Pyogenic Discitis & Osteomyelitis

Pathophysiology In adults, the intervertebral disc is avascular. Bacteria lodge in the end-arterioles of the vertebral metaphysis (endplate). The infection sets up an osteomyelitis, then breaks through the endplate into the disc, rapidly destroying it (proteolytic enzymes). From the disc, it spreads to the adjacent vertebra.

Spread

• Hematogenous (Arterial): Nutrient arteries.
• Hematogenous (Venous): Batson's Plexus (valveless venous system connecting pelvis/bladder to spine). Explains UTI link.

Epidemiology

• Bimodal distribution (under 20yo and over 50yo).
• Lumbar is more common than thoracic, which is more common than cervical.
• Note: TB spine prefers the thoracic region.

The Intervertebral Disc

• Adult: Avascular. Nutrition via diffusion from endplates.
• Child: Vascular channels persist (explains direct discitis in children).

The Endplate

• The "Metaphysis" of the spine.
• Rich vascular loop system (slow flow) → Predisposes to bacterial lodging.

Venous Anatomy (Batson's Plexus)

• A valveless system of vertebral veins.
• Connections: It communicates freely with the pelvic, abdominal, and thoracic venous systems.
• Mechanism: Increases in intra-abdominal or intra-thoracic pressure (e.g., coughing, straining, lifting) can reverse blood flow.
• Clinical Relevance: This retrograde flow allows bacteria from the pelvis (UTI, Prostatitis) or abdomen to bypass the liver/lung filters and lodge directly in the spine.
• This explains the strong association between Urinary Tract Infections and Vertebral Osteomyelitis.

Biomechanics

• Infection weakens the anterior column (body/disc).
• Leads to Kyphosis (Gibbus deformity).
• If greater than 50% body destruction → Mechanical Instability.

The pathophysiology above notes that vascular channels persist across the child's endplate ("explains direct discitis in children") and the epidemiology is bimodal with a peak under 20 years — but paediatric discitis behaves so differently from adult vertebral osteomyelitis that it deserves separate treatment.

Why it differs

• In young children the disc still has a direct vascular supply, so infection (or a low-grade inflammatory process) can settle in the disc itself rather than starting in the endplate as it does in adults. It is usually a single lumbar level (commonly L3-4 or L4-5), low-virulence, and frequently culture- and even blood-culture-negative.

How it presents

• The classic picture is a young child (often 1 to 5 years) who refuses to walk, sit or bear weight, or who is irritable with back/abdominal/hip pain, rather than the toxic septic adult. Fever and inflammatory markers are often only mildly raised; loss of normal lumbar lordosis and reluctance to flex the spine are typical.

How it is managed (much less aggressively)

• It is largely a clinical/MRI diagnosis; routine biopsy is not required in the typical case. Management is usually immobilisation/rest and a course of anti-staphylococcal antibiotics, with the threshold for biopsy reserved for atypical, non-responding or destructive disease.
• The prognosis is good — most children recover fully, in contrast to the high residual-morbidity, deformity and mortality figures quoted for adult pyogenic vertebral osteomyelitis.

History

• Pain: Constant, non-mechanical (night pain), progressively worsening.
• Constitutional: Fever, rigors, weight loss (often absent).
• Neurology: Weakness/Numbness (Epidural abscess).

Examination

• Tenderness: Percussion tenderness is highly sensitive.
• Spasm: Paravertebral muscle spasm ("Board-like").
• Neurology: Detailed myotomal/dermatomal exam.

MRI is the gold standard here, and the post-op follow-up note already warns that "post-op changes mimic infection" — so the commonest false positive deserves to be explicit: degenerative Modic type 1 endplate change produces endplate marrow oedema that looks like early infection (T1 dark, T2/STIR bright), and distinguishing the two is a classic examiner trap.

The single most useful discriminator is the disc itself: a T2-bright disc with endplate destruction and paradiscal/epidural enhancement points to infection, whereas a T2-dark degenerate disc with intact endplates and no soft-tissue enhancement is Modic type 1 degeneration.

Antimicrobial Therapy

• Empiric: Vancomycin + Ceftriaxone (coverage for MRSA and Gram Negatives).
• Targeted: Narrow spectrum based on sensitivities.
• Duration: Traditionally 6 weeks IV. Modern evidence (OVIVA Trial) suggests Oral is non-inferior if bioavailable.

Bracing

• TLSO: Often used for pain control and to prevent deformity (kyphosis).
• Worn for 6-12 weeks while bone heals.

Antibiotics

• Continue for 6 weeks minimum.
• Stop when CRP normalizes and radiographic healing seen.

Follow-up

• Weekly CRP.
• X-rays at 6 weeks/3 months to check fusion/alignment.
• MRI only if symptoms worsen (post-op changes mimic infection).

Mortality

• High (2-20%) depending on comorbidities (Age, Diabetes).
• Higher in MRSA infections.

Recurrence

• 5-10%.
• Risk factors: Inadequate duration of Abx, Undrained abscess, Retained hardware (if loose).

Functional Outcome

• Majority (greater than 70%) have significant residual back pain.
• Functional impairment is worse than standard degenerative spine surgery.

Long-term Sequelae

• Chronic Pain: Due to facet joint destruction and altered biomechanics (kyphosis).
• Instability: Degenerative spondylolisthesis may develop years later above or below the fused level (Adjacent Segment Disease).
• Quality of Life: Studies (e.g., Carragee et al.) show scores similar to chronic heart failure.

Prognostic Factors (Poor Outcome)

• Age greater than 60.
• Concurrent Endocarditis.
• Disseminated S. aureus infection.
• Delay in diagnosis (greater than 3 months).
• Presence of neurological deficit at presentation (often permanent).

Global epidemiology

• Incidence is rising worldwide (ageing populations, IV drug use, more spinal instrumentation and intravascular devices). Reported population incidence is roughly 1 to 7 per 100,000 person-years in high-income settings.
• Strong male predominance and a peak in patients over 50 years. Staphylococcus aureus is the dominant pathogen globally; Gram-negative organisms cluster with urinary sources and IV drug use.
• In TB-endemic regions (much of South Asia, sub-Saharan Africa), Mycobacterium tuberculosis (Pott disease) is a major differential and may outnumber pyogenic cases; Brucella spondylitis is important around the Mediterranean, Middle East and Central Asia.

Registry and practice variation

• No dedicated international spondylodiscitis registry exists; arthroplasty/implant registries (NJR, AJRR, AOANJRR, SHAR) inform biofilm and revision principles that carry over to instrumented spinal infection.
• High-resource settings: routine MRI, CT-guided biopsy, prolonged targeted therapy, single-stage instrumented debridement, OPAT or early oral switch.
• Limited-resource settings: empirical therapy is often started before biopsy due to limited imaging/microbiology; high TB prevalence lowers the threshold for empirical anti-tuberculous treatment; surgical capacity for instrumented reconstruction may be constrained.

• Empirical antibiotics before biopsy: Guidelines advise holding antibiotics in stable patients to maximise culture yield, yet real-world practice often starts therapy early. The optimal antibiotic-free window before biopsy (commonly quoted as 48 to 72 hours) is not firmly evidence-based.
• Total duration: The Bernard RCT supports 6 weeks for uncomplicated disease, but undrained abscess, implant-associated infection, endocarditis or persistently raised CRP frequently prompt longer courses; the right duration for these subgroups is unresolved.
• Oral versus IV: OVIVA supports early oral switch, but uptake varies and selection of a reliably bioavailable oral agent (and adherence) remains the practical limitation.
• Single-stage instrumentation in active infection: Increasingly accepted with radical debridement, but the threshold for instrumenting, choice of anterior versus posterior approach, and use of titanium versus PEEK cages are debated.
• Role of nuclear imaging: FDG-PET/CT is useful when MRI is contraindicated or equivocal and for monitoring, but is not universally available and its role in defining cure is not standardised.