Ankylosing Spondylitis



  • Ankylosing spondylitis (AS) is an axial inflammatory spondyloarthropathy (axSpA) characterized by chronic low back pain (>3 months duration) and evidence of sacroiliitis (sclerosis, erosions, and changes in joint width) on plain radiography.
    • Terminology of “axSpA” comprises whole spectrum of patients with radiographic evidence of sacroiliitis (AS) or nonradiographic axSpA (nr-axSpA)
    • Both AS and nr-axSpA are largely similar in disease presentation and treatment received.
  • Systems affected: musculoskeletal; ophthalmic; cardiovascular; neurologic; pulmonary; gastrointestinal
  • Synonyms: Marie–Strümpell disease; “bamboo spine”


  • Peak onset typically before age of 30 years; rarely occurs after age 45 years
  • Male > female (approximately 2:1) for AS
    • Studies vary with effect of gender on AS.
    • Peripheral disease
      • Higher prevalence of anterior uveitis in men versus higher prevalence of peripheral arthritis and psoriasis in women
    • Men with family history of AS at higher risk
  • Approximately 1 in 200 people affected by AS and >1 in 100 by axSpA (axial spondyloarthritis)

Affects 0.1–0.5% of the population

Prevalence of axSpA among adults in the USA varies from 0.9% to 1.4%.

Etiology and Pathophysiology

  • Autoinflammation at sites of bacterial exposure (e.g., intestines) or mechanical stress in genetically susceptible individuals
  • Inflammation at the insertion of tendons, ligaments, and fasciae to bone (enthesopathy) causes erosion, remodeling, and new bone formation.


  • 85–95% of patients with AS are HLA-B27positive (1)[C].
  • Other genetic associations include endoplasmic reticulum aminopeptidase 1 (ERAP1) and interleukin-23 receptor (IL23R).

Risk Factors

  • Positive family history
    • Concordance rates of 63% for HLA-B27–positive monozygotic twins and 27% for dizygotic twins
  • Gut microbiome theory
    • 57–70% of patients with AS had asymptomatic intestinal inflammation in terminal ileum.
  • Current smoking (not history of smoking)

Commonly Associated Conditions

  • Peripheral arthritis
  • Enthesopathy: Achilles tendonitis, plantar fasciitis (prevalence of 35–60%)
  • Uveitis (prevalence of 6–30%)
  • Psoriasis(prevalence of 10%)
  • Dactylitis “sausage digit”
  • Inflammatory bowel disease (IBD) (prevalence of 4–6%)

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