Lateral Femoral Cutaneous Nerve

Introduction

Compression of the lateral femoral cutaneous nerve (LFCN), referred to clinically as meralgia paresthetica, is a clinical syndrome in which patients report experiencing pain, discomfort, and dysesthesia throughout the distribution of sensory innervation of the nerve. The clinical syndrome of meralgia paresthetica is aptly named, as “meros” in Latin translates to thigh, while “algo” refers to pain. The LFCN does not have motor innervation and is therefore a purely sensory afferent nerve that may be compressed anywhere from its beginning at the lumbar plexus, traveling obliquely through the abdominal cavity, and coursing beneath the inguinal ligament and into the tissue of the anterolateral thigh. Peripheral nerve stimulation (PNS) has been used since 1999, when the first peripheral nerve leads were placed percutaneously to treat patients with intractable headaches. Since then, there have been major advances in the management of chronic pain. In this chapter, we will discuss PNS and its uses in the management of LFCN entrapment.

Clinical Presentation

Meralgia paresthetica is characterized by hypesthesia, paresthesia, and pain throughout the sensory afferent distribution of the LFCN fibers. The cause of entrapment is not typically systemic, and therefore symptoms tend to present unilaterally. Like most neuropathies, patients tend to complain of a burning or “pins-and-needles” sensation beneath their skin. The pain tends to have a subacute presentation. Hyperpathia, or pain from light touch, is a common presenting complaint. If the entrapment is occurring within the abdominal cavity, the pain can be exacerbated with increases in intraabdominal pressure, such as the Valsalva maneuver.

Studies have shown various etiologies for patients who develop meralgia paresthetica; compression, trauma, surgery, infection, and exercise all lead to the development of thigh symptoms. Compression of the LFCN can occur due to physiologic processes such as pregnancy in women of reproductive age but is more often secondary to pathology. There have been recorded cases of uterine myomas, ascites, lumbar disc herniation, and psoas tumors all compressing the LFCN, leading to the development of meralgia paresthetica. Various surgeries, including a total hip arthroplasty, have been described to be the cause of LFCN entrapment. One of the most important pathologies leading to the development of meralgia paresthetica is diabetes mellitus. Diabetic polyneuropathy tends to preferentially affect distal sensory neurons, such as the LFCN, and causes damage via multiple proposed pathways that shift the balance of nerve fiber repair and damage in favor of damage. This can lead to neuropathic pain originating from the LFCN.

A positive Tinel’s sign may be identified by tapping over the lateral aspect of the inguinal ligament. Symptoms tend to be exacerbated with changes in posture and locomotion; however, depending on the area of entrapment, some patients have reported aggravation of the pain when seated. There are no motor deficits associated with meralgia paresthetica because the LFCN only contains sensory fibers. In addition to the neuropathic pain, some patients report a “deep coldness” in the area, as well as hair loss (presumably from excessive touching of the affected area of the anterolateral thigh). These clinical symptoms may lead to changes in gait that can cause secondary somatic dysfunctions that require a physician’s attention.

Anatomy

The LFCN is a somatic afferent sensory nerve that originates from the lumbar plexus, specifically from L2 and L3 ( Fig. 23.1 ). Distal to the lumbar plexus, the LFCN courses obliquely between the psoas major and quadratus lumborum muscles before passing through the fascial surface of the iliacus muscle. The LFCN then travels from the iliopubic tract through an aponeuroticofascial tunnel to the inguinal ligament, where there is significant anatomic variation. The nerve can pass either beneath or through the lateral portion of the inguinal ligament ( Fig. 23.2 ) near the anterior superior iliac spine (ASIS). A study using cadavers indicated that the LFCN can course anywhere from 6.5 cm medial to the ASIS to 6 cm lateral to it. Be aware of this variation;it is important from the standpoint of landmark-guided injections. It has been shown that entrapment of a single branch of the LFCN can lead to the classical symptoms of meralgia paresthetica.

The LFCN then enters into the thigh beneath the deep circumflex iliac vessels. It passes anterior to the sartorius but posterior to the fascia lata, where the LFCN divides into anterior and posterior branches. The posterior branch is smaller than the anterior branch and innervates the area of the greater trochanter. The posterior branch typically contains the fibers from L2. The anterior branch is responsible for the sensory innervation of the anterolateral thigh, descending to the knee. The anterior branch typically contains the fibers from L3, from which terminal branches of the nerve contribute to the patellar plexus. Clinically, there are numerous locations where the nerve can experience entrapment, and this number is only increased with the diverse anatomical variation of the LFCN.

Diagnosis