Fig. 6.37 A-F Disseminated histoplasmosis has many clinical and radiographic presentations, particularly in those who are immunocompromised. A, B A double-contrast barium enema showing two apple-core lesions (arrows) in the transverse colon of a 39-year-old HIV-positive man. The colon was otherwise normal. C-E A similar lesion mimicking a carcinoma of the colon in a different, 50-year-old patient. C shows a single narrowed area, and CT scans (D) shows the thickening of the bowel wall (solid arrows) and enlarged regional lymph nodes (open arrow). (E) The lymphadenopathy had progressed 8 months later (arrows). F Histoplasmosis mimicking tuberculosis or amebiasis in the cecum and ascending colon. (Courtesy of Dr. E. G. Balthazar et al. and AIR, 1993)

CT and MR Scanning of Pulmonary Histoplasmosis

A plain chest radiograph provides a great deal of information about histoplasmosis, but however suspicious the findings may be because of local endemicity, the diagnosis can seldom be made firmly. Plain radiographs are usually unable to completely show the complications or the extent of the disease and its resulting fibrosis. CT scanning is the best way to show calcification in the mediastinal mass, particularly the fine stippled pattern which may not be seen on plain radiographs or MRI. CT is also able to demonstrate calcification within lymph nodes or in a peripheral histoplasmoma. It can accurately visualize tracheobronchial narrowing and show the relationship to any adjacent mass. CT can also show when the peripheral lobar and alveolar changes are the result of segmental bronchial blockage. Parenchymal changes, such as pulmonary infarction, which are not clearly seen on a chest radiograph, are better seen with CT, particularly those close to the pleura. In some patients a ventilation-perfusion radioisotope study showing decreased perfusion to the same area will provide additional evidence of vascular damage. If surgery is contemplated, CT is a useful way to delineate exactly the involved area.

Magnetic resonance imaging is much less accurate than CT at demonstrating calcification, which is an important part of the diagnosis of histoplasmosis, particularly fibrosing mediastinitis. The lymphadenopathy of histoplasmosis has low signal intensity on T2-weighted MR scans. This may indicate benign adenopathy, although histopathological confirmation is required. MRI is particularly useful in assessing vascular compression, which is not clearly seen on CT scanning.

To summarize; the diagnosis of pulmonary histoplasmosis may be considered from plain radiographs, and CT scanning provides additional evidence; MRI is useful to assess vascular complications, but CT should be the first choice if scanning is indicated.

Extrapulmonary Histoplasmosis

In many patients with histoplasmosis, there will be hepatosplenomegaly with coarse miliary or punctate calcification in the liver and the spleen (sometimes found in patients who have no symptoms). CT is the most accurate method of imaging. Other imaging findings include abdominal lymphadenopathy, which on CT may be homogeneous or with more diffuse or central low density. There may be bilateral adrenal enlargement due to diffuse or localized low-density masses. The scanning pattern of hepatosplenomegaly has no specific characteristics in histoplasmosis: calcification is not always found in the abdominal viscera or lymph nodes.

Colonic strictures, mimicking carcinoma, have been reported in histoplasmosis and demonstrated with a barium enema. Other areas of segmental narrowing of the bowel may be difficult to differentiate from amebiasis or tuberculous. In histoplasmosis and tuberculosis there is usually associated regional lymphadenopathy but this seldom occurs in amebiasis: there are really very few distinguishing features on imaging. Many gastrointestinal lesions due to histoplasmosis may be incidental findings at autopsy: any part of the gastrointestinal tract may be affected, but particularly the terminal ileum and ascending colon. In such lesions there is usually patchy superficial ulceration or raised thickened plaques. Polyps, bowel perforation, and anal fissures have been reported. Many of these can be found during endoscopy but, again, there are no specific imaging characteristics. In most patients gastrointestinal lesions occur as part of known disseminated infection (Fig. 6.37) but rarely they are the first sign that histoplasmosis is beginning to spread.

Skeletal involvement by Histoplasma capsulatum var. capsulatum is very uncommon and the rare lytic lesions have no specific imaging changes. Involvement of bone marrow is a common feature of disseminated histoplasmosis in immunodeficient patients.