Femoral Neuropathy

Femoral neuropathy is dysfunction of the femoral nerve (L2 to L4), causing quadriceps weakness, knee buckling and sensory loss over the anterior thigh and medial leg (saphenous nerve). Causes are retroperitoneal haematoma (anticoagulation, haemophilia), psoas abscess, pelvic surgery and hip arthroplasty, lithotomy position, femoral artery catheterisation, diabetes (amyotrophy), trauma, tumour and prolonged hip flexion or abduction.

Pathophysiology. Compression, stretch, ischaemia or direct injury of the nerve in the pelvis, iliacus compartment or inguinal ligament.

Knee buckling, difficulty climbing stairs, wasting of quadriceps, absent knee jerk, and sensory loss of the anterior and medial thigh and medial leg. Hip flexion is weak if the lesion is proximal to the inguinal ligament (iliopsoas). Pain in the groin or thigh and a flank or groin mass in haematoma.

Red flags.

• Acute quadriceps weakness in an anticoagulated patient (retroperitoneal haematoma).
• Fever with groin or psoas pain (abscess).
• Falling haemoglobin.

Examine for a lesion proximal or distal to the inguinal ligament (iliopsoas strength, adductors spared). Urgent CT or MRI of the pelvis and abdomen when haematoma or mass is suspected. Electromyography and nerve conduction studies at 2 to 3 weeks distinguish femoral neuropathy from lumbar plexopathy and L3 or L4 radiculopathy, and measure severity. Check coagulation, glucose and HbA1c. Ultrasound of the nerve and groin.

Differential diagnosis.

• L2 to L4 radiculopathy and lumbosacral plexopathy.
• Diabetic amyotrophy, quadriceps tendon rupture and myopathy.
• Motor neuron disease and meralgia paraesthetica.

Retroperitoneal haematoma. Resuscitate, reverse anticoagulation (vitamin K, prothrombin complex concentrate, or specific reversal agents such as idarucizumab for dabigatran and andexanet for factor Xa inhibitors), correct coagulopathy, and surgical or radiological intervention if the nerve is compressed or the patient is unstable. Early decompression in selected cases.

Abscess. Antibiotics and drainage.

Other causes. Treat diabetes with glycaemic control, remove external compression, and explore for transection or after a surgical injury.

Rehabilitation. Physiotherapy for quadriceps strengthening, knee brace or splint to prevent buckling, walking aids and falls prevention.

Pain. Gabapentin 300 mg three times daily, increased as tolerated, or amitriptyline 10 to 50 mg at night.

Prognosis. Most patients with neurapraxia recover over weeks to months. Axonal injury takes 6 to 12 months.

Monitoring. Strength and electromyography at 3 months.

Referral and escalation. Refer urgently to surgery or radiology for haematoma, and to neurology or nerve surgery if no recovery by 3 to 6 months.