What is the next step in management for a patient with worsening confusion, fever, and a swollen left shoulder, with a history of IV (intravenous) drug use disorder and positive blood cultures for Methicillin-resistant Staphylococcus aureus (MRSA)?

Urgent Orthopedic Surgery Consultation and Empiric Antibiotic Modification

You must obtain an urgent orthopedic surgery consultation immediately for suspected septic arthritis of the acromioclavicular joint with possible osteomyelitis, and simultaneously broaden antibiotic coverage while preparing for surgical drainage.

Immediate Actions Required

1. Urgent Orthopedic Surgery Consultation

• Orthopedic surgery consultation is mandatory for this patient with clinical signs of septic arthritis (swollen, erythematous, exquisitely tender AC joint with fever and worsening sepsis) in the setting of MRSA bacteremia 1, 2
• Septic arthritis of the AC joint requires surgical drainage as the mainstay of therapy whenever feasible, particularly with ongoing bacteremia and clinical deterioration 1
• The AC joint is an uncommon site for septic arthritis, but IV drug users are at particularly high risk, and S. aureus (including MRSA) is the causative organism in 60-70% of cases in this population 1, 3, 4, 2

2. Diagnostic Imaging - MRI is Preferred

• MRI of the left shoulder with gadolinium enhancement should be obtained urgently to define the extent of joint involvement, assess for concomitant osteomyelitis, and rule out deeper soft tissue abscess or glenohumeral joint involvement 4, 2
• MRI is superior to CT for early diagnosis of septic arthritis and can detect bone marrow edema, joint effusion, and periarticular soft tissue involvement that will guide surgical planning 4, 2
• Contrast-enhanced CT is an acceptable alternative if MRI is contraindicated or unavailable, but provides less detailed soft tissue characterization 2

3. Joint Aspiration and Microbiologic Confirmation

• Aspiration of the AC joint should be performed (ideally by orthopedics or interventional radiology) to obtain synovial fluid for Gram stain, culture, and cell count before any antibiotic modification 2
• Blood cultures should be repeated immediately given clinical deterioration and increased vasopressor requirements 1

Antibiotic Management Modification

Current Regimen Assessment

• The patient is on vancomycin and piperacillin-tazobactam (Zosyn), which provides appropriate MRSA coverage, but vancomycin alone may be insufficient for complicated MRSA infections with persistent bacteremia or metastatic foci 1
• The clinical deterioration on day 3 with worsening confusion, high fever (104°F), and increased vasopressor requirements suggests either inadequate source control or the need for adjunctive therapy 1

Recommended Antibiotic Modification

• Add rifampin 600 mg PO/IV once daily to the current vancomycin regimen once repeat blood cultures are obtained, as combination therapy with rifampin is recommended by some experts for MRSA osteomyelitis and complicated infections 1
• Rifampin should only be added after clearance of bacteremia is documented, so obtain repeat blood cultures now and add rifampin once these are negative 1
• Continue vancomycin at 15-20 mg/kg IV every 8-12 hours with trough levels maintained at 15-20 mg/L for complicated MRSA infections 1, 5
• Discontinue piperacillin-tazobactam once MRSA is confirmed as the sole pathogen and there is no evidence of polymicrobial infection, as it provides no additional benefit for MRSA 1

Treatment Duration and Monitoring

Duration of Therapy

• A minimum 8-week course of antibiotics is recommended for MRSA osteomyelitis, with some experts suggesting an additional 1-3 months of oral rifampin-based combination therapy if debridement is not performed 1
• For septic arthritis with concurrent bacteremia, 4-6 weeks of antimicrobial therapy should be administered if there is persistent bacteremia (>72 hours) after source control or evidence of metastatic infection 1

Monitoring for Complications

• Obtain transthoracic echocardiogram (TTE) immediately, and if negative or inadequate, proceed to transesophageal echocardiogram (TEE) to evaluate for infective endocarditis, which is common in IV drug users with S. aureus bacteremia 1
• The presence of persistent bacteremia (>72 hours), new heart block, or persistent fevers are indications for early valve replacement surgery evaluation 1
• Monitor for other metastatic foci including vertebral osteomyelitis, epidural abscess, or septic emboli to lungs given the patient's IV drug use history 1, 6

Critical Pitfalls to Avoid

• Do not delay surgical consultation while waiting for imaging—clinical diagnosis of septic arthritis in a deteriorating patient with MRSA bacteremia is sufficient to warrant urgent orthopedic evaluation 1, 2
• Do not add rifampin before obtaining repeat blood cultures, as it should only be added after bacteremia clearance is documented 1
• Do not assume the AC joint is the only site of infection—IV drug users with MRSA bacteremia have high rates of endocarditis and other metastatic foci that require systematic evaluation 1
• Do not use vancomycin monotherapy indefinitely for complicated MRSA infections—consider alternative agents (daptomycin 6 mg/kg IV daily, linezolid 600 mg IV twice daily) if clinical deterioration continues despite adequate vancomycin levels 1, 7