Ankylosing Spondylitis

AS is a crucial diagnosis in young adults presenting with chronic back pain, often before age 45. The defining symptom is inflammatory back pain: worse in the morning/after rest, improving with exercise, and waking the patient in the second half of the night. Untreated, chronic inflammation leads to new bone formation (syndesmophytes) and eventual spinal fusion ('bamboo spine'). While predominantly axial, AS can cause asymmetrical peripheral arthritis, enthesitis (e.g., Achilles tendonitis), and significant extra-articular manifestations including acute anterior uveitis (most common), apical lung fibrosis, aortic regurgitation, and inflammatory bowel disease. Diagnosis relies on clinical features, imaging (MRI for early sacroiliitis, X-ray for later changes), and HLA-B27 testing. Management focuses on intensive physiotherapy and NSAIDs, with biologics (TNF-inhibitors, IL-17 inhibitors) for refractory disease.

AS is an inflammatory enthesitis, not primarily a synovitis. Inflammation occurs at tendon/ligament insertions into bone. The IL-17/IL-23 cytokine axis is central to its pathogenesis. HLA-B27's role is complex, potentially involving aberrant antigen presentation or misfolding leading to an endoplasmic reticulum stress response. Chronic inflammation at the sacroiliac joints and vertebral body corners (Romanus lesions) triggers osteoproliferation, forming syndesmophytes that bridge vertebrae, leading to ankylosis. This new bone formation is a key distinguishing feature from other inflammatory arthropathies.

Patients present with insidious onset, deep-seated lower back/buttock pain. Clinical examination reveals reduced spinal mobility (e.g., restricted lumbar flexion on Schober's test, reduced chest expansion). In advanced disease, patients develop a characteristic 'question mark' posture (loss of lumbar lordosis, increased thoracic kyphosis). Enthesitis (e.g., plantar fasciitis, Achilles tendonitis) and dactylitis ('sausage digits') are common. Acute anterior uveitis requires urgent ophthalmology referral. A critical complication is spinal fracture, even with minor trauma, due to the rigid, osteoporotic 'bamboo spine'. Always consider AS in young patients with chronic back pain, especially if it has inflammatory features.

- Bedside: Schober's test (normal >5cm increase in lumbar flexion), chest expansion (<2.5cm abnormal). These are crucial for OSCEs.
- Bloods: ESR/CRP (often elevated but can be normal), HLA-B27 (positive supports diagnosis but not diagnostic alone, as 5-10% of general population are positive).
- Imaging:
- Pelvic X-ray: Look for bilateral sacroiliitis (erosions, sclerosis, joint space narrowing/fusion). Grade 2 or more is diagnostic.
- Spinal X-ray: Look for squaring of vertebral bodies, syndesmophytes, and 'bamboo spine' (late feature).
- MRI Sacroiliac joints: Gold standard for early diagnosis, detecting active inflammation (bone marrow oedema) before X-ray changes.
- Monitoring: BASDAI (Bath Ankylosing Spondylitis Disease Activity Index) and ASDAS (AS Disease Activity Score) are used to assess disease activity and guide treatment decisions.

The cornerstone is non-pharmacological: daily, specific physiotherapy and exercise to maintain spinal mobility and posture. Pharmacological management:
- First-line: NSAIDs (e.g., Naproxen, Ibuprofen) taken regularly, not just on demand. Trial at least two different NSAIDs at maximum tolerated dose for adequate periods.
- If NSAIDs fail (persistent high disease activity, e.g., BASDAI ≥4): Biologics are indicated. These include TNF-alpha inhibitors (e.g., Adalimumab, Etanercept, Infliximab) or IL-17 inhibitors (e.g., Secukinumab, Ixekizumab).
- DMARDs (e.g., Methotrexate, Sulfasalazine) are generally ineffective for axial disease but may be used for significant peripheral arthritis.