Clinical-immunologic correlates: a differential diagnostic update. Hypersensitivity pneumonitis

Hypersensitivity pneumonitis : a broader perspective

Hypersensitivity pneumonitis (HP) represents a group of lung disorders caused by the inhalation of a wide variety of organic particles by susceptible individuals. HP occurs mainly in nonsmokers, but smoking may promote an insidious and chronic disease. The prevalence of HP is difficult to estimate accurately since several antigens can produce the disease, but the range spans infancy to old age. Regardless of the causative antigen or its environmental setting, the clinical manifestations are essentially the same. Three different clinical presentations have been recognized: acute, subacute, and chronic. In the acute form, patients show flu-like symptomatology, followed by dyspnea and dry cough. Symptoms subside a few hours or days later. The subacute and chronic forms result from recurrent low-level antigen exposure and are characterized by progressive dyspnea and dry cough. Other constitutional symptoms such as fatigue, anorexia, and weight loss can be apparent. Fever may occur in the subacute form. Importantly, chronic HP may evolve insidiously or may result from repeated acute/subacute episodes. Recurrent acute, subacute, and chronic HP may progress to irreversible lung fibrosis or provoke emphysematous changes.HP can be difficult to identify, and precise diagnosis requires a history of exposure and a constellation of clinical, imaging, laboratory, bronchoalveolar lavage and pathologic findings. General laboratory tests show an increase of acute phase reactants. Specific precipitating antibodies, when present, are evidence of antigen exposure, and are a hallmark for diagnosis. Chest radiograph usually reveals widespread ground-glass attenuation, and nodular or reticulonodular shadowing. High-resolution CT features include diffuse or patchy ground-glass opacities with small poorly defined nodules and air trapping. Pulmonary function tests are characterized by a predominantly restrictive ventilatory defect with loss of lung volume and hypoxemia at rest that worsens with exercise. Bronchoalveolar lavage reveals a significant increase in lymphocytes, mostly over 40%. In the acute form there is also an increase in neutrophils. Antigen-induced lymphocyte proliferation, and environmental or laboratory-controlled inhalation challenge, may be used for diagnostic purposes and can help to establish a diagnosis of insidious forms of HP. In subacute or chronic cases, lung biopsy may be necessary. Typical findings include bronchiolitis, lymphocytic alveolitis, and loosely formed granulomas, although occasionally other morphologic patterns such as nonspecific interstitial pneumonia may exist. Treatment focuses on avoiding further exposure to the offending antigen(s). Corticosteroids are recommended in subacute and chronic forms. The usual regimen consists of initial high doses of systemic corticosteroid (e.g. prednisone 0.5-1.0 mg/kg/day), followed by gradual tapering.

Mixed infiltrative and obstructive disease on high-resolution CT: differential diagnosis and functional correlates in a consecutive series

Fourteen of 400 consecutive patients having high-resolution computed tomography (HRCT) with expiratory images showed findings of infiltrative lung disease on inspiratory HRCT and air trapping on expiratory CT. Diagnoses included hypersensitivity pneumonitis, sarcoidosis, atypical infection, and pulmonary edema. The extent of infiltrative abnormalities and air trapping were correlated with pulmonary function tests (PFT) in 11 patients. PFT indicated a mixed pattern in five, an obstructive pattern in three, and a restrictive pattern in three. Forced expiratory volume (FEV) in 1 second/forced vital capacity (FVC) correlated significantly with the extent of air-trapping (r = 0.60; p = 0.05). The extent of infiltrative abnormalities correlated significantly and negatively with forced vital capacity (r = -0.82, p = 0.002), FEV1 (r = -0.59, p = 0.05), total lung capacity (TLC) (r = -0.67, p = 0.05), and DLCO (r = -0.75, p = 0.02). Findings of lung infiltration on inspiratory HRCT scans and air trapping on expiratory CT correlated respectively with PFT measures of restrictive and obstructive lung disease.

Shortness of breath secondary to a pet cockatiel

A case of hypersensitivity pneumonitis in a 40-year-old patient secondary to a pet bird is presented. The clinical presentation, diagnosis, and management of hypersensitivity pneumonitis are reviewed.

Expiratory high-resolution CT scan

Although high-resolution CT scan has proved most useful in the diagnosis of infiltrative lung disease, its use in the diagnosis of airway and obstructive lung diseases has recently been emphasized. In particular, the use of dynamic expiratory or postexpiratory CT scans, usually in combination with an inspiratory high-resolution CT scan study, has proved useful in the diagnosis and assessment of obstructive lung diseases. This article reviews the use of expiratory CT scan in the diagnosis of lung disease, including the various CT scan techniques that can be used, normal and abnormal expiratory CT scan findings, and the use of expiratory CT scan in a variety of obstructive diseases.

Hypersensitivity pneumonitis

The lung is constantly exposed to a wide variety of environmental insults. In its defense against these environmental challenges, however, the lung responds through a limited number of pathophysiologic mechanisms. This is well illustrated by a group of diseases which are collectively referred to as hypersensitivity pneumonitis. This syndrome includes a very large number of different diseases. However, in the United States, only farmer's lung, bird-breeder's lung, and ventilation hypersensitivity pneumonitis occur with any significant frequency. Each of these is characterized by flu-like symptoms, in conjunction with a pneumonitis consisting of lymphocytic granulomatous infiltration of the alveoli and terminal bronchioles. This disease is caused by the inhalation of antigenic material which usually originates from the dusts of organic material. A host of different dusts and antigens have been described in conjunction with hypersensitivity pneumonitis but each leads to the same characteristic clinical syndrome. Thus, each of the diseases shares similar clinical features but differs primarily with respect to the nature of the exposure and causative antigens. The clinical features, pathogenesis, course, prognosis, and treatment of these related diseases are reviewed.