Anatomy

The lateral femoral cutaneous nerve (LFCN), composed of the L2/L3 branches of the lumbar plexus, is a pure sensory nerve. It emerges at the lateral border of the psoas major muscle and descends laterally and obliquely towards the pelvis. In the pelvis, the nerve lies within aduplication of the fascia iliaca and on top of the iliacus muscle. The nerve takes a remarkable sharp turn and courses inferiorly before it enters the thigh via the muscular space, lacuna musculorum, underneath or even through a gap of the inguinal ligament in variable distance to the anterior superior iliac spine (ASIS). It then arrives on top of sartorius muscle (SM) in most cases, deep to the fascia lata. Importantly, the main trunk (the anterior branch or division) stays subfascial approximately 7-10 cm below ASIS (Figure 1).

Figure 1. Anterior view of the lateral femoral cutaneous nerve which is exposed after dissection

ASIS = anterior superior iliac spine LFCN = lateral femoral cutaneous nerve (with distal dye stain) FL = fascia lata (opened in this dissection to expose the nerve) SM = sartorius muscle underneath the nerve

The anterior branch (division) provides cutaneous sensory supply to the anterolateral aspect of the thigh as far down as the knee. The posterior branch or division pierces the fascia lata more proximal than the anterior portion and supplies the lateral side of the thigh from the greater trochanter to the mid thigh. It also supplies part of the gluteal skin. Please see Clinical Pearls section regarding important anatomical variations.

Sonoanatomy

Sonographically, the lateral femoral cutaneous nerve is usually a relatively hyperechoic nerve structure best seen in the proximal thigh (several cm inferior to the anterior superior iliac spine) within a fat filled hypoechoic space. This space is found between the sartorius muscle (SM) medially and the tensor fascia lata muscle (TFL) laterally. The nerve is covered by the fascia lata, thus the nerve is subfascial (Figure 2).

Note that the nerve is not subcutaneous.

Figure 2. Sonogram showing the lateral femoral cutaneous nerve in the fat filled space

LFCN = lateral femoral cutaneous nerve TFL = tensor fascia lata muscle SM = sartorius muscle

Scanning Technique

• Positionthe patient supine.
• After skin and transducer preparation, place a linear 38-mm high frequency (12 MHz or higher) transducer transverse on the skin surface at least 3 fingerbreadths below the ASIS to obtain the best possible view of the subcutaneous tissue, fascia lata, the muscle layers and the nerve (Figure 3).

Figure 3. A high frequency linear transducer is placed transverse ~ 3 finger-breadths below ASIS

ASIS = anterior superior iliac spine

• Because the nerve is small and superficial, it is advisable to use the highest frequency transducer e.g., up to 17 MHz whenever possible especially in slim subjects.
• Optimize machine imaging capability. Select appropriate depth of field (within 1-2 cm), focus range (usually within 1 cm) and gain.
• Visualize the nerve in the subfascial plane below the ASIS in the transverse view. The nerve usually appears hyperechoic relative to the surrounding hypoechoic adipose tissue within the often triangular space.
• Close to the ASIS, the nerve lies on the surface of the sartorius muscle. As the LFCN traverses caudally, it tends to lie in the groove between the sartorius muscle and tensor fascia lata muscle.
• Apply very gentle transducer pressure on the skin surface so that the subfascial space holding the LFCN is not completely collapsed. In the very slim individuals, it is beneficial to apply copious amount of gel on the skin surface to build up a "standoff pad" for acoustic coupling between the transducer and the skin surface.

Nerve Localization

Palpable external landmarks that help to locate the LFCN are :

1. the ASIS (Figure 4);
2. the inguinal crease; and
3. a palpable groove between the SM and TFL muscle approximately 3 fingerbreadths caudad to ASIS (Figure 5).

Figure 4. Finger pointing at the anterior superior iliac spine, an obvious external landmark



Figure 5. Initial transducer position over the groove between TFL and SM ~ 3 finger-breadths below the ASIS

ASIS = anterior superior iliac spine LFCN = lateral femoral cutaneous nerve TFL = tensor fascia lata muscle SM = sartorius muscle O = ideal site to start scanning

The palpable groove between the SM and TFL muscle can be accentuated by asking the subject to actively flex the thigh.

A number of internal landmarks also help to locate and consequently trace the nerve proximally. They are:

1. the SM medial and caudad to ASIS (Figure 6);
2. the TFL muscle lateral and caudad to ASIS (Figure 6);
3. a fat filed space between the SM and TFL muscle.

Figure 6. Sonogram showing internal landmarks caudad to the ASIS

TFL = tensor fascia lata muscle SM = sartorius muscle

Also at the level of ASIS, thebony shadow of ASIS, the ilioinguinal ligament and the iliacus muscle are important internal landmarks for nerve localization (Figure 7).

Figure 7. Sonogram showing internal landmarks at the ASIS level

ASIS = anterior superior iliac spine IL = inguinal ligament ILM = iliacus muscle

The LFCN is a small nerve which can be challenging to locate and identify. This is particularly true near the ASIS. It is therefore advisable to trace the nerve starting distally and scan progressively cephalad.

After an initial palpation (ASIS and the groove between SM and TFL muscle), move the transducer (oriented transverse) to a site at least 3 fingerbreadths caudad to ASIS and search for the LFCN in the hypoechoic fat filled space deep to the fascia lata between the SM (medially) and TFL muscle (laterally).

This superficial fat filled groove is often flat, almond like or triangular in shape. It is interesting to note that similar compartments containing sensory nerves are commonly found throughout the body and they are called "fat filled flat tunnels."

The apparent size of this space is greatly influenced by the amount of transducer pressure applied to the skin. Again heavy transducer pressure will obliterate the space making visualization of the content difficult.

Within the space, the LFCN often has a honeycomb appearance and is regularly hyperechoic compared to the surround fat. It is often oval or spindle shaped.

Aim to identify the fascia lata, the superior border of this space (Figure 8). Note that the nerve is subfascial and not subcutaneous.

Figure 8. Sonogram showing LFCN in the groove

FL = fascia lata LFCN = lateral femoral cutaneous nerve TFL = tensor fascia lata muscle SM = sartorius muscle

Once identified the nerve, move the transducer cephalad towards the ASIS to trace the LFCN to the anterior surface of the SM as the muscle becomes smaller and smaller at its attachment to the ASIS.

The echogenicity and shape of the LFCN may change depending on the level of scanning and the amount of fatty and/or connective tissue in a given individual.

More cephalad near to the ASIS, the nerve may become hypoechoic and more flat as it is now situated within connective tissue layers that have a minimum amount of hypoechoic adipose tissue (Figure 9). In case of two or more LFCNs or early branching of anterior division, each of these small nerves may appear as hypoechoic "bubble" (flat or round in shape) with white boundaries.

Figure 9. Sonogram showing a very flat LFCN at the level of ASIS

LFCN = a very flat lateral femoral cutaneous nerve lying on top of the sartorius muscle near ASIS SM = the tendinous part of the sartorius muscleclose to itsattachment at the ASIS

Note that it is technically challenging to image the LFCN immediately medial to or at the ASIS as the nerve crosses the inguinal ligament and makes almost a 900 turn from above the ASIS to below the ASIS (yellow broken line in Figure 10). Additionally, the nerve courses oblique in the pelvis before reaching the thigh (Figure 10). Due to numerous connective tissue layers and bone artifacts in this region, it is sometimes impossible to visualize the LFCN at this site.

Figure 10. The oblique course of the LFCN above the ASIS in the pelvis (yellow broken line)

LFCN = lateral femoral cutaneous nerve Yellow dotted line = oblique pelvic course of LFCN above ASIS

For this reason, it is necessary to change the transducer angle for imaging at or above ASIS. Casting the ultrasound beam to insonate the nerve at 90 degrees at this location can be difficult. This is the reason for a suboptimal image.

Needle Insertion Approach

• Ultrasound guided LFCN block is considered a basic level (level I) block.
• The ideal site for blockade of the LFCN for diagnosis or treatment of meralgia paresthetica is next to the ASIS and not below. Nevertheless, scanning started at least 3 finger breaths below the ASIS helps to locate the nerve and then trace it proximally to the ASIS level.
• Either an out-of-plane or in-plane technique is suitable for this block (Figure 11).

Figure 11. Out of plane needle insertion approach for LFCN block

• With the patient lying supine up, a 22 G, 5 cm needle is advanced in the superficial plane to enter the fat filled space containing the LFCN. This space becomes progressively smaller near the ASIS.
• It is important to use the hydrolocation technique to localize the needle tip.
• Remember the optimal needle tip location is just underneath the fascia lata.

Local Anesthetic Injection

It is often sufficient to block the nerve with a small volume of local anesthetic, as little as 0.5 mL, for diagnostic purpose.

The goal is to raise the fascia lata at the time of local anesthetic injection (Figure 12 and Figure 13).

Figure 12. LFCN pre injection

LFCN = lateral femoral cutaneous nerve

Figure 13. LFCN post injection

LFCN = lateral femoral cutaneous nerve FL = fascia lata

Clinical Pearls

Course of the Lateral Femoral Cutaneous Nerve

The whole course of the LFCN as well as its formation is highly variable among individuals. Thus the LFCN may not be found immediately medial to the ASIS. Other practical important locations to locate the LFCN are:

1. the nerve may be far more medially located, e.g., halfway between the ASIS and the pubic symphysis or tubercle; in this case, the LFCN is sandwiched between fascia lata and fascia iliaca before it comes on top of SM;
2. the nerve may course over the iliac crest lateral to the ASIS;
3. very rarely, the LFCN runs through or deep to the substance of the sartorius muscle (e.g., if the nerve is arising from the femoral nerve). In these cases, tendinous part of the sartorius muscle may be mistaken for the nerve.

The LFCN may also divide into 2 or 3 small branches as it leaves the pelvis and reaches the anterior surface of the sartorius muscle. If this happens, one would see 2 or 3 tiny hypoechoic "bubbles" which make visualization very challenging because of their small size.

Because of all the anatomical variants mentioned above, ultrasound detection of the LFCN can be challenging thus block failure may happen.

Please note that regardless of anatomical variations, the key to success for locating the LFCN is to first locate the nerve distally in the fat filled space and then trace the nerve proximally towards the ASIS.

The LFCN Proximal to ASIS (an Alternate Approach)

It is also possible to view the nerve above the ASIS and the inguinal ligament. The transducer is positioned extremely oblique (Figure 14) and more medially in the lower abdomen. This transducer angulation is required to orient the beam at ~900 to the nerve as the nerve courses anteriorly and laterally on top of the iliacus muscle (see transducer position above ASIS in Figure 14).

Figure 14. Optimal transducer position above and below ASIS

LFCN = lateral femoral cutaneous nerve Yellow dotted line = oblique pelvic course of LFCN above ASIS Red line + TP = optimal transducer position above ASIS

Because the nerve is deep in this region, a greater-than-normal transducer pressure is required to compress the abdominal wall tissues so that the transducer can be closer to the nerve and the ilacus muscle (Figure 15).

Figure 15. Transducer position for scanning the LFCN above the ASIS

ASIS = anterior superior iliac spine

The LFCN often appears flat and hypo or hyper-echoic within the iliac fascia on top of the iliacus muscle (Figure 16). This is the start of its intrapelvic course upwards.

Figure 16. Sonogram showing the LFCN above the inguinal ligament and ASIS

ASIS = anterior superior iliac spine ILM = iliacus muscle

Pitfalls in Nerve Localization

It is possible to visualize a number of hyperechoic lines superficial to the fascia lata. These are lines within the subcutaneous adipose layer that may be mistaken for the fascia lata leading to a search for the LFCN in the wrong compartment.

It is also common to visualize hyperechoic structures within the sartorius muscle. It is important not to mistake the hyperechoic tendinous part of the sartorius muscle for the LFCN. Differentiation is possibleby performing a systematic dynamic scan to trace the nerve in the fat filled space below the ASIS from distal to proximal and vice versa.

Video Gallery

Philip Peng MBBS, FRCPC
Associate Professor
Department of Anesthesia
Toronto Western Hospital
Toronto ON, Canada

Selected References

• Tumber PS, Bhatia A, Chan VW. Ultrasound-guided lateral femoral cutaneous nerve block for meralgia paresthetica. Anesth Analg. 2008;106:1021-2.
• Ng I, Vaghadia H, Choi PT, Helmy N. Ultrasound imaging accurately identifies the lateral femoral cutaneous nerve. Anesth Analg. 2008;107:1070-4.
• Bodner G, Bernathova M, Galiano K, Putz D, Martinoli C, Felfernig M. Ultrasound of the lateral femoral cutaneous nerve: normal findings in a cadaver and in volunteers. Reg Anesth Pain Med. 2009;34:265-8.
• Shteynberg A, Riina LH, Glickman LT, Meringolo JN, Simpson RL.Ultrasound guided lateral femoral cutaneous nerve (LFCN) block: safe and simple anesthesia for harvesting skin grafts. Burns. 2013;39:146-9.
• Fowler IM, Tucker AA, Mendez RJ.Treatment of meralgia paresthetica with ultrasound-guided pulsed radiofrequency ablation of the lateral femoral cutaneous nerve. Pain Pract. 2012;12:394-8.
• Tagliafico A, Serafini G, Lacelli F, Perrone N, Valsania V, Martinoli C. Ultrasound-guided treatment of meralgia paresthetica (lateral femoral cutaneous neuropathy): technical description and results of treatment in 20 consecutive patients. J Ultrasound Med. 2011;30:1341-6.