Radiology of acute diffuse lung disease in the immunocompetent host

Summary The differential diagnosis of diffuse acute lung disease in the immunocompetent patient includes hydrostatic and permeability edema (with or without DAD), infectious pneumonia, acute eosinophilic pneumonia, hypersensitivity pneumonitis, pulmonary hemorrhage, and acute interstitial pneumonitis. Although there is much overlap among the radiologic appearance of these diseases, the combination of several CT or radiographic findings can suggest the correct diagnosis. CT findings of diffuse opacity with a gravitational gradient are most consistent with ARDS from an extrapulmonary source. Acute hydrostatic pulmonary edema may have a similar appearance on CT but typically also shows evidence of septal and bronchovascular thickening. Acute interstitial pneumonitis may also have a similar appearance to ARDS but may be differentiated by evidence of fibrosis as manifest by traction bronchiectasis and architectural distortion. Infections are a common cause of ARDS. The CT scans of patients with ARDS caused by pulmonary infection may show nondependent dense opacities rather than a gravitational gradient. Less severe infections may be suggested by the presence of segmental opacities or patchy centrilobular nodules and centrilobular branching structures (impacted bronchioles). Diffuse, ill-defined centrilobular nodules, especially when combined with ground-glass opacities or air trapping, favor hypersensitivity pneumonitis. The presence of lobular opacities is nonspecific and may be caused by infections and several other disease states. Recognition of concurrent radiologic or clinical findings may suggest the correct diagnosis. The combination of lobular opacities and septal thickening can be seen in hydrostatic edema, eosinophilic pneumonia, and pulmonary hemorrhage. Pleural effusions, however, or extensive septal thickening are much more common in the first 2 diseases. Evolution of lung opacities from alveolar to interstitial over time would instead suggest the diagnosis of pulmonary hemorrhage. Diffuse acute lung disease in immunocompetent hosts is most often caused .by pulmonary edema (cardiogenic or noncardiogenic) and or infection. Despite the imaging findings described in this article, our ability as radiologists to differentiate between these conditions is imperfect. The radiologist's input is most important in those cases in which the diagnosis of pulmonary edema or infection is clouded by conflicting clinical findings. Less common diseases, such as AIP, acute eosinophilic pneumonia, and hypersensittivity pneumonitis may not be initially considered by clinicians. It will continue to be our role to raise the possibility of these diagnoses when characteristic radiologic findings are detected.