Chest Imaging

Histoplasmosis

Histoplasma capsulatum was discovered by Samuel Darling during the Panama Canal construction in the early 1900s. By the 1930s this organism was recognized as a cause of human disease in the central region of the United States. The organism is frequently found in the droppings of birds or bats and outbreaks of the disease are common when widespread construction disturbs the ground where organisms dwell. The organism may be found throughout the world but is most frequently identified in North America. Human to human transmission has not been reported.

Infections begin after inhalation of Histoplasma spores. A focal area of infection in the lung is followed by macrophage ingestion and ipsilateral hilar and subsequent mediastinal lymph node involvement. Symptoms may not occur until 20 weeks after infection and simulate influenza with fever, cough, myalgia and headache.

Chest radiographs have a variable appearance. In the "epidemic" type of the disease multiple poorly marginated nodules up to 1 cm in diameter are seen throughout the lungs often with bilateral hilar and occasionally mediastinal mediastinal lymphadenopathy (Fig.1). In time these nodules may calcify producing the well marginated 34 mm in diameter uniform calcified nodules of "old" histoplasmosis. Hilar and mediastinal lymph nodes may also calcify and occasionally multiple splenic calcifications can be identified on chest film as well (Fig.2). Less commonly a primary pneumonia simulating bacterial disease may be identified. Histoplasmomas are usually solitary, well marginated nodules approximately 12 cm in diameter which may simulate neoplasm. A calcified central "target" calcification with a small surrounding soft tissue opacity is very typical of histoplasmoma (Fig.3). If patients develop chronic disease changes of fibrosis and/or cavitation, volume loss in the upper lobes may simulate postprimary tuberculosis. Finally histoplasmosis may be a cause of fibrosing mediastinitis, which can lead to obstruction of pulmonary vessels, bronchi or constrictive pericarditis. CT scanning may be of benefit in identifying calcification within the otherwise nondescript soft tissue mediastinal opacity.

In immunosuppressed individuals histoplasmosis may disseminate, producing miliary or overwhelming air-space disease in the lungs. Other organs may be involved and hepatosplenomegaly may be identified on the chest film.

Diagnosis generally requires identifying the organism within tissue samples.

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Fig.1

Multiple poorly marginated nodular opacities are seen in both lungs. These are typical of endemic histoplasmosis. In addition, right and perhaps left hilar adenopathy are also appreciated.
Histoplasmosis, Fig.1
Histoplasmosis, Fig.2
Histoplasmosis, Fig.3