What causes sciatica to develop?

How Sciatica Develops

Sciatica develops through a dual mechanism requiring both mechanical nerve root compression and chemical inflammation, most commonly from lumbar disc herniation at the L4-L5 or L5-S1 levels. 1

Primary Pathophysiological Mechanisms

Mechanical Compression

• Lumbar disc herniation is the most common cause of sciatica, accounting for the vast majority of cases, with the herniated disc material directly compressing the nerve root within the spinal canal. 2
• The compression typically occurs at the L4-L5 and L5-S1 levels, where the nerve roots exit the spinal canal and form the sciatic nerve distribution. 3
• Spinal stenosis—narrowing of the spinal canal from degenerative changes, disc herniations, and malalignment—can also cause nerve root compression leading to sciatica. 3

Chemical Inflammation

• Proinflammatory substances secreted by the nucleus pulposus (the inner disc material) are essential for nerve root pain to develop, even when mechanical compression is present. 4
• Tumor necrosis factor-alpha (TNF-alpha) is the key mediator that causes functional and structural nerve root abnormalities, and this chemical component explains why compression alone doesn't always produce symptoms. 5, 4
• The intervertebral disc is immunogenic, meaning when disc material herniates and contacts nerve tissue, it triggers an inflammatory cascade that sensitizes the nerve root. 4

Why Both Mechanisms Are Required

• Mechanical compression without inflammation often remains asymptomatic, which explains why many patients have disc herniations on MRI but no pain—57% of symptomatic patients with low back pain and 65% with radiculopathy show disc herniation, compared to 20-28% of asymptomatic individuals. 3
• Conversely, the chemical inflammatory mediators require preexisting or concomitant mechanical injury to the nerve root to produce the characteristic sciatic pain. 4
• This dual mechanism explains several clinical observations: large herniations aren't always symptomatic, severe pain can occur without imaging evidence of compression, and conservative therapy is often effective as inflammation resolves. 4

Less Common Causes

• Epidural pathology including abscess or hematoma can compress nerve roots and cause sciatica. 3
• Primary or metastatic tumors in the extradural or intradural extramedullary spaces can encroach upon the spinal canal. 3
• Infection, neoplasm, and degenerative spinal disease represent alternative etiologies. 6
• Traumatic causes such as avulsion fractures (though uncommon, especially in children) can compress the sciatic nerve. 6

Clinical Implications of the Pathophysiology

• The size and type of disc herniation, location, and presence of nerve root compression do not correlate well with patient outcomes, reflecting the variable contribution of the chemical inflammatory component. 3
• Disc herniations can undergo spontaneous resorption as the inflammatory process resolves, explaining why conservative management is often successful. 4
• The favorable natural history—with most pain improving within 2-4 weeks—reflects resolution of the inflammatory component even when mechanical compression persists. 2