Description

• Ankylosing spondylitis (AS) is an axial inflammatory spondyloarthropathy (axSpA) characterized by evidence of sacroiliitis (sclerosis, erosions, and changes in joint width) on plain radiography.
• System(s) affected: musculoskeletal; eyes; cardiac; neurologic; pulmonary
• Synonym(s): Marie–Strümpell disease; “bamboo spine”

Epidemiology

• Onset usually in early 20s; rarely occurs after age 40 years
• Male > female (approximately 2 to 3:1)

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

Prevalence
~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.
• Inflammation-independent pathways of bony changes have also been hypothesized (2).

• 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

• HLA-B27
  • 1–8% of HLA-B27–positive adults have AS.
• 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 and cardiac conduction defects