Ultrasound-guided lateral femoral cutaneous nerve block is utilized as a diagnostic and therapeutic maneuver in the evaluation and treatment of pain thought to be mediated via the lateral femoral cutaneous nerve. The most common pain syndrome mediated via the lateral femoral cutaneous nerve is the entrapment neuropathy meralgia paresthetica, which is also known as Bernhardt-Roth and skinny pants syndrome. The patient suffering from lateral femoral cutaneous neuralgia will complain of burning pain, paresthesias, and numbness that radiate into the lateral thigh and does not go below the knee (Fig. 117.1). The symptomatology of lateral femoral cutaneous neuralgia is made worse when sitting or squatting for long periods. This painful condition may also occur secondary to compression on the lateral femoral cutaneous nerve by wide belts, low cut pants, or tool pouches. Patients suffering from meralgia paresthetica exhibit tenderness over the lateral femoral cutaneous nerve at the origin of the inguinal ligament at the anterior superior iliac spine as well as a positive Tinel sign over the lateral femoral cutaneous nerve as it passes beneath the inguinal ligament. Careful sensory examination of the lateral thigh reveals a sensory deficit in the distribution of the lateral femoral cutaneous nerve, and no motor deficit should be present (see Fig. 117.1).

Ultrasound-guided lateral femoral cutaneous nerve block can be utilized to provide surgical anesthesia for skin graft harvest procedures from the lateral thigh and to relieve tourniquet pain for lower extremity surgery. Ultrasound-guided lateral femoral cutaneous 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 lower abdominal and groin pain are subserved by the lateral femoral cutaneous nerve. If destruction of the lateral femoral cutaneous nerve is being contemplated, ultrasound-guided lateral femoral cutaneous nerve block with local anesthetic can provide prognostic information as to the extent of sensory deficit the patient will experience following nerve destruction. Ultrasound-guided lateral femoral cutaneous nerve block with local anesthetic may also be used to provide postoperative pain relief following skin grafting procedures, iliac crest bone harvesting procedures, and surgical procedures of the lateral thigh.

Electromyography can distinguish lateral femoral cutaneous nerve entrapment from lumbar plexopathy, lumbar radiculopathy, and diabetic polyneuropathy. Ultrasound imaging may help confirm the diagnosis (Fig. 117.2). Plain radiographs of the hip and pelvis are indicated in all patients who present with lateral femoral cutaneous 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 tumor, infection, or retroperitoneal hematoma is suspected (Fig. 117.2). The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

The lateral femoral cutaneous nerve is one of the nerves derived from the lumbar plexus and comprises nerve fibers from posterior divisions of the L2 and L3 nerves (Fig. 117.3). The nerve leaves the lateral aspect of the middle of the psoas muscle and courses laterally and inferiorly to pass over the iliacus muscle toward the anterior superior iliac spine. The nerve then passes beneath the ilioinguinal nerve at the level of the anterior superior iliac spine and then passes under the inguinal ligament and then travels beneath the fascia lata and over the sartorius muscle, where it divides into an anterior and a posterior branch (Fig. 117.4). The anterior branch provides limited cutaneous sensory innervation over the anterolateral thigh as well as providing communicating fibers to the anterior cutaneous branch of the femoral nerve and with the infrapatellar branch of the saphenous nerve. The posterior branch provides cutaneous sensory innervation to the lateral thigh from just above the greater trochanter to the knee.

Ultrasound-guided lateral femoral cutaneous 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. 117.5). A total of 5 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 anterior superior iliac spine and inguinal ligament are identified by visual inspection and palpation (Fig. 117.6). A linear
high-frequency ultrasound transducer is placed in a plane perpendicular with the inguinal ligament with the superior aspect of the transducer lying over the anterior superior iliac spine and the inferior aspect of the transducer pointed directly at the pubic symphysis, and an ultrasound survey scan is obtained (Fig. 117.7). The hyperechoic anterior superior iliac spine and its acoustic shadow is identified (Fig. 117.8). The ultrasound transducer is then slowly moved in an inferomedial direction along the course of the inguinal ligament until the hyperechoic honeycomb-appearing lateral femoral cutaneous nerve appears lying beneath the fascia lata and on top of the sartorius muscle (Fig. 117.9). Medial to the lateral femoral cutaneous nerve is the femoral nerve, artery, and vein, which can be easily visualized with color Doppler (Fig. 117.10).