What is likely the cause of chronic unilateral low back pain and stiffness that improves with movement?

Chronic Unilateral Low Back Pain with Morning Stiffness that Improves with Movement

The most likely cause is inflammatory low back pain, specifically early axial spondyloarthropathy (including ankylosing spondylitis), which characteristically presents with morning stiffness that improves with exercise rather than rest. 1, 2

Key Diagnostic Features of Inflammatory Low Back Pain

The clinical presentation described matches the classic pattern of inflammatory spine disease rather than mechanical low back pain:

Five Cardinal Features of Inflammatory Low Back Pain

• Improvement with exercise (not rest) is the most distinguishing feature separating inflammatory from mechanical causes 1, 2
• Morning stiffness or stiffness after periods of inactivity is characteristic 1
• Insidious onset rather than acute injury 2
• Age younger than 40 years at symptom onset (mean age 24 years for ankylosing spondylitis) 2
• Pain at night, particularly awakening during the second part of the night only 1

Additional Supporting Features

• Alternating buttock pain suggests sacroiliac joint involvement 1
• Unilateral presentation can occur early in the disease course before bilateral involvement develops 1

Differential Considerations

While the presentation strongly suggests inflammatory pathology, other causes to consider include:

Mechanical Causes (Less Likely Given Improvement with Movement)

• Facet joint syndrome typically worsens with extension and improves with rest, opposite to this presentation 1
• Nonspecific mechanical low back pain generally improves with rest rather than activity 1, 3
• Sacroiliac joint dysfunction can cause unilateral pain but typically worsens rather than improves with movement 4

Diagnostic Approach

Clinical Assessment

• Document the five inflammatory parameters systematically: improvement with exercise, pain at night, insidious onset, age <40 years, no improvement with rest 2
• Assess for associated conditions: inflammatory bowel disease, psoriasis, uveitis, or family history of spondyloarthropathy 2
• Examine for sacroiliac joint tenderness and perform provocative maneuvers 1

Laboratory Testing

• HLA-B27 testing can support the diagnosis but is neither necessary nor sufficient for diagnosis 2
• Inflammatory markers (ESR, CRP) may be elevated but can be normal in early disease 5

Imaging Considerations

• Plain radiographs of the sacroiliac joints and lumbar spine should be obtained first to look for sacroiliitis or spondylitis 6
• MRI of the sacroiliac joints and spine is the most sensitive test for early inflammatory changes before radiographic abnormalities develop 1, 6
• Criteria for diagnosing early ankylosing spondylitis before radiographic changes are evolving 1

Critical Clinical Pitfall

Do not dismiss this as nonspecific mechanical low back pain based solely on the absence of radiographic findings. Early axial spondyloarthropathy can present years before structural changes appear on plain films 1. The pattern of improvement with exercise rather than rest is the key distinguishing feature that should prompt evaluation for inflammatory causes 2.

Management Implications

• Immediate referral to rheumatology is warranted given the strong clinical suspicion for inflammatory disease 2
• NSAIDs are first-line therapy and can be both diagnostic and therapeutic, as dramatic response supports the diagnosis 2
• Structured exercise programs are essential, as they align with the pathophysiology of improvement with movement 2
• Early diagnosis and treatment can prevent long-term structural damage and disability 2