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Ankylosing Spondylitis (AS)

Basics

  • Axial inflammatory spondyloarthropathy with chronic low back pain >3 months.
  • Sacroiliitis evident on radiography.
  • Encompasses radiographic AS and non-radiographic axial spondyloarthritis (nr-axSpA).
  • Affects musculoskeletal, ophthalmic, cardiovascular, neurologic, pulmonary, and gastrointestinal systems.
  • Also known as Marie-Strümpell disease or “bamboo spine.”

Epidemiology

  • Onset usually before age 30; rare after 45.
  • Male predominance (~2:1).
  • Prevalence: 0.1-0.5% for AS; 0.9-1.4% for axSpA in US adults.
  • Peripheral disease shows gender differences in symptoms.

Etiology & Pathophysiology

  • Autoinflammation at bacterial exposure sites or mechanical stress in genetically predisposed.
  • Enthesitis causes erosion, remodeling, and new bone formation.
  • Genetics: 85-95% HLA-B27 positive; also ERAP1 and IL23R gene associations.
  • Gut microbiome may contribute; 57-70% show asymptomatic intestinal inflammation.
  • Smoking increases risk.

Risk Factors

  • Positive family history.
  • HLA-B27 positive monozygotic twins have 63% concordance.
  • Active smoking.
  • Possible gut microbiome dysbiosis.

Common Associated Conditions

  • Peripheral arthritis, enthesopathy (Achilles tendonitis, plantar fasciitis).
  • Uveitis (6-30%), psoriasis (10%), dactylitis.
  • Inflammatory bowel disease (4-6%).

Diagnosis

History

  • Inflammatory back pain: insidious onset, <45 years, >3 months duration.
  • Pain worse at rest, improves with activity, night pain causing awakening.
  • Alternating buttock/hip pain, fatigue, low-grade fever.
  • Enthesitis symptoms: tendon and fascia pain; dactylitis; iritis.

Physical Exam

  • SI joint tenderness, lumbar lordosis loss, limited spinal motion.
  • Tenderness at tendon insertion sites.
  • Extra-articular: uveitis, psoriasis, IBD.

Differential Diagnosis

  • Mechanical back pain.
  • Other inflammatory arthritis.
  • Osteoarthritis, discitis, infection.
  • Vertebral fractures.
  • Fibromyalgia.

Diagnostic Tests & Interpretation

  • ESR and CRP may be mildly elevated or normal.
  • HLA-B27 testing supportive.
  • Imaging:
  • Pelvic x-ray AP view for sacroiliitis.
  • MRI if radiographs negative or equivocal.
  • Spine lateral view to rule out other causes.
  • Diagnosis per 2009 ASAS criteria or 1984 modified New York criteria.

Treatment

General Measures

  • Symptom control, maintain flexibility, posture, work ability.
  • Aggressive physical therapy recommended.

Medications

  • First-line: NSAIDs (naproxen, celecoxib, ibuprofen).
  • Caution with cardiovascular, GI, renal risks.
  • Intra-articular corticosteroids for peripheral symptoms; systemic steroids not recommended.
  • Second-line: Biologics
  • TNF inhibitors: infliximab, etanercept, adalimumab, certolizumab, golimumab.
  • IL-17 inhibitors: secukinumab, ixekizumab.
  • JAK inhibitors: tofacitinib.
  • DMARDs generally ineffective for axial disease; may help peripheral arthritis.

Precautions

  • Screen for TB, hepatitis B before biologics.
  • Update immunizations prior to biologics.
  • Avoid live vaccines once on biologics.

Issues for Referral

  • Rheumatology for diagnosis and biologic management.
  • Specialty referral for extra-articular manifestations.

Surgery and Procedures

  • Assess cervical spine stability before intubation.
  • Hip replacement for refractory pain/disability.
  • Spinal osteotomy for severe deformity.

Ongoing Care

  • Monitor posture, range of motion every 6-12 months.
  • Disease activity scores (BASDAI, ASDAS).
  • Regular CRP/ESR monitoring.
  • Screen for osteoporosis via DEXA.

Patient Education

  • Maintain activity and posture.
  • Recommended exercises: swimming, water aerobics, tai chi, walking.
  • Arthritis Foundation and Spondylitis Association of America resources.

Prognosis

  • Variable progression.
  • Early diagnosis and treatment improve outcomes.

Complications

  • MSK: osteoporosis, spinal fusion with kyphosis, fractures, cauda equina syndrome (rare).
  • Pulmonary: restrictive lung disease, upper lobe fibrosis (rare).
  • Cardiac: conduction defects, aortic insufficiency, pericarditis (rare).
  • Eye: uveitis, cataracts.
  • Renal: IgA nephropathy, amyloidosis (<1%).
  • GI: microscopic ileal and colonic ulcerations (mostly asymptomatic).