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INVITED COMMENTARY |
1 Department of Medical Imaging, Presbyterian Medical Center, Philadelphia, Pennsylvania
Over the past 2 decades, the rate of infections due to the NTMB has risen sharply. Although most of the increased prevalence of disease relates to the AIDS epidemic, there have been increases in the nonimmunocompromised population as well. Pulmonary disease catches our attention because of the frequency with which we encounter it on chest radiographs. However, skin and soft-tissue infections can be just as devastating. For example, Buruli ulcer of the leg in young children in Africa results in limb amputation in approximately 10% of cases (1).
For the most part, the NTMB are saprophytes, and their ecology is significantly different from that of M tuberculosis (2). They are quite hardy and have been cultured from water, soil, dust, and aerosols. They may survive wide ranges of temperature, pH, and salinity. They are able to resist disinfectants such as chlorine, formaldehyde, and glutaraldehyde (3).
Despite the widespread occurrence of NTMB, there are considerable geographic and temporal variations in the pattern of infection by these organisms (2). Before 1980, most patients had pulmonary disease, skin infections, or cervical lymphadenitis. In the United States, M kansasii and M avium-intracellulare accounted for the majority of infections. These infections occurred predominantly in older men, and 60%–70% of the patients had chronic obstructive lung disease. In Japan, the majority of NTMB infections were caused by M avium. In Virginia, M kansasii infections were four times more frequent from 1970 to 1974 than from 1975 to 1979. In San Francisco, the incidence of M kansasii infection in 1980 was 0.52 cases per 100,000 persons. By 1996, the incidence had risen to only 0.75 cases per 100,000 immunocompetent patients (4).
The epidemiology of NTMB infections has changed remarkably as a result of the AIDS epidemic. NTMB disease was reported in patients with AIDS in 1982, and the rate of such disease has risen sharply since. It is estimated that 25%–50% of patients with AIDS in the United States and Europe are infected with NTMB (5). These infections are frequently disseminated. The most common organisms are M avium-intracellulare followed by M kansasii, but rates of infection with other mycobacteria are also increasing. For example, in San Francisco County and the two adjacent counties, the prevalence of M kansasii infection in HIV-positive patients is now 153 times that in the general population and the prevalence in patients with AIDS is 862 times that in the general population (4). The portal of entry for these infections is uncertain but is most likely the gastrointestinal tract or respiratory tract. On the other hand, in Africa, patients with AIDS are rarely infected with these organisms (6).
In the past 2 decades, there has been improved recognition of nonpulmonary clinical syndromes associated with NTMB in immunocompetent patients. These syndromes include atypical cervical lymphadenitis (7), catheter-related sepsis (8), periocular infections associated with foreign bodies (9), tenosynovitis (10), osteomyelitis (11), and skin and soft-tissue infections (12). The imaging findings in these disorders are not specific, and radiologists serve to identify the location of disease more than the specific disease process. Radiologists practicing in geographic regions with high frequencies of these diseases should be more aware of the possibility of these infections and their radiographic appearances. Finally, NTMB have been cultured recently from some patients with cystic fibrosis (13). Whether this finding heralds a significant future problem for these patients is unknown.
In the preceding article , Erasmus et al present the radiographic findings in patients with the pulmonary syndromes. The classic appearance results primarily from infection with M kansasii. Nonclassic infection, which manifests as bilateral lower lung bronchiectasis, is most frequently associated with M avium-intracellulare. Pulmonary infections in patients with esophageal disease are usually due to M chelonae-fortuitum. Although M avium-intracellulare is the most frequently identified NTMB in patients with AIDS, infections with other NTMB have also increased markedly. There is some variance in the radiographic appearances between immunocompromised patients and nonimmunocompromised patients. This variance occurs not only when the organisms are different between the groups but also when patients infected with the same organism are compared. A direct comparison of 23 HIV-negative and 12 HIV-positive patients with M kansasii infection showed apical nodules and cavities in the HIV-negative group and air-space infiltrates, miliary nodules, and lymph node enlargement in the HIV-positive group (14). Although pulmonary NTMB infections are still relatively infrequent, when radiologists seek or are provided with the appropriate history in such cases, a relatively specific diagnosis can often be suggested.
References
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