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Ankylosing Spondylitis

Summary

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  • Chronic inflammatory arthritis primarily affecting the axial skeleton
  • Characterised by sacroiliitis, spinal fusion, and enthesitis
  • Imaging shows bamboo spine appearance and syndesmophytes on radiographs1

Pathophysiology

  • Autoimmune disorder with strong genetic association (HLA-B27)
  • Chronic inflammation of the spine and sacroiliac joints
  • Progressive ossification of ligaments and joint capsules
  • Enthesitis at sites of ligament and tendon attachment

Demographics

  • Typically affects young adults (20-40 years old)
  • Male predominance (2-3:1 male to female ratio)
  • Prevalence: 0.1-1.4% of general population
  • Higher prevalence in HLA-B27 positive individuals

Diagnosis

  • Clinical criteria: chronic back pain, morning stiffness, limited spinal mobility
  • Laboratory findings: elevated ESR, CRP, and HLA-B27 positivity
  • Imaging findings: sacroiliitis, syndesmophytes, bamboo spine
  • ASAS (Assessment of SpondyloArthritis international Society) criteria

Imaging

Radiographs

  • Sacroiliitis: erosions, sclerosis, and joint space narrowing
  • Syndesmophytes: bony outgrowths bridging vertebral bodies
  • Bamboo spine: complete fusion of vertebral bodies
  • Squaring of vertebral bodies
  • Enthesitis: whiskering at sites of ligament attachment

MRI

  • Early detection of active inflammation
  • Bone marrow oedema in sacroiliac joints and vertebral corners
  • Fat metaplasia in chronic stages
  • Erosions and sclerosis in advanced disease

CT

  • Best for syndesmophytes, ankylosis and fractures

Neuroradiological complications

  • The rigid, ankylosed ("bamboo") spine behaves as a long bone: even minor trauma causes highly unstable, often transdiscal three-column fractures (carrot-stick fractures), typically in the lower cervical spine, with a high risk of epidural haematoma and cord injury — CT with a low threshold for MRI is mandatory
  • Andersson lesion: a mobile pseudarthrosis through a fracture or non-fused segment
  • Cauda equina syndrome with dorsal dural ectasia and arachnoid diverticula scalloping the posterior elements is a late complication

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Treatment

  • NSAIDs first-line; TNF or IL-17 inhibitors for active disease
  • Surgical stabilisation for unstable fractures or progressive deformity

Differential diagnosis

Differential Diagnosis Differentiating Feature
Diffuse idiopathic skeletal hyperostosis (DISH) Affects older population; no sacroiliitis; flowing ossification of anterior longitudinal ligament
Psoriatic arthritis Presence of psoriasis; asymmetric sacroiliitis; more peripheral joint involvement
Reactive arthritis Asymmetric sacroiliitis; peripheral asymmetric arthritis; no syndesmophytes
Enteropathic arthritis May have asymmetric sacroiliitis without bamboo spine; sacroiliac joint asymmetry
Scheuermann's disease Thoracic kyphosis with anterior vertebral body wedging; no sacroiliitis; no syndesmophytes

  1. Braun et al. Ankylosing spondylitis. 2007. Lancet (London, England) - Open in new tab. ↩