Bilateral Knee Infectious Arthritis Caused by Proteus mirabilis Mimicking Reactive Arthritis in an Elderly Patient with Metastatic Cancer: A Case Report.

BACKGROUND

Infectious arthritis is an uncommon but serious condition in older adults and may be easily misdiagnosed due to atypical clinical presentations. Proteus mirabilis is a rare causative pathogen of infectious arthritis, and its manifestations can resemble reactive arthritis, particularly in elderly patients with malignancy or immunocompromised status.

CASE PRESENTATION: A 74-year-old woman with advanced endometrial cancer was admitted with fever and urinary tract infection caused by Proteus mirabilis, complicated by septic shock. After initial improvement with intravenous antibiotic therapy, she developed recurrent fever accompanied by bilateral knee swelling and pain. Reactive arthritis was initially suspected, and empiric glucocorticoid therapy resulted in transient symptomatic relief. However, magnetic resonance imaging of both knees demonstrated bone destruction consistent with infectious arthritis. Repeated joint irrigation combined with prolonged antibiotic therapy led to clinical improvement, resolution of fever, and recovery of knee function. Follow-up blood cultures were negative, and the patient remained clinically stable.

CONCLUSION

This case highlights the diagnostic challenges of differentiating infectious arthritis from reactive arthritis in elderly patients with cancer. In older adults with Proteus mirabilis bacteremia who develop acute joint symptoms, infectious arthritis should be carefully considered, even when initial response to glucocorticoids is observed. Early imaging evaluation and repeated microbiological assessment are essential to avoid delayed diagnosis and improve outcomes in the aging population.