Ever had a patient complaining of numbness, or worse yet, a burning sensation to the lateral thigh? This seemingly unexplained burning pain very often means that the patient could be suffering from lateral femoral cutaneous neuralgia. This neuralgia, which was first described in 1878 and named "meralgia paresthetica" from the Greek terms "meros" meaning thigh and "algos" meaning pain, is caused by compression or injury to this nerve.
The lateral femoral cutaneous nerve is a pure sensory nerve. It innervates the lateral aspect of the thigh. It originates from the 2nd and 3rd ventral rami of the lumbar region. The nerve will course from its posterior origin through the lateral border of the psoas muscle towards the anterior superior iliac spine. It then courses behind the inguinal ligament. There it divides into anterior and posterior nerves. These innervate the skin from the greater trochanter and lateral thigh down to the knee.
As previously stated, the usual cause of meralgia paresthetica is due to compression or injury to the lateral femoral cutaneous nerve close to the inguinal ligament. Tumors can also contribute to this painful syndrome; however, there are some cases where an etiology is not identified.
The treatment for meralgia paresthetica is a lateral femoral cutaneous nerve block. This is both diagnostic and therapeutic. It is diagnostic because if the pain resolves after the block then usually the problem is local. Should a good block occur as noted by decreased sensation over the lateral thigh but pains persist, then other causitive factors need to be investigated. Other causes include: fractures of the anterior iliac spine, iliac bone graft, hip arthroplasty, spinal stenosis, laparoscopic herniorrhaphy, laparoscopic cholecystectomy, appendectomy, abdominal aortic aneurysm repair, diabetes, hypothyroidism, pelvic tumor, psoas tumor, lumbar metastasis, iliac metastasis, tight trousers, seat belt trauma, waist belts, obesity, ascites, pelvic inflammatory disease, SLE, hemophilia.
Additionally, it is important to note that the lateral femoral cutaneous nerve block has other uses. It could be done to give regional aesthesis for skin grafting from the upper thigh or muscle biopsy. It can be used for post operative analgesia after hip surgery.
The classic block of the lateral femoral cutaneous nerve is the fan wise technique. While the patient is supine, an area 2 cm medial and 2 cm caudad from the anterior iliac spine is marked. Alcohol or betadine prep is used to sterilize the skin. A skin wheal can be done for anesthesia although most patients tolerate procedure well without the skin wheal. A 22, 25 or 27 gauge, 1 ½ inch needle is placed at the marked site and it is advanced until a "pop" is felt which is the needle piercing the fascia. The mixture of local anesthetic plus steroid is injected. Although some physicians prefer to inject only local anesthetic it is felt that the steroid injected could help potentiate the decrease in pain by decreasing inflammation of the nerve.
The local anesthetics used by physicians vary but 1% lidocaine or .25% bupivicaine x 5–10 cc are usually utilized. The steroid could be methyl prednisolone 40 mg. If subsequent injections are used then decreasing the amount of depomedrol is recommended. A successful block is confirmed by decreased pinprick sensation over the lateral thigh. Other not so common techniques are the transinguinal technique and nerve stimulator technique.
Contraindications to the lateral femoral cutaneous nerve block are infection at site of injection, sepsis, and history of allergy to the injected medications. Should there be a coagulopathy, then a 27 gauge needle can be used for the block. Risks and benefits need to be reviewed/discussed with the patient prior to performing the procedure.
Complications, which are rare but possible, include neuritis to the lateral femoral cutaneous nerve from needle trauma; allergy to injected drugs; drug toxicity; hematoma formation; femoral nerve block or developing infection at the injection site. With proper technique and appropriate injective volumes, these complications are rarer.
Lateral femoral cutaneous neuralgia is a commonly encountered problem amongst clinical practices. While some of these cases cause mild pains and pains that are self limited, other cases can cause moderate to severe pain. It is these cases that require further treatment. Lateral femoral cutaneous nerve block is simple to do for the physicians versed in procedural medicine. It is diagnostic and therapeutic. It can be repeated if necessary. It is a simple and effective way of treating this problem.
