What is the diagnostic approach and treatment for a patient presenting with symptoms of ankylosing spondylitis, including a history of arthritis, eye problems, skin conditions, or gastrointestinal issues?

Ankylosing Spondylitis: Clinical Features and Diagnostic Approach

Clinical Features

Ankylosing spondylitis (AS) presents with inflammatory back pain starting before age 45, lasting more than 3 months, characterized by improvement with exercise but not rest, and morning stiffness. 1

Axial Manifestations

• Inflammatory back pain occurs in 70-80% of patients, with onset in late adolescence or early adulthood, almost always before age 45 2, 1
• Pain improves with exercise but worsens with rest, distinguishing it from mechanical back pain 2
• Morning stiffness is a cardinal feature 3
• Progressive loss of spinal mobility develops over time, with potential for complete spinal fusion 3

Peripheral and Extra-articular Manifestations

• Asymmetrical peripheral arthritis affects large joints, particularly lower extremities 1
• Enthesitis (inflammation at tendon/ligament insertion sites) commonly involves the Achilles tendon and plantar fascia 1
• Acute anterior uveitis occurs in approximately 25-40% of patients and requires urgent ophthalmologic evaluation 4
• Inflammatory bowel disease (Crohn's disease or ulcerative colitis) is associated with AS 4
• Psoriasis may coexist as part of the spondyloarthritis spectrum 2
• Cardiovascular complications including aortitis and conduction abnormalities can develop 5

Prognostic Indicators

• Hip involvement is the most important predictor of severe disease and poor functional outcome 3
• Younger age at onset, male sex, elevated ESR, smoking, and poor response to NSAIDs predict worse outcomes 3

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Diagnostic Approach

Begin with plain radiographs of the pelvis (AP view including sacroiliac joints) as the first-line imaging test, combined with HLA-B27 testing in patients with chronic inflammatory back pain starting before age 45. 1, 2

Step 1: Clinical Screening in Primary Care

Screen patients with chronic back pain (>3 months duration) starting before age 45 using inflammatory back pain criteria OR HLA-B27 testing. 3

• Inflammatory back pain has 75% sensitivity and increases post-test probability to 14% when present 3
• HLA-B27 positivity has 90% sensitivity and 90% specificity, increasing post-test probability to 32% in patients with chronic back pain 2, 3
• Refer to rheumatology if either inflammatory back pain OR positive HLA-B27 is present, as this identifies the majority of AS patients while maintaining practical simplicity for primary care 3

Step 2: Imaging Strategy

Order plain radiographs of the pelvis first; if negative but clinical suspicion remains high, proceed to MRI of the sacroiliac joints. 1, 2

Plain Radiography

• AP pelvis radiograph (including both sacroiliac joints) is the standard initial test with 90% sensitivity and specificity for detecting sacroiliitis 2
• Radiographic sacroiliitis may take several years to develop, causing diagnostic delay 3
• Add AP and lateral lumbar spine and lateral cervical spine radiographs to assess disease extent once diagnosis is established 1

MRI When Radiographs Are Negative

• MRI of sacroiliac joints detects bone marrow edema (active inflammation) years before radiographic changes appear, with 90% sensitivity and specificity 2, 6
• MRI is critical for diagnosing "pre-radiographic AS" or early axial spondyloarthritis 3, 7
• MRI can quantify spinal inflammation using scoring systems like ASspiMR, with thoracic spine showing strongest correlation with disease activity 8

Step 3: Laboratory Testing

Obtain HLA-B27, ESR, and CRP as part of the diagnostic workup. 1, 2

• HLA-B27 is positive in approximately 90% of AS patients but also in 6-8% of the general population, making it useful for screening but not diagnostic alone 1
• ESR and CRP are elevated in many but not all AS patients; normal values do not exclude the diagnosis 1, 5
• Acute phase reactants help monitor disease activity but correlate imperfectly with clinical symptoms 8

Step 4: Apply Diagnostic Criteria

Use the modified New York Criteria for established AS or ASAS criteria for early axial spondyloarthritis. 1, 2

Modified New York Criteria (for established AS)

• Radiographic sacroiliitis PLUS at least one clinical criterion (inflammatory back pain, limitation of lumbar spine motion, or limited chest expansion) 1

ASAS Criteria (for early disease)

• Sacroiliitis on imaging (radiographs or MRI) PLUS at least one spondyloarthritis feature (inflammatory back pain, arthritis, enthesitis, uveitis, dactylitis, psoriasis, Crohn's/colitis, good response to NSAIDs, family history, HLA-B27, elevated CRP) 2

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Disease Monitoring After Diagnosis

Monitor disease activity using the ASAS core set at individualized intervals based on symptoms and treatment, with reassessment every 4-6 weeks when initiating or changing therapy. 1, 5

ASAS Core Set Components

• Physical function: Bath AS Functional Index (BASFI) 3
• Pain: Visual analog scale for spinal pain 3
• Spinal mobility: Chest expansion, modified Schober test, occiput-to-wall distance, Bath AS Metrology Index (BASMI) 3
• Patient global assessment: Overall disease activity on visual analog scale 3
• Morning stiffness: Duration and severity 3
• Peripheral joints and entheses: Swollen joint count and enthesitis assessment 3
• Acute phase reactants: ESR or CRP 3

Imaging Follow-up

• Radiographic monitoring of the spine is not needed more frequently than every 2 years, though syndesmophytes can develop within 6 months in some patients 3

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Common Pitfalls and Caveats

Diagnostic Delays

• The average delay from symptom onset to diagnosis is 5-10 years, primarily because radiographic sacroiliitis takes years to develop 3, 7
• Do not wait for radiographic changes if clinical suspicion is high—proceed directly to MRI 1, 2
• One-third of patients have ambiguous back pain characteristics that are neither clearly inflammatory nor mechanical; in these cases, use HLA-B27 testing to guide referral decisions 3

Screening Limitations

• Using inflammatory back pain alone misses 25% of AS patients (75% sensitivity) 3
• Combining multiple screening parameters (e.g., inflammatory back pain PLUS HLA-B27) increases specificity but decreases sensitivity to 68%, missing more patients 3
• For primary care screening, use single parameters (inflammatory back pain OR HLA-B27) to maximize sensitivity 3

Extra-articular Manifestations

• Acute anterior uveitis requires same-day ophthalmology referral to prevent vision loss 4
• Screen for inflammatory bowel disease symptoms (diarrhea, abdominal pain, blood in stool) as this affects treatment choices 4
• Cardiovascular risk is increased 1.6-1.9 times; address traditional risk factors aggressively 5

Fracture Risk

• Patients with advanced AS have brittle, osteoporotic spines prone to fracture with minimal trauma 4
• Plain radiographs have poor sensitivity for detecting spinal fractures in ankylosed spines; use CT with multiplanar reformats if fracture is suspected 2
• MRI without contrast is needed if neurological symptoms suggest spinal cord or nerve root injury 2