How do I differentiate infectious spondylodiscitis from an inflammatory spondyloarthropathy in a patient with back pain and disc or vertebral body abnormalities on imaging, and what initial work‑up and management should be undertaken?

Differentiating Infectious Spondylodiscitis from Inflammatory Spondyloarthropathy

Order MRI of the spine without and with IV contrast immediately—this is the definitive test with 96% sensitivity and 94% specificity for infectious spondylodiscitis and will distinguish it from inflammatory spondyloarthropathy through specific imaging patterns. 1

Imaging Algorithm

First-Line Imaging: MRI with and without IV contrast

• MRI protocol must include: T1-weighted sequences, T2-weighted with fat suppression or STIR sequences, followed by contrast-enhanced T1-weighted sequences in axial and sagittal planes with fat suppression 1
• Critical technical point: Always obtain precontrast images first—comparing pre- and post-contrast sequences is essential to confirm true enhancement versus intrinsic T1 hyperintensity 2
• Sensitivity/specificity: MRI achieves 96% sensitivity, 94% specificity, and 92% accuracy for detecting spine infection 1, 3
• Diffusion-weighted imaging (DWI): Add this sequence to help differentiate acute infectious spondylitis from reactive (Modic type 1) vertebral endplate changes and to identify abscesses 1

When MRI Cannot Be Performed

• Alternative option: Combined gallium-67/Tc-99m bone scan provides approximately 78% sensitivity and 81% specificity for spinal infection 3
• CT limitations: CT has only 79% sensitivity and critically low (6%) sensitivity for epidural abscess—do not rely on CT alone for diagnosis 1

Key Imaging Features That Distinguish the Two Conditions

Infectious Spondylodiscitis Shows:

• Disc involvement: Abnormal signal within the intervertebral disc with adjacent vertebral endplate involvement 1
• Endplate erosion: Destruction of opposing endplates of two contiguous vertebrae 3
• Paraspinal soft tissue: Peripherally enhancing fluid collections (abscesses) in paraspinal muscles or epidural space 1, 4
• Enhancement pattern: Epidural enhancement combined with abnormal lab values predicts positive biopsy for spondylodiscitis 1
• Rapid disc destruction: Particularly in pyogenic infection, though tuberculous infection may show relative disc preservation early 3
• Single or multiple levels: Approximately 10% of cases involve non-contiguous multilevel disease 4

Inflammatory Spondyloarthropathy Shows:

• Sacroiliac joint involvement: Begin imaging with MRI of SI joints, as isolated spine involvement without SI joint disease occurs in only 6-23% of cases 1
• Posterior element changes: Erosive changes or enthesopathic bone formation in facet joints and posterior elements 1
• Disc preservation: Inflammatory changes at facet joints and entheses rather than disc destruction 1
• No abscess formation: Absence of peripherally enhancing fluid collections or paraspinal abscesses 1
• Symmetric involvement: More symmetric inflammatory changes compared to infection 1
• Fat-suppressed sequences required: Standard spine MRI protocols for disc disease may not include the fat suppression necessary to detect inflammatory features—specifically request evaluation for axial spondyloarthropathy 1

Essential Laboratory Work-Up

For Suspected Infection:

• Blood cultures: Obtain immediately before starting antibiotics 4
• Inflammatory markers: Measure CRP and ESR for baseline and treatment monitoring 4
• Image-guided biopsy: Perform CT-guided or fluoroscopy-guided percutaneous biopsy to identify the causative organism 1, 4
• Tuberculin testing: If epidemiologic risk factors present (endemic region, immunocompromised), perform PPD or interferon-γ release assay—the latter offers higher sensitivity in immunocompromised patients 3

For Suspected Inflammatory Arthropathy:

• HLA-B27 testing: Consider in patients with inflammatory back pain and suggestive imaging 1
• Inflammatory markers: May be elevated but typically less dramatically than in acute infection 1

Clinical Features That Guide Diagnosis

Red Flags for Infection:

• Risk factors: Diabetes mellitus, IV drug use, cancer, HIV, dialysis, recent spinal intervention or surgery 2, 4
• Fever and constitutional symptoms: More common in infection than inflammatory arthropathy 4, 5
• Acute neurological deficits: Suggest epidural abscess with cord compression—requires immediate surgical evaluation 4
• Elevated inflammatory markers: CRP and ESR typically markedly elevated in acute infection 4, 5

Features Suggesting Inflammatory Arthropathy:

• Inflammatory back pain pattern: Morning stiffness >30 minutes, improvement with exercise, age <40 years at onset 1
• Chronic progressive course: Symptoms over months to years rather than acute/subacute presentation 1
• Extra-articular manifestations: Uveitis, psoriasis, inflammatory bowel disease 1

Management Algorithm

If Infection Confirmed or Highly Suspected:

1. Immediate surgical consultation if neurological deficits present with documented spinal cord compression 4
2. Start broad-spectrum IV antibiotics after obtaining blood cultures 4
3. Monitor treatment response with serial CRP/ESR—clinical improvement typically precedes radiographic resolution 4
4. Follow-up MRI to assess treatment response, though imaging findings lag behind clinical improvement 1, 4

If Inflammatory Arthropathy Confirmed:

1. Radiography of SI joints and symptomatic spine regions for baseline structural assessment 1
2. Rheumatology referral for consideration of TNF-α antagonist therapy 1
3. Follow-up imaging with radiography every 2 years for structural progression monitoring 1

Critical Pitfalls to Avoid

• Do not order MRI with contrast only: Precontrast images are mandatory for accurate interpretation 1, 2
• Do not rely on plain radiographs alone: Findings lag 2-8 weeks behind clinical symptoms in infection and have poor sensitivity for early disease 1
• Do not miss multilevel involvement: Image the entire spine if infection suspected, as 10% of cases are non-contiguous 4
• Do not delay treatment for imaging: If neurological deficits present, initiate treatment immediately while arranging urgent imaging 4
• Do not use standard disc protocol MRI for spondyloarthropathy: Specifically request fat-suppressed sequences (STIR or T2 fat-sat) to detect inflammatory changes 1
• Do not assume single-level vertebral body involvement excludes infection: Atypical presentations with single vertebral involvement can occur in early spondylodiscitis 6