What is the likely cause of unilateral back pain with stiffness?

Unilateral Back Pain with Stiffness: Likely Causes

Unilateral back pain with stiffness is most commonly caused by nonspecific mechanical low back pain (affecting >85% of cases), but the unilateral presentation with morning stiffness should prompt consideration of ankylosing spondylitis, particularly in younger patients, or facet joint pathology in older adults. 1

Primary Diagnostic Considerations

Nonspecific Mechanical Low Back Pain

• This accounts for more than 85% of all low back pain presentations and cannot be reliably attributed to a specific anatomical structure. 1
• Unilateral presentation suggests localized musculoskeletal strain, myofascial pain, or facet joint involvement rather than central disc pathology. 2
• Stiffness is a common feature but typically improves with activity throughout the day in mechanical causes. 1

Ankylosing Spondylitis (Red Flag for Inflammatory Cause)

• Morning stiffness that improves with exercise, alternating buttock pain, and awakening due to back pain during the second part of the night are highly suggestive features. 1
• Prevalence ranges from 0.3% to 5% in primary care patients with back pain. 1
• Younger age is a key risk factor, and early diagnosis (before radiographic changes) is evolving. 1
• This diagnosis is critical because specific treatments are available that can prevent long-term disability. 1

Facet Joint Pain

• Facet joint pathology commonly presents as unilateral back pain with localized stiffness. 2
• More common in older adults with degenerative changes. 2
• Pain typically worsens with extension and rotation toward the affected side. 2

Critical Red Flags to Exclude

Before attributing symptoms to benign mechanical causes, you must systematically exclude serious pathology:

Malignancy Indicators

• History of cancer increases posttest probability from 0.7% to 9%. 1, 3
• Age >50 years, unexplained weight loss, or failure to improve after 1 month warrant immediate evaluation. 1, 4

Infection Risk Factors

• Fever, recent infection, intravenous drug use, or immunocompromised status require urgent assessment. 4, 3
• Spinal infections account for only 0.01% of cases but have high morbidity if missed. 4

Neurologic Compromise

• Assess for cauda equina syndrome: urinary retention (90% sensitivity), fecal incontinence, saddle anesthesia, or motor deficits at multiple levels. 1, 4
• Radiculopathy symptoms (sciatica in typical nerve root distribution) suggest disc herniation or spinal stenosis. 1

Diagnostic Algorithm

Step 1: Characterize the Pain Pattern

• Ask specifically about morning stiffness duration and whether it improves with exercise versus rest. 1
• Improvement with exercise suggests inflammatory causes like ankylosing spondylitis. 1
• Improvement with rest suggests mechanical causes. 1

Step 2: Age-Based Risk Stratification

• Younger patients (<40 years) with morning stiffness: strongly consider ankylosing spondylitis. 1
• Older patients (>50 years): consider facet joint disease, compression fracture (especially with osteoporosis or steroid use), or malignancy. 1, 3

Step 3: Red Flag Assessment

• Screen systematically for cancer, infection, fracture, and neurologic compromise as outlined above. 4, 3
• If any red flags present: obtain urgent MRI without and with contrast plus laboratory studies (CBC, ESR, CRP). 4

Step 4: Physical Examination Focus

• Palpate for localized tenderness over facet joints (paraspinal, 2-3 cm lateral to midline). 2
• Assess range of motion: pain with extension/rotation suggests facet pathology. 2
• Perform straight-leg raise test if radicular symptoms present (91% sensitivity for disc herniation). 1
• Document any motor weakness, sensory deficits, or reflex changes. 1, 3

Common Pitfalls to Avoid

• Do not delay imaging when red flags are present—immediate MRI is indicated rather than the usual 4-6 week waiting period for nonspecific pain. 4, 3
• Do not dismiss morning stiffness in younger patients as simple muscle strain—this is a cardinal feature of ankylosing spondylitis. 1
• Avoid routine imaging for nonspecific low back pain without red flags, as it does not improve outcomes. 3
• Do not assume degenerative changes on imaging explain symptoms—they correlate poorly with clinical presentation. 5

When Specific Diagnosis Cannot Be Made

• If no red flags and no inflammatory features are present, classify as nonspecific low back pain. 1, 5
• Psychosocial factors (depression, job dissatisfaction, passive coping) are stronger predictors of outcomes than physical findings. 3
• Consider referred pain from non-spinal sources (pancreatitis, nephrolithiasis, aortic aneurysm) if pain pattern is atypical. 1