How do you differentiate between true sciatica and referred pain?

Differentiating True Sciatica from Referred Pain

True sciatica is distinguished from referred pain by the presence of leg pain radiating below the knee in a dermatomal distribution, accompanied by objective neurological findings (motor weakness, sensory changes, or reflex abnormalities), whereas referred pain from the lower back typically does not extend below the knee and lacks neurological deficits. 1, 2

Key Clinical Features That Distinguish True Sciatica

Pain Distribution Pattern

• True sciatica: Pain radiates down the leg below the knee into the foot and toes, following a specific nerve root distribution (dermatomal pattern) 2, 3
• Referred pain: Pain typically remains above the knee or is diffuse in the buttock and thigh without following a dermatomal pattern 4
• The distinction based on pain extending below versus above the knee is critical for classification 1, 5

Neurological Examination Findings

True sciatica presents with objective neurological deficits in a nerve root distribution: 6, 2

• L4 nerve root: Knee weakness and diminished knee reflex 6
• L5 nerve root: Great toe and foot dorsiflexion weakness 6
• S1 nerve root: Foot plantarflexion weakness and diminished ankle reflex 6
• Sensory changes: Dermatomal numbness or paresthesias in the affected nerve root distribution 2

Referred pain lacks these objective neurological findings and presents only with pain without motor weakness, sensory deficits, or reflex changes 4

Diagnostic Physical Examination Maneuvers

Straight Leg Raise (SLR) Test

• Positive test: Reproduces the patient's radiating leg pain when the leg is raised between 30-70 degrees with knee extended 6, 2
• Sensitivity: 91% for detecting lumbar disc herniation causing nerve root compression 6
• Specificity: Only 26%, meaning many positive tests occur without true nerve root compression 6
• Important caveat: A negative SLR does not rule out sciatica, particularly in patients over 60 years old where sensitivity drops below 50% 6

Crossed Straight Leg Raise Test

• Highly specific finding: Pain reproduced in the affected leg when raising the unaffected leg 6, 2
• Sensitivity: 29% (less sensitive) 6
• Specificity: 88% (highly specific for disc herniation) 6
• When positive, this strongly indicates true nerve root compression rather than referred pain 2

Clinical Algorithm for Differentiation

Step 1: Assess Pain Distribution

• Does pain radiate below the knee into foot/toes? → Suggests true sciatica 2, 3
• Pain remains above knee or diffuse in buttock/thigh? → Suggests referred pain 4

Step 2: Perform Focused Neurological Examination

• Test motor strength at each nerve root level (L4, L5, S1) 6
• Assess deep tendon reflexes (knee and ankle) 6
• Map sensory distribution for dermatomal patterns 2
• Presence of objective deficits = true sciatica with radiculopathy 2
• Absence of deficits = either referred pain or sciatica without radiculopathy 1

Step 3: Perform SLR Testing

• Standard SLR positive with leg pain below knee = supports sciatica 6
• Crossed SLR positive = highly specific for nerve root compression 6
• Both tests negative does NOT exclude sciatica if clinical presentation is consistent 6

Step 4: Classify Patient into Diagnostic Category

The American College of Physicians recommends classifying patients into three categories: 1

1. Nonspecific low back pain (referred pain): No radicular features, pain does not extend below knee, no neurological deficits
2. Back pain with radiculopathy/sciatica: Pain below knee in dermatomal distribution with or without objective neurological findings
3. Back pain with specific spinal cause: Red flag symptoms requiring urgent evaluation

Common Pitfalls to Avoid

Terminology Confusion

• The term "sciatica" is often misused by patients and clinicians to describe any lower back pain radiating to the leg 4, 5
• Critical distinction: Most leg pain from the back is referred pain, NOT true sciatica related to nerve root compression 4
• The 2023 NeuPSIG working group recommends discouraging the term "sciatica" without further specification and using "spine-related leg pain" as an umbrella term 5

Over-reliance on SLR Test

• The modest specificity (26%) means many positive SLR tests are false positives 6
• Clinical diagnosis should not rely solely on SLR but must incorporate pain distribution and neurological examination 6
• The combination of radicular pain pattern, dermatomal sensory changes, and motor weakness establishes diagnosis even without positive SLR 6

Mixed Pain Syndromes

• Chronic sciatica may involve both nociceptive and neuropathic pain components (mixed pain syndrome) 7
• Different pathophysiological mechanisms can coexist: mechanical compression, inflammatory mediators, and degenerative disc changes 7
• This complexity explains why some patients with true sciatica may not respond to NSAIDs alone and may require neuropathic pain medications 7

When Imaging Is Indicated

Imaging is NOT routinely needed for initial differentiation between true sciatica and referred pain, as this is primarily a clinical diagnosis 1, 2

Immediate MRI is indicated for: 8, 2

• Severe or progressive neurological deficits
• Suspected cauda equina syndrome (urinary retention has 90% sensitivity) 1
• Red flag symptoms (history of cancer, unexplained weight loss, fever, IV drug use) 1

Delayed MRI (after 4-6 weeks) is appropriate for: 8, 2

• Persistent radicular symptoms despite conservative management
• Consideration of surgical intervention
• Persistent positive SLR test with functional limitations