What is the differential diagnosis (ddx) for back pain in a patient with unknown demographic and medical history?

Differential Diagnosis of Back Pain

Diagnostic Triage Framework

Categorize all patients with back pain into one of three groups: (1) nonspecific mechanical low back pain (85-97% of cases), (2) back pain with radiculopathy or spinal stenosis, or (3) back pain from serious underlying pathology requiring urgent intervention. 1, 2, 3

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Category 1: Nonspecific Mechanical Low Back Pain (Most Common)

This accounts for over 85% of primary care presentations and arises from spinal structures including bone, ligaments, discs, joints, and surrounding soft tissues. 3, 4

Key Features:

• Pain worsens with activity and improves with rest 3
• No red flag symptoms present 2
• No neurologic deficits 1
• Self-limited course in 90% of patients within one month 1, 5

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Category 2: Back Pain with Neurologic Involvement

Herniated Disc with Radiculopathy (4% prevalence)

• Back and leg pain in lumbar nerve root distribution (sciatica) 1
• Over 90% occur at L4/L5 or L5/S1 levels 1
• Positive straight-leg-raise test between 30-70 degrees (91% sensitivity, 26% specificity) 1
• Assess L4 nerve root: knee strength and reflexes 1
• Assess L5 nerve root: great toe and foot dorsiflexion strength 1
• Assess S1 nerve root: foot plantarflexion and ankle reflexes 1

Spinal Stenosis (3% prevalence)

• Pseudoclaudication: leg pain with walking/standing, relieved by sitting or spinal flexion 1, 2
• Age older than 65 years (positive likelihood ratio 2.5) 1
• Radiating leg pain (positive likelihood ratio 2.2) 1

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Category 3: Serious Underlying Conditions (Red Flags)

Cauda Equina Syndrome (0.04% prevalence) - URGENT

Urinary retention has 90% sensitivity and is the most frequent finding. 1, 2

• Fecal incontinence 2, 3
• Saddle anesthesia 2, 3
• Motor deficits at multiple levels 1, 2
• Without urinary retention, probability is only 1 in 10,000 1

Malignancy (0.7% baseline prevalence)

History of cancer increases posttest probability from 0.7% to 9% (positive likelihood ratio 14.7). 1, 2, 3

• Unexplained weight loss (positive likelihood ratio 2.7) 1, 2
• Failure to improve after 1 month (positive likelihood ratio 3.0) 1, 2
• Age older than 50 years (positive likelihood ratio 2.7) 1, 2

Spinal Infection (0.01% prevalence)

• Fever 1, 2
• Intravenous drug use 1
• Recent infection 1
• Immunocompromised status 2

Vertebral Compression Fracture (4% prevalence)

• Older age 1
• History of osteoporosis 1
• Chronic steroid use 1

Ankylosing Spondylitis (0.3-5% prevalence in chronic low back pain)

Morning stiffness that improves with exercise is pathognomonic for inflammatory spondyloarthropathy. 6, 3

• Younger age (typically <45 years) 1, 6
• Morning stiffness across whole back 6, 3
• Improvement with exercise and movement 1, 6
• Alternating buttock pain 1, 6
• Awakening due to back pain during second part of night only 1, 6

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Non-Spinal Causes to Consider

Always consider referred pain from extra-spinal sources: 1

• Pancreatitis 1
• Nephrolithiasis 1
• Aortic aneurysm 1
• Endocarditis 1
• Viral syndromes 1

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Critical Pitfalls to Avoid

Do not delay imaging when red flags are present—immediate MRI is indicated rather than waiting 4-6 weeks. 2, 3

Avoid routine imaging for nonspecific low back pain without red flags, as it does not improve outcomes and may lead to unnecessary interventions. 2, 3, 7

Do not overlook inflammatory causes in younger patients (<45 years) with chronic symptoms and morning stiffness, as early diagnosis allows for disease-modifying treatment. 3

Psychosocial factors (depression, passive coping, job dissatisfaction, disputed compensation claims, somatization) are stronger predictors of outcomes than physical findings or pain severity. 1, 2