapies to improve patient outcomes.