Ankylosing Spondylitis
Summary
- Chronic inflammatory arthritis primarily affecting the axial skeleton
- Characterised by sacroiliitis, spinal fusion, and enthesitis
- Imaging shows bamboo spine appearance and syndesmophytes on radiographs
Pathophysiology
- Autoimmune disorder with strong genetic association (HLA-B27)
- Chronic inflammation of the spine and sacroiliac joints
- Progressive ossification of ligaments and joint capsules
- Enthesitis at sites of ligament and tendon attachment
Demographics
- Typically affects young adults (20-40 years old)
- Male predominance (2-3:1 male to female ratio)
- Prevalence: 0.1-1.4% of general population
- Higher prevalence in HLA-B27 positive individuals
Diagnosis
- Clinical criteria: chronic back pain, morning stiffness, limited spinal mobility
- Laboratory findings: elevated ESR, CRP, and HLA-B27 positivity
- Imaging findings: sacroiliitis, syndesmophytes, bamboo spine
- ASAS (Assessment of SpondyloArthritis international Society) criteria
Imaging
Radiographs
- Sacroiliitis: erosions, sclerosis, and joint space narrowing
- Syndesmophytes: bony outgrowths bridging vertebral bodies
- Bamboo spine: complete fusion of vertebral bodies
- Squaring of vertebral bodies
- Enthesitis: whiskering at sites of ligament attachment
MRI
- Early detection of active inflammation
- Bone marrow oedema in sacroiliac joints and vertebral corners
- Fat metaplasia in chronic stages
- Erosions and sclerosis in advanced disease
CT
- Detailed assessment of bony changes
- Useful for evaluating fracture risk in advanced disease
- Superior for detecting syndesmophytes and ankylosis
Treatment
- Non-steroidal anti-inflammatory drugs (NSAIDs) as first-line treatment
- Disease-modifying antirheumatic drugs (DMARDs) for peripheral arthritis
- Biologic agents: TNF inhibitors, IL-17 inhibitors
- Physical therapy and exercise to maintain spinal mobility
- Surgery for severe deformities or fractures
Differential diagnosis
| Differential Diagnosis | Differentiating Feature |
|---|---|
| Diffuse idiopathic skeletal hyperostosis (DISH) | Affects older population; no sacroiliitis; flowing ossification of anterior longitudinal ligament |
| Psoriatic arthritis | Presence of psoriasis; asymmetric sacroiliitis; more peripheral joint involvement |
| Reactive arthritis | Asymmetric sacroiliitis; peripheral asymmetric arthritis; no syndesmophytes |
| Enteropathic arthritis | May have asymmetric sacroiliitis without bamboo spine; sacroiliac joint asymmetry |
| Scheuermann's disease | Thoracic kyphosis with anterior vertebral body wedging; no sacroiliitis; no syndesmophytes |