Ankylosing Spondylitis (AS)

Characteristics of Back Pain

Distinguishing inflammatory from mechanical back pain is critical:

1. Inflammatory Back Pain

   • Onset: Age <40, insidious onset >3 months

   • Improves with exercise, worsens with rest

   • Morning stiffness >45 minutes

   • Night pain (often in second half of night)

   • Alternating buttock pain may occur

   • Good symptomatic response to NSAIDs

2. Mechanical Back Pain

   • Any age, often acute onset

   • Worsens with activity, improves with rest

   • Stiffness short-lived, typically <30 minutes in the morning

   • Usually no major improvement with NSAIDs

Clinical Presentation

• Chronic back pain in individuals <45 years of age, lasting >3 months

• Morning stiffness that significantly improves with exercise or NSAIDs

• Extra-axial features:

  • Asymmetric oligoarthritis (predominantly lower limbs)

  • Enthesitis: Inflammation where tendons insert into bone (e.g. Achilles tendon, plantar fascia, chest wall)

  • Dactylitis (“sausage digit”)

Extra-Articular Features

• Eye: Acute anterior uveitis (~40%); presents with unilateral eye pain, redness, photophobia

• Lung: Apical pulmonary fibrosis, typically late stage (~15%)

• Heart: Aortic regurgitation (~10%); conduction abnormalities

• Bone: Osteopenia/osteoporosis → risk of vertebral fractures

• Bowel: Inflammatory bowel disease (~10%)

Pathogenesis

• Chronic inflammation of sacroiliac joints and the spine leads to erosion followed by bony fusion(syndesmophytes).

• HLA-B27 positivity: ~85–90% of AS patients carry the gene, but only ~5% of HLA-B27 carriers develop AS.

• Risk factors: Family history, male sex, HLA-B27 positivity, smoking (which worsens prognosis).

Diagnosis

1. Clinical Suspicion:

   • Age <45, insidious onset of chronic back pain lasting >3 months.

   • Inflammatory features (morning stiffness, improvement with exercise, etc.).

   • Extra-articular manifestations (e.g. uveitis, enthesitis).

2. Imaging:

   • X-ray of the pelvis (AP view) to assess for sacroiliitis.

     • Bilateral sacroiliitis (grade ≥2) or unilateral sacroiliitis (grade ≥3) is diagnostic.

     • Bamboo spine in advanced disease, where vertebral bodies show marginal syndesmophytes.

   • MRI of the sacroiliac joints: Can detect early inflammatory changes (bone marrow oedema) before X-ray changes appear.

3. Laboratory Tests:

   • ESR/CRP: Elevated in ~50–70% of cases (indicates ongoing inflammation).

   • HLA-B27: Supportive but not definitive; a negative test does not exclude AS.

Investigations

• X-ray: Pelvis (AP view) for sacroiliitis; spine X-ray if indicated.

• MRI: Sacroiliac joints if suspicion is high but X-ray is inconclusive (early disease).

• Bloods: ESR, CRP, HLA-B27.

• Additional tests if required (e.g. ECG or echocardiogram for cardiac involvement).

Treatment

Non-Pharmacological Measures

1. Exercise & Physiotherapy

   • Stretching and range-of-motion exercises reduce stiffness.

   • Hydrotherapy can be particularly beneficial.

   • Consider supervised physiotherapy programmes.

2. Smoking Cessation

   • Smoking exacerbates the disease and is associated with worse outcomes.

3. Patient Education

   • Understanding the chronic nature of AS, importance of adherence to exercise and medication.

Pharmacological Therapies

1. NSAIDs

   • First-line for symptom relief and possible disease-modifying effects.

   • Monitor for gastrointestinal, renal, and cardiovascular side effects.

2. Biologics

   • TNF inhibitors (e.g. etanercept, adalimumab) or IL-17 inhibitors (e.g. secukinumab) if insufficient response to NSAIDs.

   • Require rheumatology input; indications typically include high disease activity or progressive structural damage.

3. DMARDs

   • Generally limited role for axial disease, but can be used for peripheral arthritis.

   • Sulfasalazine, methotrexate for peripheral joint involvement.

Complications

• Spinal Ankylosis: Advanced bony fusion can cause severe stiffness and reduced chest expansion.

• Fractures: Osteopenia or osteoporosis can lead to vertebral fractures with minimal trauma.

• Neurological: Risk of cauda equina syndrome due to dural ectasia or spinal lesions.

• Cardiovascular: Aortic regurgitation, increased cardiovascular risk due to chronic inflammation.

Notes

• NSAIDs: Symptom-relieving and may slow radiographic progression in early disease.

• Early Diagnosis: Requires a high index of suspicion, especially in younger patients with unexplained back pain.

• Monitoring: Regular assessment of spinal mobility (e.g. Schober’s test), chest expansion, and posture.

• Referral: To rheumatology if diagnosis is uncertain, poor response to initial therapy, or consideration of biologic treatment.

When to Refer

• Diagnostic uncertainty or suspicion of other spondyloarthropathies.

• Poor response to NSAIDs or significant side effects limiting treatment.

• Extra-articular complications: Uveitis, cardiac involvement (valvular disease), or severe peripheral arthritis needing advanced therapies.

• Biologics: Require specialist initiation and monitoring.