What examination should be done next in a patient with back pain, worsening in the morning and improving within an hour after movement, with a positive Schober test, suggestive of ankylosing spondylitis?

Eye Examination for Uveitis (Option A)

The next examination should be an eye examination to screen for uveitis, as this patient's clinical presentation is highly suggestive of ankylosing spondylitis, and acute anterior uveitis occurs in up to 40% of AS patients and requires immediate ophthalmologic evaluation if symptomatic. 1

Clinical Reasoning

This patient presents with classic inflammatory back pain characteristics that strongly suggest ankylosing spondylitis:

• Morning stiffness improving within an hour after movement is pathognomonic for inflammatory back pain, distinguishing it from mechanical causes 2
• Positive Schober test indicates reduced lumbar spine mobility, a hallmark of AS 2
• The pattern of pain worsening in the morning and improving with activity has 75% sensitivity and 75% specificity for axial spondyloarthropathy 1

Why Eye Examination is Critical

Uveitis screening is the most important next step because:

• Acute anterior uveitis occurs in 15-40% of AS patients and may precede spinal symptoms or occur during apparent remission 1, 3, 4
• While uveitis has only 15% sensitivity as a screening parameter, it has 98% specificity with a likelihood ratio of 7.3, meaning when present, it strongly confirms the diagnosis 1
• Immediate referral to an ophthalmologist is mandatory for symptoms of uveitis (eye pain or redness) to prevent vision-threatening complications 5
• Uveitis can cause significant morbidity including vision loss if not promptly identified and treated 4

Why Not the Other Options

CNS examination for weakness (Option B) is not the priority because:

• Neurologic complications in AS typically occur late in disease with advanced ankylosis and spinal fractures 1
• There are no red flags in this presentation suggesting acute neurologic compromise

CVS examination for mitral regurgitation (Option C) is not appropriate because:

• Aortic valve involvement, not mitral regurgitation, is the cardiac manifestation of AS, occurring in up to 80% of patients with long-standing disease 1
• Cardiac complications develop after years of disease progression, not at initial presentation 4
• The patient needs diagnosis confirmation first before screening for late complications

Recommended Diagnostic Algorithm

After eye examination, proceed with:

1. HLA-B27 testing - the single most valuable laboratory test with 90% sensitivity, 90% specificity, and 32% post-test probability 1, 2

2. Inflammatory markers (ESR/CRP) - though only 50% sensitive, they help assess disease activity 1

3. Imaging of sacroiliac joints:

   • X-rays initially for established disease (80% sensitivity/specificity) 1
   • MRI if X-rays normal - detects early inflammation years before radiographic changes with 90% sensitivity/specificity 1, 2

4. Referral to rheumatologist for definitive diagnosis and treatment initiation 5

Critical Pitfall to Avoid

Do not delay uveitis screening - even asymptomatic patients with suspected AS should have baseline ophthalmologic evaluation, as uveitis can present acutely and requires urgent treatment to prevent permanent vision loss 5, 4. The examination is low-cost, easy to interpret, and has high specificity for confirming spondyloarthropathy 1.