eMedicine Specialties > Neurology > Electromyography and Nerve Conduction Studies

Femoral Mononeuropathy

Author: Elizabeth A Sekul, MD, Associate Professor, Department of Neurology, Medical College of Georgia
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Updated: Jan 5, 2010

Introduction

Background

Femoral neuropathies can occur secondary to direct trauma, compression, stretch injury, or ischemia. Femoral neuropathy causes weakness predominantly of the quadriceps, which results in difficulty with ambulation.

Pathophysiology

Knowledge of femoral nerve anatomy is essential to understanding the mechanism of its injury and to localizing the lesion.

The femoral nerve is part of the lumbar plexus. It is formed by L2-4 roots and reaches the front of the leg by penetrating the psoas muscle before it exits the pelvis by passing beneath the medial inguinal ligament to enter the femoral triangle just lateral to the femoral artery and vein. Approximately 4 cm proximal to passing beneath the inguinal ligament, the femoral nerve is covered by a tight fascia at the iliopsoas groove. The nerve can be compressed anywhere along its course, but it is particularly susceptible within the body of the psoas muscle, at the iliopsoas groove, and at the inguinal ligament.

The main motor component innervates the iliopsoas (a hip flexor) and the quadriceps (a knee extensor). The motor branch to the iliopsoas originates in the pelvis proximal to the inguinal ligament. The sensory branch of the femoral nerve, the saphenous nerve, innervates skin of the medial thigh and the anterior and medial aspects of the calf.

Frequency

United States

Femoral mononeuropathies account for approximately 1% of all mononeuropathies seen in the author's active electrodiagnostic laboratory.

Race

No racial predilection has been noted.

Sex

No gender preponderance is known.

Age

Femoral mononeuropathy is reported in all age groups.

Clinical

History

• Patients with femoral neuropathy complain of difficulty with stairs and frequent falling secondary to "knee buckling."
  • This weakness is typically of acute or subacute onset.
  • This contrasts with a myopathic process in which the weakness is subacute to chronic in onset and bilateral in nature.
• Acute severe pain in the groin, thigh, and/or lower abdomen may occur if the neuropathy is associated with a retroperitoneal hematoma. Otherwise the associated pain is usually mild and located near the inguinal ligament.
• Patients may complain of medial leg and calf numbness. Sensory symptoms in saphenous nerve distribution are rare with injury to the main trunk of the femoral nerve.

Physical

• Weakness of the quadriceps muscle and decreased patellar reflex are the most striking examination findings.
• If the neuropathy is advanced and chronic, wasting of the quadriceps may be noted.
• In some patients, the iliopsoas muscle is involved. In such cases, the lesion must be above the inguinal ligament, as the motor branch to this muscle comes off before the inguinal ligament.
• In isolated femoral neuropathies, the thigh adductors are normal. Although the thigh adductors share common lumbar roots with the muscles innervated by the femoral nerve, they are innervated by the obturator nerve along with the sciatic nerve and therefore are spared. Sensory deficits consist of numbness of the medial thigh and the anteromedial calf.
• If a retroperitoneal hematoma is present, hip extension may cause pain.

Causes

• The femoral nerve is predisposed to compression within the psoas muscle. This commonly is associated with hemorrhage into this muscle due to hemophilia, anticoagulation therapy, or trauma.¹
• Direct trauma to the femoral nerve can occur as a result of penetrating wounds or fractures of the hip or pelvis.
• Lithotomy positioning during delivery or gynecological/urological procedures also has been associated with compressive femoral neuropathies.² In this position, the sharp flexion of the hip can compress the nerve at the inguinal ligament. Excessive hip abduction and external rotation cause additional stretch on the nerve.
• Compression of the femoral nerve also can be due to aortic or iliac aneurysms or tumors.
• Diabetic patients have an unusual predilection for femoral and proximal mononeuropathies. Abnormal femoral nerve conduction has been found in diabetic patients with no clinical femoral nerve involvement.³ The etiology is suspected to be an inflammatory vasculitis.^4,5
• Iatrogenic causes of femoral mononeuropathy include direct pressure or trauma to the nerve during pelvic, abdominal, or spinal surgery or focal damage at the femoral triangle due to a difficult femoral line placement.^6,7 Rarely, femoral neuropathy may complicate hip arthroplasty.⁸

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