Meralgia Paresthetica—A Common Cause of Thigh Pain
Meralgia paresthetica (MP), a common condition seen by both primary care physicians and specialists, is easily misdiagnosed because it can mimic other disease processes. Often, merely the awareness of the condition and knowledge of a few key differentiating factors helps the clinician quickly recognize this disorder.
MP is a mononeuropathy caused by impingement of the lateral femoral cutaneous nerve (LFCN), which supplies sensation to the lateral aspect of the thigh. When impingement or entrapment of the nerve occurs, the patient can experience numbness, burning, stabbing, and aching along the well-delineated path of the LFCN, from the front of the thigh to just above the knee (Figure 1).
The most common cause of impingement of the LFCN is entrapment of the nerve under the inguinal ligament (Figure 2), which can occur spontaneously or develop after an injury.1 Causes of LFCN nerve entrapment can be divided into 3 categories: mechanical, metabolic, and iatrogenic. Pregnancy (or any condition that increases abdominal pressure), obesity, wearing tight clothing/belts in the waist area, different leg lengths, and pubic symphysis (pelvic girdle) dysfunction are common mechanical issues leading to MP.2 It also has been recognized that carrying items such as a wallet or a cell phone in the front and side pockets of pants can cause unintentional compression of the nerve. Metabolic causes include neuropathy (from diabetes or alcoholism), hypothyroidism, and lead poisoning. Clinicians should ask patients about occupational and living conditions that may have exposed them to lead paint. Prolonged traction during spine surgery or injury to the nerve during retroperitoneal dissection are common iatrogenic causes; therefore, a surgical history is important to consider.3
It is imperative to differentiate MP symptoms from other causes of pain and nerve discomfort that can have similar clinical presentations. Included in the differential are:
- Spinal nerve radiculopathy at L1-L34
- Malignancy or metastasis to the iliac crest5
- Uterine fibroids or pelvic mass that compress the nerve6
- Avulsion fracture of the anterior superior iliac spine (ASIS)7,8
- Chronic appendicitis9
Hip pain that presents with neurological, urogenital, or gastrointestinal symptoms should prompt a more focused evaluation to rule out MP.10 Although symptoms may vary between patients, the differential diagnosis of MP is limited, and it usually can be distinguished through clinical evaluation and diagnostic workup.
The patient’s medical history can elicit key information to direct the clinician to the diagnosis of MP. For example, a thorough discussion of the type of clothing the patient wears (tight), use of thick belts, or work apparel that can lead to pinching at the ASIS can elicit whether symptoms are worse during their use. Another clue is if symptoms worsen with prolonged standing and hip extension and improve with sitting.11
The pelvic compression test is a diagnostic tool that involves placing the patient in the lateral recumbent position while an external downward force is applied with the examiner’s hands over the lateral aspect of the ASIS. The pressure is held for 45 seconds to determine if the patient’s symptoms improved. Symptom amelioration is considered a positive test result and helps to rule out lumbosacral radicular pain.12 In a study conducted by Nouraei et al, 19 out of 20 patients who had abnormal nerve conduction studies also had a positive pelvic compression test.12 Based on these results, the pelvic compression test has a sensitivity of diagnosing MP of 95%. The same study found that a negative pelvic compression test was found in 14 out of 15 patients, yielding specificity of 93.3%. The course of the LFCN, from the abdomen to the thigh, can be variable, with 5 separate anatomic subtypes described.13
Sensory testing with light touch or monofilament testing can be performed with the patient in the supine position to determine the presence of numbness or paresthesia along the LFCN distribution. The LFCN is a sensory nerve only, so the patient will not have any resulting motor weakness in true MP. The Tinel’s sign also can be used to elicit tenderness and sensitivity over the lateral aspect of the inguinal ligament just medial to the ASIS (where the LFCN typically runs).14 In the LFCN distribution, often there is a small area of hair loss on the thigh due to persistent rubbing of the area by the patient.10
A diminished or absent cremasteric reflex (in male patients) can indicate L1 nerve damage, which would eliminate MP as a possible diagnosis. Manual muscle testing of myotomes L2-L5 is critical, because again, LFCN pathology will not cause motor dysfunction in any of lower extremity muscles. The patellar tendon reflex will be normal, and a straight leg test also should be performed to rule out lumbar radiculopathy.10 If a motor or sphincter dysfunction is present on exam, the patient’s symptoms likely are secondary to a spinal cord or nerve root lesion.
Plain radiographs (x-rays) of the pelvis and hip (AP and frog leg views) should be obtained first to rule out osteoarthritis of the hip joint or bone metastasis to the ileum.5 Magnetic resonance imaging or computed tomography can rule out disc herniation, nerve lesions, annular tears, or other spinal pathology that may be causing radiculopathy. A pelvic ultrasound should be obtained in women of childbearing age with a history of prolonged menstrual bleeding to rule out uterine fibroids.6 A detailed history also will indicate a history of alcoholism or lead exposure.
During the initial work-up, the clinician should order blood work to test for thyroid function, vitamin B12 and folate levels, and serum lead levels because these are common causes of MP.3 A complete blood count may be ordered to evaluate for macrocytic anemia and diabetes in patients with neuropathy symptoms.
A nerve test (blockade) may be both diagnostic and therapeutic in patients suspected of having MP. Using a nerve stimulator, the LFCN can be located and injected with a local anesthetic. Relief of the numbness and pain confirms the diagnosis of MP. Newer methods of conducting nerve blocks include landmark-based and ultrasound-guided techniques.14 The landmark approach can present a challenge because the normal course of the LFCN and anatomy can vary in patients. This method involves insertion of a needle, 2.5 cm medial to the ASIS and caudal to the inguinal ligament. Lidocaine can be injected when a “loss of resistance” or “pop” is felt as the needle goes through the fascial layer.3,8,15