lateral femoral cutaneous nerve

ANATOMY: The LFCN (also called the lateral cutaneous nerve of the thigh) arises from the dorsal divisions of the L2 and L3 ventral rami of the lumbar plexus. It emerges from the lateral border of the psoas major and crosses the iliacus obliquely toward the anterior superior iliac spine (ASIS). It passes under or through the inguinal ligament, about 1 cm medial to the ASIS, and enters the thigh over or through the sartorius. It then divides into an anterior branch and a posterior branch. The anterior branch supplies the anterolateral thigh down to the knee. The posterior branch supplies the lateral thigh from the greater trochanter to the mid-thigh. The nerve is purely sensory, with no motor function, so there is no weakness and no reflex loss.

CLINICAL SIGNIFICANCE: Meralgia paresthetica (Bernhardt-Roth syndrome) is the classic LFCN lesion. It is a compressive mononeuropathy at the inguinal ligament or ASIS. Features: - Burning pain, paresthesias, tingling, or numbness over the anterolateral thigh. - Symptoms worsen with standing or walking and with hip extension. They improve with sitting or hip flexion. - Strength and deep tendon reflexes are normal, because the nerve is sensory only. - Cause: entrapment, usually from obesity, pregnancy, tight belts, tight pants, or a tool belt. Other causes are seat belt injury, diabetes mellitus, and iatrogenic injury from surgery near the ASIS (e.g., iliac crest bone graft harvest, anterior hip approach, laparoscopic hernia repair). - Diagnosis is mostly clinical. Pain relief after a local anesthetic block medial to the ASIS supports it. Nerve conduction studies or ultrasound can help in unclear cases. - Treatment: remove the cause (weight loss, loose clothing), then NSAIDs, neuropathic agents (gabapentin, TCAs), and local steroid injection. Surgical decompression or neurectomy is rarely needed.

DIFFERENTIAL (board favorites): - Femoral nerve (L2–L4): quadriceps weakness, diminished patellar reflex, and anterior thigh and medial leg sensory loss (saphenous branch). - L2–L3 radiculopathy: back pain and motor or reflex changes. - Obturator nerve: medial thigh sensory loss and weak adduction. - Retroperitoneal hematoma or mass, or psoas pathology: can compress the lumbar plexus.

HIGH-YIELD PEARLS: - Root values: L2–L3 for the LFCN, L2–L4 for the femoral and obturator nerves, and L1 for the ilioinguinal and iliohypogastric nerves. - Landmark: ASIS and inguinal ligament. - Classic vignette: an obese patient, a pregnant patient, or a patient with a recent tight belt, vest, or tool belt who has lateral thigh burning without weakness. - "Pure sensory" is the key discriminator from femoral neuropathy.