Radiology Resident Case of the Week
While there is no clinical syndrome characteristic of nocardiosis, this is a typical presentation. It is most often a disease of the immunocompromised, seen in transplant patients (on immunosuppresive agents such as cyclophosphamide), with chronic steroid administration (as in this patient), as well as in AIDS, diabetes, and alcoholism. Nocardia may complicate a variety of diseases including pulmonary alveolar proteinosis, sarcoidosis, ulcerative colitis, and intestinal lipodystrophy. Rarely, patients with no predisposing factors can be affected.
The mode of transmission is unclear, but the respiratory tract is the most likely portal of entry. Onset of symptoms is often insidious. The patientŐs temperature tends to gradually rise. Pleuritic chest pain develops as the infection spreads to the pleura and chest wall. The patient may complain of night sweats. Auscultation may reveal rales and consolidation, and palpation may reveal a soft tissue mass from chest wall invasion. Bronchocutaneous fistulae have been reported.
Nocardia is relatively sensitive to the sulfonamides.
The leukocyte count may be normal to slightly elevated. Weight loss, anemia, and finger-clubbing may occur with chronic infection. The skin and subcutaneous tissues are most often involved. Pulmonary infection may present as diffuse or nodular infiltrates, cavitary lesions, or pleural effusion. Ring-enhancing lesions may be seen on chest CT. The initial chest film may be normal despite positive sputum cultures. Direct extension to surrounding blood vessels, pleura, and chest wall is seen. Dissemination, hematogenous (most often to brain) or by fistula formation, is not uncommon.
Etiology/Pathophysiology
Nocardiosis is a disease of humans and cattle, caused by species of the family Nocardeaceae. The most important causative species is Nocardia asteroides, which has a worldwide distribution. N. brasilensis, confined to Central and South America, is another important cause of human pulmonary infections. The organism is aerobic, non-motile, and non-sporulating. It is also found in grasses, straw, and grains. In tissue, it is a gram-positive, delicately branching, filamentous organism. It is usually acid-fast, with weak decolorizing agents, which may be useful in distinction from Actinomyces. A low index of suspicion in the setting of immunocompromising disease is critical for appropriate staining of specimens, and for adequate retention of culture media (up to 21 days) if the specific diagnosis is to be made.
Pathology:
Affected tissues contain numerous yellow-white abscesses with a fibrinous exudate. Polymorphonuclear leukocytes, plasma cells, lymphocytes and histiocytes are found within the abscesses. Inhibition of phagocytic and lysosomal function, reduction in intracellular lysosomal acid phosphatase, and production of superoxide dismutase and catalase by Nocardia growing within macrophages promotes virulence of the organism. Absence of sulfur granules aids in distinction from actinomycosis.
Miscellaneous
The "opportunistic" infections seen in the immunocompromised are normally of low pathogenicity in man. Decreased host resistance in this special population relates to impaired phagocytosis and cell-mediated immune response, as well as an altered humoral response, which may result from either the primary disease or from the immunosuppresive therapy. A high index of suspicion in this population with uncommon disease manifestations will be required to identify and appropriately treat these diseases.
Imaging
The imaging appearance of central nervous system infections in the immunocompromised patient is more a function of host response than infective organism. In patients unable to localize the infection, lesions may be subtle - poorly circumscribed and poorly enhancing. As might be expected, the absence of a vigorous host response predicts a poorer outcome. The typical "ring" enhacing lesion of Nocardia infection, then, reflects localization of the infection by the host, and is an early finding in the progressive formation of abscesses in the host still capable of mounting a functional, albeit impaired, response.
DDX
Metastasis (lung, breast, colon, rectum, renal, melanoma)
Multifocal infection (tuberculosis, histoplasmosis, candidiasis, actinomycosis, nocardiosis, aspergillosis)
Septic emboli (e.g. Klebsiella pneumoniae)
Parasitic disease (cysticercosis, toxoplasmosis)
Lymphoma (especially in transplant patients or those with AIDS)
Radiation necrosis (in the appropriate setting)
Sarcoidosis
Key references
1. Hasleton PS (ed.). SpencerŐs Pathology of the Lung (5th ed.).
New York: McGraw-Hill, pp. 1028-34, 1996.
2. Fraser RG, Pare JAP (eds.). Diagnosis of Diseases of the Chest (2nd ed.). Philadelphia: WB Saunders Company, pp. 237-9, 1989.
3. Raby NF, Forbes G, Williams R. Nocardia Infection in Patients with Liver Transplants or Chronic Liver Disease: Radiologic Findings. Radiology 174:713-6, 1990.
4. Enzmann DR, Brant-Zawadzki M, Britt RH. CT of Central Nervous System Infections in Immunocompromised Patients. AJR 135:263-7, 1980.
5. Reeder MM, Bradley WG. Gamuts in Neuroradiology. New York: Springer-Verlag, pp. 41, 42, 47, 48, 1993.
ACR Code
13.205
Keywords:
Nocardiosis, immunocompromised, ring-enhancing lesions, pneumonia, disseminated disease, central nervous system infection