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.
  • Systems affected: musculoskeletal; ophthalmic; cardiovascular; neurologic; pulmonary
  • Synonyms: Marie–Strümpell disease; “bamboo spine”


  • Peak age of onset 20 to 30; rarely occurs after age 40 years
  • Male > female (approximately 2 to 3:1)

Age- and gender-adjusted rate of 6.3 to 7.3/100,000 person-years

~0.55% for AS and ~1.4% for all axSpA in the United States (1)

Etiology and Pathophysiology

  • Autoinflammation at sites of bacterial exposure (e.g., intestines) or mechanical stress in genetically susceptible individuals (2)
  • 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.
  • Other genetic associations include endoplasmic reticulum aminopeptidase 1 (ERAP1), interleukin-23 receptor (IL23R), and gene deserts on chromosome 2p15 and 21q22 (2).

Risk Factors

Positive family history

  • HLA-B27–positive child of a parent with AS has a 10–30% risk of developing the disease.

Commonly Associated Conditions

  • Peripheral arthritis (30%)
  • Enthesopathy (29%): Achilles tendonitis, plantar fasciitis
  • Uveitis (25–35%)
  • Psoriasis (10%)
  • Dactylitis “sausage digit” (6%)
  • Inflammatory bowel disease “IBD” (4%) (3)[A]
  • Peripheral spondyloarthritis (SpA): psoriatic arthritis, reactive arthritis, IBD-related arthritis, juvenile idiopathic arthritis
  • Aortitis, aortic regurgitation, and cardiac conduction defects

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