Diagnosis of Ankylosing Spondylitis
Clinical Diagnostic Criteria
Diagnose ankylosing spondylitis (AS) based on chronic inflammatory back pain starting before age 45, combined with radiographic evidence of sacroiliitis, following the modified New York Criteria. 1
Key Clinical Features to Elicit
- Inflammatory back pain pattern: Nocturnal back pain that wakes the patient, prolonged morning stiffness (>1 hour), diurnal variation with improvement through the day, and excellent response to NSAIDs 2, 3
- Age of onset: Symptoms typically begin in late adolescence or early adulthood, almost always before age 45 4, 2
- Duration: Chronic symptoms lasting >3 months are required 5
- Location: Low back and/or hip pain, often bilateral sacroiliac joint involvement 4, 2
Physical Examination Findings
Physical examination is often unrevealing in early disease, but assess for: 3
- Spinal mobility: Modified Schober's test (normal >5 cm), chest expansion (normal >5 cm), occiput-to-wall distance, and lateral spinal flexion 1
- Peripheral manifestations: Asymmetrical arthritis of peripheral joints, enthesitis (heel pain, Achilles tendinitis) 1, 6
- Extra-articular features: Acute anterior uveitis, psoriasis, inflammatory bowel disease 1, 2
Imaging Strategy
Initial Imaging
- Plain radiographs of the pelvis (AP view including sacroiliac joints) are the standard first imaging test 1, 6, 7
- Look for bilateral sacroiliitis with erosions, sclerosis, or fusion 6, 7
- Critical pitfall: Conventional radiographs are often normal in early disease, with a typical diagnostic delay of 5-7 years from symptom onset 5, 4
When Plain Films Are Normal
If clinical suspicion is high but pelvic radiographs are normal, obtain MRI of the sacroiliac joints to detect early inflammatory lesions (bone marrow edema). 1, 2, 3
- MRI is the most sensitive imaging technique for detecting early inflammatory sacroiliitis before structural damage appears on plain films 2, 6, 3
- MRI can detect active inflammation that precedes radiographic changes by years 6, 3
- Important caveat: Bone marrow edema lesions can occur in individuals without axial spondyloarthritis; interpret MRI findings in clinical context 1
Spinal Imaging
- Obtain AP and lateral radiographs of the lumbar spine and lateral cervical spine when assessing disease extent 1
- Look for syndesmophytes, squaring of vertebral bodies, and bamboo spine in advanced disease 1, 6
- Do not obtain routine repeat spine radiographs at scheduled intervals (e.g., every 2 years) in stable patients, as radiographic progression is slow 1
Laboratory Testing
Essential Tests
- HLA-B27: Positive in approximately 90% of AS patients, but also present in 6-8% of the general population 7
- Acute phase reactants: ESR and CRP are elevated in many but not all patients 1, 4
- Complete blood count 5
Critical Diagnostic Pitfalls
Do not rule out AS based solely on negative HLA-B27 or normal inflammatory markers (ESR/CRP). 5
- HLA-B27 is of limited value in AS associated with psoriasis or inflammatory bowel disease, where it is often negative 7
- The diagnostic utility of HLA-B27 testing depends on pre-test probability and varies among ethnic groups 7
- Normal inflammatory markers do not exclude active disease 5, 4
Disease Monitoring After Diagnosis
Monitor disease activity using the ASAS core set, which includes: 1
- Patient-reported outcomes: Bath Ankylosing Spondylitis Functional Index (BASFI), Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), visual analog scales for pain and patient global assessment 1
- Physical examination: Spinal mobility measurements (chest expansion, modified Schober, occiput-to-wall distance, BASMI) 1
- Laboratory tests: ESR or CRP 1
- Imaging: Radiographic monitoring may not be needed more often than every 2 years, though syndesmophytes can develop within 6 months in some patients 1
The frequency of monitoring should be individualized based on symptoms, severity, and treatment, but reassess at 4-6 week intervals when initiating or changing therapy 1, 8
Treatment Overview
First-Line Therapy
NSAIDs at maximum tolerated doses are first-line pharmacological treatment for AS with pain and stiffness. 1, 5
- Approximately 75% of patients show good or very good response within 48 hours of full-dose NSAID therapy 5
- Continuous NSAID use is preferred over on-demand dosing for patients who respond well 5
- For patients with gastrointestinal risk, use non-selective NSAIDs plus gastroprotective agents or selective COX-2 inhibitors 1
Non-Pharmacological Treatment
All patients require patient education and regular exercise; individual or group physical therapy should be considered. 1, 5
- Home exercise improves function in the short term 1
- Structured physical therapy programs are often more beneficial than home exercises alone 5
- Smoking cessation is essential as smoking worsens disease progression 5
When to Escalate Therapy
Anti-TNF treatment (adalimumab, etanercept, infliximab) should be given to patients with persistently high disease activity despite conventional treatments (NSAIDs and physical therapy). 1, 5, 9, 10
- TNF inhibitors achieve ASAS 20 response in approximately 60% of patients by 12 weeks 1, 9, 10
- There is no evidence to support obligatory use of DMARDs before or concomitant with anti-TNF treatment in axial disease 1
What NOT to Use
Do not use conventional DMARDs (methotrexate, sulfasalazine, leflunomide) for purely axial disease—they are ineffective. 1, 8, 5
- Sulfasalazine may only be considered if peripheral arthritis is present 1, 5
- Systemic corticosteroids for axial disease are not supported by evidence 1, 8
- Corticosteroid injections directed to local sites of musculoskeletal inflammation may be considered 1
Alternative Biologics
Secukinumab or ixekizumab (IL-17 inhibitors) are recommended for patients with contraindications to TNF inhibitors (heart failure, demyelinating disease) or primary non-responders to TNF inhibitors. 1