What is the primary cause of sciatica?

Primary Cause of Sciatica

The primary cause of sciatica is nerve root compromise due to mechanical pressure or inflammation, most commonly from a herniated lumbar disc at the L4/L5 and L5/S1 levels. 1

Understanding Sciatica

Sciatica is not a diagnosis but a symptom characterized by:

• Pain radiating down the leg below the knee in the distribution of the sciatic nerve
• Often accompanied by sensory impairment, weakness, or diminished reflexes in the affected nerve root distribution
• Typically the most common symptom of lumbar radiculopathy

Anatomical Basis

The pathophysiology of sciatica involves:

1. Disc Herniation (Most Common Cause)

   • Over 90% of symptomatic lumbar disc herniations occur at L4/L5 and L5/S1 levels 1
   • Herniation involves the nucleus pulposus protruding through the fibrous outer covering of the intervertebral disc 1
   • This protrusion can directly compress adjacent nerve roots

2. Other Structural Causes

   • Spinal stenosis (narrowing of the spinal canal) 1
   • Posterior intervertebral osteoarthritis 2
   • Tumors (less common) 2
   • Infections or inflammation (uncommon) 1

Dual Mechanism of Pain

Modern understanding of sciatica recognizes both mechanical and chemical components:

1. Mechanical Component

   • Direct compression of the nerve root by herniated disc material
   • Compression causes neural ischemia and mechanical deformation

2. Chemical/Inflammatory Component

   • Proinflammatory substances released by nucleus pulposus tissue 3
   • Tumor necrosis factor-alpha (TNF-α) appears to be a key mediator 3
   • These substances can cause nerve root pain even without significant compression
   • Explains why:
     • Disc surgery doesn't always relieve pain
     • Large herniations aren't always symptomatic
     • Pain severity doesn't always correlate with herniation size 3

Clinical Presentation and Diagnosis

The diagnosis of sciatica primarily relies on:

• History: Pain radiating below the knee into foot and toes, often worse than accompanying back pain 4

• Physical Examination:

  • Straight-leg-raise test (high sensitivity at 91% but modest specificity at 26%) 1
  • Crossed straight-leg-raise test (more specific at 88% but less sensitive at 29%) 1
  • Neurological examination focusing on L4, L5, and S1 nerve root function 1

• Imaging:

  • Not necessary in most cases during first 6 weeks of symptoms 1, 4
  • MRI is preferred when imaging is indicated (better visualization of soft tissues) 1, 4
  • Indicated when symptoms persist beyond 6-8 weeks of conservative treatment or when red flags suggest serious pathology 4

Clinical Course and Management

The natural history of sciatica is generally favorable:

• Most patients improve within 2-4 weeks with or without treatment 4
• Conservative management is first-line for most patients 4, 5

Important Considerations

• Cauda equina syndrome is a rare but serious complication requiring emergency evaluation
• Urinary retention is the most sensitive finding (90%) for cauda equina syndrome 1
• Other red flags requiring prompt evaluation include progressive neurological deficits, suspected cancer, infection, or fracture 1

Remember that while disc herniation is the most common cause of sciatica, the pain mechanism involves both mechanical compression and chemical inflammation of the nerve root, which explains the variable relationship between imaging findings and symptom severity.