Overview
This lecture explains the anatomy of the sciatic nerve, its role in sciatica, and how its structure and branches relate to clinical symptoms in patients.
Sciatic Nerve Anatomy
- The sciatic nerve is the largest nerve in the leg, formed from L4, L5, S1, S2, and S3 spinal nerve roots.
- Irritation often occurs at L4, L5, and S1 levels, commonly due to disc herniation.
- The nerve exits through the sciatic notch and passes deep to the piriformis muscle.
Causes & Epidemiology of Sciatica
- Sciatic nerve irritation can be due to various structures in the gluteal (buttock) region, not just piriformis muscle.
- Deep gluteal pain syndrome is now preferred over "piriformis syndrome" as the cause is often multifactorial.
- Deep gluteal pain syndrome accounts for only 0.3–6% of sciatica cases.
- Sciatica typically causes pain, pins and needles, numbness, or burning in the back of the leg, not the front.
Sciatic Nerve Branches and Clinical Relevance
- Sciatic nerve splits into the tibial nerve and common peroneal (fibular) nerve at the posterior knee.
- Tibial nerve supplies key muscles for plantar flexion (e.g., gastrocnemius, soleus); weakness here suggests tibial nerve involvement.
- Common peroneal nerve divides into deep and superficial peroneal nerves (also called fibular nerves).
- Deep peroneal nerve supplies anterior tibia muscles (dorsiflexors); dysfunction leads to foot drop.
- Superficial peroneal nerve supplies lateral leg muscles (evertors); weakness means loss of eversion strength.
- Mnemonic: ADLs—Anterior Deep (deep peroneal), Lateral Superficial (superficial peroneal).
Clinical Signs and Dermatomes
- Ankle reflex tests S1-S2; a reduction indicates possible sciatica.
- L4 dermatome: medial tibia; L5: fibular region to first toe; S1: lateral foot.
- L5 and S1 dermatomes most often affected in sciatica (pins and needles, numbness, especially lateral foot).
Key Terms & Definitions
- Sciatic nerve — the largest nerve in the leg, formed from L4-S3 spinal nerves.
- Piriformis syndrome / Deep gluteal pain syndrome — pain caused by structures compressing the sciatic nerve in the gluteal region.
- Tibial nerve — branch of the sciatic nerve, supplies muscles for foot plantar flexion.
- Common peroneal (fibular) nerve — sciatic nerve branch splitting into nerves for dorsiflexion and eversion.
- Dermatome — area of skin supplied by a single spinal nerve root.
- Foot drop — inability to dorsiflex the foot, a sign of deep peroneal nerve dysfunction.
Action Items / Next Steps
- Review the anatomy and branching of the sciatic nerve.
- Practice identifying symptoms corresponding to tibial and peroneal nerve involvement.
- Learn the dermatomal patterns for L4, L5, and S1.