A 71–year–old man with chronic lymphocytic leukemia (CLL) and chronic obstructive pulmonary disease presented with fatigue, fever, and right knee pain and swelling. He denied right knee trauma. He also reported dyspnea and cough productive of clear sputum. Imaging studies of the right lower extremity revealed a large, aggressive–appearing lesion of the proximal tibia, associated pathologic fracture of the tibial plateau, and right knee effusion. The radiographic appearance of the right lower extremity was concerning for malignancy of the proximal tibia, with intra–articular extension to the right knee. Biopsy of the right tibial lesion revealed acute osteomyelitis, but no evidence of malignancy. Staining and culture revealed modified acid–fast, gram–positive, branching, filamentous rods, consistent with Nocardia infection. On further questioning of the patient, it was discovered that he had been diagnosed with Nocardia pneumonia and empyema at an outside hospital 6 weeks previously and started on appropriate antibiotics. Subsequent chest imaging at our hospital revealed multifocal airspace disease, with characteristics suggestive of atypical infection. He underwent right tibia debridement and right knee washout. Microbiologic and gross intraoperative findings confirmed the diagnosis of Nocardia osteomyelitis and septic arthritis.
Nocardiosis is an uncommon gram–positive bacterial infection caused by aerobic actinomycetes in the genus Nocardia. It is typically regarded as an opportunistic infection, although approximately one–third of infected patients are immunocompetent. The lungs are the primary site of infection in more than two–thirds of cases, but Nocardia is well–known for its ability to disseminate to any organ, especially the central nervous system and skin. Despite this tendency for dissemination, Nocardia osteomyelitis is rare, having been reported fewer than 10 times. Patients with leukemia may have discrete bony leukemic lesions. Those who have been treated for CLL with chemotherapy and/or immunotherapy are also at higher risk for primary bone cancers and cancers of other primary sites with bony metastases. In this case, the radiographic appearance of Nocardia osteomyelitis mimicked that of a bony neoplastic process.
Hospitalists are, with increasing frequency, encountering patients with complicated orthopedic and oncologic conditions through our roles in surgical co–management and hospitalist–run oncology teams. The purpose of reporting this case is to highlight a rare manifestation of an atypical bacterial infection and to remind hospitalists to maintain broad differential diagnoses when evaluating complaints of immunocompromised patients.