What is axial spondyloarthritis?

Axial Spondyloarthritis (axSpA)

Axial spondyloarthritis (axSpA) is a chronic inflammatory disorder primarily affecting the sacroiliac joints and spine, characterized by inflammatory back pain, morning stiffness, and potential extra-musculoskeletal manifestations that significantly impairs quality of life and physical function if not diagnosed and treated promptly.

Definition and Classification

Axial spondyloarthritis comprises a spectrum of inflammatory conditions affecting the axial skeleton that were historically classified separately, including:

• Ankylosing spondylitis (radiographic axSpA or r-axSpA)
• Non-radiographic axSpA (nr-axSpA)
• Psoriatic spondyloarthritis
• Reactive arthritis
• Enteropathic spondyloarthritis
• Juvenile spondyloarthritis
• Undifferentiated spondyloarthritis 1

The prevalence of axSpA is estimated to be between 0.9% to 1.4% in the United States adult population 1, with global prevalence varying from 0.14% in Latin America to 0.25% in Europe 1.

Clinical Presentation

Key Clinical Features

• Inflammatory back pain (present in 70-80% of patients) characterized by:

  • Insidious onset before age 45
  • Symptoms lasting >3 months
  • Morning stiffness >30 minutes
  • Pain at night/early morning
  • Improvement with exercise but not with rest
  • Alternating buttock pain 1, 2

• Musculoskeletal manifestations:

  • Limitation of lumbar spine motion in sagittal and frontal planes
  • Limitation of chest expansion
  • Peripheral arthritis (commonly in large joints in an oligoarticular, asymmetric pattern)
  • Enthesitis (inflammation at insertion of tendons/ligaments into bone)
  • Dactylitis 2, 3

• Extra-musculoskeletal manifestations:

  • Uveitis (inflammatory eye disease)
  • Psoriasis
  • Inflammatory bowel disease 1, 3

Peripheral manifestations such as arthritis and enthesitis have been reported more frequently in Latin America than in Europe or the USA 1.

Pathogenesis

The pathogenesis involves genetic predisposition (strong association with HLA-B27, positive in 74-89% of patients), gut microbial dysbiosis, and entheseal trauma, leading to immune cell infiltration of the sacroiliac joints and entheseal insertion areas in the spine 1, 3.

Diagnostic Approach

Laboratory Testing

• HLA-B27 testing (sensitivity 90-95% in ankylosing spondylitis patients) 2
• Inflammatory markers (ESR/CRP) have limited sensitivity (only 50%) and normal values do not rule out axSpA 2, 3

Imaging

1. Radiography of sacroiliac joints - first-line imaging modality, looking for:

   • Erosions
   • Sclerosis
   • Joint space narrowing
   • Ankylosis 2

2. MRI of sacroiliac joints - can detect early inflammatory changes:

   • Should include T1 sequences and fat-suppressed fluid-sensitive sequences
   • Bone marrow edema is the hallmark of active sacroiliitis 2
   • Higher sensitivity (78%) and specificity (88%) than radiography 3

3. CT of sacroiliac joints - helpful when radiographs are equivocal 2

4. Spine imaging - to assess for syndesmophytes and other spinal changes in established disease 2

Diagnostic Criteria

The American College of Rheumatology recommends:

• For established ankylosing spondylitis: Radiological criterion (sacroiliitis grade ≥2 bilaterally or grade ≥3 unilaterally) plus at least one clinical criterion 2

• For axSpA in patients with back pain ≥3 months and age of onset <45 years:

  • Either sacroiliitis on imaging (MRI or radiographs) plus ≥1 SpA feature, or
  • HLA-B27 positive plus ≥2 other SpA features 2

Disease Activity Assessment

• ASDAS-CRP (Ankylosing Spondylitis Disease Activity Score) - recommended for measuring disease activity 2
• BASDAI (Bath Ankylosing Spondylitis Disease Activity Index) - measures fatigue, pain, stiffness, and discomfort 2
• BASFI (Bath Ankylosing Spondylitis Functional Index) - measures functional ability 2

Impact on Quality of Life

Patients with axSpA experience substantial deterioration in health-related quality of life due to:

• Impaired physical function
• Decreased work productivity
• Limited social interactions
• Increased risk of depression and anxiety 1, 3

Common Pitfalls in Diagnosis

• Delayed diagnosis - average delay from symptom onset to diagnosis is 7-10 years 2, 4
• Over-reliance on radiographs - can miss early disease 2
• Ignoring inflammatory back pain patterns 2
• Misinterpreting normal inflammatory markers - normal ESR/CRP doesn't rule out axSpA 2
• Referral to inappropriate specialists - only 37% of patients with ankylosing spondylitis in the USA are diagnosed by rheumatologists 4

Referral Recommendations

Patients with chronic low back pain should be referred to a rheumatologist when they have:

• Inflammatory back pain characteristics
• HLA-B27 positivity
• Evidence of sacroiliitis on imaging 2, 4

To reduce diagnostic delay, non-rheumatologist healthcare professionals should refer patients with back pain and ≥1 of 3 SpA features (HLA-B27 positivity, current inflammatory back pain, or x-ray/MRI evidence of sacroiliitis) to a rheumatologist 4.

Early diagnosis and appropriate treatment are crucial for reducing disease severity, preventing structural damage, and improving quality of life in patients with axial spondyloarthritis 3, 5.