Indian Journal of Anaesthesia

: 2018  |  Volume : 62  |  Issue : 12  |  Page : 997--998

Aberrant femoral nerve anatomy: No longer a cause of block failure when using ultrasound guidance

R Sripriya, T Sivashanmugam 
 Department of Anaesthesiology, Mahatma Gandhi Medical College and Research Institute, SBV University, Pillayarkuppam, Puducherry, India

Correspondence Address:
Dr. R Sripriya
Department of Anaesthesiology, Mahatma Gandhi Medical College and Research Institute Pillayarkuppam, Puducherry - 607 402

How to cite this article:
Sripriya R, Sivashanmugam T. Aberrant femoral nerve anatomy: No longer a cause of block failure when using ultrasound guidance.Indian J Anaesth 2018;62:997-998

How to cite this URL:
Sripriya R, Sivashanmugam T. Aberrant femoral nerve anatomy: No longer a cause of block failure when using ultrasound guidance. Indian J Anaesth [serial online] 2018 [cited 2021 Mar 2 ];62:997-998
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A 25-year-old American Society of Anesthesiologists class 1, male patient [body mass index (BMI) 20 kg/m2] was referred to our pain clinic for insertion of femoral nerve catheter for mobilisation of a posttraumatic stiff knee joint. On Ultrasound (US) scanning [HFL-50 (15–6 MHz) probe of X-Porte Ultrasound system (FUJIFILM Sono Site, Inc, Bothell, USA)] of the femoral triangle, the femoral nerve (FN) was not appreciated in its usual location lateral to the femoral artery. As FN exhibits the property of anisotropy, the probe was tilted in all directions to optimize visualisation and scanning was continued proximally and distally to see if any structure was consistently detected. While doing so, we detected a honeycombed structure emerging much laterally between two groups of muscles, which when traced proximally above the inguinal ligament was seen to wind medially behind a chunk of muscle [[Figure 1]b and Video 1]. Here, another bulky muscle was seen between the nerve and the external iliac artery, which based on its location, was most likely the psoas major muscle. The chunk of muscle around which the FN was winding was probably an abnormal band of iliacus or psoas muscle. Ten milliliter 2% lignocaine with adrenaline was injected around this structure and a block consistent to the area of distribution of FN was achieved. This confirmed that the structure was in fact the FN. A caudal to cephalic, infra-inguinal FN catheter was inserted. The other side could have been scanned to detect if the variation was bilateral, which was not done.{Figure 1}


The FN arises from the posterior division of the second, third, and fourth lumbar ventral rami. The nerve emerges from the lateral border of the psoas major muscle, descends in the groove between the iliacus-psoas major muscles, and enters the thigh below the inguinal ligament lateral to the femoral sheath [Figure 1]a and [Figure 2]a. Anatomical variations in the formation and branching pattern of FN are not infrequent. Astik and Dave have described anatomical variations of lumbar plexus as high as 25% in cadaveric studies and Anloague et al. have reported variations in 35% cadaveric dissections.[1],[2] The variations described are abnormally long L2 root, early division of the FN, origin of lateral cutaneous nerve of thigh from the FN, and origin of nerve to pectineus from the FN in iliac fossa. One of the several variations is the splitting of the FN nerve into two slips by psoas major or accessory slips of iliacus muscle. Although the FN splits above the inguinal ligament, the two parts join below the inguinal ligament to form a single nerve.{Figure 2}

One of the main advantages of using US guidance for peripheral nerve blocks is the ability to detect anatomical variations and appropriately target the LA deposition. Easily identifiable structures using US are the blood vessels (anechoic round structures-pulsatile in case of artery and compressible in case of vein) and bone (hyperechoic periosteum with post-acoustic shadow). As novices in US-guided peripheral nerve blocks, we depend on these anatomical landmarks for identifying nerves located in proximity to them. In the femoral triangle, the orientation of structures from medial to lateral are the femoral vein, femoral artery, and the FN [Figure 2]b. These anatomical landmarks only serve as surrogate markers. US guidance enables us to directly appreciate the honeycombed appearance of the FN located deep to the hyperechoic fascia iliaca lining the ilio-psoas compartment. Further, the structure can be traced back and forth for its consistent location to be identified as nerve. However, the entire course of the FN cannot be traced proximally due to the presence of intestines in the pelvic cavity [Figure 2]b. As our knowledge and skills in US-guided regional anaesthesia has evolved over time, we are now able to step up from surrogate markers for locating nerves to identifying nerves based on their innate characteristics despite anatomical variation.

In our case, although the course was altered, we could not identify a split suprainguinal FN. The splitting of the nerve above the inguinal ligament is another important consideration while planning for a suprainguinal fascia iliaca compartment block where one or more components located deeper to the surface may be easily spared.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given his consent for his images and other clinical information to be reported in the journal. The patient understands that name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.


1Astik RB, Dave UH. Anatomical variations in formation and branching pattern of the femoral nerve in iliac fossa: A study in 64 human lumbar plexuses. Peoples J Sci Res 2011;4:14-9.
2Anloague PA, Huijbregts P. Anatomical variations of the lumbar plexus: A descriptive anatomy study with proposed clinical implications. J Man Manip Ther 2009;17:e107-14.