Meralgia Paresthetica

Etiology

The condition is caused by injury or entrapment of the lateral femoral cutaneous nerve (LFCN) and is classified as spontaneous or iatrogenic. Spontaneous causes include diabetes mellitus, lead poisoning, alcohol use disorder, and hypothyroidism resulting in an isolated neuropathy of the LFCN. Mechanical causes due to pressure on the LFCN include external direct pressure from tight seat belts, belts, or restrictive clothing. Additional mechanical causes include increased intra-abdominal pressure from obesity, pregnancy, or tumors. Leg length discrepancy, degenerative defects of the pubic symphysis, or possibly a rare bone tumor located at the iliac crests near the anterior superior iliac spine may also contribute.

Iatrogenic causes due to surgical intervention or direct nerve injury include hip replacement surgery, spine surgery, laparoscopic inguinal repair, pelvic osteotomy, surgery in the area of the anterior superior iliac spine, iliac crest bone graft harvesting, acetabular fracture surgery, laparoscopic myomectomy, laparoscopic cholecystectomy, vein harvesting for coronary bypass surgeries, or bariatric surgery.[2]

Epidemiology

There is no defined association between gender, but the condition is typically more common in women than men and is also more common in the military. It is most common in the fourth or fifth decade of life but can occur in all ages and is more typical in pregnant patients or those with obesity. Studies have reported an incidence of approximately 3 to 4 out of 10,000 person-years. Carpal tunnel syndrome is also associated with an increased risk of meralgia paresthetica.[2][3]

Pathophysiology

The LFCN is derived from a combination of lumbar nerve roots (posterior divisions of L2/L3 spinal nerves) and sometimes adjacent peripheral nerves. It emerges from the lateral aspect of the psoas muscle, passes under the iliac fascia, then crosses the anterior surface of the iliacus muscle as it travels toward the anterior superior iliac spine. It then passes under, through, or above the inguinal ligament and divides distally into the anterior and posterior divisions. The anterior branch provides sensory innervation of the anterior thigh to the knee, while the posterior branch provides innervation from the lateral thigh to the greater trochanter. The LFCN passes under the inguinal ligament in most patients but may enter the thigh lateral to the anterior superior iliac spine or in a more medial position near the inguinal ligament insertion on the pubis in a small fraction of patients.[4]

Injury to the lateral femoral cutaneous nerve occurs due to external compression or from internal pressure such as from obesity, pregnancy, or tumors. Injury may also occur during surgery as the nerve enters the anterior thigh past the inguinal ligament. Metabolic causes from diabetes, alcohol, or lead poisoning may also result in an isolated nerve injury. Diabetic nerve injury of the LFCN may occur due to swelling from decreased axoplasmic transport or impaired sodium-potassium ATP activity via the sorbitol pathway activation by glucose.[4][5]

Histopathology

Chronic meralgia paresthetica is associated with specific pathologic findings on LFCN biopsy, including multifocal fiber loss, loss of large myelinated fibers, thinly myelinated profiles, regenerating nerve clusters, perineurial thickening, and subperineurial edema. Several cases also reported focal nerve indentation at the inguinal ligament and intraneural or epineurial inflammation.[5]

History and Physical

Patients commonly present with causalgia or burning pain, paresthesias, and hypesthesia over the upper lateral thigh. Symptoms are typically unilateral. Onset is usually subacute over days to weeks. Patients will typically point to or rub their outer thigh when describing symptoms and may have associated hair loss over the area secondary to repeated rubbing. Symptoms are typically unchanged from seating or standing position. However, symptoms may be associated with prolonged hip extension, such as walking or rising from a seated position or prolonged standing. Symptoms may also be relieved by hip flexion movements such as sitting. More rarely, symptoms are worsened by the Valsalva maneuver or other causes of increased intra-abdominal pressure.[6]

Pertinent history includes tight-fitting clothing, recent trauma, recent weight gain, or pregnancy. The area of paresthesia or numbness is usually able to be distinguished on the exam. Symptoms are purely sensory since the LFCN does not contain motor fibers. 

Diagnostic maneuvers include the pelvic compression test in which the patient lies on their unaffected side, and the examiner applies downward pressure on the patient’s ilium/pelvis for approximately 45 seconds. A test is positive if symptoms are reduced and has a reported sensitivity and specificity of 95% and 93%, respectively. Findings not associated with meralgia paresthetica include motor deficits, abnormal lower extremity reflexes, other sensory losses outside the LFCN distribution, or other neurologic symptoms.[7]

Evaluation

Diagnosis is based primarily on the history and physical exam, including purely sensory neurological changes without motor involvement in the anatomical location of the upper thigh. Evaluation should include a complete lower extremity neurologic examination. Deficiencies may be noted with pinprick, and light touch over an approximately 10" x 6” oval area over the anterolateral thigh; however, deficiencies may also be noted more anteriorly or medially based on a patient’s specific anatomy. A lower extremity neurologic exam is normal with preserved deep tendon reflexes, motor strength, negative straight leg raise, and without sacroiliac, back, or hip abnormalities.[8]

Plain radiographs are not required in the diagnosis, given a consistent history and physical exam. Electrodiagnostic studies for the LFCN produce variable results, are difficult to perform on overweight patients, and are typically not indicated in the diagnosis unless used to rule out radiculopathy or plexopathy. Nerve blockade via injection of the lateral femoral cutaneous nerve (in which relief of pain confirms the diagnosis) is rarely used for diagnosis to distinguish from lumbosacral root pain.

The clinician may consider blood tests if a metabolic etiology is suspected. Imaging studies such as a pelvic x-ray may be indicated if a tumor or osteoarthritis is suspected. Likewise, an ultrasound or MRI may be ordered to investigate for a pelvic or retroperitoneal tumor.[2]

Treatment / Management

Meralgia paresthetica is a typically benign and self-limited condition with frequent spontaneous remission. Treatment focuses on patient reassurance and ways to reduce pressure and irritation over the nerve and groin region. This includes patient education that the condition is benign, counseling the patient to avoid tight-fitting garments, and discussion of weight loss if obesity is a contributing factor. Icing the area may be helpful in reducing local nerve irritation and inflammation of acute symptoms.[4]

Medications that may be helpful include NSAIDs, topical capsaicin, lidocaine, or tacrolimus for epidermal dysesthesia or cutaneous hypersensitivity.  Abdominal exercises may reduce pressure, but physical therapy is not a proven modality in treatment. Patients should be reexamined with a repeat neurologic exam if symptoms persist beyond 1 to 2 months with conservative treatments, at which point anticonvulsants such as gabapentin, phenytoin, or carbamazepine may be helpful for treating the patient’s neuropathic pain.[4]

Anesthesiologist referral for a nerve block injection with a local anesthetic or glucocorticoid may also be considered.[9](B3)

Surgery is rare but may be considered in chronic refractory cases and is performed via either a surgical release procedure with decompression of the LFCN (which may include transposition of the LFCN approximately 2 cm medially away from ASIS) or via a nerve transection procedure. The nerve decompression procedure preserves sensory function but is generally less successful than the sectioning procedure, which results in permanent anesthesia and is reserved only for patients with intractable pain. Approximately 80% of patients reported symptomatic improvement in a retrospective series of 167 patients who had a surgical nerve release or transection.[10] (B2)

Several case reports have also noted improvement of refractory cases with pulsed radiofrequency nerve ablation of the lateral femoral cutaneous nerve, electroacupuncture, and kinesiology taping.[11]

Differential Diagnosis

Differential diagnosis of meralgia paresthetica may include lumbar radiculopathy, which is not commonly associated with localized symptoms and usually includes back pain. Other causes of anterolateral thigh pain to consider include abdominal masses, pelvic tumors, metastases in the iliac crest, avulsion fractures, hip osteoarthritis, and chronic appendicitis.[2]

Prognosis

Approximately 85% of patients with meralgia paresthetica report spontaneous recovery with conservative management. Cases due to iatrogenic causes typically improve within 3 months, and pregnancy-associated meralgia paresthetica typically improves after delivery.[2][4]

Complications

Complications of meralgia paresthetica are secondary to the surgical transection of the lateral femoral cutaneous nerve, which results in permanent anesthesia of the anterolateral thigh.[4]