What Causes Sciatica
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 Mechanism
The development of sciatica requires two components working together:
Mechanical compression alone is insufficient to produce symptoms—this explains why 20-28% of asymptomatic individuals have disc herniations on MRI without any pain, compared to 57% of patients with low back pain and 65% with radiculopathy who show disc herniation. 1
Chemical inflammation must accompany mechanical compression for nerve roots to become symptomatic, with tumor necrosis factor-alpha (TNF-alpha) identified as a key mediator in this inflammatory process. 2, 3
Specific Causes of Sciatic Nerve Compression
Most Common Cause
- Lumbar disc herniation at the L4-L5 and L5-S1 levels is the predominant cause, where nerve roots exit the spinal canal and form the sciatic nerve distribution. 1, 4
Other Structural Causes
Spinal stenosis causes bony constriction of the cauda equina and emerging nerve roots through narrowing of the spinal canal from degenerative changes, disc herniations, and malalignment. 1, 4
Neoplasms (primary or metastatic tumors) in the extradural or intradural extramedullary spaces can encroach upon the spinal canal and compress nerve roots. 1, 4
Epidural pathology including abscess or hematoma can directly compress nerve roots. 1, 4
Hemorrhage around the nerve roots can cause compression. 4
Infection or inflammation in the spine can lead to nerve compression. 4
Critical Clinical Pitfall
Do not assume that imaging findings correlate with symptoms. Many patients have disc herniations visible on MRI but remain completely asymptomatic because mechanical compression without the inflammatory component does not produce pain. 1 Conversely, severe pain may be present without imaging evidence of significant nerve root compression when inflammation is the dominant factor. 3
Distinguishing True Sciatica from Referred Pain
True sciatica (radiculopathy) involves pain radiating below the knee into the foot and toes in the sciatic nerve distribution, often accompanied by sensory impairment, motor weakness, diminished deep tendon reflexes, and positive straight-leg-raise test. 4, 5
Referred pain from the lower back radiating to the leg is commonly mislabeled as sciatica but is neither related to disc herniation nor nerve root compression—this distinction is critical because epidemiology, clinical course, and treatment differ substantially. 2