Normal Lumbar Nerve Roots & the Sciatic Nerve
Five nerve roots merge painlessly into the body's thickest nerve.
- L4–S3: Roots forming sciatic nerve (five nerve root levels)
- ~2 cm: Sciatic nerve width (thickest nerve in body)
- ~1 m: Nerve length (buttock to foot)
- 100%: Foramen clearance (normal) (no root contact)
Root anatomy
Five lumbosacral roots exit through open neural foramina.
Nerve formation
Roots braid together into one sciatic nerve trunk.
Signal conduction
Healthy myelin carries signals fast and pain-free.
Normal conduction velocity runs about 40–70 meters per second.
Mild Disc Bulge or Bone Spur Contact
A small bulge starts touching the nerve root's edge.
- Disc bulge: Typical cause (or early osteophyte)
- <30%: Foramen narrowing (mild encroachment)
- Often none: Symptom onset (or mild ache)
- High: Reversibility (with conservative care)
Disc degeneration
Annulus fibers weaken and the disc begins to bulge.
Bone spur growth
Facet joints may grow small bony overgrowths.
Early nerve contact
Mild pressure barely alters conduction at this stage.
Most mild bulges never progress to symptomatic sciatica.
Radicular Pain Down the Sciatic Distribution
Rising pressure inflames the root and pain travels downward.
- Buttock→leg: Pain pathway (along nerve distribution)
- 30–55%: Foramen narrowing (moderate encroachment)
- Sitting, bending: Common trigger (raises disc pressure)
- Sharp, burning: Pain character (shooting quality)
Mechanical irritation
Direct pressure deforms and inflames nerve fibers.
Inflammatory chemicals
Disc material releases irritants that sensitize the root.
Referred pain pattern
Pain follows the nerve, not the compression site.
Pain often radiates below the knee, unlike simple back strain.
Sensory Loss and Motor Weakness Emerge
Sustained pressure starts blocking sensory and motor signals.
- 55–80%: Foramen narrowing (marked encroachment)
- Numbness: Sensory change (tingling, pins & needles)
- Foot drop risk: Motor change (weak dorsiflexion)
- Diminished: Reflex effect (ankle or knee jerk)
Sensory fiber block
Large fibers fail first, dulling touch and vibration.
Motor fiber block
Weakness appears in muscles the root supplies.
Duration matters
Longer compression deepens both deficits over weeks.
Deficits present over six weeks warrant prompt evaluation.
Severe Nerve Compromise & Permanent Risk
Near-total compression threatens lasting nerve damage.
- >80%: Foramen narrowing (severe encroachment)
- Emergency: Cauda equina risk (if bladder/bowel affected)
- Discectomy: Surgical consideration (or decompression)
- Permanent deficit: Untreated outcome (possible nerve death)
Ischemic injury
Severe pressure cuts blood flow to the nerve root.
Axonal loss
Prolonged ischemia can kill nerve fibers permanently.
Red flag symptoms
Bladder or bowel changes signal a surgical emergency.
Cauda equina syndrome requires decompression within 24–48 hours.