What are the initial diagnostic steps and tests for a patient presenting with sciatica?

Diagnostic Approach for Sciatica

Begin with a focused clinical history and physical examination to classify the patient into one of three categories: nonspecific low back pain, radiculopathy/sciatica, or back pain with serious underlying pathology requiring urgent evaluation. 1

Initial Clinical Assessment

Key Historical Features to Elicit

• Pain distribution: True sciatica presents as lower limb pain radiating below the knee into the foot and toes, distinguishing it from referred pain that stops above the knee 2, 3
• Duration and onset: Document symptom duration, as most acute sciatica improves within 2-4 weeks regardless of treatment 2
• Red flag symptoms requiring urgent evaluation include:
  • Progressive or severe neurologic deficits at multiple levels 1
  • Urinary retention (90% sensitive for cauda equina syndrome) 1
  • Fecal incontinence or bladder dysfunction 1
  • Fever with back pain (suggests infection) 1, 4
  • History of cancer (increases cancer probability from 0.7% to 9%) 1
  • Unexplained weight loss 1
  • Age >50 years combined with failure to improve after 1 month 1
  • Intravenous drug use (infection risk) 4

Physical Examination Findings

• Neurological testing is essential to document motor deficits, sensory changes, and reflex abnormalities, though dermatomes are unreliable for precise localization 5
• Straight leg raise test assesses nerve root mobility and pain generation, though it tests mobility rather than directly correlating nerve root pathology 5
• Palpation along the peripheral nerve may help identify inflammatory nerve involvement when electrophysiology is normal 5

Imaging Strategy

When NOT to Image Initially

• Conservative management without imaging is appropriate for patients with typical sciatica symptoms, no red flags, and symptom duration <6-8 weeks 2, 3
• The clinical course of acute sciatica is generally favorable, with most cases improving spontaneously 2

When to Obtain Imaging

Order MRI lumbar spine without and with IV contrast when:

• Red flag symptoms are present suggesting infection, malignancy, or cauda equina syndrome 1, 4
• Severe symptoms persist beyond 6-8 weeks of conservative treatment 2
• Progressive neurologic deficits develop 1

MRI is the preferred imaging modality because it visualizes soft tissues (disc herniations, nerve roots, epidural space) better than CT and avoids ionizing radiation, with 96% sensitivity and 94% specificity for spine infection 4, 6, 2

Plain Radiographs Have Limited Utility

• Plain radiographs are not recommended as initial imaging for typical sciatica, as they cannot visualize disc herniations or nerve root compression 2
• Consider radiographs only when evaluating for vertebral fracture, alignment issues, or hardware integrity in post-surgical patients 6

Laboratory Testing

When to Order Labs

Obtain laboratory studies when red flags suggest infection or systemic disease:

• ESR (erythrocyte sedimentation rate): Highly sensitive marker for spine infection, though nonspecific 4
• CRP (C-reactive protein): More specific than ESR and rises rapidly with infection; CRP >100 mg/L indicates very high suspicion for active spinal infection requiring urgent intervention 4
• Complete blood count: May show leukocytosis, but can be normal in up to 40% of spine infections 4, 7
• Blood cultures (two sets): Essential before starting antibiotics when infection is suspected 4, 7
• Tuberculosis testing (PPD or interferon-gamma release assay) in HIV patients or those from endemic areas 7
• Brucella serology if epidemiologic risk factors present 4, 7

Critical Pitfall to Avoid

• Do not rely solely on WBC count to rule out infection, as it may be normal in 40% of cases; ESR and CRP are more reliable markers 4, 7

Differential Diagnosis Beyond Disc Herniation

While disc herniation causes the vast majority of sciatica cases 2, consider these alternative diagnoses when clinical features are atypical:

• Spinal stenosis: Presents with pseudoclaudication (leg pain with walking, relieved by sitting/flexion) 1
• Vertebral osteomyelitis/discitis: Fever, elevated ESR/CRP, risk factors for infection 4, 7
• Tumor or metastatic disease: History of cancer, unexplained weight loss, age >50, failure to improve after 1 month 1
• Compression fracture: Older age, osteoporosis, steroid use 1
• Extraspinal causes: Piriformis syndrome, intrapelvic mass, sacroiliitis, coxarthrosis—consider when pain pattern is atypical 8

Diagnostic Algorithm Summary

1. Perform focused history and examination to identify red flags and classify pain pattern 1
2. If no red flags and typical sciatica: Begin conservative management without imaging for 6-8 weeks 2, 3
3. If red flags present: Order MRI lumbar spine with and without contrast immediately 4, 6, 2
4. If infection suspected: Obtain ESR, CRP, blood cultures, and CBC before imaging 4, 7
5. If symptoms persist >6-8 weeks: Order MRI to guide further management decisions 2

Special Consideration for Cauda Equina Syndrome

• Urinary retention is the key finding (90% sensitivity); without it, probability is only 1 in 10,000 1
• Emergent MRI and surgical consultation are mandatory when cauda equina is suspected 1