Can asthma cause eosinopenia?

Asthma and Eosinopenia: Understanding the Relationship

Asthma does not typically cause eosinopenia; rather, asthma is generally associated with normal or elevated eosinophil counts, particularly during exacerbations. The relationship between asthma and eosinophil counts is complex and varies based on phenotype, treatment status, and disease activity.

Eosinophils in Asthma: The Normal Pattern

Asthma is typically characterized by:

• Eosinophilic inflammation in the airways 1
• Normal or elevated blood eosinophil counts, especially during exacerbations 2, 3
• Sputum eosinophilia in many (but not all) patients 1

According to the CHEST guidelines, asthma is "typically, but not exclusively, associated with eosinophilic inflammation, and non-invasive inflammatory markers such as blood or sputum eosinophil counts and fractional exhaled nitric oxide (FENO) can provide additional evidence to support the need for corticosteroid treatment" 1.

Asthma Phenotypes and Eosinophil Patterns

Asthma is heterogeneous, with different inflammatory patterns:

1. Eosinophilic asthma:

   • Characterized by elevated eosinophil counts in blood and/or sputum
   • Typically responds well to corticosteroid therapy 1
   • Associated with higher risk of exacerbations 2

2. Non-eosinophilic asthma:

   • Approximately 40-50% of patients with mild exacerbations may have non-eosinophilic inflammation 4
   • May show neutrophilic inflammation instead 1
   • Often less responsive to standard corticosteroid therapy

3. Mixed inflammatory patterns:

   • Some patients show both eosinophilic and neutrophilic inflammation 1

When Might Eosinophil Counts Be Low in Asthma?

While asthma doesn't cause eosinopenia, low eosinophil counts in asthmatic patients may occur in several situations:

1. Treatment effect:

   • Corticosteroid therapy (inhaled or systemic) effectively reduces eosinophil counts 1
   • Patients on maintenance therapy may have normal or low eosinophil counts despite active disease

2. Neutrophilic asthma phenotype:

   • Some patients with refractory asthma demonstrate neutrophilic rather than eosinophilic inflammation 1
   • These patients may have normal or low eosinophil counts even during exacerbations

3. Stress response:

   • Acute stress (including severe asthma attacks) can temporarily reduce eosinophil counts through stress-induced cortisol release

4. Concurrent infections:

   • Certain viral or bacterial infections can cause transient eosinopenia

Eosinophil Subtypes in Asthma

Recent research has identified different eosinophil subtypes that may explain some variations in eosinophil patterns:

• Patients with asthma have a higher proportion of inflammatory eosinophils (iEos; Siglec-8+CD62LloIL-3Rhi) compared to patients with COPD 5
• These inflammatory eosinophils express more IL-5 receptors and may respond differently to targeted therapies 5

Clinical Implications

When evaluating eosinophil counts in asthma:

• Normal or elevated counts are more typical than eosinopenia

• Low counts in untreated asthma should prompt consideration of:

  • Non-eosinophilic asthma phenotype
  • Alternative or additional diagnoses
  • Potential confounding factors (recent infections, stress, medications)

• Monitoring eosinophil counts can help guide therapy, particularly for:

  • Assessing corticosteroid response
  • Identifying candidates for targeted biologics
  • Evaluating disease activity

Key Distinctions

It's important to distinguish between:

1. Asthma - typically associated with normal or elevated eosinophils
2. Non-asthmatic eosinophilic bronchitis (NAEB) - characterized by chronic cough, sputum eosinophilia, but normal airway hyperresponsiveness 1
3. Eosinophilic granulomatosis with polyangiitis (EGPA) - characterized by asthma, eosinophilia >10%, and systemic vasculitis 1

In summary, while asthma itself does not cause eosinopenia, eosinophil counts in asthmatic patients can vary widely based on phenotype, treatment status, and disease activity. Understanding these patterns can help guide appropriate diagnosis and management.