Meralgia paresthetica is a painful mononeuropathy of the lateral femoral cutaneous nerve (LFCN), presenting as unpleasant paresthesia, pain, and numbness in the anterolateral thigh.
First described by Bernhardt in 1878, the symptom complex initially comprised of pain, numbness, tingling, and paresthesia in the anterolateral thigh that was not associated with a surgical procedure. However, Roth was credited for using the term meralgia paresthetica, which was derived from the Greek words Bmeros and Balgos,, meaning thigh and pain. Hager was the first to associate meralgia paresthetica with compression of the LFCN.
Meralgia paresthetica is a painful mononeuropathy of the lateral femoral cutaneous nerve (LFCN), commonly due to focal entrapment of the nerve as it passes through the inguinal ligament. Most patients may respond to conservative treatments and painkillers. Injection treatments including pulsed radiofrequency ablation of the nerve may help patients who do not make progress with simpler treatments.
At Pain Spa Dr Krishna is very experienced in performing interventional pain treatments. Dr Krishna always performs Lateral femoral cutaneous nerve blocks under ultrasound guidance for greater accuracy and improved safety. Dr Krishna has performed a number of LFCN ablations under ultrasound guidance with excellent results.
The incidence of the disease is approximately 4.3 per 10,000 person years based on a large case-control study in general practice setting. Spontaneous meralgia paresthetica can occur in any age group but is most prevalent in the age group 41 to 60 years. There was no consensus to sex predominance. Pregnancy and carpel tunnel syndrome are associated with increased occurrence of meralgia paresthetica. The disease also affects paediatric population.
Many aetiological factors can account for the presence of meralgia paresthetica and can be categorized as mechanical, metabolic, and iatrogenic factors. In most of the patients with meralgia paresthetica, there is no attributable cause.
Compressive effect from uterine fibroid or tumor in iliac crest has been implicated as one of the causes of meralgia paresthetica. The risk of meralgia paresthetica is higher in obese and pregnant patients or those with conditions associated with an increase in intra-abdominal pressure and the protrusion of the abdomen. This may be related to the close relationship between the LFCN and the iliac fascia, and thus, the protruding anterior abdominal wall will result in the traction of the iliac fascia and the LFCN. Wearing of belts, corset, and tight low-waist trousers like hip huggers can also result in direct pressure on the nerve. This is of particular significance when the LFCN runs an aberrant course above and lateral to the inguinal ligament.
Meralgia paresthetica has been reported after a variety of orthopaedic surgeries. Because the nerve courses around the iliac bone near the anterior superior iliac spine (ASIS), the LFCN is at risk after surgery in the vicinity such as iliac crest bone grafting, pelvic fixation, and pelvic osteotomy. Meralgia parestheticais a well-known complication after spine surgery. Mirovsky and Neuwirth noted a 20% complication rate in patients undergoing spine procedures. The mechanisms for the LFCN injury include bone grafting from the ASIS, pressure on the nerve while on prone position, and retraction of the psoas muscle in retroperitoneal approaches (the LFCN travels lateral to the psoas muscle in pelvis).
Meralgia paresthetica has also been reported after a large variety of nonorthopedic surgeries such as laparoscopic cholescystectomy, laparoscopic myomectomy, coronary bypass grafting, aortic valve surgeries, bariatric surgery , cesarean section and laparoscopic hernia repair.
The LFCN is a pure sensory nerve arising from the second and third lumbar nerve roots. Emerging from the lateral border of psoas major muscle, the LFCN runs across the iliacus muscle traveling toward the ASIS. The nerve travels under the inguinal ligament medial and close to the ASIS. On entering the anterior compartment of the thigh region, this nerve runs in a lateral and caudad direction and divides into anterior and posterior branches. The diameter of LFCN at the level of inguinal ligament is approximately 3mm.
The most common presentation is a burning and tingling sensation on the anterior and lateral aspects of the thigh as far as the knee. Numbness is a late sign and is rarely the only presentation. Bilateral presentation is uncommon (20%). Pain or unpleasant tingling sensation can be aggravated by standing and hip extension and relieved by sitting. Physical examination may reveal tenderness over the lateral aspect of the inguinal ligament with Tinel sign elicited at the site of entrapment. Hypoesthesia over the area innervated by the LFCN is commonly found with or without allodynia. Clues suggestive of the etiology may be revealed during the examination, such as the surgical scars or raised intra-abdominal pressure.
Because the LFCN is a sensory nerve, the presence of dermatomal sensory loss and motor and sphincter dysfunctions should alert the clinicians to the spinal etiology.
The presence of red flags such as weight loss and appetite change and acute onset of severe pain on pressure may suggest metastasis at the iliac crest or avulsion fracture of ASIS, respectively.
The diagnosis of meralgia paresthetica is mainly on clinical. In situations of uncertainty, both electrophysiological test and diagnostic nerve block can be useful.
When the diagnosis is still uncertain after history and physical examination (eg, atypical sensory distribution of presentation), sensory nerve conduction test can be very useful. The method involves either by stimulating the LFCN as it exits the pelvis near ASIS and recording potentials distally or by stimulating distally along the course of the nerve and recording proximally in the region of ASIS. One limitation of the nerve conduction test is that it evaluates mainly large myelinated axons. Because neuropathic pain in the entrapment syndrome is often due to the disease of small myelinated AC and C fibers, nerve conduction test can be normal in patients whose condition principally affects the small fibers.
Although the use of the LFCN block has not been compared with other diagnostic tests, it can be a useful diagnostic tool to ascertain the diagnosis after initial evaluation.
In general, the LFCN block is performed with either landmark or imaging-guided techniques. The former can be assisted with nerve stimulator and the latter mainly refers to the use of ultrasound.
The technique of application of ultrasound to the LFCN blockade has been well published. In cadavers, the success rate of needle contact with the LFCN was 84% under ultrasound guidance as opposed to 5% with landmark-based technique.
The LFCN appears as one or more hyperechoic or hypoechoic structures in the short-axis view. If the nerve cannot be identified, the alternative is to look for the LFCN in the plane between the tensor fascia lata and the sartorius muscle. Once the LFCN has been identified, a needle is advanced in plane with the ultrasound probe. Alternatively, the needle can be advanced out-of-plane using a nerve- stimulating needle to confirm placement.
In general, the LFCN block is performed with either landmark or imaging-guided techniques. The former can be assisted with nerve stimulator and the latter mainly refers to the use of ultrasound.
The technique of application of ultrasound to the LFCN blockade has been well published. In cadavers, the success rate of needle contact with the LFCN was 84% under ultrasound guidance as opposed to 5% with landmark-based technique.
The LFCN appears as one or more hyperechoic or hypoechoic structures in the short-axis view. If the nerve cannot be identified, the alternative is to look for the LFCN in the plane between the tensor fascia lata and the sartorius muscle. Once the LFCN has been identified, a needle is advanced in plane with the ultrasound probe. Alternatively, the needle can be advanced out-of-plane using a nerve- stimulating needle to confirm placement.
Most of the patients will respond to the conservative therapy. These include mainly lifestyle modification and pharmacological therapy.
Lifestyle modification includes weight reduction and avoidance of tight low-cut pants. Obesity doubles the risk of meralgia paresthetica, probably due to the increase in intra-abdominal pressure. Tight fitting low-cut pants with the waistline resting at hip level may cause meralgia paresthetica, especially in a thin person with an anomalous pathway for the LFCN.
Nonsteroidal anti-inflammatory agents are usually used as the initial pharmacological treatment. The use of tricyclic antidepressants and gabapentinoids for neuropathic pain has been well documented.
Ultrasound guided injection with local anaesthetic and steroid can be of diagnostic and therapeutic value in patients with meralgia paresthetica, especially if conservative management has failed.
Pulsed radiofrequency ablation of Lateral femoral cutaneous nerve can be performed under ultrasound guidance. This can result in long term pain relief lasting 6 months and up to 2 years.
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