A 56-year-old man comes to the office for evaluation of right leg pain and numbness. The pain started 2 days ago during an 8-hour car ride during which the patient was a backseat passenger in a small car. Midway through the car ride, he began to have numbness and burning pain over the lateral aspect of the right thigh. The patient has tried stretching to relieve the pain, but it has only worsened and he is now unable to wear a belt due to the discomfort. He has type 2 diabetes mellitus and has gained 11.3 kg (25 lb) over the past year. BMI is 42 kg/m². On examination, lower extremity strength is 5/5 bilaterally. The right leg has a large area of numbness over the upper lateral thigh. Straight leg raise is negative. Reflexes are symmetrical and intact. Compression of which of the following nerves is the most likely cause of this patient's presentation?

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11ec160b_35bd_91a0_9249_2f55ae36291e_MD0001_00 This patient has pain and decreased sensation in the distribution of the lateral femoral cutaneous nerve (LFCN), consistent with meralgia paresthetica (MP). The LFCN is a branch of the lumbar plexus, originating from L2-L3. It runs lateral to the psoas muscle and exits the abdomen beneath the inguinal ligament. MP is a compression mononeuropathy of the LFCN at the inguinal ligament, typically caused by tight clothing (eg, belts) or by injury during surgery (eg, hip arthroplasty). Those with obesity, increased lumbar lordosis (eg, pregnancy), or diabetes mellitus are at increased risk. MP presents with pain, paresthesia, and numbness in the lateral thigh above the knee. However, strength is normal because the LFCN does not contain motor fibers. The diagnosis is usually based on clinical findings. Most patients are managed with weight loss and avoidance of tight clothing. (Choice A) The ilioinguinal nerve primarily provides sensory innervation to the upper medial thigh and genital region. It accompanies the spermatic cord through the superficial inguinal ring; injury during pelvic or hernia surgery can lead to chronic suprapubic or groin pain. (Choice C) The obturator nerve is typically injured in anterior hip dislocation or pelvic surgery. Sensory symptoms occur in the medial thigh and are often associated with weakness in the adductor muscles. (Choice D) The pelvic splanchnic nerves (S2-S4) provide parasympathetic innervation to the hindgut, bladder, and urinary sphincters and regulate peristalsis, bladder emptying, and defecation. They are commonly involved in cauda equina syndrome (eg, lumbar epidural metastasis) but are not vulnerable to external compression and would not cause lateral thigh symptoms. (Choice E) The sciatic nerve arises from the L4-S3 nerve roots. Compression of the nerve (or contributing nerve roots) can occur at the spine (eg, herniated disc), piriformis muscle, greater sciatic foramen, or posterior thigh. Sciatica is a classic (though nonspecific) syndrome characterized by back and buttock pain that radiates down the posterior thigh to the foot; associated features include lower extremity weakness and a positive straight leg raise test. Educational objective: Meralgia paresthetica is caused by compression of the lateral femoral cutaneous nerve, typically due to tight clothing. It presents with pain, paresthesia, and numbness in the lateral thigh without motor weakness. Risk factors include obesity, pregnancy, and diabetes mellitus.

A 56-Year-Old Man Comes to the Office for Evaluation of Right