Ultrasound-Guided Lateral Femoral Cutaneous Nerve Block for Meralgia Paresthetica

To the Editor: The lateral femoral cutaneous nerve (LFCN) is a purely sensory nerve derived from the second and third lumbar roots and innervates the skin of the upper and outer thigh. Blockade of the LFCN may be attempted for treating cases of meralgia paresthetica when patients do not respond to oral medications or conservative measures.1 A 57-yr-old woman with obesity, type 1 diabetes mellitus, and essential hypertension presented with a 10-mo history of burning pain in the lateral area of her left thigh that developed after a decompressive laminectomy for central spinal canal stenosis at L4–5, L5–S1 level. Examination of her thigh revealed hypoesthesia and a complaint of dysesthesia when touching the skin over the left lateral thigh. She consented to a trial of a LFCN block with local anesthetic and steroid. Ultrasonography (SonoSiteMicroMaxx®, SonoSite Inc., Bothell, WA) was used to identify the active site-inhibited factor VIIa (ASIS). With the ultrasound probe (6–13 MHz) scanning transversely inferior to the ASIS, the sartorius muscle was identified. The LFCN is superficial to the sartorius in a tissue plane deep to the fascia lata but superficial to the fascia iliaca (Fig. 1A). A 22-gauge (2.5 inch) Braun Stimuplex needle was inserted in-line with the ultrasound probe and the location of the lateral femoral cutaneous nerve confirmed as the patient experienced a reproducible paresthesia at a level of 1 mA current at 1 HZ frequency of stimulation. A test dose of 1 mL did not increase any pain. The injectate was continually visualized by ultrasound (Fig. 1B) as it spread around the nerve circumferentially and in a cephalad manner, and a total volume of 10 mL was injected (9 mL of bupivacaine 0.25% with 1:200,000 epinephrine along with 40 mg of methylprednisolone). The patient reported a 50% reduction in pain intensity as well as numbness in the distribution of the LFCN, lasting for 72 h. A repeat nerve block was performed 3 wk later using 5 mL of bupivacaine 0.25% with 1:200,000 epinephrine along with 20 mg of methylprednisolone was injected. The patient developed numbness of the left lateral thigh lasting 24 h, but her pain relief was rated as greater than 50% reduction in pain at her last follow up 8 wk after injection. She is now able to walk without support.Figure 1.: A, Ultrasonographic view of LFCN before injection. (FL = fascia lata, FI = fascia iliaca, Sart. M. = sartorius muscle, n = LFCN). B, Ultrasonographic view of LFCN after injection. (FL = fascia lata, FI = fascia iliaca, Sart. M. = sartorius muscle, n = LFCN, LA = local anesthetic and steroid spreading around LFCN).In conclusion, the LFCN may be identified, deposition of the local anesthetic around the nerve can be visualized, and intraneural injection can be avoided in a situation in which there is preexisting nerve dysfunction.2 ACKNOWLEDGMENTS The authors thank Sherif Abbas for his help in preparation of the manuscript. Paul S. Tumber, MD, FRCPC Anuj Bhatia, FRCA, MD Vincent W. Chan, MD, FRCPC Department of Anesthesiology Toronto Western Hospital University Health Network 399 Bathurst St. Toronto, Ontario, Canada [email protected]

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