Schematic diagram showing the pathway of a typical course of lateral femoral cutaneous nerve. (Reprinted with permission from Philip Peng Educational Series)
The lateral femoral cutaneous nerve (LFCN) is a purely sensory nerve from L2 and L3 nerve roots. Emerging from the lateral border of the psoas major muscle, LFCN runs across the iliacus muscle toward the anterior superior iliac spine (ASIS) and passes deep into the inguinal ligament (Fig. 10.1). Before it reaches the level of ASIS, it crosses under the deep circumflex artery.
The LFCN usually emerges from under the inguinal ligament (IL) medial to ASIS at a variable distance (average 29 mm; range 0–73 mm) (Fig. 10.2). A few anatomical variations that one should bear in mind in scanning the LFCN: it may course over or through the IL rather than under it, it may branch before crossing the IL in up to 28% of cases, and it may pass over or posterior the ASIS in 4–29%. The average diameter of LFCN was found to be 3.2 ± 0.7 mm although there is no established normal value for cross-sectional area. Once the LFCN passes under the IL, it usually enters the thigh superficial to the sartorius muscle confined by the fascia lata, although in 22% of cases the LFCN may pass through the sartorius muscle or at times medial to it. Thereafter the LFCN is visualized consistently in a “fatty groove” between the tensor fascia lata muscle and the sartorius muscle. It will normally divide into an anterior and a posterior branch and will pierce the fascia lata in order to innervate the skin. The skin innervated by the LCFN has been characterized in details, covering the lateral aspect of the thigh approximately 8 cm below the greater trochanter and above the tibiofemoral joint line (Fig. 10.3).
Nerves at the inguinal area. (Reprinted with permission from Philip Peng Educational Series)
The skin area supplied by LFCN. (Reprinted with permission from Dr. Danilo Jankovic)
Patient Selection
The diagnosis of MP is mainly on clinical ground. 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, the presence of which theoretically disqualifies the use of the term of meralgia. 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, raised intra-abdominal pressure or low rise jean (hip-hugger). 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. In that case, spine imaging (magnetic resonance imaging) should be arranged. 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.
In the situation of uncertainty, both electrophysiological test and diagnostic nerve block can be useful. Limitation of nerve conduction test is that it evaluates mainly large myelinated axons and the test can be normal in patients whose condition principally affects the small myelinated A δ and C fibers.
Ultrasound Scanning
Position: Supine.
Probe: High-frequency linear 6–18 MHz use the highest for slim individual.
Scanning at the fat-filled grove between sartorius and tensor fascia lata. (Reprinted with permission from Philip Peng Educational Series)
Scan 1
One of the starting points is the fat-filled groove between the sartorius (SAR) and tensor fascia lata (TFL) (Fig. 10.4). At this site, the LFCN is consistently found approximately10cm distal to ASIS. Because the adipose tissue is typically hypoechoic, the nerve with the connective tissue can be easily seen in this region.
Scan 2
There are two drawbacks of the technique described above. First, injection even with 5 mL at this location will miss the proximal branches. While this can be remedied by injecting higher volume or advancing the needle proximally during injection to enhance proximal spread, the injection site is supposedly at or close to the site of pathology, i.e., inguinal ligament, in chronic pain setting. Second, pulsed radiofrequency lesion is commonly used to prolong the analgesic duration and this requires the precise location of the nerve at or close to the site of pathology.
