Histoplasmosis, caused by the fungus Histoplasma cap-sulatum, is a common disease that often produces a radiographic appearance simulating tuberculosis.
The primary form of histoplasmosis is usually relatively benign and often passes unnoticed. Chest radiographs may demonstrate single or multiple areas of pulmonary infiltration that are most often in the lower lung and are frequently associated with hilar lymph node enlargement. Although this pattern simulates the primary complex of tuberculosis, pleural effusion rarely occurs with histoplasmosis. Hilar calcification often develops in adults. A common manifestation of pulmonary histoplasmosis is a solitary, sharply circumscribed, granulomatous nodule (histoplasmoma), which is usually less than 3 cm in diameter and is most often in a lower lobe. Central rounded calcification within the mass (target lesion) is virtually pathognomonic of this disease. Multiple soft tissue nodules scattered throughout both lungs may simulate miliary tuberculosis. These shadows may clear completely or may fibrose and persist, often appearing on subsequent chest radiographs as widely disseminated punctate calcifications.
The more chronic form of histoplasmosis is characterized by zones of parenchymal consolidation, often large and with a loss of lung volume, that usually develop in an upper lobe. Cavitation is common, and the radiographic appearance closely simulates reinfection tuberculosis.
Histoplasmosis can incite progressive fibrosis in the mediastinum. This can cause obstruction of the superior vena cava, pulmonary arteries, and pulmonary veins, as well as severe narrowing of the esophagus.
Blastomycosis is a granulomatous disease caused by the yeastlike fungus Blastomyces dermatitidis, which is found in the soil and is inhaled into the respiratory tract. The pulmonary manifestations of blastomycosis are variable and nonspecific. In acute disease, patchy areas of air space consolidation simulate pneumonia. Cavitation and miliary nodules infrequently occur. In chronic disease, fibronodular changes in the upper lobe, sometimes with cavitation, can simulate tuberculosis. Blastomycosis may appear as a solitary pulmonary mass that, when associated with unilateral lymph node enlargement, may closely mimic a bronchogenic carcinoma.
Coccidioidomycosis is a granulomatous infection caused by the fungus Coccidioides immitis that is endemic in the southwestern United States, especially in the San loaquin valley of California.
Pulmonary involvement usually begins as a fleeting area of patchy pneumonia that is often accompanied by ipsilateral hilar adenopathy and, less frequently, by pleural effusion. Unilateral or bilateral hilar and mediastinal adenopathy occasionally develop even in the absence of parenchymal disease. Disseminated coccidioidomycosis produces a diffuse nodular pattern simulating miliary tuberculosis.
Although the primary disease usually clears completely, residual active parenchymal infection may persist. Well-defined nodules of varying sizes may occur singly or in groups. These nodules infrequently calcify and may excavate to form thin- or thick-walled cavities. Thin-walled cavities without surrounding reaction are suggestive of coccidioidomycosis. The thick-walled cavities are nonspecific and simulate those of tuberculosis. The pulmonary inflammation may persist as a chronic pneumonia or may progress to fibronodular disease.
Aspergillosis is almost always a secondary infection in which the fungus colonizes a damaged bronchial tree, pulmonary cyst, or cavity of a patient with underlying lung disease. It may also be an opportunistic invader in patients with debilitating disease and those on antibiotic or immunosuppressive therapy.
The radiographic hallmark of secondary aspergillosis is a pulmonary mycetoma, a conglomeration of intertwined fungal hyphae matted together with fibrin, mucus, and cellular debris within a pulmonary cavity. The fungus is almost always a pure saprophyte, and the underlying cavity can usually be ascribed to other causes, such as tuberculosis, bronchiectasis, sarcoidosis, histoplasmosis, a bronchial cyst, a chronic bacterial abscess, or a carcinoma. The mycetoma appears as a solid homogeneous rounded mass within a spherical or ovoid cavity; the fungal mass is separated from the wall of the cavity by a crescent-shaped air space. Air-fluid levels are not seen within the cavities. The intracavitary fungus ball occurs much more commonly in the upper than in the lower lobes, presumably because it so frequently occupies a tuberculous cavity. The cavities are usually thin-walled and are often contiguous to a pleural surface, which may be thickened. The mycetoma may move within the cavity when the patient changes position. Calcification of the mycelial mass can appear as scattered small nodules, a fine peripheral rim, or an extensive process involving most of the fungus ball.
In patients with severe debilitating diseases, such as reticuloendothelial malignancies, and those on steroid, immunosuppressive, or antibiotic therapy, secondary aspergillosis may cause single or multiple areas of pulmonary consolidation with poorly defined margins. Cavitation may occur. Miliary spread throughout the lungs is very rare.
Cryptococcosis is a granulomatous disease caused by the yeastlike fungus Cryptococcus neoformans, which lives in soil, particularly that contaminated by pigeon droppings. Although primary cryptococcal infections occur, the organism is most frequently an opportunistic invader that infects debilitated patients with chronic reticuloendothelial system disease (leukemia, lymphoma) or patients undergoing steroid or antibiotic therapy.
The major pulmonary manifestation of cryptococcosis is a single, fairly well circumscribed mass that is usually in the periphery of the lung and is often pleural-based. This radiographic pattern may be indistinguishable from carcinoma or other chronic pulmonary granulomas. Cavitation is relatively uncommon compared with its frequency in other mycoses. An ill-defined infiltrate may simulate pneumonia, and widely disseminated disease may produce a miliary pattern of diffuse nodules.
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