bronchiolitis obliterans
Summary
Bronchiolitis obliterans (BO) is a rare, irreversible obstructive lung disease caused by inflammation and fibrosis of the small airways (bronchioles), leading to their narrowing or complete obliteration. It presents with progressive dyspnea, dry cough, and wheezing that are often refractory to bronchodilators. Classic causes include post-infectious injury, inhalational toxin exposure (e.g., 'popcorn lung' from diacetyl), and chronic lung transplant rejection.
Detail
Pathophysiology: Injury to the bronchiolar epithelium (from infection, toxic inhalation, autoimmune disease, or alloimmune reaction in transplant) triggers an inflammatory response with fibroblast proliferation and submucosal/peribronchiolar fibrosis. This results in concentric narrowing or complete luminal obliteration of small airways (<2mm), causing fixed airflow obstruction.
Etiologies: 1. Post-infectious (especially adenovirus, Mycoplasma, respiratory syncytial virus) particularly in children 2. Toxic inhalation exposures (diacetyl in flavoring - 'popcorn lung', nitrogen dioxide, sulfur dioxide, ammonia) 3. Chronic lung allograft dysfunction after lung transplantation (most common late complication, forms of chronic rejection) 4. Post-bone marrow/hematopoietic stem cell transplantation (graft-versus-host disease manifestation) 5. Connective tissue diseases (rheumatoid arthritis) 6. Drug-induced (rare)
Clinical presentation: Progressive dyspnea, dry cough, wheezing unresponsive to bronchodilators, occurring weeks to months after inciting event. Exam may reveal mid-inspiratory squeaks/crackles.
Diagnosis: PFTs show obstructive pattern (decreased FEV1/FVC) without significant bronchodilator response; air trapping and hyperinflation. High-resolution CT (HRCT) with expiratory views shows mosaic attenuation pattern, air trapping, bronchial wall thickening, and bronchiectasis. Gold standard is lung biopsy showing concentric fibrosis obliterating bronchiolar lumen (constrictive bronchiolitis), though biopsy is often not performed given patchy distribution and clinical risk.
Distinguish from: Bronchiolitis obliterans organizing pneumonia (BOOP, now called cryptogenic organizing pneumonia) - this is a DIFFERENT entity with intraluminal polyps of granulation tissue in alveolar ducts/alveoli, presents with restrictive pattern, responds well to steroids, and has better prognosis. BO proper is largely irreversible and steroid-refractory.
Treatment: Largely supportive; immunosuppression (steroids, azathioprine) often tried but limited efficacy. In transplant patients, augmented immunosuppression. Lung retransplantation may be considered. Prognosis is generally poor with progressive decline in lung function.
High-yield board points: Associate with popcorn lung (diacetyl exposure), chronic lung transplant rejection, and post-viral injury in children. Remember distinction from BOOP/COP which is more treatable.
Sources
- Robbins and Cotran Pathologic Basis of Disease
- First Aid for the USMLE Step 1
- Harrison's Principles of Internal Medicine
- UpToDate: Bronchiolitis obliterans syndrome
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