What is the differential diagnosis (ddx) for lower back pain in an adult patient with no significant medical history?

Differential Diagnosis for Lower Back Pain

The vast majority (>85%) of adult patients presenting with lower back pain have nonspecific mechanical low back pain that cannot be attributed to a specific anatomical cause, but a focused history and physical examination must systematically rule out the small minority of serious conditions requiring urgent intervention. 1, 2

Three-Category Diagnostic Framework

The American College of Physicians recommends triaging all patients into one of three broad categories to guide management 1, 2:

1. Nonspecific Mechanical Low Back Pain (>85% of cases)

• Pain without identifiable specific disease or spinal abnormality 1, 2
• Worsens with activity and improves with rest (mechanical pattern) 3
• No red flag symptoms present 1
• Most patients improve substantially within the first month 1

2. Back Pain with Radiculopathy or Spinal Stenosis (~7% combined)

Herniated disc with radiculopathy (4% prevalence):

• Sciatica following a lumbar nerve root distribution 1
• Positive straight leg raise test (91% sensitivity, 26% specificity) 1
• Dermatomal sensory changes and motor weakness in specific nerve root distribution 2
• Associated paraspinal muscle spasm 2, 3

Spinal stenosis (3% prevalence):

• Pseudoclaudication: bilateral leg pain and weakness with walking/standing, relieved by sitting or spinal flexion 1, 4
• Age >65 years (positive likelihood ratio 2.5) 1
• Most improve within 4 weeks with noninvasive management 2

3. Back Pain with Specific Underlying Pathology (<2% combined)

Red Flag Conditions Requiring Immediate Evaluation

Cauda equina syndrome (0.04% prevalence) - SURGICAL EMERGENCY:

• Urinary retention (90% sensitivity - most frequent finding) 1, 2
• Fecal incontinence 1, 2
• Saddle anesthesia 2, 5
• Motor deficits at multiple levels 1, 2
• Requires immediate MRI or CT and urgent neurosurgical consultation 2

Vertebral malignancy (0.7% prevalence):

• History of cancer (positive likelihood ratio 14.7; increases posttest probability from 0.7% to 9%) 1, 2
• Age >50 years (positive likelihood ratio 2.7) 1
• Unexplained weight loss (positive likelihood ratio 2.7) 1
• Failure to improve after 1 month (positive likelihood ratio 3.0) 1

Vertebral compression fracture (4% prevalence):

• Older age with history of osteoporosis 1, 2
• Chronic steroid use 1, 2
• Midline tenderness to palpation in high-risk patients 2
• Plain radiography is appropriate initial imaging 2

Spinal infection (0.01% prevalence):

• Fever 1, 3
• Intravenous drug use 1, 3
• Recent infection 1, 3
• Immunocompromised status 3

Ankylosing spondylitis/Axial spondyloarthritis (0.3-5% prevalence):

• Younger age (<45 years) 1, 3
• Morning stiffness >30 minutes that improves with exercise 1, 3
• Alternating buttock pain 1
• Awakening due to back pain during second part of night only 1
• Pain worsens with rest, improves with movement (inflammatory pattern) 3

Non-Spinal Causes to Consider

Referred pain from visceral sources 1:

• Pancreatitis 1
• Nephrolithiasis 1
• Abdominal aortic aneurysm 1
• Endocarditis 1
• Viral syndromes 1

Psychosocial "Yellow Flags" Predicting Chronicity

These factors are stronger predictors of outcomes than physical findings or pain severity 1, 2:

• Depression 1, 2
• Passive coping strategies 1, 2
• Job dissatisfaction 1, 2
• Higher baseline disability levels 1, 2
• Disputed compensation claims 1
• Somatization 1

Critical Diagnostic Pitfalls to Avoid

Do NOT routinely order imaging for nonspecific low back pain without red flags - this does not improve outcomes and may lead to unnecessary interventions 1, 2. A single lumbar spine radiograph delivers gonadal radiation equivalent to daily chest x-rays for over 1 year 1, 2.

Do NOT miss cauda equina syndrome - the probability drops to 1 in 10,000 in patients without urinary retention, but delayed surgical decompression causes permanent neurologic disability 1, 2.

Do NOT overlook inflammatory causes in younger patients with chronic symptoms and morning stiffness - early diagnosis allows access to highly effective TNF-blocking agents 2, 3.

Do NOT dismiss cancer risk in patients with prior malignancy - the posttest probability jumps from 0.7% to 9% in this population 1, 2, 3.

When to Image

Immediate imaging (MRI preferred over CT) is indicated for 1:

• Severe or progressive neurologic deficits 1
• Suspected cauda equina syndrome 1, 2
• Red flags suggesting malignancy, infection, or fracture 1

Delayed imaging (at 4-6 weeks) only if patient is surgical candidate 2:

• Persistent radiculopathy or spinal stenosis symptoms despite conservative management 1
• Consideration for epidural steroid injection 1

MRI is preferred over CT for better soft tissue visualization and avoidance of radiation exposure 2.