Lateral Femoral Cutaneous Nerve Entrapment

The lateral femoral cutaneous nerve (LFCN) is a pure sensory nerve that is vulnerable to compression. Its course begins at the lumbosacral plexus, travels down through the retroperitoneum, under the inguinal ligament, and into the subcutaneous tissue of the thigh. Meralgia paraesthetica (MP), also known as Bernhardt-Roth syndrome refers to nerve compression causing the clinical syndrome of pain, dysaesthesia, or both in the anterolateral thigh. MP was first described by Werner Hager in 1885. It was later named "meralgia," with the etymology stemming from the Greek words "meros" (thigh), and "algos" (pain). This article deals with the clinical syndrome of LFCN entrapment. Full discussion of injection treatments and the anatomy are discussed elsewhere (See Lateral Femoral Cutaneous Nerve Injection and Lateral Femoral Cutaneous Nerve.)
==Aetiopathophysiology==
The LFCN can be injured anywhere along its course, either in the retroperitoneum or pelvis. Nevertheless, there are two common areas of entrapment. 
*The iliopubic tract (IPT): This is a region of dense connective tissue. It is at the junction of the anterior lamina of the iliac fascia, and the transversalis fascia that invests the transversus abdominis. The LFCN always courses deep to and adjacent to the IPT. Pseudo-neuromas can form proximal to the IPT and is a clue for compression at this site.
*Passage from pelvis to thigh: The nerve makes a 100 ± 10° turn as it passes into the thigh. This makes it susceptible to entrapment, compression, and traction injury. The angle increases with movement and hip extension. Notably, lithotomy position with hip flexion and abduction does not increase the strain on the LFCN. There is increased risk of entrapment at this site with obesity, pregnancy, ascites, tight clothing, seat belts, braces, scoliosis, direct trauma, leg length discrepancy, and muscle spasm.
Meralgia paraesthetica has been described in all anatomical variations of the LFCN. However, it is most susceptible with the following nerve courses: posterior to the ASIS across the iliac crest, medial to the iliac crest superficial to sartorius origin, and medial to the ASIS within the sartorius. LFCN entrapment is less likely with its more medial variants. Repetitive trauma to the LFCN can be indicated by significant enlargement proximal to the inguinal ligament, or pseudo-neuromas (pseudo-ganglions) at the inguinal ligament.
==Epidemiology and Risk Factors==
Nontraumatic MP has an incidence of 3.4–4.3/10,000 person-years. It is significantly linked to carpal tunnel syndrome, pregnancy, obesity, and diabetes mellitus (DM). Diabetes mellitus confers a 7 times higher rate of MP. Conversely, for those without DM, patients with MP are twice as likely to be lateral diagnosed with DM. There is a male predominance.
==Clinical Features==
===History===
Symptoms are classically subacute on onset, and involve burning pain, dysaesthesia (paraesthesia and hypoaesthesia), or both. The affected area is the anterolateral thigh. Pain can be felt anywhere from the anterior to the lateral hip, the anterior and lateral thigh, and distally down to the anterior knee. Some patients feel a coldness, deep muscle aching, profoundly reduced sensation, or hair loss in the area due to stroking it. The symptoms are purely sensory in nature, because the LFCN doesn't have any motor fibres. Symptoms tend to be aggravated by prolonged standing and walking. Relieving factors are typically sitting down, but some patients describe sitting worsening the pain. Patients may modify their gait to reduce symptoms and develop secondary hip, knee, and calf pain.
The clinician should thoroughly assess for red flag symptoms that could suggest something more sinister such as tumours and vasculitis (see differential diagnosis below).
The occupational, exercise, and trauma history that is relevant to LFCNE is below.Trescot, Andrea. Peripheral nerve entrapments : clinical diagnosis and management. Switzerland: Springer, 2016.
Category 	History
Compression 	Obesity
Compression 	Pregnancy
Compression 	Abdominal masses (uterine myoma, retroperitoneal lipofibrosarcoma)
Compression 	Ascites, large abdomen
Compression 	Tight garments or seat belts, especially in thin individuals
Compression 	Leg length discrepancy
Compression 	Lumbar herniated disk
Compression 	Psoas tumor/infection/spasm
Trauma 	“Hip-checked”
Surgery 	Laparoscopic appendectomy, cholecystectomy, hernia repair
Surgery 	Iliac crest graft
Surgery 	Lumbar sympathetic block or neurolysis
Surgery 	Femoral artery catheterization
Surgery 	Occurs in 20 % of spinal surgeries
Surgery 	Total hip arthroplasty
Surgery 	Ilioinguinal repair of a pelvic fracture
Infection/inflammation	Diabetes mellitus
Infection/inflammation	Periostitis of the ilium
Infection/inflammation	Retrocecal tumor
Infection/inflammation	Appendicitis
Exercise 	Strenuous abdominal or lower body exercise
===Examination===
The patient should have characteristic sensory changes over the anterolateral thigh, without any motor findings or reflex abnormalities. There may be reduced sensation to cold and pin prick over the affected area. Due to anatomical variations, abnormalities may be present only over the lateral thigh. There may be tenderness and a positive Tinel's sign adjacent to the ASIS. The most reliable physical finding is ASIS tenderness. Standing, lying straight, and hip extension may be provocative. Sitting or lateral pelvic compression may relieve the pain. Screening hip, lumbar, and sacroiliac joint examinations should be performed.
==Investigations==
===Imaging===
;Radiographs
:Usually unnecessary when symptoms and signs are classical and there are no red flags. X-rays can be helpful in detecting spondylolisthesis, spinal stenosis, and disc disease.
;MRI
:Can evaluate for space-occupying lesions compressing the LFCN, and showing peripheral nerve changes in clinically uncertain cases. It is rare to see the LFCN on axial views because it has a nearly horizontal intrapelvic course where it runs along the anterior surface of the iliacus. Once it leaves the pelvis it can be seen on axial views. 3T MRI has a 94% predictive value in diagnosing meralgia paraesthetica.
===Nerve Conduction Studies===
Sensory nerve conduction study results are highly variable. They are also technically difficult due to the variation in location of the LFCN, and the increased body habitus of the typical patient. Electromyography is not generally indicated because there are no motor fibres, however combined EMG and NCS can be used on occasion to evaluate for radiculopathy and plexopathy.
==Diagnosis==
The diagnosis can usually be made on classical history and examination findings. If there is doubt then a diagnostic ultrasound guided LFCN injection and/or MRI can be done.
==Differential Diagnosis==
Differential 	Features
L2 or L3 radiculopathy 	Low back pain, sensory and/or motor abnormalities below the knee, MRI findings of disc herniation.
Pelvic or iliac crest mass 	Tumour history, genetic risk (HNPCC, BRCA), bloating, constitutional symptoms, palpable pelvic mass, MRI or US findings
Chronic appendicitis 	RLQ pain, constitutional symptoms, CT or US findings
Superior gluteal nerve entrapment 	Pain from the lateral buttock down to the knee, tender mid-lateral buttock; no sensory abnormalities
Femoral neuropathy 	Quadriceps weakness
Hemangioma of pelvis 	Seen on imaging
Inguinal hernia 	Detected on examination, seen on standing if small, positive cough impulse
Hip joint pain 	Restricted range of motion, pain referred to medial thigh, x-ray findings of osteoarthritis.
Greater trochanteric pain syndrome	Lateral thigh pain, Greater trochanteric or hip abductor tenderness, pain relief after local anaesthetic injection,
The clinician must consider other pathologies such as those in the abdomen, lumbar spine, pelvis, and hip. There are also a variety of different peripheral nerve entrapments that can cause thigh pain. Red flag conditions to consider include pelvic masses, chronic appendicitis, caecal tumours, and retroperitoneal sarcomas. These can all cause pain and dysaesthesia in the LFCN distribution. Other conditions that can cause symptoms are haemangiomas, uterine myomas, periostitis, and traction during retroperitoneal procedures.
Any entrapment neuropathy must be differentiated from other mononeuropathies especially vasculitis where the nerve can become ischaemic or infarct. Vasculitis is usually acute in onset and is usually self-resolves over 6 weeks. Entrapment neuropathies generally start slowly, are gradually progressive, and persistent.
Lumbar radiculopathy can cause leg numbness, but it is very unusual for it to cause sensory changes such a localised area seen in LFCN entrapment. Also back/buttock pain are often present. However, LFCN irritation can refer pain both distally, and proximally into the buttock, which could lead to an incorrect diagnosis of lumbar radiculopathy.
==Treatment==
Assuming there is no red flag condition, then modifiable factors should be addressed. Most patients can achieve good pain relief from non-interventional treatment and NSAIDs. The most important modifiable factor is weight loss in those with large abdomens. The patient should be instructed to avoid tight belts or clothes around the waist. Good glucose control should be achieved in diabetes. Alcohol intake should be minimised. If the patient does a lot of long-distance walking or cycling then this could be stopped for a short period. Neuromodulatory such as TCAs drugs can be considered. Topical local anaesthetics and TENS are sometimes helpful.
;Injections
:Lateral femoral cutaneous nerve injection can be performed as both a diagnostic and therapeutic intervention. Injection can be considered for symptoms that persist for 1-2 months in spite of conservative measures. In some cases repeated injections are required. Typical injectate options are corticosteroid and dextrose 5%. Dextrose 5% has been shown to be superior to corticosteroid in a randomised controlled trial out to 6 months. Hydrodissection should be performed.
;Neurolysis
:More invasive procedures such as radiofrequency neurotomy has been described but is not done in NZ to the authors knowledge. Neurolysis causes persistent numbness and has a risk of anaeesthesia dolorosa. There is also a risk of damage to the surrounding femoral and obturator nerves.
;Surgery
:Surgical decompression has been described but it is not universally successful and recurrence can still occur.
==Follow Up and Prognosis==
Assuming there is no red flag condition, the condition is often treatable, but recurrence is common.
==Summary==
*The LFCN has variable anatomy and therefore clinical presentation
*Symptoms may be classic - pain and/or dysaesthesia affecting the cutaneous distribution of the LFCN.
*There is a strong association with diabetes mellitus
*Most cases can be managed with conservative management.
==References==
Category:Pelvis, Hip and Thigh Conditions