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Hypersensitivity pneumonitis, subacute to chronic phase.


Hypersensitivity pneumonitis (HP) (otherwise known as extrinsic allergic alveolitis) is a hypersensitivity reaction resulting from exposure to certain environmental allergens. It is commonly associated with interstitial lung disease. Those affected commonly belong to occupational groups routinely exposed to birds, hay, or certain reactive chemical species. HP can be classified into acute, subacute, or chronic forms depending on the frequency, length, and intensity of exposure, severity of symptoms and the presence of leukocytosis. The acute form is characterized by fever, flu-like symptoms, and leukocytosis. The subacute form is characterized by the insidious onset of productive cough, dyspnea, and fatigue over weeks to months. The chronic form is characterized by long-term progressive dyspnea, cough, weight loss, and fatigue.

Diagnosis is usually based on exposure history and evidence of fibrosis on imaging, bronchoalveolar lavage with lymphocytosis, positive inhalation challenge testing. If there is uncertainty regarding diagnosis, a lung biopsy is needed to confirm HP. Histopathologic features of HP include poorly formed noncaseating granulomas, multinucleated giant cells near respiratory or terminal bronchioles, chronic cellular bronchiolitis, and chronic cellular pneumonitis with lymphoplasmacytic infiltrates. HP can also be fibrotic and will predominantly show features of usual interstitial pneumonia (UIP) and/or fibrotic nonspecific interstitial pneumonia (NSIP) though the aforementioned features can also be present here. Multinucleated giant cells with cholesterol clefts or Schaumann bodies support the diagnosis of chronic/fibrotic HP. It is important that the pathologist be informed of the clinical context as the diagnosis of HP requires careful clinicopathological correlation.

The differential for HP includes hot tub lung, inhalation fever, organic dust toxic syndrome, chronic bronchitis, asthma, and airway-centered interstitial fibrosis. The prognosis for HP worsens with each classification, usually with longer length of symptoms correlating to a worse prognosis. Avoiding ongoing exposure often leads to resolution of symptoms for patients with acute or mild HP. For those with the chronic variant and evidence of fibrosis and reduced lung function, a trial of oral steroids is worthwhile although management is primarily supportive, including smoking cessation, seasonal influenza and pneumococcal vaccinations, pulmonary rehabilitation and supplemental oxygen. Once severe lung damage has occurred, few treatment options remain and HP may ultimately require lung transplantation.



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