Hypersensitivity pneumonitis (HP), also known as extrinsic allergic alveolitis in the United Kingdom, is an allergic lung disease that results from the inhalation of antigens contained in a variety of organic dusts. Farmer's lung, the most well known HP syndrome, results from the inhalation of fungal organisms (thermophilic actinomycetes) that grow in moist hay. Many other HP syndromes also result from exposure to fungi, and they are usually named after the setting in which the exposure occurs or the organic substance involved.
Pathologic features--HP is characterized by interstitial alveolitis, cellular bronchiolitis, and noncaseating epitheloid granulomas.2, 68, 69 The diagnosis of HP is confirmed by the presence of all three of these findings, but all three are not invariably present. Thus, the diagnosis often rests on a combination of the proper clinical scenario, characteristic radiographic findings, and suggestive histopathologic findings. The radiographic and pathologic findings in the various etiologies of HP are similar, thus the disease may be classified into acute, subacute, and chronic stages regardless of the responsible antigen.
Clinical findings--An acute, heavy exposure to an offending antigen may produce fever, chills, dry cough, and dyspnea. Long-term exposure can produce shortness of breath with few or minimal systemic symptoms. Recurrent exposure may result in recurrent acute episodes, possibly superimposed on chronic changes. Chronic HP may present with progressive respiratory impairment. It is not uncommon that, despite extensive questioning, a history of an exposure to an offending antigen cannot be elicited. Thus, a tissue diagnosis is frequently required.
Radiographic findings--Chest radiographs: In the acute stage, heavy exposure to an enticing antigen may result in ill-defined, bilateral air space disease. These findings reflect alveolar inflammation due to neutrophils, eosinophils, lymphocytes, and large mononuclear cells and/or obstructive pneumonitis. Patients are rarely seen at this stage.
The acute phase may resolve in several days. Then, in the subacute phase, a fine nodular pattern may develop on the chest radiograph (figure 14 -- 34KB). The nodular pattern correlates with alveolitis; interstitial infiltration; small, poorly defined granulomas; and cellular bronchiolitis. These histologic findings are usually most apparent in a peribronchiolar distribution. The classic radiographic appearance of HP is recurrent, transient areas of consolidation superimposed on a fine, nodular pattern. In the majority of patients, however, the chest radiograph is normal.70 The subacute presentation may be seen following a heavy acute exposure or with repeated, low-level exposures.
In the chronic phase, fibrosis develops months or years following the initial exposure. The fibrosis can be patchy in distribution, often with a mid-lung predominance.70
HRCT: HRCT findings in the acute phase of HP include bilateral air space consolidation and small, (1 to 3 mm diameter) ill-defined nodules.68 Patients are uncommonly imaged in this phase.
HRCT findings of HP in the subacute phase include ground glass opacities with poorly defined centrilobular nodules.71-74 Occasionally, the poorly defined centrilobular nodules may be the predominant finding (figure 15 -- 56KB). HP centrilobular nodules are commonly mid- and lower-lung predominant, although a diffuse distribution may occur. Acute and subacute HP may be very difficult to distinguish from DIP. Centrilobular nodules favor diagnosis of the latter. Areas of decreased lung attenuation on inspiratory HRCT images (mosaic perfusion) may be seen in subacute HP, and air trapping may be demonstrated on postexpiratory scans.75-77 In a few cases, air trapping on postexpiratory scans may be the only finding of HP (figure 16 -- 43KB).75 Occasionally the finding of ground glass opacity, normal lung, and mosaic perfusion on the same inspiratory scan image (the "headcheese" sign) may reflect the combination of alveolitis and air flow obstruction that characterize this disease (figure 17 -- 68KB).78
Chronic HP is characterized by fibrosis on HRCT (figure 18 -- 60KB) and (figure 19 -- 52KB), although the findings of active disease may be superimposed. Fibrosis in HP often shows a mid-lung predominance, although the findings may be distributed evenly throughout the upper, mid, and lower lungs.79, 80 Relative sparing of the lung bases, seen in a majority of cases of chronic HP, may allow chronic HP and UIP to be distinguished (figure 18 -- 60KB).31 The headcheese sign (figure 19 -- 52KB) may be seen in chronic HP also. HRCT is more sensitive than chest radiographs in the assessment of patients with HP,70, 71, 80 although the sensitivity of HRCT is not 100%.31
Sarcoidosis may also be considered as an interstitial lung disease characterized by granulomas. The clinical and radiographic features of sarcoidosis have been described extensively elsewhere1-3 and will not be considered further here.
Although early in its course, granulomas may be seen histologically, the radiographic presentation (in particular the HRCT manifestations) of Langerhans cell histiocytosis are dominated by pulmonary cysts. Thus, this disorder will be considered under ILDs characterized by cyst formation.
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