Sciatica: L4, L5 and S1 nerve root pain distribution
Sciatica is pain or paraesthesia (pins and needles) from compression of the sciatic nerve or its L4 to S3 roots, and the area affected shows the root.
Remember
Match the area of skin to the root: the front and inner leg is L4, the outer leg and top of the foot is L5, and the back of the leg into the sole is S1.
Sciatica is pain or paraesthesia (tingling, or pins and needles) that arises when the sciatic nerve, or one or more of the spinal nerve roots feeding it (L4 to S3), is compressed. It usually starts in the lower back and buttock and then spreads down the leg.
Each root supplies its own strip of skin, called a dermatome, so the area of pain or tingling tells you which root is irritated. Learn the three common patterns.
- L4: the front of the thigh and knee, and the inner (medial) leg.
- L5: the outer (lateral) thigh and leg, and the top (dorsum) of the foot.
- S1: the back of the thigh and leg, and the sole of the foot.
The neighbouring roots are the main traps, so check where the symptoms end up: the top of the foot is L5, whereas the sole is S1. Dermatome maps vary a little between people, so these are classic patterns rather than exact borders.
A paracentral disc herniation (a slipped disc bulging just off the midline) bulges backwards and to one side, so it usually compresses the root that is on its way down to leave at the next level. At the L5/S1 disc that root is S1, so an L5/S1 herniation typically gives pain down the back of the leg.
Numbness around the anus or genitals (the saddle area) with bladder or bowel disturbance points away from simple sciatica, towards cauda equina syndrome: compression of the bundle of nerve roots at the base of the spinal canal, which is an emergency.
Key facts
- L4 pattern
- the front of the thigh and knee, and the inner (medial) leg.
- L5 pattern
- the outer (lateral) thigh and leg, and the top (dorsum) of the foot.
- S1 pattern
- the back of the thigh and leg, and the sole of the foot.
- Top versus sole
- the top of the foot is L5, whereas the sole is S1.
- Front of the thigh
- the femoral nerve, which supplies the front of the thigh, is not part of the sciatic nerve; pain on the front and inner thigh only, without the leg, is a pattern of the femoral nerve and the roots higher up the spine (L2 to L4) and is not sciatica, whereas L4 sciatica also reaches the knee and the inner leg.
- Disc level
- at the L5/S1 disc a paracentral herniation usually compresses S1.
Exam traps
- Pain down the back of the leg that reaches the sole is S1 rather than L5, because L5 runs down the outer leg to the top of the foot, and symptoms that reach the sole point lower, to S1.
- L5 and S1 both give pain below the knee, but pain down the outer leg to the top of the foot is L5, not S1, because S1 runs down the back of the leg to the sole.
- At the L5/S1 disc, the L5 root is the exiting root, which leaves the spinal canal above the disc, so it is compressed mainly if the herniation is far lateral (out to the side, where the root exits); a paracentral herniation typically affects S1.
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