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Spondylodiscitis

Spondylodiscitis is characterised by infection involving the intervertebral disc and adjacent vertebrae.

Epidemiology

Spondylodiscitis has a bimodal age distribution, which many authors consider these essentially separate entities:

  • paediatric
  • older population ~ 50 years

Clinical presentation

The typical presentation is usually fever and back pain. Patients are often bacteraemic with sources such as endocarditis and intravenous drug use. 

Pathology

In the paediatric age group infection often starts in the intervertebral disc itself (direct blood supply still present) whereas in adults infection is thought to begin at the vertebral body endplate, extending into the intervertebral disc space and then into the adjacent vertebral body endplate.

Risk factors
  • remote infection (present in ~25%)
  • spinal instrumentation
  • advanced age
  • intravenous drug use
  • immunosuppression
  • long-term systemic administration of steroids
  • diabetes mellitus
  • organ transplantation
  • malnutrition
  • cancer
Biochemical markers

Elevated serum CRP and/or ESR.

Aetiology
  • Staphylococcus aureus (most common)
  • Streptococus viridans (IVDU & immunocompromised)
  • Gram negative organisms
  • Mycobacterium tuberculosis (Pott's disease)
  • Less common agents
    • fungal
      • Cryptococcus neoformans,
      • candida species
      • Histoplasma capsulatum
      • Coccidioidesimmitis
    • Burkholderia pseudomallei (i.e., melioidosis) - diabetic patients from northern Australia and parts of South-east Asia
    • Brucellae spp
Location
  • can occur anywhere in the vertebral column but more commonly involves lumbar spine

Radiographic features

Plain film

Plain radiography is insensitive to the early changes of discitis/osteomyelitis, with normal appearances being maintained for up to 2-4 weeks. Thereafter disc space narrowing and irregularity or ill-definition of the vertebral endplates can be seen. In untreated cases, bony sclerosis may begin to appear in ~ 10-12 weeks.

CT

CT findings are similar to plain film, but more sensitive to earlier changes. Additionally surrounding soft tissue swelling, collections and even epidural abscesses may be evident.

MRI

MRI is the imaging modality of choice due to very high sensitivity and specificity. It is also useful in differentiating between pyogenic infection, tuberculous and fungal infections or a neoplastic process.

Signal characteristics include

  • T1
    • low signal in disc space (fluid)
    • low signal in adjacent endplates (bone marrow oedema)
  • T2 - (fat sat or STIR especially useful)
    • high signal in disc space (fluid)
    • high signal in adjacent endplates (bone marrow oedema)
    • loss of low signal cortex at endplates
    • high signal in paravertebral soft tissues
  • T1 C+ (Gd)
    • peripheral enhancement around fluid collection(s)
    • enhancement of vertebral endplates
    • enhancement of perivertebral soft tissues
    • enhancement around low density center indicates abscess formation (hard to distinguish inflammatory phlegmon from abscess without contrast)
  • DWI
    • ​hyperintense in acute stage
    • hypointense in chronic stage

DWI sequence can help to distinguish acute from chronic stage of the disease 7.

Nuclear medicine

A bone scan and white cell scan (WBC) may be used to demonstrate increased uptake at the site of infection, and are more sensitive than plain film and CT, however lack specificity. Not infrequently, WBC scan demonstrates cold spots, a nonspecific finding. The classic appearance on multiphase bone scans is increased blood flow and pool activity and associated increased uptake on the standard delayed static images 15. 67Ga-citrate has been used with some success, but is hampered by inferior dosimetric and imaging characteristics (high effective dose, long half-life time, poor spatial resolution) 14-15.

PET and PET/CT

18F-FDG PET has been demonstrated to possess high sensitivity in detecting spondylodiscitis, as such infectious spondylodiscitis can virtually be excluded in a negative scan. Dual imaging with PET/CT may thus become the imaging modality of choice, especially in patients with prior surgery and/or implants, where MR is contraindicated or hampered by artefact 8-11. Specificity is not as high 10,16, but monitoring of treatment results is possible 9.

Non-FDG PET/CT with 68Ga-citrate (an emerging, generator-based tracer) has shown promising results in pilot studies/small series 12-13.

Differential diagnosis

Possible imaging differential considerations include

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