Lateral femoral cutaneous nerve
Injury or entrapment of the lateral femoral cutaneous nerve is also known as meralgia paresthetica. It is derived from the Greek word meros, meaning thigh, and algo, meaning pain. It is a syndrome of paresthesia and pain in the lateral and anterolateral thigh. This syndrome is seen most commonly in individuals aged 20-60 years, but it can occur in people of all ages.
Anatomy
This nerve arises from the ventral primary rami of L2-4 where they divide into anterior and posterior branches. The dorsal portions fuse to form the lateral femoral cutaneous nerve in the midpelvic region of the psoas major. The nerve then courses over the iliacus toward the ASIS. The nerve travels posterior to the inguinal ligament and superior to the sartorius muscle at the iliac crest region and divides into anterior and posterior branches. The anterior branch comes off 10 cm distal to the inguinal ligament in line with the ASIS, and supplies cutaneous sensation to the lateral thigh, including just proximal to the patella. It then communicates with cutaneous branches of the femoral nerve and saphenous nerve to form the patellar plexus. The posterior branch pierces the fascia lata posteriorly and laterally, and divides into multiple, small branches that supply the skin from the greater trochanter to the midthigh.7
Etiology
Entrapment usually occurs at the inguinal ligament. The peak incidence for this condition is in middle age. Differential diagnoses include lumbar radiculopathies and discogenic or nerve root problems at L2 and L3. The entrapment may be from intrapelvic causes, extrapelvic causes, or mechanical causes. Intrapelvic causes include pregnancy, abdominal tumors, uterine fibroids, diverticulitis, or appendicitis. Injury has been described in cases of abdominal aortic aneurysm. Examples of extrapelvic causes include trauma to the region of the ASIS (eg, from a seatbelt in a motor vehicle accident), tight garments, belts, girdles, or stretch from obesity and ascites. Mechanical factors include prolonged sitting or standing and pelvic tilt from leg-length discrepancy. Diabetes can also cause this neuropathy in isolation or in the clinical setting of a polyneuropathy.
Clinical
Symptoms include anterior and lateral thigh burning, tingling, and/or numbness, that increase with standing, walking, or hip extension. Symptoms may also increase with lying prone. Symptoms usually are unilateral but may be bilateral in rare cases. The symptoms usually improve with sitting unless compressive forces, such as tight belts or garments, remain.
Physical examination findings may be completely normal. Findings may include hyperesthesia over the lateral thigh (usually in a smaller area than the symptoms). Pain can be produced by pressure medial to the ASIS. A positive Tinel sign may be present over the ASIS or inguinal ligament.
Diagnosis of this entrapment may again be based on an injection of local anesthetic near the inguinal ligament or ASIS. Spontaneous recovery usually is expected. Electrodiagnostic testing may be performed for diagnosis. With nerve conduction studies, the technique includes using a bar electrode for recording and reference. This can be performed with either antidromic (conduction against the direction of sensory fiber conduction) or orthodromic (conduction in the direction of nerve conduction) methods. The antidromic study is usually easier to perform, although response may be absent bilaterally on occasion. The response is small and difficult to obtain in obese patients.
A needle stimulation electrode may needed. The sensory response is absent in 71% of patients with meralgia paresthetica and is prolonged in 24% of patients with this condition. Electromyographic test results with needle are normal in patients with this diagnosis, which may help to differentiate it from an upper lumbar radiculopathy. Technically, the sensory test is a difficult study and a response must be present on the opposite side to determine entrapment. It may be nearly impossible to obtain a response in an obese patient or a patient with a large abdomen without using a needle for stimulation. Unfortunately, the test may be difficult for the patient to tolerate because of the large amount of current (with respect to more peripheral nerves) that is required to stimulate a nerve that lies under adipose tissue.
Treatment
Treatment may include the injection of local anesthetic agents, as previously noted. A steroid can also be used to prolong the effects of the local anesthetic and reduce inflammation. Oral medications, such as NSAIDs, antiseizure medications (gabapentin [Neurontin]), tricyclic antidepressants, and tramadol may be used, as may capsaicin cream and topical lidocaine. One must also instruct patients on ways to prevent further irritation of the nerve. These may include avoidance of hip extension, prolonged standing, and compressive garments. The use of ice and a TENS unit may also be helpful. Surgical exploration may be required if the above treatment options are not helpful. This includes transection of the nerve, or decompression with or without neurolysis. Anatomical variations of the nerve and neuromas can occur and lead to recurrence.8