Hypersensitivity Pneumonitis



Hypersensitivity pneumonitis or extrinsic al­lergic alveolitis occurs in individuals who have developed an abnormal sensitivity to some or­ganic agent. Four to six hours following exposure in a sensitized subject there is the onset of cough, dyspnea, fever, and malaise; wheezing is usually absent. Diffuse crackles are heard on auscultation and the x-ray reveals nodular or reticulonodular infiltrates with relative sparing of the apices. In most cases there is gradual resolution of these symptoms, which recur on subsequent exposure. The duration of symptoms may gradually increase with repeated exposure and eventually result in the development of pulmonary fibrosis and re­strictive lung disease.

A vast array of substances can cause this dis­order , the prototype being farmer’s lung, a hypersensitivity to thermophilic actino-mycetes, fungus-like organisms found in moldy hay. Patients with these disorders have serum pre­cipitins to specific proteins, although the pres­ence of precipitins does not, by itself, define the disease, since 50 per cent of similarly exposed subjects develop precipitins but remain asymp­tomatic.

The treatment is to remove or avoid the offend­ing agent. Occasionally in the acute situation cor­ticosteroids are required. The efficacy of corti­costeroids in the chronic phase, once fibrosis has set in, however, is less clear, although a trial in symptomatic patients is usually worthwhile.