What is the diagnosis and management for a patient with left foot drop, loss of sensation, elevated C-Reactive Protein (CRP), and a history of Intravenous (IV) drug use?

Suspected Septic Arthritis or Osteomyelitis with Peroneal Neuropathy in an IV Drug User

This patient most likely has septic arthritis of the ankle/foot or osteomyelitis with secondary peroneal nerve compression, and requires immediate hospitalization, blood cultures, imaging with MRI, and empiric broad-spectrum IV antibiotics covering MRSA and gram-negative organisms. 1, 2

Most Likely Diagnoses

Primary Infectious Process

• Septic arthritis or osteomyelitis is the leading concern given the triad of IV drug use (major risk factor for hematogenous seeding), elevated CRP (indicating active infection/inflammation), and neurological deficit 1, 2
• IV drug users have significantly increased risk of spine and joint infections due to hematogenous spread of bacteria, particularly Staphylococcus aureus (including MRSA) and gram-negative organisms 1
• The foot drop with sensory loss suggests either direct nerve compression from an abscess/infected joint or L5 radiculopathy from spinal infection (discitis/osteomyelitis) 3, 4

Secondary Neurological Complication

• Peroneal neuropathy causing foot drop can result from compression at the fibular head, but the combination with elevated inflammatory markers and IV drug use history makes infectious etiology more likely than mechanical compression alone 3, 4
• L5 radiculopathy from spinal infection (discitis-osteomyelitis) is another critical consideration, as this is the most common cause of foot drop overall and frequently occurs in IV drug users 3, 4

Immediate Assessment Required

Clinical Examination Priorities

• Assess for systemic sepsis: Obtain vital signs immediately including temperature, heart rate, respiratory rate, and blood pressure to identify fever, tachycardia, or hemodynamic instability 2
• Examine the entire lower extremity and spine: Look for focal tenderness over the lumbar spine (L4-L5), sacroiliac joints, ankle, or foot; assess for warmth, erythema, swelling, or fluctuance suggesting abscess 1, 2
• Neurological localization: Test L5 dermatome sensation (dorsum of foot, first web space), ankle dorsiflexion strength, great toe extension, and ankle inversion to differentiate L5 radiculopathy from isolated peroneal neuropathy 3, 4
• Vascular assessment: Palpate dorsalis pedis and posterior tibial pulses, assess capillary refill, and note skin temperature/color, as IV drug users may have concurrent vascular complications 1, 2

Laboratory Workup

• Blood cultures (2 sets) before antibiotics, as bacteremia is common in IV drug users with musculoskeletal infections 2, 5
• Complete blood count with differential: Check for leukocytosis with left shift, though absence does not exclude infection 1, 6
• Inflammatory markers: CRP and ESR are already elevated; these support infection but are nonspecific 1, 5
• Comprehensive metabolic panel: Assess for metabolic derangements and renal function before contrast imaging 2

Critical pitfall: Up to 50% of patients with severe infections may have normal white blood cell counts, so do not rely on WBC alone to exclude serious infection 1, 6

Imaging Strategy

First-Line Imaging

• Plain radiographs of the left foot, ankle, and lumbar spine as initial screening for osteomyelitis, foreign bodies, or gas in soft tissues 1, 2
• Plain films may be negative early in osteomyelitis (changes take 10-14 days to appear), so negative films do not exclude infection 1

Definitive Imaging

• MRI of the lumbar spine AND left lower extremity is the gold standard for diagnosing both spinal infection (discitis-osteomyelitis) and peripheral osteomyelitis/soft tissue abscess 1, 2
• MRI has superior sensitivity and specificity for osteomyelitis and can identify epidural abscess, which is a neurosurgical emergency 1
• If MRI is contraindicated or unavailable, consider PET scan, leukocyte scintigraphy, or SPECT as alternatives 1

Critical consideration: The location of infection determines management—spinal infection may require neurosurgical intervention, while peripheral infection may need orthopedic debridement 1, 2

Immediate Management

Hospitalization Decision

• Admit immediately given the combination of elevated CRP, IV drug use history, and neurological deficit—this patient meets criteria for severe infection requiring inpatient management 1, 2
• The IWGDF/IDSA guidelines recommend hospitalization for patients with severe infections or moderate infections with relevant comorbidities (IV drug use qualifies) 1, 2

Empiric Antibiotic Therapy

• Initiate broad-spectrum IV antibiotics immediately after obtaining blood cultures, covering MRSA and gram-negative organisms 2
• Recommended regimens include:
  • Vancomycin PLUS piperacillin-tazobactam, OR
  • Vancomycin PLUS cefepime plus metronidazole 2
• IV drug users have high rates of MRSA, so vancomycin (or alternative MRSA coverage) is essential 2

Surgical Consultation

• Urgent orthopedic or spine surgery consultation within 24 hours if imaging confirms osteomyelitis, septic arthritis, or abscess requiring debridement 2
• Obtain deep tissue or bone specimens intraoperatively for culture and histology—these are superior to superficial swabs 1, 2

Specific Investigations Algorithm

If Spinal Infection Suspected (L5 Radiculopathy Pattern)

1. MRI lumbar spine with and without contrast to evaluate for discitis-osteomyelitis, epidural abscess, or paraspinal abscess 1
2. If epidural abscess identified, immediate neurosurgical consultation for potential decompression 1
3. CT-guided biopsy of vertebral body if diagnosis uncertain and patient stable 1

If Peripheral Infection Suspected (Isolated Peroneal Neuropathy)

1. MRI of left lower extremity to evaluate for soft tissue abscess, septic arthritis, or osteomyelitis compressing peroneal nerve at fibular head 1, 2
2. Ultrasound of popliteal fossa and fibular head region can identify fluid collections or nerve compression 4
3. Nerve conduction studies and EMG after acute infection treated to assess nerve damage and prognosis for recovery 3, 4

Important caveat: Do not delay antibiotics or imaging to perform electrodiagnostic studies—these are adjuncts for later prognostication, not acute diagnosis 3, 4

Antibiotic Duration and Monitoring

• Soft tissue infections: Typically require 1-2 weeks of IV therapy, then transition to oral antibiotics once systemically stable and afebrile for 24-48 hours 2
• Osteomyelitis: Requires minimum 4-6 weeks of therapy (often 6-8 weeks for vertebral osteomyelitis) 1, 2
• Narrow antibiotics based on culture results when available—do not continue broad-spectrum coverage unnecessarily 2
• Monitor CRP weekly: Failure of CRP to decline suggests treatment failure, inadequate source control, or resistant organism 1, 5

Common Pitfalls to Avoid

• Missing spinal epidural abscess: This is a neurosurgical emergency that can cause permanent paralysis if not decompressed urgently 1
• Attributing foot drop to "habitual leg crossing" in an IV drug user with elevated CRP—this is infection until proven otherwise 3, 4
• Relying on normal WBC to exclude infection: Inflammatory markers (CRP/ESR) are more sensitive than WBC in this population 1, 6
• Inadequate source control: Antibiotics alone are insufficient if there is undrained abscess or necrotic bone requiring debridement 2
• Superficial wound cultures: If surgical debridement performed, obtain deep tissue or bone specimens—superficial swabs are unreliable 1, 2