Ankylosing Spondylitis

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Description

  • 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.
    • Assessment of Spondyloarthritis International Society (ASAS) published within an updated consensus that axial spondyloarthritis and AS are equivalent and can be used interchangeably, therefore referred to now as axSpA (1).
  • Systems affected: musculoskeletal (MSK); ophthalmic; cardiovascular; neurologic; pulmonary; gastrointestinal (GI)
  • Synonyms: Marie-Strümpell disease; “bamboo spine”; Bekhterev disease

Epidemiology

  • Peak onset typically during 3rd decade of life; rarely occurs after age 45 years
  • Male > female (approximately 2 to 3:1) for axSpA
    • Studies vary with effect of gender on axSpA.
    • Peripheral disease
      • Higher prevalence of anterior uveitis in men versus higher prevalence of peripheral arthritis and psoriasis in women

Incidence

  • Approximately 7 per 100,000 affected
  • Higher incidence in Europe and United States given presence of HLA-B27 gene; more studies are needed for this to assess incidence of axSpA in other areas.

Prevalence

Prevalence of axSpA among adults varies from 0.3 to 1.4% although highly influenced by presence of HLA-B27.

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.

Genetics

  • Majority of patients with axSpA are HLA-B27 positive.
  • Other genetic associations include endoplasmic reticulum aminopeptidase 1 (ERAP1), 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 35–60%)
  • Uveitis (prevalence 6–30%)
  • Psoriasis (prevalence 10%)
  • Dactylitis “sausage digit”
  • Inflammatory bowel disease (IBD) (prevalence of 4–6%)

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