Ankylosing Spondylitis: Definition and Clinical Features
Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease characterized by sacroiliitis, enthesitis, and a marked propensity for sacroiliac joint and spinal fusion, with universal involvement of sacroiliac joint inflammation or fusion and more prevalent spinal ankylosis. 1
Key Characteristics
- Classification: AS is a form of axial spondyloarthritis (SpA), belonging to the spondyloarthritis family of diseases that share clinical, genetic, and immunologic features 1
- Prevalence: Affects 0.1-0.5% of the population 1
- Genetic Association: High prevalence of HLA-B27 (positive in 74-89% of patients) 2
- Primary Sites of Inflammation:
- Sacroiliac joints (universal involvement)
- Spine (vertebral column)
- Entheses (sites where tendons or ligaments attach to bone)
Clinical Manifestations
Cardinal Symptoms
- Inflammatory Back Pain (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 2
Disease Progression
- Early Disease: Inflammation of sacroiliac joints and spine
- Advanced Disease: Progressive spinal stiffness, fusion, and deformity 3
- End-Stage: Can lead to severe kyphosis, resulting in debilitating functional impairment 3
Extra-articular Manifestations
- Uveitis (eye inflammation)
- Psoriasis
- Inflammatory bowel disease 2
Diagnostic Features
Radiographic Findings
- Sacroiliitis: Erosions, sclerosis, joint space narrowing, and ankylosis 2
- Spinal Changes: Syndesmophytes (bony growths), vertebral fusion, "bamboo spine" appearance
Laboratory Findings
- HLA-B27: Positive in 74-89% of AS patients 2
- Inflammatory Markers: Elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) in active disease
Imaging
- Radiography: First-line imaging for sacroiliac joints 2
- MRI: Can detect early inflammatory changes before radiographic damage
- Bone marrow edema is the hallmark of active sacroiliitis 2
Disease Classification
- Radiographic axial SpA (AS): Meets modified New York criteria with definite radiographic sacroiliitis
- Non-radiographic axial SpA: Chronic back pain and features suggestive of SpA but without radiographic sacroiliitis 1
Complications
- Structural Damage: Spinal fusion and ankylosis
- Fractures: Increased susceptibility to vertebral column fractures due to weakened spine 4
- Spinal Cord Injury: Higher risk due to unstable nature of fractures in the ankylosed spine 4
- Functional Limitations: Reduced mobility, difficulty with daily activities, and decreased quality of life 3
Disease Burden
- Diagnostic Delay: Average 7-10 years from symptom onset to diagnosis 2
- Work Impact: Can interfere with work and schooling 1
- Quality of Life: Significant reduction in quality of life, especially in advanced cases 3
Understanding the defining features of AS is crucial for early diagnosis and appropriate management to prevent disease progression and maintain quality of life for affected individuals.