How do I diagnose sciatica?

Diagnosing Sciatica

Diagnose sciatica primarily through clinical assessment: obtain a focused history of leg pain radiating below the knee in a lumbar nerve root distribution, perform straight-leg-raise testing, and conduct a targeted neurologic examination of lower extremity strength, reflexes, and sensation. 1, 2

Clinical History Assessment

Obtain these specific historical elements:

• Pain distribution: Ask whether pain radiates down the leg below the knee into the foot and toes in a typical lumbar nerve root distribution—this history has high sensitivity for herniated disc 1, 3
• Pain severity comparison: Determine if leg pain is worse than back pain, which has diagnostic value for nerve root compression 4
• Symptom frequency and duration: Document how long symptoms have been present and their pattern 2
• Onset characteristics: Note whether onset was sudden or gradual (non-sudden onset has diagnostic significance) 4
• Previous episodes: Ask about prior symptoms, treatments received, and response to those treatments 2

Red Flag Screening

Immediately assess for these urgent conditions requiring emergent evaluation:

• Cauda equina syndrome: Ask about urinary retention (90% sensitivity), fecal incontinence, or bladder dysfunction 1, 2
• Progressive neurologic deficits: Inquire about rapidly worsening weakness or motor deficits at multiple levels 2
• Cancer risk factors: History of cancer (increases probability from 0.7% to 9%), unexplained weight loss, age >50 years, or failure to improve after 1 month 1
• Infection risk factors: Fever, intravenous drug use, or recent infection 1
• Compression fracture risk: Older age, osteoporosis history, or corticosteroid use 1

Physical Examination

Perform this focused neurologic assessment:

Nerve Root Tension Testing

• Straight-leg-raise test: Flex the hip with knee extended between 30-70 degrees—reproduction of the patient's leg pain has 91% sensitivity but only 26% specificity for herniated disc 1, 2
• Crossed straight-leg-raise test: Lift the unaffected leg—reproduction of sciatica is more specific (88%) but less sensitive (29%) for herniated disc 1, 2

Neurologic Examination by Nerve Root Level

• L4 nerve root: Test knee extension strength and patellar reflex 1, 2
• L5 nerve root: Test great toe and foot dorsiflexion strength 1, 2
• S1 nerve root: Test foot plantarflexion strength and ankle reflexes 1, 2
• Sensory distribution: Map the distribution of sensory symptoms or deficits 1

Diagnostic Categorization

Classify the patient into one of three categories: 2

1. Nonspecific low back pain
2. Back pain with radiculopathy or spinal stenosis (sciatica)
3. Back pain with another specific spinal cause

Imaging Indications

Do not routinely order imaging in the first 6-8 weeks unless red flags are present. 3

When to Image:

• Evidence of underlying pathology (infection, malignancy, cauda equina syndrome) 3
• Severe symptoms not improving after 6-8 weeks of conservative treatment 3
• Consideration of surgical intervention 3

Preferred Imaging Modality:

MRI is the preferred imaging study because it visualizes soft tissues better than CT, does not expose patients to ionizing radiation, and can detect both nerve root compression and inflammation 2, 3

Common Diagnostic Pitfalls

• Imaging-symptom mismatch: Images may not correlate with symptoms in approximately 28% of cases—do not rely solely on imaging findings 5
• Dermatome unreliability: Dermatomes are unreliable for precise localization—use multiple examination findings together 5
• Referred pain confusion: Many patients and clinicians use "sciatica" to describe any leg pain, but true sciatica is radicular pain below the knee, not referred pain from the lower back 6
• Normal electrophysiology: Nerve conduction studies may be normal in purely inflamed nerve roots without compression 5

Psychosocial Assessment

Assess psychosocial factors and emotional distress, as these are stronger predictors of outcomes than physical examination findings or pain severity. 1, 2 This identifies patients at risk for delayed recovery.