Applied Radiology

Diagnosis

Disseminated Nocardia asteroides infection with pulmonary and cerebral abscesses

Findings

An initial chest radiograph showed parenchymal consolidation in the right lower lobe along with a loculated right-sided effusion (Figure 1). A subsequent chest radiograph showed increasing pleural effusion (Figure 2). An initial noncontrast CT scan showed parenchymal consolidation with possible tiny cavitations as well as a right-sided effusion; a subsequent CT showed increased pleural effusion (Figure 3). Cranial MRI, performed with and without contrast, revealed a multiseptated ring-enhancing cystic lesion (3 * 2 cm) in the right frontal lobe with associated peripheral edema and mass effect upon adjacent cerebral sulci and the right lateral ventricle (Figure 4).

Discussion

Nocardia asteroides is a gram-positive, partially acid-fast, soil-born aerobic actinomycete that causes both localized and disseminated infection. Edmund Nocard first described Nocardia infection in 1888.¹ Nocard originally isolated the organism on the island of Guadeloupe and named it Streptothrix farcinica from cattle afflicted with "bovine farcy." Cattle with bovine farcy developed a granulomatous disease with multiple abscesses, draining sinuses, pulmonary involvement, and emaciation leading to death. Within 2 years of Nocard, Eppinger described the first human case as a pseudotuberculosis syndrome with pulmonary disease and cerebral abscesses and renamed the organism Cladothrix asteroids.¹ The organism was subsequently renamed Nocardia asteroides by Blanchard in 1896.¹ Currently, numerous subgroups of Nocardia have been classified. The two that account for a vast majority of the human disease seen are N asteroides and N braziliensis. Nocardia asteroides accounts for 86% of systemic nocardiosis in humans, 98% of cerebral nocardial abscesses, and approximately 2% of all cerebral abscesses.²

Nocardiosis is a relatively uncommon bacterial infection but is strongly associated with immunosuppression. The majority of infections occur in patients with weakened cell-mediated immunity. Patients commonly include those who have received bone-marrow or solid-organ transplantations and are on immunosuppressive therapy, those with HIV/AIDS, and those with malignancies. Other immunocompromised states include alcoholism, hypogamma-globulinemia, chronic granulomatous disease, chronic obstructive pulmonary disease, pulmonary alveolar proteinosis, diabetes, sarcoidosis, tuberculosis, and systemic lupus erythematosis.²

Nocardia asteroides infection is acquired from the environment through inhalation into the respiratory tract, which leads to pulmonary disease. Subsequent hematogenous dissemination occurs from the lungs, which leads to cerebral abscess formation as well as cutaneous skin lesions. Dissemination, with cerebral abscess formation, occurs in approximately 15% to 44% of patients with systemic nocardiosis. Mortality rates are reported at approximately 30% with current early diagnosis and treatment regimens. This is in sharp contrast to previously reported mortality rates of 78% to 90%.² Mortality is significantly higher in patients with multiple abscesses and is believed to be a function of the patient's degree of immunosuppression.

Nocardia asteroides cerebral abscesses typically appear on CT and MRI as nonspecific ring-enhancing lesions. These consist of a central liquid or gelatinous necrotic cavity surrounded by a contrast-enhancing smooth fibrotic capsule. An uncommon, but characteristic, finding in cerebral abscesses is multiple concentric rims with varying signal intensities on MRI. Multiple rims may be the result of phagocytosis by macrophages in the capsule. Pyhtinen and coworkers³ suggest that a multiple-rim pattern on MRI is specific for infection and excludes other processes, such as cancer metastasis or infarct.³

Pulmonary manifestations described on chest radiograph include, in decreasing order of frequency, pleural effusions, pulmonary consolidations, cavitating masses, and pulmonary nodules. None of these allows the diagnosis to be made with certainty. Sputum cultures are required for diagnosis. Repeated samples are often required before a positive culture for Nocardia is obtained.⁴

Medical management with trimethoprim/sulfametho-xazole, an agent that readily penetrates the central nervous system, is the first-line treatment for nocardiosis. Second-line treatments include imipenem/cilastatin, amikacin, cefo-taxime, minocycline, and other antibiotics. Nocardia species are generally resistant to penicillin and require 6 months to 1 year of antibiotic treatment. Surgical management for cerebral nocardia infection is often based upon the patient's clinical and immune status and the number and size of lesions. In immunocompromised patients and those with multiple abscesses, a more aggressive surgical approach, such as craniotomy and excision, is indicated to maximize likelihood of survival. In non-immunocompromised patients or those with a single lesion, stereotactic aspiration followed by antibiotic therapy may be sufficient.⁵ Fleetwood et al² suggest stereotactic or open biopsy of nonspecific cerebral lesions for diagnostic and therapeutic purposes, even in cases in which a presumptive diagnosis has been made based on sputum cultures. This is based on the possibility of multiple organism infections that commonly occur in immunocompromised patients. Medical treatment would be based on susceptibility results.²

The patient in this case was switched from trimethoprim/ sulfamethoxazole to imipenem and amikacin upon admission. Following his cerebral MRI, he underwent a craniotomy with resection of a multicystic frontal lobe abscess. Specimen pathology confirmed Nocardia infection. The patient was discharged in stable condition on long-term antibiotic therapy.

1. Wilson JP, Turner HR, Kirchner KA, Chapman SW. Nocardial infections in renal transplant patients. Medicine. 1989;68:38-57.

2. Fleetwood IG, Embil JM, Ross IB. Nocardia asteroides cerebral abscess in IC hosts: Report of three cases and review of surgical recommendations. Surg Neurol. 2000;53:605-610.

3. Pyhtinen J, Paakko E, Jartti P. Cerebral abscess with multiple rims on MRI. Neuroradiology. 1997;39:857-859.

4. Raby N, Forbes G, Williams R. Nocardia infection in patients with liver transplants or chronic liver disease: Radiologic findings. Radiology. 1990;174(3 pt 1):713-716.

5. Mamelak AN, Obana WG, Flaherty JF, Rosenblum ML. Nocardial brain abscess: Treatment strategies and factors influencing out-come. Neurosurgery. 1994;354:622-628.