Hypersensitivity pneumonitis (HP) or extrinsic allergic alveolitis (EAA) is a syndrome caused by the repetitive inhalation of antigens from the environment in susceptible or sensitized people. Common antigens include molds, bacteria, bird droppings, bird feathers, agricultural dusts, bioaerosols and chemicals from paints or plastics. Hypersensitivity pneumonitis may eventually lead to interstitial lung disease.
Signs and symptoms
Hypersensitivity pneumonitis (HP) can be categorized as acute, subacute, and chronic based on the duration of the illness.
Subacute
Patients with subacute HP gradually develop a productive cough, dyspnea, fatigue, anorexia, weight loss, and pleurisy. Symptoms are similar to the acute form of the disease, but are less severe and last longer. Findings may be present in patients who have experienced repeated acute attacks.
Chronic
In chronic HP, dose of the antigen tends to be low volume but for a longer duration. In general, acute HP is suspected to be attributed to a type III hypersensitivity while the subacute and chronic types are suspected to be caused by T cell infiltration and granuloma formation. It has been recommended that the questionnaire administered should be relevant to the region in which the exposure has potentially occurred.
Detailed exposure assessments are warranted in the cause of damp indoor environments which have the potential to propagate mold throughout the dwelling. The decision to enlist an industrial hygienist should be made based on the answers to evidence based questions during the environmental assessment. !! Specific antigen !! Exposure
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! Bird fancier's lung <span style="font-size:87%;"> <br>Also called bird breeder's lung, pigeon breeder's lung, and poultry worker's lung</span>
| Avian proteins
| Feathers and bird droppings
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! Bagassosis <span style="font-size:87%;"> <br>Exposure to moldy molasses</span>
| Thermophilic actinomycetes
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! Malt worker's lung
| Aspergillus clavatus
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! Wine-grower's lung
| Botrytis cinerea mold
| Moldy grapes
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! Woodworker's lung
| Alternaria, Penicillium spp
| Wood pulp, dust
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Of these types, Farmer's Lung and Bird-Breeder's Lung are the most common. "Studies document 8-540 cases per 100,000 persons per year for farmers and 6000-21,000 cases per 100,000 persons per year for pigeon breeders. High attack rates are documented in sporadic outbreaks. Prevalence varies by region, climate, and farming practices. HP affects 0.4–7% of the farming population. Reported prevalence among bird fanciers is estimated to be 20-20,000 cases per 100,000 persons at risk." Three-density pattern (head cheese sign) is radiological sign which shows a region of the lung with three or more different types of attenuation which can be typical for the fibrotic type. Cholesterol clefts or asteroid bodies are present within or outside granulomas.
When fibrosis develops in chronic hypersensitivity pneumonitis, the differential diagnosis in lung biopsies includes the idiopathic interstitial pneumonias. This group of diseases includes usual interstitial pneumonia, non-specific interstitial pneumonia and cryptogenic organizing pneumonia, among others.
Differential diagnosis
Organic dust toxic syndrome presents similarly with fevers, chills a few hours after exposure to bioaerosols with toxins from fungi, however this is not a true hypersensitivity reaction because it occurs on initial exposure without a preceding sensitization. Unlike asthma, hypersensitivity pneumonitis targets lung alveoli rather than bronchi.
Similarly, sarcoidosis has noncaseating granuloma formation, however hilar adenopathy is often seen on chest radiographs. Corticosteroids such as prednisolone may help to control symptoms but may produce side-effects. In addition to steroids, other immunosuppressants (e.g., Azathioprine, Mycophenolic acid) and anti-fibrotic agents (e.g., Nintedanib) may be used although their effectiveness is unclear.
Additional images
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Image:Hypersensitivity pneumonitis high mag.jpg|High magnification micrograph of hypersensitivity pneumonitis showing granulomatous inflammation. Trichrome stain.
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See also
- Byssinosis
- Dust pneumonia
- Pneumonitis