BLOCK AT A GLANCE
Block of the lateral femoral cutaneous nerve distal to the anterior superior iliac spine.
Indications: Analgesia for surgery on the anterolateral thigh; skin grafting, muscle biopsy, meralgia paresthetica
Goal: Local anesthetic spread around the nerve superficial or lateral to the sartorius muscle
Local anesthetic volume: 3 to 10 mL
The lateral femoral cutaneous nerve (LFCN) block is a commonly performed technique to provide cutaneous anesthesia or analgesia to the anterolateral aspect of the thigh. The block also can help diagnose and treat meralgia paresthetica. This mononeuropathy of the LFCN is manifested by pain, dysesthesia, or numbness in the area supplied by the LFCN. Ultrasound (US) facilitates clear identification of the nerve and ensures needle placement in the correct fascial plane.
The LFCN is a small sensory nerve arising from the dorsal divisions of L2-L3. After emerging from the lateral border of the psoas major muscle, it courses under the fascia iliaca laterally toward the anterior superior iliac spine (ASIS). The nerve then passes under the inguinal ligament and travels distally over the sartorius muscle into the thigh, where it divides into two branches (i.e., anterior and posterior) to provide innervation to the anterolateral aspect of the thigh (Figure 26-1). There are many anatomical variations of the nerve with regards to its entrance into the thigh.
Anatomy of the lateral femoral cutaneous nerve.
CROSS-SECTIONAL ANATOMY AND ULTRASOUND VIEW
At the level of the ASIS, the LFCN is located just medial to the insertion of the sartorius muscle. As the sartorius muscle descends in a lateral to a medial direction across the anterior thigh, the LFCN travels superficially from its medial to its lateral border. A few centimeters distally, the LFCN is located between the sartorius and tensor fasciae latae muscles. On US, the nerve appears as a small hyperechoic neural structure 0.5 to 2 cm below the skin surface between the fascia lata and superficial fascia of the sartorius, or within a fat-filled hypoechoic space between the sartorius and tensor fascia latae muscles (Figure 26-2).
Transducer position and sonoanatomy of the lateral femoral cutaneous nerve.
DISTRIBUTION OF ANESTHESIA AND ANALGESIA
The LFCN is a purely sensory nerve that provides cutaneous innervation to the anterolateral thigh (Figure 26-3). The innervation territory of the LFCN is highly variable.
Expected sensory distribution of the lateral femoral cutaneous nerve (highlighted in red).