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.
  • 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”

Epidemiology

• 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)

Incidence
Affects 0.1–0.5% of the population

Prevalence
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.

Genetics

• 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%)