Ultrasound-guided femoral nerve block is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of lower-extremity pain thought to be mediated via the femoral nerve. The most common pain syndrome mediated via the femoral nerve is diabetic amyotrophy, which is a proximal neuropathy commonly seen in diabetic patients, which presents as neuropathic burning pain in the hip and thigh and weakness and wasting of quadriceps muscles.

Ultrasound-guided femoral nerve block can also be utilized to provide surgical anesthesia for the hip and lower extremity when combined with lateral femoral cutaneous, sciatic, and obturator nerve block or lumbar plexus block when the patient would not tolerate general anesthesia and/or the hypotension secondary to the blockade of the sympathetic nerves associated with spinal or epidural anesthesia. Ultrasound-guided femoral nerve block with local anesthetics can be employed as a diagnostic maneuver when performing differential neural blockade on an anatomic basis to determine if the patient’s lowerextremity pain is subserved by the femoral nerve. If destruction of the femoral nerve is being contemplated, ultrasound-guided femoral nerve block with local anesthetic can provide prognostic information as to the extent of motor and sensory deficit the patient will experience following nerve destruction.

Femoral nerve block with local anesthetic may be used to palliate acute pain emergencies, including femoral neck and shaft fractures, and for postoperative pain relief while waiting for pharmacologic methods to become effective (Fig. 116.1). Femoral nerve block with local anesthetic and steroid is occasionally used in the treatment of persistent lower-extremity pain when the pain is thought to be secondary to inflammation or when entrapment of the femoral nerve as it passes under the inguinal ligament is suspected. Femoral nerve block with local anesthetic and steroid is also indicated in the palliation of pain and motor dysfunction associated with diabetic femoral amyotrophy. Destruction of the femoral nerve is occasionally used in the palliation of persistent lower extremity pain secondary to invasive tumor that is mediated by the femoral nerve and has not responded to more conservative measures.

Electromyography can distinguish femoral nerve entrapment from lumbar plexopathy and lumbar radiculopathy. Plain radiographs of the hip and pelvis are indicated in all patients who present with femoral neuralgia to rule out occult bony pathology. Based on the patient’s clinical presentation, additional testing may be warranted, including a complete blood count, uric acid level, erythrocyte sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging (MRI) of the lumbar spine and lumbar plexus and retroperitoneum is indicated if herniated disc, tumor, or hematoma is suspected. The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

The femoral nerve is derived from the posterior branches of the L2, L3, and L4 nerve roots (Fig. 116.2). The nerve fibers enter the psoas muscle where they fuse together within the muscle body and then descend laterally between the psoas and iliacus muscles. The femoral nerve provides motor innervation to the iliacus muscle as it descends toward the iliac fossa. The nerve then passes just lateral to the femoral artery, lying on top of the iliacus muscle and beneath the fascia iliaca as it travels beneath the inguinal ligament with the artery, vein, and nerve enclosed in the femoral sheath (Fig. 116.3). It is at this point that the nerve can consistently be identified with ultrasound scanning and is amenable to ultrasound-guided nerve block. The femoral nerve provides motor innervation to the sartorius, quadriceps femoris, and pectineus muscles and also provides sensory fibers to the knee joint as well as the skin overlying the anterior thigh (see Fig. 116.2).

Ultrasound-guided femoral nerve block can be carried out by placing the patient in the supine position with the arms resting comfortably across the patient’s chest (Fig. 116.4). A total of 7 mL of local anesthetic is drawn up in a 12-mL sterile syringe. If the painful condition being treated is thought to have an inflammatory component, 40 to 80 mg of depot steroid is added to the local anesthetic. The inguinal creased
on the affected side is identified, and a linear high-frequency ultrasound transducer is placed in an oblique plane perpendicular with the inguinal ligament. An ultrasound survey scan is obtained (Fig. 116.5). The iliacus muscle is identified with the femoral nerve lying between the muscle and the pulsatile femoral artery (Fig. 116.6). The femoral vein lies medial to the femoral artery and is easily compressible by pressure from the ultrasound transducer (Fig. 116.7). Color Doppler can be utilized to aid in the identification of the femoral artery and vein (Fig. 116.8). When these anatomic structures are clearly identified on oblique ultrasound scan, the skin is prepped with anesthetic solution, and a 3½-inch, 22-gauge needle is advanced from the lateral border of the ultrasound transducer and advanced utilizing an in-plane approach with the trajectory being adjusted under real-time ultrasound guidance so that the needle passes just on top of the iliacus muscle until the needle tip is resting beneath the fascia iliacus in proximity to the femoral nerve (Fig. 116.9). At that point, after careful aspiration, a small amount of solution is injected under real-time ultrasound imaging to utilize hydrodissection to reconfirm the position of the needle tip. Once the position of the needle tip is reconfirmed, after careful aspiration, the remainder of the solution is slowly injected under ultrasound guidance. There should be minimal resistance to injection. The needle is then removed, and a sterile pressure dressing and ice pack are placed at the injection site.