Case Presentation: A 78 year old man with a past medical history of diabetes on insulin and chronic back pain initially presented with several days of diarrhea. On admission he was found to have Clostridium difficile colitis that improved slowly on oral vancomycin. On hospital day 7 he developed acute debilitating back pain and was febrile to 38.4 celsius. A thorough exam did not reveal skin lesions or phlebitis except various ecchymoses on the abdomen attributed to prophylactic subcutaneous enoxaparin injections since admission. His lower extremity motor exam was newly limited by pain. Magnetic resonance imaging (MRI) of the total spine with contrast on hospital day 8 showed lumbar degenerative disease similar to prior imaging from several months ago. Infectious workup revealed Methicillin-resistant Staphylococcus aureus (MRSA) bacteremia.Despite adequate treatment with intravenous vancomycin, his blood cultures were persistently positive for the subsequent 7 days. Both transthoracic and transesophageal echocardiograms were negative for endocarditis. Because of the persistent bacteremia and lack of an apparent source, a CT scan of face, chest, and abdomen was performed, which showed a right anterior abdominal subcutaneous thickening with a 1.2 centimeter hyperdensity. Upon further inquiry, the patient recalled potentially dislodging a needle when he self administered insulin prior to admission. He also received a gallium scan that revealed thoracic spine inflammation, and the repeat MRI done on hospital day 17 confirmed osteomyelitis. The needle was retrieved by interventional radiology. His subsequent blood cultures remained negative and he was discharged with 8 weeks of intravenous vancomycin.
Discussion: Determining the source and complications of MRSA bacteremia has a direct impact on management. Skin and soft tissue, as well as catheter related infections remain the most common sources, followed by pulmonary, endovascular and osteoarticular infections. However, the source can remain unidentified despite appropriate workup in up to 25% of the cases. Insulin injection has not been identified as a common source, but cases involving needle reuse have been reported. Similarly, subcutaneous injections have been shown to cause skin and soft tissue infections in drug users when needle and skin hygiene is not observed. Staphylococcus aureus is notorious for forming microfilms on foreign material, so the embedded insulin needle is likely to act as a sanctuary site, which explains both the delayed onset of infectious symptoms and prolonged bacteremia.Although MRI has been the preferred imaging modality to diagnose vertebral osteomyelitis with sensitivity and specificity greater than 90%, it has been shown to lack sensitivity in the very early stage of disease. In some case studies, expert opinion has called for repeat MRI in 1 to 3 weeks if there is high clinical suspicion despite initial negative results. In our patient who developed acute back pain distinct from his chronic symptoms in the setting of bacteremia, clinical suspicion is high and repeat imaging is warranted despite the initial false negative study.
Conclusions: 1. Achieving source control is imperative in patients with bacteremia. Thorough history taking and physical exam is crucial to identify unusual sources.2. MRI can lack sensitivity in the very early stage of spinal osteomyelitis. If clinical suspicion is high, repeat imaging is warranted to ensure proper diagnosis.
