38 Lateral Femoral Cutaneous Nerve Block

The lateral femoral cutaneous nerve (LFCN) is a sensory nerve derived from the second and third lumbar nerve roots. The nerve is a branch of the lumbar plexus that provides cutaneous sensation from the lateral aspect of the thigh. The nerve emerges from the lumbar plexus to travel across the iliacus muscle and rise up toward the anterior-superior iliac spine (ASIS). The nerve usually enters the anterior thigh medial to the ASIS and then crosses over the sartorius muscle from medial to lateral.

Anatomic variation of the LFCN is common. The nerve may consist of as many as four branches where it exits the pelvis. The nerve passes medial to the ASIS by a variable distance and may cross the pelvic bone to reach the anterior thigh. Rather than passing over the sartorius, in some patients the LFCN can pass through the sartorius muscle. The estimated incidence of this ranges between 3% and 22%. Although the LFCN is referred to as a discrete nerve, this designation can indicate a collective set of nerves. Like other cutaneous nerves, the LFCN branches extensively when it enters the subcutaneous tissue. In rare cases (≈7%), the LFCN is absent, and its territory is covered by the ilioinguinal and femoral nerves.

High-resolution ultrasound imaging can identify the LFCN superficial to the sartorius muscle in the proximal thigh.^1,² The nerve has a characteristic medial to lateral course over this muscle. The nerve and muscle are best visualized just medial and distal to the ASIS. As with other small nerves, it is necessary to scan along the length of the nerve to confirm nerve identity. The best imaging technique is to slide the transducer along the known course of the nerve with the nerve viewed in short axis. LFCNs that pass through the sartorius are easier to image because the nerve is surrounded by hypoechoic muscle that provides better acoustic contrast than echobright subcutaneous tissue. Although ultrasound imaging of the LFCN has been reported, the nerve is normally small (1-3 mm in diameter), and sonographic visualization can be difficult within the echobright subcutaneous tissue of the anterior thigh.

LFCN block can be used alone or in conjunction with other lower extremity blocks. It is useful for skin graft harvests and surgical procedures with lateral incisions of the thigh. It is one of the few lower extremity blocks for weight-bearing patients (this group also includes ankle block and saphenous block). LFCN block can improve thigh tourniquet tolerance when combined with other lower extremity blocks.³

If adequate volume is administered, separate LFCN block is not normally necessary after ultrasound-guided femoral block. However, as volume reduction for femoral blocks becomes more feasible because of more targeted injections, sparing of the other nerves such as the LFCN will occur.

Ultrasound imaging may be useful for the diagnosis and treatment of meralgia paresthetica (from Greek meros for “thigh,” and algos for “pain”). Meralgia can result from mechanical stretch or compression injury of the LFCN. Abnormal nerve morphology has been described in patients with meralgia paresthetica. Fusiform enlargement of the LFCN, loss of fascicular discrimination, and hyperemia can all occur in this condition.⁴^–⁶