The lateral femoral cutaneous nerve (LFCN) divides into approximately two to five branches innervating the lateral and upper aspects of the thigh. Many studies have described how the variable anatomy of the lateral femoral cutaneous nerve makes it challenging to perform an effective landmark-based technique block. Ultrasound guidance, however, allows more accurate needle insertion into the appropriate fascial plane where lateral femoral cutaneous nerve passes through, allowing for a higher success rate.
The lateral femoral cutaneous nerve typically is located between the tensor fasciae latae (TFLM) and sartorius (SaM) muscles, 1 to 2 cm medial and inferior to the anterior superior iliac spine (ASIS) and 0.5 to 1.0 cm deep to the skin surface (Figure 42.4-1). Ultrasound imaging of the lateral femoral cutaneous nerve yields an oval hypoechoic structure in cross-sectional view. The lateral edge of the sartorius muscle is a useful landmark, and as such, it can be relied on throughout the procedure. (Figures 42.4-2A and B). The LFCN branches sometimes may be seen across the anterior margin of the TFL.
FIGURE 42.4-1. Cross-sectional anatomy of the lateral femoral cutaneous nerve (LFCN). Shown are LFCN, sartorius muscle (SaM), and tensor fascia latae muscle (TFLM).
FIGURE 42.4-2. (A) Ultrasound anatomy of the lateral femoral cutaneous nerve (LFCN). (B) Labeled ultrasound anatomy of the LFCN.
Distribution of Blockade
Block of the lateral femoral cutaneous nerve provides anesthesia or analgesia in the anterolateral thigh. There is a large variation in the area of sensory coverage among individuals because of the highly variable course of LFCN and its branches.
Equipment needed is as follows:
• Ultrasound machine with linear transducer (6–18 MHz), sterile sleeve, and gel
• Standard nerve block tray
• Syringe(s) with 10 mL of local anesthetic (LA)
• 50-mm, 22- to 24-gauge needle
• Sterile gloves
Landmarks and Patient Positioning
Block of the lateral femoral cutaneous nerve is performed with the patient in the supine or lateral position. Palpation of the anterior-superior spine provides the initial landmark for transducer placement; the transducer is first positioned at 2 cm inferior and medial to the ASIS and adjusted accordingly. Typically, the nerve is identified slightly more distally in its course. If nerve stimulator is used, precise identification of the LFCN may be confirmed by eliciting a tingling sensation on the lateral side of the thigh.
The goal is to inject local anesthetic in the plane between the tensor fasciae latae and the sartorius muscle, typically 1 to 2 cm medial and inferior to the anterior-superior iliac spine.
With the patient supine, the skin is disinfected and the transducer placed immediately inferior to the ASIS, parallel to the inguinal ligament (Figure 42.4-3). The tensor fasciae latae and the sartorius muscle should be identified. The nerve should appear as a small hypoechoic oval structure between the tensor fasciae latae and the sartorius muscle in a short-axis view. A skin wheal is then made on the lateral aspect of the transducer, and the needle is inserted in-plane in a lateral to medial orientation, through the subcutaneous tissue and the tensor fasciae latae muscle (Figure 42.4-4A). A pop may be felt as the needle tip enters the plane between the tensor fascia latae and sartorius muscles. After gentle aspiration, 1 to 2 mL of LA is injected to verify the needle tip position. When the injection of the LA appears to be intramuscular, the needle is withdrawn or advanced 1 to 2 mm and another bolus is administered. This is repeated until the correct position is achieved by visualizing the spread of the LA in the described plane between the tensor fasciae latae and sartorius muscles (Figure 42.4-4B).
FIGURE 42.4-3. Transducer position and needle insertion to accomplish a lateral femoral cutaneous nerve (LFCN) block.
FIGURE 42.4-4. (A) Simulated needle path (1) to block the LCFN. (B) Simulated needle path (1) and local anesthetic spread (area shaded in blue) to anesthetize the LFCN.
• An out-of-plane technique can also be used for this nerve block. Because the needle tip may not be seen throughout the procedure, small boluses of local anesthetic (0.5–1 mL) are injected as the needle is advanced, to confirm the exact position.
In an adult patient, 5 to 10 mL of LA is usually sufficient for successful blockade. In children, a volume of 0.15 mL/kg per side is adequate for effective analgesia.
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