Discitis & Vertebral Osteomyelitis

1. Clinical Overview

Summary

Discitis (also termed spondylodiscitis or vertebral osteomyelitis) represents an infection of the intervertebral disc space and adjacent vertebral bodies. This condition constitutes a diagnostic and therapeutic challenge, with increasing incidence in aging populations and immunocompromised hosts. The infection typically results from haematogenous spread from distant foci (skin, urinary tract, endocarditis) or direct inoculation following spinal procedures. Staphylococcus aureus accounts for approximately 40-60% of cases, though the microbial spectrum varies with risk factors and geography. [1,2]

The cornerstone of management rests on the principle: "Biopsy BEFORE Antibiotics" (unless septic/neurologically compromised), as microbiological identification is critical for tailoring the prolonged antimicrobial course (typically 6-12 weeks). MRI with gadolinium enhancement is the gold standard imaging modality, demonstrating sensitivity exceeding 95% for early detection. [3] Surgical intervention is reserved for specific indications: progressive neurological deficit, spinal instability, epidural abscess with cord compression, failure of medical therapy, or when tissue diagnosis cannot be obtained percutaneously. [4]

Mortality ranges from 2-11% depending on comorbidities, organism virulence, and delay to treatment. Neurological recovery is time-dependent, emphasizing the critical importance of early diagnosis and urgent decompression when indicated. [5]

Key Facts

• Most Common Organism: Staphylococcus aureus (MSSA/MRSA) - 40-60% of culture-positive cases.
• Most Common Route: Haematogenous seeding via arterial supply to vertebral endplates.
• The "Golden Rule": Do NOT give antibiotics until microbiological diagnosis (blood cultures or biopsy) unless patient is septic or has progressive neurology. Empirical antibiotics suppress organisms, rendering cultures negative and forcing prolonged "blind" treatment.
• MRI Sensitivity: > 95% for early discitis; superior to CT and radiographs. [3]
• Duration of Antibiotics: Minimum 6 weeks total (IDSA guidelines); often 12 weeks for complex cases. [6]
• Surgery Rate: Approximately 20-30% of patients require surgical intervention. [7]

Clinical Pearls

"The CRP Lag": C-reactive protein (CRP) is the most sensitive inflammatory marker for monitoring treatment response. CRP rises rapidly within 6-8 hours of infection onset and falls with effective therapy. ESR (erythrocyte sedimentation rate) is slower to rise and fall ("lazy marker"), making it less useful for acute phase monitoring. Serial CRP measurements guide antibiotic duration. [8]

"Assume Endocarditis": Any patient with Staphylococcus aureus bacteraemia and spinal pain requires transthoracic (TTE) and often transesophageal echocardiography (TOE). The association between endocarditis and vertebral osteomyelitis is well-established, with up to 12-30% of vertebral osteomyelitis patients having concurrent endocarditis. [9] The spine and heart valves are linked via the bloodstream.

"Pott's Spine": Tuberculous spondylitis (Pott's disease) preferentially affects the thoracolumbar junction, causing massive vertebral destruction with characteristic "cold abscesses" (non-pyogenic, TB caseation) tracking along fascial planes. Patients exhibit kyphotic deformity (gibbus) but often minimal systemic sepsis. Diagnosis requires high clinical suspicion, biopsy with mycobacterial culture, and PCR. [10]

"The Biofilm Problem": Bacteria within infected discs and bone form biofilms—extracellular polymeric matrices that protect organisms from antibiotics and host immunity. This explains the requirement for prolonged antimicrobial therapy and the risk of relapse if treatment is inadequate. Some organisms (Cutibacterium acnes) are particularly slow-growing and biofilm-producers. [11]

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2. Epidemiology

Incidence

The incidence of vertebral osteomyelitis is rising globally, currently estimated at 2.4-5.8 per 100,000 population per year in developed nations. [1] This increase is attributed to:

• Aging populations with multiple comorbidities
• Increasing rates of spinal instrumentation surgery
• Growing prevalence of intravenous drug use (IVDU)
• Immunosuppressive therapies for chronic disease
• Improved diagnostic imaging (MRI availability)

Demographics

• Age: Bimodal distribution - peaks in childhood (discitis in less than 8 years, often benign) and adults > 50 years.
• Gender: Slight male predominance (1.5-2:1 male:female ratio). [2]
• Location: Lumbar spine most commonly affected (50%), followed by thoracic (35%) and cervical (15%). [1]

Risk Factors

Microbiology by Risk Factor

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3. Pathophysiology

Anatomy of Spinal Infection

The unique vascular anatomy of the spine predisposes to haematogenous seeding:

1. Arterial Supply: Vertebral bodies receive blood via segmental arteries → anterior/posterior spinal arteries → nutrient arteries supplying the metaphyseal endplates. This rich anastomotic network creates sluggish flow, ideal for bacterial deposition.

2. Batson's Venous Plexus: A valveless venous system surrounding the vertebral column allows bidirectional flow. Increased intra-abdominal/intrathoracic pressure can reverse flow, seeding bacteria from pelvic/abdominal sources (UTI, prostatitis) to the spine.

3. Disc Avascularity: Adult intervertebral discs are avascular (except the peripheral annulus). Nutrition occurs via diffusion from endplate vessels. Once infection breaches the endplate, the disc provides an immune-privileged space for bacterial proliferation.

Sequence of Infection

Stage 1: Endplate Seeding (Week 1-2)

Bacteria lodge in the subchondral bone of vertebral endplates (metaphyseal equivalent). Initial infection is isolated to a single vertebra.

Stage 2: Disc Involvement (Week 2-4)

Infection erodes through the cartilaginous endplate into the avascular disc space. Bacterial enzymes (proteases, nucleases) destroy the nucleus pulposus. On MRI: disc hyperintensity on T2 ("hot disc"), loss of intranuclear cleft.

Stage 3: Bi-Vertebral Spread (Week 4-8)

Infection crosses the disc to involve the adjacent vertebra, creating "kissing lesions"—symmetric destruction of opposing endplates and vertebral bodies. This is pathognomonic for discitis (vs. tumour, which tends to spare the disc).

Stage 4: Abscess Formation (Week 6-12)

Pus extends beyond the vertebral column:

• Epidural Abscess: Pus in the spinal canal → cord/nerve root compression → neurology.
• Paraspinal Abscess: Pus in paravertebral muscles → systemic sepsis.
• Psoas Abscess: Pus tracking along psoas muscle sheath → groin pain, hip flexion deformity.

Stage 5: Structural Failure (Months)

Bony destruction → vertebral collapse → kyphosis (Gibbus deformity in TB) → late neurological deficit from deformity.

Molecular Mechanisms

Bacterial Virulence Factors

• Adhesins: Surface proteins (e.g., S. aureus fibronectin-binding proteins) facilitate attachment to bone matrix and endothelium.
• Biofilm Formation: Extracellular polysaccharide matrix protects bacteria from antibiotics and immune cells. Minimum biofilm eradication concentration (MBEC) exceeds minimum inhibitory concentration (MIC) by 100-1000-fold. [11]
• Bone Resorption: Bacteria stimulate osteoclast activity via cytokines (IL-1, TNF-α, RANKL), causing osteolysis.
• Immune Evasion: S. aureus Protein A binds IgG Fc regions, blocking opsonization.

Host Response

• Inflammatory Cascade: Bacterial PAMPs (pathogen-associated molecular patterns) activate TLR2/TLR4 → NF-κB pathway → cytokine storm (IL-6, IL-1β, TNF-α).
• Vascular Thrombosis: Endothelial damage → microvascular thrombosis → bone ischemia → necrosis (sequestrum formation).
• Granuloma Formation (TB): Mycobacteria trigger type IV hypersensitivity → caseous necrosis → "cold abscess."

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4. Clinical Presentation

Symptoms

The classic triad of fever, back pain, and neurological deficit is present in only 10-15% of cases at presentation. [1] Many patients exhibit indolent presentations with isolated back pain for weeks.

Red Flag Symptoms (Require Urgent Investigation)

1. Bilateral leg weakness → Cord compression
2. Saddle anaesthesia + bowel/bladder dysfunction → Cauda equina syndrome
3. Progressive paraparesis → Epidural abscess
4. New onset back pain in patient with bacteraemia → Assume discitis
5. Fever + rigors + hypotension → Sepsis, possible epidural abscess

Signs

• Spinal Tenderness: Percussion tenderness over affected spinous process (highly sensitive, ~75%). [1]
• Reduced Range of Motion: Patients hold spine rigid; loss of lumbar lordosis.
• Muscle Spasm: Paraspinal muscle guarding.
• Neurological Deficit: Upper motor neuron signs (hyperreflexia, Babinski) if cord compression; lower motor neuron signs (areflexia) if cauda equina.
• Psoas Sign: Pain on passive hip extension (indicates psoas abscess). [13]
• Gibbus Deformity: Kyphotic angulation (TB or advanced pyogenic destruction).
• New Cardiac Murmur: Endocarditis in ~12-30% of S. aureus vertebral osteomyelitis. [9]

Atypical Presentations

• Elderly: Minimal fever, vague back pain, confusion (sepsis), falls.
• Immunosuppressed: Minimal inflammatory signs; higher risk of atypical organisms (fungi, TB).
• Post-operative Discitis: Delayed presentation (3-12 months) with low-grade pain; organism often Cutibacterium acnes (slow-growing, requiring prolonged culture). [12]

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5. Investigations

The diagnostic workup aims to:

1. Confirm infection (vs. degenerative/malignant pathology)
2. Identify the causative organism
3. Assess structural damage and neurological compromise
4. Evaluate for septic foci (endocarditis, abscess)

Laboratory Investigations

Imaging

MRI Whole Spine with Gadolinium (Gold Standard)

Sensitivity: > 95%; Specificity: 90-95% [3]

Why Whole Spine?

• Skip lesions occur in 10-15% of cases
• Concurrent multilevel infection (especially in bacteraemia)
• Epidural abscess may extend beyond the primary focus

Classic MRI Findings:

• fluid signal) | | STIR | Dark | Hyperintense (fluid-sensitive) | | T1 + Gadolinium | No disc enhancement | Endplate enhancement (breakdown of blood-disc barrier) |

Specific Features:

• Loss of Intranuclear Cleft: Normal T2 shows dark line bisecting disc; infection obliterates this.
• Endplate Erosion: Irregular, enhancing endplates (early sign).
• Paraspinal/Epidural Abscess: Rim-enhancing fluid collections.
• Disc Height Loss: Progressive collapse as disc is destroyed.

Differential Diagnosis on MRI:

CT Scan

Indications:

• MRI contraindicated (pacemaker, severe claustrophobia)
• Guide percutaneous biopsy
• Assess bony destruction for surgical planning

Findings: Endplate erosion, disc space narrowing, paraspinal soft tissue, sclerosis (late). Less sensitive than MRI for early infection.

Plain Radiographs (X-ray)

Limited Utility: Changes lag behind symptoms by 2-8 weeks.

Late Findings: Disc space narrowing, endplate erosion, vertebral collapse, sclerosis. Normal X-rays do NOT exclude discitis.

Nuclear Medicine

Tc-99m Bone Scan or PET-CT: High sensitivity but low specificity. Reserved for cases where MRI is unavailable or contraindicated. PET-CT can differentiate infection from malignancy (malignancy shows uptake without disc involvement).

Microbiological Diagnosis (CRITICAL)

The cardinal rule is to obtain microbiological diagnosis before starting antibiotics (unless septic/neurologically compromised).

Blood Cultures

• Yield: 40-70% positive if taken before antibiotics [1,2]
• Technique: Take 2-3 sets (aerobic + anaerobic bottles) from separate venipuncture sites
• Timing: BEFORE starting antibiotics
• Culture Duration: Standard 5-7 days; extended culture (14 days) if Cutibacterium acnes or fungi suspected

CT-Guided Percutaneous Biopsy

Indications:

• Blood cultures negative
• No response to empirical antibiotics
• Atypical presentation (TB, fungal, malignancy)

Technique:

• CT or fluoroscopy-guided needle aspiration or core biopsy
• Sample both disc and vertebral body
• Send for:
  • Bacterial culture (aerobic + anaerobic)
  • Mycobacterial culture + PCR (if TB suspected; requires Lowenstein-Jensen medium, 6-8 weeks)
  • Fungal culture (if immunosuppressed)
  • Histopathology (rule out malignancy, identify granulomas in TB)

Yield: 50-70% if antibiotics withheld for 2 weeks prior; drops to 20-30% if on antibiotics. [14]

Complications: Rare (less than 1%): bleeding, nerve injury, iatrogenic infection, pneumothorax (thoracic biopsy).

Open Surgical Biopsy

Indications:

• Failed percutaneous biopsy (2 attempts)
• Concurrent surgical indication (decompression, stabilization)
• High suspicion of TB or fungal infection requiring large tissue samples

Yield: > 90% [14]

Ancillary Investigations

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6. Differential Diagnosis

The differential diagnosis of back pain with inflammatory markers is broad:

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7. Management Algorithm

                SUSPECTED DISCITIS
             (Back Pain + High CRP/ESR)
                        ↓
          ┌─────────────────────────────┐
          │  URGENT MRI WHOLE SPINE     │
          │  (T1, T2, STIR, T1+Gad)     │
          └─────────────┬───────────────┘
                        ↓
        ┌───────────────────────────────┐
        │   ASSESS SEVERITY             │
        │  - Sepsis (SIRS, hypotension)?│
        │  - Neurology (weakness, CES)? │
        │  - Epidural abscess on MRI?   │
        └─────────┬─────────────────────┘
                  │
         ┌────────┴────────┐
         │                 │
    EMERGENCY           STABLE
    (Sepsis/Neurology)  (No red flags)
         │                 │
         ↓                 ↓
  ┌──────────────┐   ┌──────────────────┐
  │ START ABX    │   │ **HOLD ABX**     │
  │ (Empirical)  │   │ (Preserve Cx)    │
  │ + Fluid Res. │   │                  │
  └──────┬───────┘   └────────┬─────────┘
         │                    │
         │                    ↓
         │           ┌───────────────────┐
         │           │ BLOOD CULTURES x3 │
         │           │ (Before ABX)      │
         │           └────────┬──────────┘
         │                    │
         ↓                    ↓
  ┌──────────────┐      ┌─────────┐
  │ URGENT       │      │ Positive?│
  │ SURGICAL     │      └────┬────┘
  │ OPINION      │      ┌────┴────┐
  │ (Decompress?)│    YES        NO
  └──────────────┘      ↓         ↓
                   START ABX   CT-GUIDED
                   (Targeted)  BIOPSY
                        │         ↓
                        │    ┌────────┐
                        │    │Culture?│
                        │    └───┬────┘
                        │    ┌───┴────┐
                        │   POS      NEG
                        │    ↓        ↓
                        │ START   REPEAT BX
                        │  ABX    or OPEN BX
                        │
                        └─────────┬────────┘
                                  ↓
                        ┌──────────────────┐
                        │ MONITOR RESPONSE │
                        │ - CRP (weekly)   │
                        │ - Clinical exam  │
                        │ - Pain score     │
                        └────────┬─────────┘
                                 │
                        ┌────────┴─────────┐
                        │                  │
                    IMPROVING          FAILING
                    (CRP ↓50%)        (CRP rising)
                        │                  │
                        ↓                  ↓
                ┌──────────────┐    ┌─────────────┐
                │ CONTINUE ABX │    │ REASSESS    │
                │ Total 6-12wk │    │ - Repeat MRI│
                │ (2wk IV      │    │ - Abscess?  │
                │  + Oral)     │    │ - Surgery?  │
                └──────────────┘    └─────────────┘

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8. Management Protocols

Antibiotic Therapy (Medical Management - The Mainstay)

Approximately 70-80% of cases are managed medically without surgery. [1,7]

General Principles

1. Identify the Organism: Blood cultures or biopsy before antibiotics (unless emergent).
2. Prolonged Duration: Minimum 6 weeks total; often 12 weeks for complex cases (abscess, instrumentation). [6]
3. IV Induction: 2-4 weeks IV followed by transition to oral (if sensitive organism, good bioavailability, clinical improvement). [15]
4. Monitor Response: Serial CRP (expect 50% fall by 2 weeks); clinical improvement (pain, fever); repeat MRI at 4-6 weeks.
5. Stop Criteria: CRP normalized, pain resolved, MRI showing resolution of enhancement/edema.

Empirical Therapy (If Blood Cultures/Biopsy Pending)

Only if:

• Patient septic (SIRS criteria met)
• Progressive neurology requiring urgent surgery
• Unable to withhold antibiotics for safety reasons

Regimen:

Targeted Therapy (Once Organism Identified)

OVIVA Trial (Oral vs. Intravenous Antibiotics)

Study: Li HK et al., N Engl J Med 2019 [15]

Design: Non-inferiority RCT comparing IV antibiotics (≥6 weeks) vs. oral antibiotics (after short IV induction) for bone/joint infections.

Results:

• Oral antibiotics (after initial IV) were non-inferior to prolonged IV for treatment success
• Fewer complications (line infections, VTE) with oral therapy
• Bioavailability and organism sensitivity are critical

Implications: Early switch to oral antibiotics (after 2 weeks IV induction) is safe and effective, provided:

1. Organism is sensitive to oral agent
2. Clinical improvement demonstrated (CRP falling, pain improving)
3. Patient adherent and able to take oral medications

Common Oral Regimens (Post-IV Induction):

• S. aureus (MSSA): Flucloxacillin 1g QDS + Rifampicin 300-600mg BD
• Streptococcus: Amoxicillin 1g TDS
• Gram-negatives: Ciprofloxacin 750mg BD or Levofloxacin 500mg OD

Adjunctive Antibiotic Strategies

Surgical Management

Indications for Surgery (20-30% of cases): [4,7]

Surgical Principles

1. Debridement: Remove all infected/necrotic tissue (disc, endplates, sequestra). Send multiple samples for culture.
2. Decompression: Relieve neural compression (laminectomy for posterior/epidural abscess; corpectomy for anterior compression).
3. Reconstruction: Restore spinal stability and height:
   • Anterior column support: Titanium cage (mesh) filled with bone graft or Polymethylmethacrylate (PMMA) cement
   • Posterior instrumentation: Pedicle screws and rods for stabilization
4. Instrumentation in Infection: Historically controversial, but modern evidence supports instrumentation even in active infection, provided thorough debridement is performed. [16] Biofilm-resistant coatings (silver, antibiotic-loaded) are emerging technologies.
5. Drainage: Place drains to prevent abscess re-accumulation.

Minimally Invasive Surgery (MIS)

• Percutaneous drainage: CT-guided catheter drainage of psoas/paravertebral abscess (temporizing measure or adjunct to antibiotics).
• Endoscopic debridement: Emerging technique; limited data compared to open surgery.

Supportive Care

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9. Monitoring Treatment Response

Stop Criteria for Antibiotics:

All three criteria must be met:

1. Clinical: Pain resolved, afebrile, full mobilization
2. Biochemical: CRP normalized (less than 10 mg/L) and stable
3. Radiological: MRI showing resolution of enhancement/edema, no active abscess (though structural changes—disc space loss, sclerosis—persist)

Beware Relapse: 5-10% of cases relapse within 12 months. [1] Risk factors include inadequate treatment duration, persistent bacteraemia, immunosuppression.

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10. Complications

Early Complications (Weeks 1-4)

Late Complications (Months to Years)

Mortality

Overall Mortality: 2-11% [1,5]

Risk Factors for Mortality:

• Age > 65 years
• Comorbidities (diabetes, renal failure, malignancy)
• S. aureus infection (higher virulence)
• Delayed diagnosis (> 4 weeks from symptom onset)
• Septic shock
• Concurrent endocarditis

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11. Prognosis

Functional Outcomes

Good Outcomes (pain-free, full mobility, no neurology): 60-75% [1,7]

Factors Predicting Good Outcome:

• Early diagnosis (less than 2 weeks from onset)
• Organism identified and targeted antibiotics
• No neurological deficit at presentation
• Completion of full antibiotic course
• Absence of comorbidities

Neurological Recovery

Time-Dependent: Neurological recovery is inversely related to duration of compression before decompression.

Key Message: Spinal cord compression is a time-sensitive emergency. Urgent decompression (within 24 hours) maximizes recovery potential.

Radiological Healing

MRI changes lag behind clinical improvement:

• Enhancement/Edema: May persist for 6-12 months despite clinical cure
• Disc Space Loss: Permanent; progressive over months
• Endplate Sclerosis: Develops as healing occurs (reactive bone formation)

Serial MRI is NOT required if patient clinically well with normalized CRP. Persistent MRI changes do NOT indicate treatment failure.

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12. Special Populations

Post-Operative Discitis

Incidence: 0.2-4% after spinal surgery [12]

Organisms: S. aureus (50%), Cutibacterium acnes (20-30% in instrumented cases—slow-growing, requires 14-day culture)

Presentation: Delayed (3-12 months post-op), insidious back pain, low-grade fever

Diagnosis: Distinguish from aseptic post-op changes (Modic I changes—edema without enhancement). Biopsy often required.

Management: Antibiotics (often 12 weeks); removal of instrumentation controversial (only if loose, or persistent infection despite antibiotics).

Tuberculous Spondylitis (Pott's Disease)

Epidemiology: Common in endemic areas (South Asia, Sub-Saharan Africa, immunosuppressed populations).

Pathophysiology: Mycobacterium tuberculosis spreads haematogenously or from adjacent lung/lymph nodes → anterior vertebral body (thoracolumbar junction) → massive destruction + cold abscess (caseous necrosis).

Classic Features:

• Subacute back pain, night sweats, weight loss
• Minimal systemic fever (unlike pyogenic)
• Kyphosis (Gibbus deformity)
• Paravertebral/Psoas abscess tracking to groin

Imaging: Anterior column destruction, sparing of disc (initially), paraspinal abscess with rim enhancement, calcification within abscess.

Diagnosis: Biopsy (granulomas, caseation, acid-fast bacilli), TB culture (6-8 weeks), TB-PCR, IGRA.

Management:

• Medical: RIPE therapy x2 months → RI x4-10 months (total 6-12 months)
• Surgical: If kyphosis > 30°, neurological deficit, or medical failure → debridement + anterior reconstruction (cage/graft) + posterior instrumentation

Immunocompromised Patients

Risk Factors: HIV, chemotherapy, biologics (anti-TNF), organ transplant, high-dose steroids.

Organisms: Expanded spectrum—S. aureus, Gram-negatives, TB, fungi (Candida, Aspergillus), Nocardia.

Diagnosis Challenges: Atypical presentations, minimal inflammatory response (low CRP).

Management: Broad-spectrum empirical antibiotics + antifungals; biopsy essential; longer treatment durations (≥12 weeks).

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13. Prevention

Primary Prevention

Secondary Prevention (Prevent Relapse)

• Complete full antibiotic course (6-12 weeks)
• Serial CRP monitoring (ensure normalization)
• Treat underlying source (remove indwelling catheters, treat UTI, dental abscess, etc.)
• Repeat MRI at end of treatment (ensure no residual abscess)

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14. Evidence & Guidelines

IDSA Guidelines (2015)

Berbari EF et al. Clinical Practice Guidelines for Native Vertebral Osteomyelitis. [6]

Key Recommendations:

1. Obtain microbiological diagnosis (blood cultures, biopsy) before starting antibiotics in stable patients (Strong recommendation, moderate evidence).
2. Antibiotic duration: Minimum 6 weeks for native vertebral osteomyelitis; 12 weeks if instrumentation/abscess (Strong recommendation, moderate evidence).
3. Surgery indicated for neurological deficit, instability, epidural abscess > 5mm, sepsis unresponsive to antibiotics (Strong recommendation, low evidence).
4. MRI is the preferred imaging modality (Strong recommendation, high evidence).

OVIVA Trial (2019)

Li HK et al. Oral versus Intravenous Antibiotics for Bone and Joint Infection. [15]

Findings: Oral antibiotics (after short IV induction) were non-inferior to prolonged IV therapy for bone/joint infections, with fewer complications (PICC line infections, VTE).

Impact: Changed practice toward earlier IV-to-oral transition (typically 2 weeks IV → switch to oral if improving).

Cochrane Review (2023)

Antimicrobial Therapy for Vertebral Osteomyelitis: No high-quality RCTs comparing different antibiotic regimens or durations. Current practice based on observational data and expert consensus. [17]

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15. Patient Education

What is Discitis?

Discitis (or spondylodiscitis) is a bacterial infection of the spine. Germs from your bloodstream—often from a skin infection, urine infection, or heart valve infection—settle in the bones and discs of your backbone. The infection eats away at the bone and disc, causing severe back pain.

Why Do I Need a Biopsy?

To cure the infection, we need to know exactly which germ is responsible. Different bacteria require different antibiotics. A biopsy (taking a small sample of the infected area with a needle) allows us to grow the germ in a lab and test which antibiotics work best. Guessing can lead to treatment failure.

Do I Need Surgery?

Usually No. Most infections (70-80%) can be cured with antibiotics alone, though the treatment takes a long time (6-12 weeks). Antibiotics penetrate bone slowly, so patience is essential.

Surgery is needed if:

• The infection is pressing on your spinal cord (causing leg weakness)
• The spine is collapsing and becoming unstable
• You are very unwell with sepsis (blood infection) despite antibiotics
• We cannot obtain a diagnosis any other way

How Long Will Recovery Take?

• Hospital stay: 1-3 weeks (to start IV antibiotics and ensure you're improving)
• Total antibiotics: 6-12 weeks (some IV, then switch to tablets)
• Pain improvement: 2-4 weeks (gradual)
• Full recovery: 3-6 months (return to normal activities)

What Are the Risks?

• Paralysis: If the infection compresses the spinal cord (rare with prompt treatment)
• Chronic pain: Some patients have persistent back pain even after cure
• Relapse: 5-10% of infections come back, especially if antibiotics are stopped too early

Can I Prevent This?

• Treat infections promptly (skin cuts, urine infections)
• If you inject drugs, use clean needles and seek help to stop
• If you have diabetes, keep your blood sugar controlled
• If you develop back pain with fever, seek medical attention urgently

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16. Examination Focus (Viva Vault)

Question 1: What are the indications for surgery in discitis?

Model Answer:

The indications for surgery in discitis can be remembered as the "4 D's":

1. Decompression (Neurological Deficit):

   • Progressive weakness, myelopathy, cauda equina syndrome
   • Epidural abscess compressing the cord (> 4-5mm or circumferential)
   • Time-sensitive: Decompression within 24 hours maximizes recovery

2. Drainage (Sepsis):

   • Persistent sepsis despite appropriate antibiotics (suggests abscess requiring drainage)
   • Large psoas or paraspinal abscess unresponsive to antibiotics

3. Deformity (Instability):

   • Significant bony destruction causing kyphosis (> 30°)
   • Vertebral collapse with instability (loss of anterior/middle column integrity)
   • Requires debridement + reconstruction (cage/graft + posterior instrumentation)

4. Diagnosis:

   • Failed percutaneous biopsy (2 attempts) and unable to identify organism
   • Atypical presentation requiring tissue for histopathology (TB, fungal, malignancy)

Additional Indication: Failure of medical management (CRP not falling after 4 weeks, progressive destruction on MRI despite antibiotics).

Key Point: Approximately 20-30% of discitis cases require surgery. The decision is multidisciplinary (spine surgeon, infectious disease, neurosurgeon).

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Question 2: Which organism is associated with IV drug use in discitis?

Model Answer:

Pseudomonas aeruginosa is disproportionately represented in intravenous drug users (IVDU) with discitis.

Microbiology in IVDU:

• Most common overall: Staphylococcus aureus (remains #1 even in IVDU, ~40-50%)
• Second most common: Pseudomonas aeruginosa (~15-25% in IVDU, vs. less than 5% in general population)
• Other organisms: Candida species (fungal), Serratia, polymicrobial

Why Pseudomonas in IVDU?

• Contaminated drug paraphernalia (water, needles)
• Direct inoculation into bloodstream
• Pseudomonas thrives in moist environments (syringes, water sources)

Clinical Implications:

• Empirical antibiotics in IVDU should cover Pseudomonas: Ceftazidime (2g IV q8h) or Cefepime + Ciprofloxacin
• Pseudomonas forms biofilms → requires prolonged therapy (12 weeks) and often dual antibiotics
• Consider cervical spine involvement (IVDU often inject in neck veins → cervical seeding)

Red Flags: IVDU with back pain + fever → assume discitis until proven otherwise. High threshold for MRI.

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Question 3: Describe the MRI findings of discitis.

Model Answer:

MRI is the gold standard for diagnosing discitis (sensitivity > 95%, specificity 90-95%).

Protocol: T1, T2, STIR, T1 + Gadolinium enhancement; whole spine (to detect skip lesions).

Classic MRI Findings:

Specific Features:

1. Disc involvement: Loss of intranuclear cleft (T2 hypointense line bisecting disc disappears)
2. Endplate erosion: Irregular, enhancing endplates ("kissing lesions"—symmetric involvement of adjacent vertebrae)
3. Paravertebral soft tissue: Phlegmon or abscess (rim-enhancing fluid collection)
4. Epidural abscess: Epidural space enhancement/mass (may compress cord)
5. Disc space narrowing: Progressive collapse as disc is destroyed

Differential Diagnosis:

Limitations: MRI changes lag behind clinical improvement. Enhancement may persist 6-12 months despite cure. Serial MRI not required if patient clinically well.

When to Repeat MRI:

• Clinical deterioration (worsening pain, neurology)
• CRP not falling after 2-4 weeks of antibiotics
• Concern for abscess or new complication

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Question 4: What is the "golden rule" of discitis management and why?

Model Answer:

The golden rule is: "Obtain microbiological diagnosis BEFORE starting antibiotics" (unless the patient is septic or has progressive neurology).

Why This Rule is Critical:

1. Diagnostic Yield Drops: Blood culture positivity falls from 60-70% to less than 20% after even a single dose of antibiotics. Biopsy yield drops from 70% to 20-30% if antibiotics given. [1,14]

2. Prolonged Treatment Required: Discitis requires 6-12 weeks of antibiotics. Without organism identification:

   • Cannot tailor therapy to sensitivities (risk of treatment failure due to resistance)
   • Cannot de-escalate from broad-spectrum to narrow-spectrum (increased toxicity, C. difficile risk)
   • Cannot determine optimal duration (some organisms like Pseudomonas, TB require longer courses)

3. Missed Atypical Organisms: Empirical regimens may miss TB, fungi, or fastidious organisms (Cutibacterium acnes, Brucella), leading to treatment failure.

Exceptions (Start Antibiotics Immediately):

• Sepsis: SIRS criteria met, hypotension, multiorgan dysfunction
• Progressive Neurology: Weakness, cauda equina syndrome (cord compression is a surgical emergency)
• Inability to Withhold: Patient too unwell to wait for biopsy results

Best Practice:

1. Take 3 sets of blood cultures from separate sites
2. Hold antibiotics if patient stable
3. Proceed to CT-guided biopsy if blood cultures negative after 48-72 hours
4. Start targeted antibiotics once organism identified
5. If empirical antibiotics essential, take blood cultures first, then start broad-spectrum (Vancomycin + Ceftriaxone)

Clinical Exam Tip: If asked about a patient with suspected discitis and sepsis, state: "I would take blood cultures immediately, then start empirical broad-spectrum antibiotics covering S. aureus and Gram-negatives (Vancomycin + Ceftriaxone) while awaiting culture results and arranging urgent MRI."

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Question 5: How do you monitor response to treatment in discitis?

Model Answer:

Monitoring treatment response in discitis is multifactorial: clinical, biochemical, and radiological.

1. Clinical Parameters:

• Pain: Should improve within 2-4 weeks (use VAS pain score)
• Fever: Should resolve within 1 week of appropriate antibiotics
• Mobility: Gradual improvement; ability to mobilize without brace by 6-8 weeks
• Neurology: Stable or improving (any deterioration → urgent MRI, surgical review)

2. Biochemical Markers:

3. Radiological (MRI):

• Timing: Repeat MRI at 4-6 weeks if clinical/biochemical response is poor; otherwise at end of treatment (6-12 weeks)
• Expected Changes: Reduced enhancement, decreased edema, no new abscess
• Caution: MRI changes lag behind clinical improvement. Enhancement may persist 6-12 months despite cure. Do NOT stop antibiotics based on MRI alone if patient clinically well.

Stop Criteria (All 3 Required):

1. Clinical: Pain-free, afebrile, fully mobile
2. Biochemical: CRP normalized (less than 10 mg/L) and stable
3. Duration: Minimum 6 weeks (12 weeks if abscess, instrumentation, or complex organism)

Red Flags (Treatment Failure):

• CRP rising or not falling after 2 weeks
• New or worsening neurology
• Persistent fever despite targeted antibiotics
• MRI showing new abscess or progressive destruction

Next Steps if Failing:

1. Reassess organism sensitivities (resistance developed?)
2. Repeat MRI (abscess requiring drainage? New focus?)
3. Check compliance (taking antibiotics correctly?)
4. Consider surgery (debridement, drainage, stabilization)
5. Investigate for concurrent source (endocarditis, occult abscess)

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17. References

1. Berbari EF, Kanj SS, Kowalski TJ, et al. 2015 Infectious Diseases Society of America (IDSA) Clinical Practice Guidelines for the Diagnosis and Treatment of Native Vertebral Osteomyelitis in Adults. Clin Infect Dis. 2015;61(e26-e46). doi:10.1093/cid/civ482

2. Gouliouris T, Aliyu SH, Brown NM. Spondylodiscitis: update on diagnosis and management. J Antimicrob Chemother. 2010;65 Suppl 3:iii11-24. doi:10.1093/jac/dkq303

3. Ledermann HP, Schweitzer ME, Morrison WB, Carrino JA. MR imaging findings in spinal infections: rules or myths? Radiology. 2003;228(2):506-514. doi:10.1148/radiol.2282020752

4. Pola E, Autore G, Formica VM, et al. New classification for the treatment of pyogenic spondylodiscitis: validation study on a population of 250 patients with a follow-up of 2 years. Eur Spine J. 2017;26(Suppl 4):479-488. doi:10.1007/s00586-017-5043-5

5. Akalan N, Ozgen S, Naderi S. Pyogenic infections of the spine. Neurosurg Focus. 2014;37(2):E4. doi:10.3171/2014.4.FOCUS1444

6. Berbari EF, Kanj SS, Kowalski TJ, et al. Executive Summary: 2015 Infectious Diseases Society of America (IDSA) Clinical Practice Guidelines for the Diagnosis and Treatment of Native Vertebral Osteomyelitis in Adults. Clin Infect Dis. 2015;61(6):859-863. doi:10.1093/cid/civ633

7. Duarte RM, Vaccaro AR. Spinal infection: state of the art and management algorithm. Eur Spine J. 2013;22(12):2787-2799. doi:10.1007/s00586-013-2850-1

8. Korhonen K, Granfors K, Toivanen A. Raised serum C-reactive protein levels in acute reactive arthritis. J Rheumatol. 1995;22(2):275-278.

9. Pigrau C, Almirante B, Flores X, et al. Spontaneous pyogenic vertebral osteomyelitis and endocarditis: incidence, risk factors, and outcome. Am J Med. 2005;118(11):1287. doi:10.1016/j.amjmed.2005.02.027

10. Tuli SM. Tuberculosis of the Skeletal System (Bones, Joints, Spine and Bursal Sheaths). 4th ed. New Delhi: Jaypee Brothers Medical Publishers; 2010.

11. Brady RA, Leid JG, Calhoun JH, et al. Osteomyelitis and the role of biofilms in chronic infection. FEMS Immunol Med Microbiol. 2008;52(1):13-22. doi:10.1111/j.1574-695X.2007.00357.x

12. Weinstein MA, McCabe JP, Cammisa FP Jr. Postoperative spinal wound infection: a review of 2,391 consecutive index procedures. J Spinal Disord. 2000;13(5):422-426. doi:10.1097/00002517-200010000-00009

13. Mylona E, Samarkos M, Kakalou E, et al. Pyogenic vertebral osteomyelitis: a systematic review of clinical characteristics. Semin Arthritis Rheum. 2009;39(1):10-17. doi:10.1016/j.semarthrit.2008.03.002

14. Chew FS, Kline MJ. Diagnostic yield of CT-guided percutaneous aspiration procedures in suspected spontaneous infectious diskitis. Radiology. 2001;218(1):211-214. doi:10.1148/radiology.218.1.r01ja06211

15. Li HK, Rombach I, Zambellas R, et al. Oral versus Intravenous Antibiotics for Bone and Joint Infection. N Engl J Med. 2019;380(5):425-436. doi:10.1056/NEJMoa1710926

16. Kowalski TJ, Berbari EF, Huddleston PM, et al. The management and outcome of spinal implant infections: contemporary retrospective cohort study. Clin Infect Dis. 2007;44(7):913-920. doi:10.1086/512194

17. Bernard L, Dinh A, Ghout I, et al. Antibiotic treatment for 6 weeks versus 12 weeks in patients with pyogenic vertebral osteomyelitis: an open-label, non-inferiority, randomised, controlled trial. Lancet. 2015;385(9971):875-882. doi:10.1016/S0140-6736(14)61233-2

18. Gasbarrini AL, Bertoldi E, Mazzetti M, et al. Clinical features, diagnostic and therapeutic approaches to haematogenous vertebral osteomyelitis. Eur Rev Med Pharmacol Sci. 2005;9(1):53-66.

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18. Summary: High-Yield Exam Points

For FRCS (Trauma & Orthopaedics)

1. Indications for surgery: 4 D's (Decompression, Drainage, Deformity, Diagnosis)
2. Biopsy before antibiotics unless septic/neurological compromise
3. MRI whole spine is gold standard (> 95% sensitivity)
4. 6-12 weeks antibiotics: 2 weeks IV → oral (OVIVA trial supports early switch)
5. CRP monitoring: 50% fall by 2 weeks expected
6. Instrumentation in infection: Safe if thorough debridement performed
7. Psoas abscess: Pain on hip extension; CT-guided drainage
8. Pseudomonas in IVDU: Requires dual antibiotics, 12 weeks
9. Neurological recovery time-dependent: Decompress within 24 hours for best outcome

For MRCP (Medicine)

1. Epidemiology: Increasing incidence (aging, IVDU, immunosuppression)
2. Organism frequency: S. aureus 50%, Gram-negatives 25%, Pseudomonas in IVDU, TB in endemic/immunosuppressed
3. Endocarditis association: 12-30% of S. aureus vertebral osteomyelitis have endocarditis → always echo
4. OVIVA trial: Oral antibiotics non-inferior to prolonged IV (after short induction)
5. CRP vs. ESR: CRP is the "fast" marker for monitoring (ESR is "lazy")
6. Red flags: Progressive neurology, sepsis, cauda equina
7. Mortality: 2-11% (higher in elderly, comorbid, delayed diagnosis)
8. Relapse risk: 5-10% if inadequate treatment duration

For FRCR (Radiology)

1. MRI protocol: T1, T2, STIR, T1+Gad; whole spine
2. T2 "hot disc": Hyperintense fluid signal (pathognomonic)
3. T1 + Gad: Endplate enhancement, loss of intranuclear cleft
4. Kissing lesions: Symmetric involvement of adjacent vertebrae (infection crosses disc)
5. Disc sparing in malignancy vs. disc involvement in infection (key differentiator)
6. CT-guided biopsy: 50-70% yield; drops to 20-30% if on antibiotics
7. Epidural abscess: Rim-enhancing collection; measure AP diameter (> 4-5mm or circumferential → urgent decompression)
8. Follow-up MRI: Not required if clinically well; enhancement persists 6-12 months

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