What are the symptoms and diagnosis of ankylosing spondylitis?

Symptoms and Diagnosis of Ankylosing Spondylitis

Key Clinical Symptoms

Inflammatory back pain (IBP) is the hallmark symptom of ankylosing spondylitis, present in 70-80% of patients, and should trigger diagnostic evaluation when chronic back pain begins before age 45 and lasts more than 3 months. 1, 2

Defining Features of Inflammatory Back Pain

The characteristic pattern includes:

• Insidious onset before age 40-45 years with chronic duration (≥3 months) 1, 2
• Prolonged morning stiffness that distinguishes it from mechanical back pain 2
• Improvement with exercise but not with rest - a critical distinguishing feature 2
• Night pain, particularly awakening in the second half of the night 2
• Alternating buttock pain indicating sacroiliac joint involvement 2

Musculoskeletal Manifestations Beyond the Spine

• Sacroiliac joint pain is typically the initial site, presenting as lower back/buttock pain 2
• Peripheral arthritis affecting large joints (most commonly knees) in an oligoarticular, asymmetric pattern occurs in 30-50% of patients 2
• Enthesitis (inflammation at tendon/ligament insertion sites) is a hallmark pathologic feature 3
• Progressive loss of spinal mobility develops with chronic disease 2

Extra-Articular Manifestations

• Uveitis (acute anterior uveitis) is the most common extra-articular manifestation 2
• Psoriasis may be present 2
• Inflammatory bowel disease can coexist 2

Diagnostic Approach

The optimal diagnostic strategy combines clinical parameters (inflammatory back pain), laboratory testing (HLA-B27), and imaging (MRI for early disease, X-rays for established disease) to achieve early diagnosis before irreversible structural damage occurs. 1

Clinical Screening Parameters

Screen only patients with chronic back pain >3 months duration and onset before age 45, as AS rarely starts after age 40 (<4% of cases) 1

The most effective clinical screening parameters are:

• Inflammatory back pain (sensitivity 75%, specificity 75%, post-test probability 14%) 1
• Good response to full-dose NSAIDs within 48 hours (sensitivity 75%, specificity 85%, post-test probability 21%) 1

Additional clinical features with lower sensitivity but high specificity:

• Uveitis (sensitivity 15%, specificity 98%) 1
• Family history of AS (sensitivity 25%, specificity 96%) 1
• Peripheral arthritis (sensitivity 40%, specificity 90%) 1

Laboratory Testing

HLA-B27 testing is the single most valuable laboratory test (sensitivity 90%, specificity 90%, post-test probability 32%) 1

Important caveats:

• HLA-B27 is present in 74-89% of AS patients but only 1% of HLA-B27 positive individuals develop AS 2, 3
• Inflammatory markers (ESR/CRP) have poor diagnostic utility (sensitivity 50%, specificity 80%) and may be normal in active disease 1, 4

Imaging Strategy

MRI is the preferred imaging modality for early/pre-radiographic disease, while conventional X-rays remain essential for established disease with structural changes. 1, 5

MRI for Early Disease

• MRI detects sacroiliac joint inflammation years before radiographic changes appear 1, 5
• Sensitivity and specificity both 90%, post-test probability 32% 1
• Use STIR sequences or gadolinium-DTPA contrast to visualize active inflammation 5
• Particularly valuable in young women, children, and for differential diagnosis 5

Conventional Radiography

• X-rays of sacroiliac joints (sensitivity 80%, specificity 80%) remain the standard for established disease 1
• Radiographic monitoring may not be needed more frequently than every 2 years in most patients, though syndesmophytes can develop within 6 months in some cases 1
• Chronic structural changes are better visualized on plain X-rays than MRI 4

Diagnostic Algorithm

For patients with chronic back pain (>3 months, onset <45 years):

1. Assess for inflammatory back pain features - if present, post-test probability is 14% 1
2. Trial of full-dose NSAIDs - good response within 48 hours increases probability to 21% 1
3. Order HLA-B27 testing - if positive, probability increases to 32% 1
4. Obtain MRI of sacroiliac joints for suspected early disease - positive findings increase probability to 32% 1
5. Refer to rheumatology when post-test probability exceeds 20-30% 1

Critical Pitfalls to Avoid

Diagnosis is frequently delayed by 5-8 years from symptom onset, highlighting the challenge of early recognition 2, 6

Common diagnostic errors:

• Waiting for radiographic sacroiliitis - inflammation may persist for years before X-ray changes appear 1
• Relying on inflammatory markers - ESR/CRP are often normal in active AS 1, 4
• Dismissing patients with negative HLA-B27 - 10-25% of AS patients are HLA-B27 negative 2, 3
• Overlooking the diagnosis in women - AS is at least twice as common in men but occurs in women and may present differently 3
• Missing the diagnosis when peripheral symptoms dominate - 30-50% have peripheral arthritis 2

Assessment Tools for Disease Monitoring

The ASAS core set should be used for clinical record keeping, including 1, 7:

• Bath Ankylosing Spondylitis Functional Index (BASFI) for physical function 1
• Modified Schober test for spinal mobility assessment 1, 7
• Patient global assessment and pain scores 1
• Assessment of peripheral joints and entheses when involved 1