Entrapment, Lateral Femoral Cutaneous Neuropathy
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Lateral femoral cutaneous neuropathy, also known as "meralgia paresthetica", is an entrapment neuropathy that produces the pure sensory syndrome of pain, paresthesias, and/or numbness along the anterolateral thigh. There is no motor component of this nerve; thus, motor deficits are not present.
The lateral femoral cutaneous nerve arises from the lumbar plexus from the L1 and L2 nerve roots. It emerges from lateral border of the psoas muscle and travels across the iliacus to the medial border of the superior iliac spine. The nerve most commonly enters thigh by traveling through or underneath the ilioinguinal ligament. After crossing the ligament, the nerve divides into an anterior branch that supplies the anterior thigh to knee, and a lateral branch that supplies the lateral thigh after piercing the fascia lata. The nerve is at risk for compression at the point it passes through or under the ilioinguinal ligament. In certain cases, it may undergo compression as it passes over the bony prominence of the pelvic brim. Any compression present at these structures may be aggravated by chronic stretching due to a thick overhanging pannus or by external compression from a tight belt.
Patient typically complain of pain, numbness, and paresthesias along the anterolateral thigh. Symptoms almost never extend below the knee. Weakness in the lower extremity is not present. Patients often relate a history of obesity, prolonged sitting with a seatbelt (e.g. truck drivers), the use of a heavy utility belt (e.g. police officers or line workers), or diabetes.
In most cases, the diagnosis is made on clinical grounds. Electrodiagnostic studies may reveal impairment in sensory conduction along the lateral femoral cutaneous nerve. EMG sampling of the upper lumbar paraspinal muscles and lumbar spinal imaging can help exclude an upper lumbar radiculopathy.
Once the diagnosis is made, a trial of noninvasive therapy is indicated. Patients are encouraged to lose weight, refrain from wearing tight belts, and to maintain better control over diabetes, as appropriate. A trial of physical therapy may be quite helpful. The neuropathic pain medications, such as the anticonvulsants and antidepressants, may be utilized. NSAIDs and opiates may be tried as well, depending upon the patient's response. Steroid and/or local anesthetic injections along the course of the nerve at the site of entrapment may be useful as well.
If nonoperative therapy is ineffective, then surgery is typically the next course of treatment. There is some controversy regarding the optimal surgical treatment. One strategy advocates an attempt at nerve decompression and/or transposition, with every attempt possible made to preserve the integrity of the nerve. An advantage of this approach is the avoidance of permanent numbness along the anterolateral thigh following nerve sectioning. Another approach advocates nerve sectioning in all cases. There have been no good comparison studies to evaluate the relative advantages and disadvantages of these two strategies.
Patients who are actually able to lose weight, achieve significant activity modification, and/or maintain better control of their diabetes may do quite nicely with nonoperative therapy. Surgical therapy is effective in the majority of patients who require it.
Numbness, and rarely, deafferentation pain or a sensory neuroma can occur after nerve sectioning. During sectioning, the nerve is divided under tension, and allowed to retract to an intrapelvic location, to minimize the likelihood of forming a painful sensory neuroma.