TABLE 23.1 Etiologic Agents of Hypersensitivity Pneumonia and Their Associated Clinical Diseases | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Hypersensitivity Pneumonia
Hypersensitivity Pneumonia
Marie-Christine Aubry, M.D.
Allen P. Burke, M.D.
Seth Kligerman, M.D.
Hypersensitivity Pneumonia
Terminology and Definition
Hypersensitivity pneumonia, also known as hypersensitivity pneumonia1,2 and extrinsic allergic alveolitis,3 is an interstitial lung disease that results from an immunologic reaction to various inhaled antigens (Table 23.1). The exposure may not be readily recognized clinically, and therefore, pathology becomes essential to the diagnosis. Although a specific combination of histologic features may lead to a histologic diagnosis of hypersensitivity pneumonia, in many instances, the pathology is not entirely specific4 and other morphologic features may be present, including the so-called bronchiolocentric interstitial pneumonias, such as airway-centered interstitial fibrosis, idiopathic bronchiolocentric interstitial pneumonia.5,6,7
Pathogenesis
The pathogenesis of hypersensitivity pneumonia is obscure. Only a small proportion of patients exposed to offending agents develop the disease. A Th1-predominant immune response has been shown in the acute phase of hypersensitivity pneumonia, whereas a Th2-predominant immune response may factor in the development of the fibrotic form of the disease, similar to other fibrosing lung diseases.8 Other pathogenetic features in common with idiopathic pulmonary fibrosis include the up-regulation of proteins involved in apoptosis in bronchiolar epithelial cells.9
Epidemiology
In the United Kingdom, the incidence of hypersensitivity pneumonia has been estimated nearly 1 per 100,000 person-years, with a threefold increased risk for death over the general population.3
In a series of interstitial lung disease from Mexico in the 1980s, after excluding patients with autoimmune collagen vascular disease and inorganic exposures, over two-thirds of patients with interstitial lung disease were diagnosed with hypersensitivity pneumonia (pigeon breeder’s lung). The high incidence was attributed to the custom of having pigeons fly freely in the home as pets.10
Clinical
Between two-thirds and 90% of patients are women, with a mean age at presentation of about 50 years for the acute form and 60 years for the fibrotic form.11 There is no association with cigarette smoking.3 Most patients have symptoms for a year or longer. In approximately twothirds of patients, an inciting antigen is found and may be derived from a wide variety of fungal, bacterial, animal, or chemical sources including birds, household mold, thermophilic bacteria linked to contaminated humidifiers, and hay.
The clinical presentation of hypersensitivity pneumonia can be acute, subacute, or chronic. Acute hypersensitivity pneumonia occurs following exposure to a large amount of antigen. Patients develop severe dyspnea, cough, fever, and chills usually within 4 to 8 hours of exposure.2 The symptoms resolve rapidly after removal of exposure and recur with re-exposure to the antigen. Typically, a biopsy is not performed as it is not necessary for the diagnosis.
The subacute and chronic presentations result from prolonged exposure to small amounts of antigen, which may not be evident to the patient, and therefore the diagnosis becomes more problematic, commonly leading to a biopsy. These patients have a more insidious onset of shortness of breath, with nonproductive cough. They may also develop fever, fatigue, and malaise. Chronic hypersensitivity pneumonia may progress into pulmonary fibrosis.2,11,12,13,14
Upon examination of the chest, tachypnea and inspiratory crackles are found in all clinical presentations of hypersensitivity pneumonia. Wheezing may occur and lead to a misdiagnosis of asthma. In chronic hypersensitivity pneumonia progressing to fibrosis, digital clubbing and manifestations of pulmonary hypertension with cor pulmonale may occur.
Pulmonary function tests are characterized by a predominantly restrictive physiology, increase residual volume due to air trapping, decrease diffusion capacity, and hypoxemia.4
Laboratory Detection of Antibodies
Specific IgG-precipitating serum antibodies are detected in 30% to 75% of patients4,10,11,15 and may be used to identify the causal antigen. They are especially useful in the acute form of hypersensitivity pneumonia. Because precipitins are markers of exposure, however, they do not indicate disease and are frequently present in asymptomatic persons. False negatives are also common due to variation in standardization of reagents and in serum antibody levels.2 In one study, the sensitivity of precipitin testing among groups of patients with known exposure was 75% for microbial antigens and 52% for avian antigen.4
