Is bronchiectasis considered an interstitial lung disease?

Bronchiectasis is Not an Interstitial Lung Disease

No, bronchiectasis is not considered an interstitial lung disease (ILD). Bronchiectasis is a distinct structural airway disease characterized by permanent dilation of bronchi and bronchioles, while ILDs primarily affect the lung interstitium 1.

Understanding Bronchiectasis

Bronchiectasis is defined by:

• Permanent dilation of bronchi with destruction of elastic and muscular components of airway walls 1
• Cardinal symptoms including chronic cough and sputum production 1
• Diagnosis confirmed by high-resolution CT (HRCT) showing dilated airways, often with airway thickening and mucus plugging 2

Bronchiectasis belongs to the family of chronic obstructive lung diseases, not interstitial lung diseases 3. It is characterized by:

• Recurrent bacterial colonization and infection leading to progressive airway injury
• Inflammation mediated by neutrophils, T lymphocytes, and monocyte-derived cytokines
• Destruction of elastic and muscular components of bronchial walls 1

Distinguishing Bronchiectasis from ILDs

Interstitial lung diseases are characterized by:

• Primary involvement of the lung interstitium (tissue between capillary endothelium and alveolar epithelium) 4
• Specific radiographic patterns such as usual interstitial pneumonia (UIP), nonspecific interstitial pneumonia (NSIP), and hypersensitivity pneumonitis 4
• Progressive fibrosis of the lung parenchyma in many forms 1

The American Thoracic Society/European Respiratory Society classification of ILDs does not include bronchiectasis as an ILD 1. The 2022 ATS/ERS/JRS/ALAT clinical practice guideline clearly separates bronchiectasis from ILDs 1.

Important Clinical Distinction

While bronchiectasis is not an ILD, it's important to note that:

1. Traction bronchiectasis can occur as a radiological feature within ILDs:

   • Increased extent or severity of traction bronchiectasis is listed as one of the radiological criteria for progressive pulmonary fibrosis in ILDs 1
   • Traction bronchiectasis represents bronchial dilation due to surrounding fibrosis, not primary bronchiectasis disease 1

2. Bronchiectasis and ILDs can coexist:

   • Some patients may have both conditions simultaneously 5
   • In a study of microscopic polyangiitis patients, both ILD and bronchiectasis were observed, with bronchiectasis antedating the vasculitis in 75% of cases 5

Clinical Implications

Understanding that bronchiectasis is not an ILD has important management implications:

• Bronchiectasis treatment focuses on airway clearance, antibiotics for exacerbations, and addressing underlying causes 2
• ILD treatment often involves anti-inflammatory and antifibrotic medications 1
• Misclassification could lead to inappropriate treatment approaches

Conditions Associated with Bronchiectasis

Bronchiectasis may be associated with:

• Prior pneumonia or infections (tuberculosis, nontuberculous mycobacteria)
• Genetic conditions (cystic fibrosis, α1-antitrypsin deficiency, primary ciliary dyskinesia)
• Autoimmune diseases (rheumatoid arthritis, inflammatory bowel disease)
• Allergic bronchopulmonary aspergillosis
• Immunodeficiency syndromes 2

Up to 38% of cases are idiopathic, with no identifiable cause 2.

In summary, bronchiectasis is a structural airway disease characterized by bronchial dilation and chronic inflammation, distinct from interstitial lung diseases which primarily affect the lung parenchyma and interstitium.