What are the differential diagnoses and diagnostic approaches for left lower back pain?

Differential Diagnoses and Diagnostic Approach for Left Lower Back Pain

Categorize the patient into one of three groups through focused history and physical examination: nonspecific low back pain (85% of cases), back pain with radiculopathy/spinal stenosis, or back pain with a specific serious spinal cause requiring urgent evaluation. 1, 2

Differential Diagnoses by Category

Nonspecific Low Back Pain (85% of cases)

• Myofascial pain - localized pain without identifiable structural cause 3
• Facet joint pain - mechanical pain worsened by extension 3
• Sacroiliac joint pain - unilateral lower back pain, may radiate to buttock 3
• Discogenic pain - axial pain from disc degeneration (note: degenerative changes on imaging correlate poorly with symptoms) 4, 3

Back Pain with Radiculopathy or Spinal Stenosis

• Symptomatic herniated disc (4% of cases) - radiating leg pain (sciatica) following dermatomal distribution with corresponding motor/sensory deficits 1, 2
• Spinal stenosis (3% of cases) - neurogenic claudication with leg pain and weakness on walking/standing, relieved by sitting or spinal flexion (pseudoclaudication) 1, 4

Serious Specific Spinal Causes (Require Urgent Evaluation)

• Malignancy (0.7% of cases) - metastatic disease or primary spinal tumors 1, 5
• Compression fracture (4% of cases) - osteoporotic or traumatic 1
• Spinal infection (0.01% of cases) - osteomyelitis, discitis, epidural abscess 1, 5
• Ankylosing spondylitis (0.3-5% of cases) - inflammatory spondyloarthropathy 1
• Cauda equina syndrome (0.04% of cases) - massive midline disc herniation causing bowel/bladder dysfunction 1, 5

Non-Spinal Causes (Referred Pain)

• Pancreatitis - epigastric pain radiating to back 2
• Nephrolithiasis - colicky flank pain radiating anteriorly 2
• Aortic aneurysm - pulsatile mass, vascular risk factors 2
• Endocarditis or systemic infections - constitutional symptoms 2

Critical Red Flags Requiring Immediate Imaging and Evaluation

Cauda Equina Syndrome (Surgical Emergency)

• Urinary retention 5
• Fecal incontinence 5
• Saddle anesthesia (perineal numbness) 5
• Motor deficits at multiple levels 5

Malignancy Indicators

• History of cancer (increases probability from 0.7% to 9%; positive likelihood ratio 14.7) 5
• Unexplained weight loss (positive likelihood ratio 2.7) 5
• Age >50 years (positive likelihood ratio 2.7) 5
• Failure to improve after 1 month (positive likelihood ratio 3.0) 5

Spinal Infection

• Fever or recent infection 1, 5
• Intravenous drug use 5
• Immunocompromised status 5

Neurological Compromise

• Rapidly progressive or severe neurologic deficits 2
• Significant trauma 2

Diagnostic Algorithm

Step 1: History Taking

• Pain characteristics: Location (localized vs. radiating), frequency (constant vs. intermittent), duration 2
• Neurological symptoms: Sciatica (radiating leg pain), motor weakness, sensory changes, pseudoclaudication 2
• Previous episodes and treatment response 2
• Screen for all red flags listed above 2, 5
• Psychosocial risk factors: Depression, job dissatisfaction, catastrophizing (predict chronic disability) 1, 2

Step 2: Physical Examination

• Neurological examination: Motor strength, sensory testing, reflexes, straight leg raise 1
• Spinal examination: Range of motion, palpation for tenderness, deformity 1
• Rectal examination if cauda equina suspected: Assess sphincter tone 5

Step 3: Diagnostic Testing Based on Category

For Nonspecific Low Back Pain (No Red Flags)

Do NOT routinely obtain imaging or diagnostic tests - this is a strong recommendation from the American College of Physicians 1, 2

• Provide reassurance and education about expected benign course 1
• Advise to remain active 1

For Suspected Serious Pathology (Red Flags Present)

Obtain urgent imaging and laboratory studies 1, 5

• MRI without and with contrast (preferred modality) for suspected infection or malignancy 5
• Laboratory studies: Complete blood count, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP) 5
• Urgent referral to oncology and spine surgery for coordinated care if malignancy confirmed 5

For Persistent Pain with Radiculopathy/Spinal Stenosis

Obtain MRI (preferred) or CT ONLY if patient is a candidate for surgery or epidural steroid injection 1, 4

• Timing: Consider imaging after 4-6 weeks of conservative management unless red flags present 5
• Do not image earlier in absence of red flags, as this does not improve outcomes 1

Common Pitfalls to Avoid

• Delaying imaging when red flags are present - immediate MRI is indicated rather than the usual 4-6 week waiting period for nonspecific back pain 5
• Over-imaging patients with nonspecific low back pain - degenerative changes correlate poorly with symptoms and do not guide treatment 4
• Missing cauda equina syndrome - though rare (0.04% prevalence), it requires immediate intervention to prevent permanent disability 5
• Attributing pain to imaging findings without clinical correlation - most degenerative changes are age-related and asymptomatic 4
• Ignoring psychosocial yellow flags - these predict chronicity and disability more than anatomical findings 1, 2