Hypersensitivity Pneumonitis
Hypersensitivity pneumonitis = extrinsic allergic alveolitis, is the result of non-IgE mediated immunological inflammation, caused by repeated inhalation of non-human protein, which can be part of plant or animal origin. The inflammation manifests in alveoli and distal bronchioles, and can present as acute, sub-acute, and chronic hypersensitivity pneumonitis. Many cases are the result of occupational exposure.
The prevalence is not exactly known, but thought to be about 2-8%. Multiple causative agents have been reported, but the most commonly reported agents are bacteria (e.g. thermophilic actinomycetes causing farmers lung) and animal proteins (causing the likes of pigeon lung). Fungi have also been reported as possible causative agents.
The exact pathogenesis is unclear but it is thought to be a cell mediated reaction, within CD3+ T lymphocytes being an important cell in inflammation. The cellular infiltrate consists of lymphocytes, plasma cells and neutrophils, with non-caseating granulomas and activated macrophages found. The inflammation tends to be bronchocentric or bronchiolocentric.
Image: “Histology of chronic hypersensitivity pneumonitis” by Mutleysmith, CC BY-SA 3.0, via Wikimedia Commons.
In acte HP there is fever, tachypnea, dyspnoea, restrictive pulmonary function tests and reduced diffusing lung capacity of carbon monoxide. In the subacute phase inflammation is less intense and there are indolent symptoms such as malaise, dyspnoea, and cough. Finally, in the chronic phase, there is very little inflammation, instead simply fibrosis left that restricts lung function.
High-resolution CT in hypersensitivity pneumonitis: diffuse ground-glass opacity with small centrilobular nodules in both lungs.
Image: “Neumonitis por hipersensibilidad” by Mluisamtz11, CC BY-SA 3.0, via Wikimedia Commons.
Diagnostic criteria can be minor or major:
- Major
- Exposure to known causes of HP
- Compatible history of dyspnoea, cough and malaise
- Infiltrates on CXR
- Minor
- Rales
- Abnormal PFTs (restrictive)
- Decreased lung capacity of carbon monoxide
- Histological changes on biopsy
- Positive inhalational challenge
Management is avoidance of causative agents as the most important aspect, as well as reducing humidity and improving ventilation. There are few medications available, oral corticosteroids are sometimes introduced as they may improve lung function slightly.