Synopsis
Recognise small-airway physiology and constrictive bronchiolitis, identify causal contexts including transplantation and inhalation injury, and coordinate evidence-limited specialist treatment without confusing organising pneumonia.
- Small airways are bronchioles below the resolution of routine imaging; substantial disease can exist before conventional spirometry becomes clearly abnormal.
- Bronchiolitis obliterans, or constrictive bronchiolitis, is fibrotic narrowing and obliteration of small airways that usually produces progressive, relatively fixed airflow obstruction.
- Important contexts include lung transplantation, allogeneic haematopoietic stem-cell transplantation, connective-tissue disease, severe inhalational injury, selected infections and drug or immune-mediated injury.
- After lung transplantation, an obstructive chronic allograft dysfunction phenotype is termed bronchiolitis obliterans syndrome when defined clinically without requiring surgical histology.
- Symptoms are often insidious exertional breathlessness and dry cough; examination may be normal or reveal wheeze, crackles or inspiratory squeaks.
- Expiratory HRCT may show mosaic attenuation and air trapping, but these findings are not specific and require inspiratory comparison, adequate expiration and clinical context.
- Small-airway indices from spirometry can be suggestive but are variable and should not independently diagnose bronchiolitis obliterans.
- Exclude reversible asthma, COPD, bronchiectasis, infection and central airway disease; a poor bronchodilator response does not by itself establish bronchiolar fibrosis.
- Treatment is cause-specific and evidence is limited. Transplant immunosuppression changes belong to the transplant centre; there is no universal community corticosteroid regimen.
- Selected post-lung-transplant patients may receive long-term azithromycin, but ECG, liver, hearing, drug-interaction, NTM and antimicrobial-resistance safeguards are essential.
Key red flags
A sustained fall from the best post-transplant expiratory function with new obstruction may represent BOS, but transplant specialists must exclude infection, acute rejection, airway stenosis and restrictive allograft dysfunction.
Progressive dry cough, wheeze or exertional dyspnoea with fixed obstruction in a person with allogeneic transplantation and chronic graft-versus-host features should prompt urgent haematology-respiratory review.
Fever, purulent sputum, pleuritic pain, rapid hypoxaemia or abrupt functional loss suggests infection, embolism, pneumothorax or acute transplant complication rather than slow bronchiolar fibrosis alone.
Investigation priorities
Confirm obstruction, trajectory and degree of reversibility.
Management branches
Progressive breathlessness or cough accompanies unexplained fixed obstruction or expiratory air trapping.
- Construct a timeline of transplantation, infection, inhalational exposure, autoimmune disease and medicines, noting onset relative to each potential insult.
- Repeat quality-assured spirometry with bronchodilator and obtain volumes, gas transfer and paired inspiratory-expiratory CT where clinically justified.
- Exclude asthma, COPD, bronchiectasis, central airway obstruction, infection, vascular disease and organising pneumonia using the pattern rather than one small-airway index.
- Refer for specialist multidisciplinary review and reserve invasive tissue sampling for a specific question likely to alter treatment.