What are the most common causes of sciatica?

Causes of Sciatica

Lumbar disc herniation, particularly at the L4-L5 and L5-S1 levels, is the most common cause of sciatica, accounting for the vast majority of cases. 1, 2

Primary Structural Causes

Disc Herniation (Most Common)

• Lumbar disc herniation at L4-L5 and L5-S1 represents the predominant cause, with 56% of herniations occurring at L4-L5 1, 3, 4
• The pathophysiology requires both mechanical nerve root compression AND chemical inflammation—compression alone often remains asymptomatic, which explains why 20-28% of asymptomatic individuals show disc herniation on MRI 3
• Pain typically radiates below the knee into the foot and toes in the sciatic nerve distribution 5, 2

Spinal Stenosis (Second Most Common)

• Narrowing of the spinal canal causes bony constriction of the cauda equina and emerging nerve roots 5, 1
• Results from degenerative changes, disc herniations, and malalignment 3
• Patients typically present with neurogenic claudication—leg pain and weakness on walking or standing that is relieved by sitting or spinal flexion 5

Less Common but Important Causes

Neoplastic Compression

• Primary or metastatic tumors in the extradural or intradural extramedullary spaces can encroach upon the spinal canal and compress nerve roots 1, 3

Infectious/Inflammatory Processes

• Epidural abscess can compress nerve roots and cause sciatica 1, 3
• Infection or inflammation in the spine leads to nerve compression 1

Hemorrhagic Compression

• Epidural hematoma around the nerve roots can cause acute compression 1, 3

Rare Causes

• Vascular compression from gluteal varicosities (extremely uncommon but should be considered when pain is provoked by sitting on the affected side rather than by posture changes or coughing) 6
• Posterior intervertebral osteoarthritis 7
• Truncular sciatica from pelvic tumoral infiltration 7

Clinical Pitfalls to Avoid

• Do not assume all disc herniations cause sciatica: 57% of symptomatic patients with low back pain and 65% with radiculopathy show disc herniation, but so do 20-28% of completely asymptomatic individuals 3
• Mechanical compression without inflammation is often asymptomatic—both components are required for symptomatic sciatica 3
• Pain projection into the S1 distribution is most common even though L4-L5 herniations are more frequent than L5-S1 herniations, making clinical localization challenging 4
• High lumbar herniations (above L4-L5) produce unreliable neurologic pictures and require imaging for accurate diagnosis 4