Description
Bronchiolitis obliterans (BO), also known as bronchiolitis obliterans syndrome or obliterative bronchiolitis, is a rare and severe lung disease characterised by the inflammation and subsequent obstruction of the bronchioles, the smallest airways in the lungs. The inflammation leads to the production of excess fibrous tissue (fibrosis), which progressively blocks off the airways. BO can occur in both children and adults, with different underlying aetiologies. It often leads to chronic and progressive lung disease, significantly impairing respiratory function.
Pathogenesis
The pathogenesis of bronchiolitis obliterans involves a significant inflammatory response that leads to the obliteration of the small airways. This is typically initiated by an injury to the lung, often due to inhalation of toxic substances, infection, or as a complication of lung transplantation. The exact mechanism of disease progression is not fully understood but is thought to involve both immune-mediated damage and an aberrant wound healing response. Following the initial lung injury, inflammatory cells infiltrate the bronchioles, leading to local damage and the proliferation of fibroblasts, which lay down fibrous tissue, ultimately leading to bronchiolar obstruction.
Epidemiology, Risk Factors, and Associations
- The exact incidence and prevalence of BO are difficult to estimate due to its diverse aetiologies. However, it is considered a rare disease.
- Major risk factors include lung transplantation (developing as a form of chronic transplant rejection), inhalation of toxic gases or particles, certain viral infections such as adenovirus (especially in children), atypical bacterial pneumonia (Mycoplasma pneumoniae), and connective tissue diseases.
- It has also been associated with certain medications, such as penicillamine and gold therapy for rheumatoid arthritis.
Clinical Features
The clinical presentation of bronchiolitis obliterans can vary, but common features include:
- Chronic and progressive dyspnoea (shortness of breath)
- Dry cough
- Wheezing
- Fatigue
Pulmonary function tests often show a pattern of obstructive lung disease, with decreased forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC), as well as reduced total lung capacity.
Complications
Swyer-James-MacLeod Syndrome
- It’s a rare complication in young patients with post-infectious obliterative bronchiolitis leading to emphysema, classically triggered by adenovirus or Mycoplasma pneumoniae. It is not a sequela of viral bronchiolitis, of which RSV is the most common aetiology.
- Imaging features include small ipsilateral hyperlucent lung or part of a lung on chest X-ray due to air-trapping and a reduction in pulmonary vascular markings. CT may show bronchiectasis and air trapping, predominantly affecting one lung.
Pathological Features
- Histologically, bronchiolitis obliterans is characterised by submucosal inflammation and fibrosis of the bronchioles leading to partial or complete luminal occlusion.
- The fibrosis may be accompanied by excessive granulation tissue, known as “organising pneumonia.”
- The process is typically patchy, with some areas of the lung affected more than others.
Radiological Features
Plain Radiography
- Usually normal or may show mild hyperinflation with attenuation of vascular markings.
CT
- High-resolution computed tomography (HRCT) is often used in the diagnosis of BO.
- Typical findings suggestive of small airway obstruction include air trapping on expiratory phase, bronchiectasis, bronchial wall thickening, and mosaic attenuation.
Grading
The severity of the disease is usually classified based on the degree of impairment in lung function, particularly FEV1.
Differential Diagnosis
- Asthma and chronic obstructive pulmonary disease (COPD) are the most common conditions that need to be differentiated from BO, given their similar clinical and functional presentations.
- Asthma: Episodic wheezing, dyspnoea, chest tightness, coughing, particularly at night or early morning, possible history of allergies. Variable airflow obstruction that is at least partially reversible, hyperresponsiveness of airways.
- COPD: Older age, history of heavy smoking, dyspnea, chronic cough with sputum production. Emphysematous changes, bronchial wall thickening, ‘dirty lungs’ on CT scan.
- Infectious Bronchiolitis: Clinical evidence of infection. Centrilobular nodules and tree-in-bud opacities. Good response to antibiotic therapy.
- Cystic Fibrosis: Common in younger age group, history of recurrent chest infections, possible history of pancreatic insufficiency (malabsorption, poor growth). Positive sweat chloride test, genetic testing showing CFTR mutation.
- Other interstitial lung disease: Progressive dyspnoea on exertion, nonproductive cough, possible history of exposure to ILD-associated drugs or occupational hazards. Interstitial markings, honeycombing, and fibrotic changes on CT scan, restrictive pattern on lung function tests.
Management
- Incurable and management is therefore largely supportive with immunosuppressive and corticosteroid therapy.