Ankylosing Spondylitis

Detailed Overview

AS typically affects young men in their 20s and 30s. The disease is strongly associated with the HLA-B27 genotype, though environmental triggers like gut dysbiosis or biomechanical stress are believed to initiate the aberrant immune response. The inflammation localizes to the entheses (where tendons and ligaments insert into bone). Over time, the body's repair mechanisms replace inflamed tissue with bone (syndesmophytes), ultimately fusing the spine into a rigid 'bamboo spine'. Extra-articular manifestations, particularly anterior uveitis, are common and can cause severe morbidity.

Epidemiology & Demographics

Prevalence is roughly 0.1% to 0.5% in the general population. Males are affected 2-3 times more frequently than females. Peak age of symptom onset is 20-30 years; it is rarely diagnosed after age 45.

Etiological Mechanism

The exact etiology is autoimmune and multifactorial. The genetic hallmark is the HLA-B27 allele, present in 85-90% of affected patients. The 'arthritogenic peptide' and 'unfolded protein response' hypotheses suggest aberrant immune targeting of joint tissue.

Primary Causes

Triggered by a combination of HLA-B27 genetic predisposition and potential environmental factors such as mechanical stress at entheses or microbiome alterations (e.g., Klebsiella infections).

• HLA-B27 Positivity: The strongest genetic predictor, though only 5% of HLA-B27 positive individuals develop the disease.
• Family History: Having a first-degree relative with AS significantly increases lifetime risk.
• Male Sex: Men typically experience more severe radiographic progression compared to women.

Inflammation originates at the subchondral bone of the SI joints and the entheses of the spinal ligaments (enthesitis). Cytokines, heavily driven by TNF-alpha and IL-17, promote an erosive osteitis. Following erosion, aberrant osteoblast activation leads to heterotopic bone formation. Progressive ossification of the annulus fibrosus forms bridging syndesmophytes between adjacent vertebral bodies, resulting in complete spinal fusion.

Characteristic Clinical Presentation

• Inflammatory Back Pain: Insidious onset of dull, aching lower back pain lasting > 3 months. Uniquely worsens with rest and significantly improves with physical activity/exercise.
• Morning Stiffness: Profound stiffness upon waking that lasts longer than 30-60 minutes.
• Extra-articular Symptoms: Unilateral eye pain, photophobia, and redness indicative of acute anterior uveitis (occurs in ~25-30% of patients).

Physical Examination Signs

• Schober Test: Decreased forward flexion of the lumbar spine (< 5 cm increase between marks on flexion).
• Chest Expansion: Restricted chest expansion (< 2.5 cm) due to costovertebral joint involvement.
• FABER (Patrick's) Test: Provokes sacroiliac joint pain.

• Spinal Fractures: The fused spine is highly osteoporotic and rigid, prone to fractures (especially cervical) even from minor trauma.
• Restrictive Lung Disease: Costovertebral fusion limits chest wall excursion, reducing vital capacity.
• Aortic Regurgitation: Inflammation of the aortic root leads to dilation and valve insufficiency.

Diagnostic Criteria & Guidelines

ASAS criteria for inflammatory back pain: Onset before age 40, insidious onset, improvement with exercise, no improvement with rest, pain at night. Diagnosis requires radiographic evidence of sacroiliitis PLUS at least one clinical feature of SpA, OR HLA-B27 positivity with at least two clinical features.

Differential Diagnosis

• Mechanical Back Pain (e.g., herniated disc) - worsens with activity, improves with rest.
• Diffuse Idiopathic Skeletal Hyperostosis (DISH) - occurs in older adults, spares SI joints.
• Psoriatic Arthritis (axial subset)
• Reactive Arthritis

Laboratory Tests & Biomarkers

• ESR and CRP: Elevated in 50-70% of patients with active disease, though normal levels do not rule out AS.
• HLA-B27: Positive in 85-90% of cases. Helpful for diagnosis but not definitive.
• Rheumatoid Factor / Anti-CCP: Negative (hence 'seronegative' spondyloarthropathy).

Imaging Modalities & Findings

• Pelvis X-Ray:
• Spine X-Ray:
• MRI Pelvis (STIR sequence):

• Non-radiographic Axial SpA
  Symptomatic inflammatory back pain without visible structural damage on conventional X-rays (requires MRI for diagnosis).
• Early AS
  Definite sacroiliitis on X-ray (e.g., bilateral grade 2 or unilateral grade 3/4) with mild spinal restriction.
• Advanced AS
  Extensive syndesmophyte formation leading to 'bamboo spine' and complete loss of spinal mobility.

Second-Line & Adjunctive Therapy

For persistently high disease activity despite maximum NSAID therapy, biologic DMARDs are indicated. TNF inhibitors (Adalimumab 40 mg SQ every 2 weeks, Etanercept 50 mg SQ weekly) or IL-17 inhibitors (Secukinumab 150 mg SQ every 4 weeks). Conventional synthetic DMARDs (Methotrexate, Sulfasalazine) are ineffective for axial disease but may be used for peripheral joint involvement.

Surgical & Procedural Management

Total Hip Arthroplasty (THA) for severe, debilitating hip joint arthritis. Spinal wedge osteotomy is rarely performed to correct severe fixed kyphosis allowing the patient to look forward.

Recommended Lifestyle Changes

• Daily stretching and spinal extension exercises to prevent kyphotic deformity.
• Strict smoking cessation, as smoking accelerates radiographic progression and functional decline.
• Firm mattress and sleeping without thick pillows to maintain cervical extension.

Patient Counseling & Advice

Educate the patient on the importance of maintaining an upright posture. Explain that any minor trauma (e.g., falling, minor car accident) requires immediate medical evaluation to rule out cervical spine fractures.

Follow-Up & Monitoring Schedule

Assess disease activity every 3-6 months using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) and ASDAS. Annual spinal X-rays to monitor structural progression. DEXA scans to monitor for secondary osteoporosis.

Preventive Strategies

No prevention for disease onset. Prevention of deformity relies entirely on early initiation of TNF/IL-17 inhibitors and lifelong adherence to physical therapy.

Lifespan is generally normal, though morbidity can be high. Early treatment with biologics halts progression in many. Poor prognostic indicators include onset < 16 years, persistent systemic inflammation, and early hip involvement.