Bronchiolitis and Small Airway Disease
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Direct answer
Bronchiolitis is inflammation and fibrosis of airways under 2 mm in diameter, which narrows or obliterates their lumen. Constrictive bronchiolitis is the form that matters most in exams, causing fixed airflow obstruction that does not reverse with bronchodilators. Key triggers are severe post-viral injury, connective tissue disease, inhaled diacetyl and chronic rejection after lung transplantation and chronic graft-versus-host disease after stem cell transplantation.
What you must remember
- Histology: concentric fibrosis of the bronchiolar wall narrows the lumen, so spirometry shows obstruction while the plain chest X-ray may look near normal.
- Spirometry: reduced FEV1 and FEV1/FVC ratio with little or no response to bronchodilators, which separates it from asthma.
- Key imaging test: expiratory high-resolution CT, showing mosaic attenuation and air trapping.
- Popcorn lung: bronchiolitis obliterans from inhaled diacetyl, a butter flavouring chemical, in workers who make microwave popcorn or flavourings.
- Drug cause: penicillamine, used in Wilson disease, cystinuria and rheumatoid arthritis, is the classic drug implicated.
- Post-viral form: constrictive bronchiolitis in children, classically after severe adenovirus infection.
- Transplant form: bronchiolitis obliterans syndrome, defined by a sustained fall in FEV1 of 20 per cent or more from the post-transplant baseline.
A stepwise route from breathlessness to diagnosis
A 34-year-old man who packs flavourings in a food factory has two years of dry cough and progressive breathlessness. His chest X-ray is near normal, but his FEV1 is reduced and does not improve after salbutamol.
- Confirm fixed obstruction: Spirometry shows a reduced FEV1/FVC ratio that does not reverse, pointing to small airway disease rather than reversible asthma.
- Take the exposure history: Ask about diacetyl or other fumes at work, drugs such as penicillamine, joint symptoms and any transplant or stem cell transplant history.
- Image on expiration: Expiratory high-resolution CT shows air trapping that inspiratory images can miss, and it is the test that points to the diagnosis.
- Plan a biopsy only if unclear: Transbronchial samples are usually too small, so surgical or video-assisted lung biopsy is reserved for doubtful cases.
- Remove the cause: Stop the exposure or the offending drug, and treat any underlying connective tissue disease.
- Treat and monitor: Inhaled bronchodilators relieve symptoms, and steroids are tried in selected causes with spirometry judging the response.
- Plan for end-stage disease: Pulmonary rehabilitation, oxygen when indicated and early referral for lung transplant assessment.
Where the exam tests the asthma trap
Small airway disease is often mistaken for something more familiar, so the examiner looks for the wrong label.
- Trap: calling the wheeze asthma and starting inhaled steroids alone Correct: fixed obstruction that fails to reverse points to constrictive bronchiolitis, and the cause must be sought.
- Trap: trusting a normal chest X-ray to exclude disease Correct: the lesion lies in airways too small to show on plain film, so expiratory CT is the key test.
- Trap: expecting a transbronchial biopsy to confirm the diagnosis Correct: the sample is too small, so the diagnosis rests on clinical, spirometric and CT features.
Frequently asked questions
Which spirometry pattern suggests constrictive bronchiolitis?
Constrictive bronchiolitis shows a reduced FEV1 and FEV1/FVC ratio that remains low after bronchodilator. This fixed obstruction, in a patient with a suitable trigger, should prompt an expiratory high-resolution CT scan of the chest.
What is popcorn lung and which exposure causes it?
Popcorn lung is bronchiolitis obliterans caused by inhaling diacetyl, a butter flavouring chemical used in microwave popcorn and food flavouring manufacture. Workers handling these flavourings are at risk, and removing the exposure is the most important step in management.
Why is bronchiolitis obliterans syndrome diagnosed by FEV1 after transplant?
Bronchiolitis obliterans syndrome after lung transplantation is diagnosed by a sustained fall in FEV1 of 20 per cent or more from the post-transplant baseline, after other causes are excluded. It reflects chronic rejection of the graft, and FEV1 is the standard measure used to track it.
Which drug is the classic cause of bronchiolitis obliterans?
Penicillamine is the classic drug cause of bronchiolitis obliterans, used in Wilson disease, cystinuria and rheumatoid arthritis. Stopping penicillamine is the first step, and the lung response to that step is then followed on spirometry.