Ankylosing Spondylitis

Ankylosing spondylitis is an autoimmune seronegative spondyloarthopathy that affects the axial skeleton and joints (shoulder and hip). It is HLA-B27 positive.

It is more predominant in young men (under the age of 40), with a male-to-female ratio of 4:1.

• Risk factors
  • HLA-B27
  • Infection – particularly with Klebsiella spp – due to molecular mimicry
  • Dysbiosis of gut microbiota – modulate the immune response
  • Smoking – induces autoimmunity by promoting inflammation and causing oxidative damage
• Pathophysiology
  • Presentation of self peptides by HLA-B27 → immune response
  • Molecular mimicry where HLA-B27 misfolds, forming a structure that the immune system recognises as foreign → immune response
  • Inflammation of the entheses due to autoreactivity → enthesitis
  • Dysregulated repair → pathological bone formation via endochondral and intramembranous ossification → mineralisation of cartilage → fusion of affected joints
• Signs and symptoms
  • Lower back pain
    • Persistent and chronic
    • Worse in the morning
    • Improves throughout the day (like most autoimmune diseases)
  • Reduced lateral flexion
  • Reduced forward flexion
  • Reduced chest expansion
  • Loss of lumbar lordosis
    • Represents fusion of lumbar spinal processes, making the patient susceptible to spinal fractures
• Physical examination
  • Schober’s test
    • A line is drawn 10 cm above and 5 cm below the dimples of Venus
    • The distance between the two lines should increase by > 5 cm when the patient bends as far forward as possible
• Differentials
  • Mechanical back pain
  • Rheumatoid arthritis
  • Psoriatic arthritis
• Investigations
  • Lumbar spinal X-ray: best initial diagnostic test
    • Sacroiliitis (subchondral erosions and sclerosis)
    • Square lumbar vertebrae
    • Syndesmophytes
    • Bamboo spine – fusion of lumbar spinal vertebral processes – is a late and uncommon finding
  • MRI to detect active inflammation
    • Bone marrow oedema
  • ESR and CRP
    • Raised
  • HLA-B27
    • Positive in 90% of patients with ankylosing spondylitis and 10% of normal patients
  • Chest X-ray
    • Apical fibrosis
• Treatment
  • Exercise and physiotherapy
  • NSAIDs (first-line)
  • DMARDs if there is peripheral joint involvement
    • TNF blockers (infliximab, adalimumab, etanercept, golimumab) are second-line
  • Local steroid injections for temporary relief
  • Hip replacement surgery to improve mobility if the hips are involved
  • Spinal osteotomy (rare)
  • Bisphosphonates may be considered due to the risk of osteoporotic spinal fractures
• Complications
  • Depression and anxiety
  • Cardiovascular
    • Aortic regurgitation
    • Aortitis
    • AV node block
    • Ischemic heart disease
  • Pulmonary
    • Apical lung fibrosis
  • Ocular
    • Anterior uveitis
    • Iridocyclitis
  • Musculoskeletal
    • Achilles tendonitis
    • Reduced spinal mobility and fragility fractures
    • Peripheral arthritis
  • Neurologic
    • Atlantoaxial subluxation
    • Cauda equina syndrome
  • Gastrointestinal
    • Inflammatory bowel disease
  • Amylodosis