Definition and epidemiology
Hypersensitivity pneumonitis (HP), formerly extrinsic allergic alveolitis, is an immune-mediated interstitial lung disease caused by inhalation of an environmental antigen in a susceptible host, producing inflammation and, over time, fibrosis of the lung parenchyma and small airways. Antigens include avian proteins (bird-fancier’s lung), thermophilic actinomycetes and fungi (farmer’s lung, humidifier lung) and various chemicals. HP is uncommon but under-recognised in children; a careful environmental and exposure history — pets, birds, mould, damp housing, hobbies — is the single most valuable diagnostic step. The modern framework divides HP into nonfibrotic (NFHP) and fibrotic (FHP) phenotypes, which carry very different prognoses.
Clinical features
Presentation varies with exposure intensity and chronicity. Nonfibrotic HP may present with recurrent flu-like episodes — fever, cough, dyspnoea and malaise hours after antigen exposure — that resolve when the exposure stops. Fibrotic HP is more insidious, with progressive exertional dyspnoea, dry cough, weight loss, fatigue and, in advanced disease, clubbing and inspiratory crackles. Symptoms that consistently improve away from a particular environment (home, school, farm) and recur on return strongly suggest HP.
Diagnosis
The 2020 ATS/JRS/ALAT guideline frames diagnosis around four domains: identification of a potential exposure/antigen; characteristic HRCT pattern (using inspiratory and expiratory imaging to detect air trapping and mosaic attenuation, classified as typical, compatible or indeterminate); BAL lymphocytosis; and histopathology when needed. Combinations of these factors yield graded diagnostic confidence levels from “HP not excluded” through to “definite HP,” ideally reached by multidisciplinary discussion. Serum-specific IgG against suspected antigens supports, but does not prove, the diagnosis. Distinguishing FHP from other fibrosing ILDs (including IPF) is the central diagnostic challenge and has direct treatment implications.
See the full criteria: Hypersensitivity Pneumonitis
Red flags
- Progressive fibrosis, traction bronchiectasis or honeycombing on HRCT (fibrotic phenotype)
- Resting or exertional hypoxaemia and falling diffusing capacity
- Failure to improve despite documented antigen removal
- Diagnostic overlap with IPF or connective-tissue-disease ILD — needs multidisciplinary review
- Pulmonary hypertension complicating advanced disease
Management overview
The single most important intervention is complete antigen identification and avoidance — removing the bird, remediating mould or damp, or changing the offending environment can halt or reverse nonfibrotic disease, and no pharmacotherapy substitutes for it. Systemic corticosteroids are used to accelerate recovery in symptomatic or progressive nonfibrotic HP, though they do not alter long-term outcome once antigen avoidance is achieved. In fibrotic HP, steroid-sparing immunosuppressant agents are often added, and antifibrotic therapy is considered for the progressive-fibrosing phenotype. Provide supplemental oxygen as needed, pulmonary rehabilitation, vaccination and longitudinal monitoring of lung function; refer advanced disease for transplant assessment. Patient and family education on sustained exposure avoidance is essential to prevent relapse.
References
- Raghu G, et al. ATS/JRS/ALAT HP guideline. Am J Respir Crit Care Med, 2020.
- Fernández Pérez ER, et al. CHEST HP guideline. CHEST, 2021.
- Nelson Textbook of Pediatrics, 21st ed.
Decision support for qualified clinicians only — verify against current primary guidelines and your clinical judgement.