Lateral femoral cutaneous nerve blockade is useful for diagnosing and treating pain in the lateral thigh, thought to be from irritation of this nerve.
After informed consent is obtained, the patient is placed in a supine position, and the anterosuperior iliac spine is palpated. The patient is prepared in a standard aseptic fashion over an area large enough to allow palpation of landmarks, and sterile technique is used throughout the procedure. A 1½-in. (4-cm), 21- to 23-gauge needle is inserted 1 cm medially and below the anterosuperior iliac spine. The needle is advanced deeply into the fascia lata toward the shelving of the iliac crest. After negative aspiration, 5 mL of local anesthetic is injected in a fanwise manner (Fig. 67-12).
This nerve can be blocked through an alternative approach by directing the needle superiorly beneath the inguinal ligament into the fascial compartment containing the nerve above the level of the inguinal ligament. This fascial compartment can be identified by directing a short, beveled needle medially to the anterosuperior iliac spine and advancing through the external oblique aponeurosis, the internal oblique muscle, and the fascia iliaca. The short, beveled needle allows the physician to feel a distinct loss of resistance or characteristic pop as the two fascial layers are penetrated. After negative aspiration, 5 mL of local anesthetic is injected to block the lateral femoral cutaneous nerve.
FIGURE 67-12. Lateral femoral cutaneous nerve block. Approach for lateral femoral cutaneous nerve injection and neural blockade.
The lateral femoral cutaneous nerve emerges along the lateral border of the psoas muscle below the ilioinguinal nerve. It runs obliquely under the iliac fascia across the iliacus muscle and enters the thigh by passing posteriorly to the inguinal ligament, just medial to the anterosuperior iliac spine. It provides cutaneous innervation to the lateral aspect of the thigh to the knee. A large area over the lateral aspect of the thigh can be easily blocked with this technique.
The lateral femoral cutaneous nerve block has no significant complications, with the rare exception of a dysesthesia if the nerve is injured during the injection. Severe pain on injection suggests the possibility of an intraneural injection, and the needle should be immediately repositioned. It is possible to block the femoral nerve inadvertently when large amounts of local anesthetic is injected, resulting in a temporary weakness of knee extension and impaired ambulation. This occurs secondary to the medial spread of local anesthetic beneath the fascia iliaca.