Sciatic Neuropathy

Sciatic neuropathy is dysfunction of the sciatic nerve (L4 to S3), causing weakness of knee flexion and all muscles distal to the knee, with sensory loss in the leg and foot (except the medial leg). The peroneal division is more vulnerable, so foot drop often predominates. Causes are hip arthroplasty and posterior hip dislocation or acetabular fracture, intramuscular injection, prolonged pressure (coma, drug overdose, surgery), trauma, piriformis syndrome, haematoma, tumour, vasculitis and diabetes.

Pathophysiology. Compression, stretch, direct injury or ischaemia of the nerve at the sciatic notch, gluteal region or thigh.

Foot drop, weakness of knee flexion, plantar flexion and toe movements, sensory loss over the lateral leg and sole, absent ankle jerk, burning pain, and trophic changes. Pain in the buttock and posterior thigh. High lesions affect hamstrings, and lesions at the thigh spare the biceps femoris short head.

Red flags.

• Acute foot drop after hip surgery, fracture or prolonged immobility.
• Buttock mass, haematoma or signs of cauda equina syndrome.

Examination with hamstring strength (exclude peroneal neuropathy and L5 radiculopathy). Electromyography and nerve conduction studies at 2 to 3 weeks localise the lesion (sciatic versus peroneal, plexus or root). MRI or ultrasound of the nerve, pelvis and hip for compression, mass or haematoma. MRI lumbar spine to exclude radiculopathy. Check glucose and vasculitis markers.

Differential diagnosis.

• Common peroneal neuropathy, L5 radiculopathy and lumbosacral plexopathy.
• Piriformis syndrome, cauda equina syndrome and motor neuron disease.
• Diabetic neuropathy and peripheral vascular disease.

Urgent causes. Decompress haematoma, reduce hip dislocation, and correct fracture. Surgical exploration for suspected laceration or injection injury, with repair or grafting.

Conservative care. Remove compression, protect the foot, ankle foot orthosis for foot drop, physiotherapy to maintain range of movement and strength, and falls prevention.

Neuropathic pain. Gabapentin 300 mg three times daily, increased to 600 to 1200 mg three times daily (adjust to eGFR), pregabalin 75 to 300 mg twice daily, or amitriptyline 10 to 75 mg at night.

Piriformis syndrome. Stretching, physiotherapy, and injection of local anaesthetic with corticosteroid or botulinum toxin if refractory.

Prognosis. Recovery is best for neurapraxia. Axonal injury regenerates about 1 mm/day, and surgery is considered if no signs of recovery at 3 to 6 months.

Monitoring. Strength and serial electromyography.

Referral and escalation. Refer to neurology and peripheral nerve surgery.