The communications between sensory and motor neurons is brought about by interneurons which comprises 99.9% of all neurons in the central nervous system. Various sensory motor communications (SMCs) are reported to show massive and progressive failure in a number of motor neuron diseases. Variations in SMCs have also been reported in peripheral nervous system [1]. The posterior femoral cutaneous nerve (PFCN) has been widely studied with context to its origin, distribution and mononeuropathies [2, 3]. However, we report a communication between PFCN and inferior gluteal nerve (IGN) and cite its embryological basis and clinical importance.

PFCN arises from the dorsal branches of S1, S2 and ventral branches of S2 and S3 sacral rami. It leaves the pelvis through the greater sciatic foramen below the piriformis muscle (PM) and descends beneath the gluteus maximus (GM) muscle with the IGN. It gives gluteal (S1–S2) and perineal (S2–S3) branches after reaching infragluteal area, communicates with the inferior rectal and posterior scrotal/labial branches of the perineal nerve, and innervates the skin of posterior and medial side of the thigh, the popliteal fossa and the proximal part of the back of the leg [4, 5]. The IGN arises from the dorsal divisions of the L5, S1 and S2 ventral rami. After leaving the pelvis through the greater sciatic notch below the PM, it divides into branches entering the deep surface of the GM muscle [4]. Various studies have been conducted to assess the course and distribution of the nerve, nerve entrapment, importance in inferior gluteal flap surgeries etc [2, 3, 5, 6].

During the routine dissection schedule of first year M.B.B.S students, the bilateral gluteal regions were studied to observe the distribution of the nerves and the variations in the gluteal region in the Department of Anatomy, All India Institute of Medical Sciences, New Delhi, India. All the cadavers were donated and written and informed consent was taken for whole body dissections for educational and research purposes. The GM muscle was cut and underlying structures were exposed after initial skin incision and fascia removal.

After a meticulous dissection of all the cadavers, SMC between PFCN and IGN was observed in the right gluteal region of a 56-year-old male cadaver of Indian origin. Both the PFCN and IGN were seen to emerge out the greater sciatic foramen at the lower border of GM below the PM (Fig. 1A). The PFCN descended at the back of thigh superficial to biceps femoris beyond this communication and terminated after supplying the skin at the back of the knee in the popliteal fossa (Fig. 1B, C). A cutaneous branch originating from the IGN was observed passing under the lower border of GM. No significant variations were noted amongst the terminal branches of the sacral plexus. The arrangement and relationship of the nerves followed the normal anatomical description on the opposite side.

Figure 1. Photograph showing right gluteal region. (A) Structures emerging below piriformis muscle after reflecting gluteus maximus muscle. (B) Sensory motor communication (SMC) between inferior gluteal nerve (IGN) and posterior femoral cutaneous nerve (PFCN) at the back of the thigh; (C) termination of PFCN after supplying the skin at the back of the knee in the popliteal fossa. SN, sciatic nerve; GS, gemellus superior muscle; OI, obturator internus muscle; GI, gemellus inferior muscle; QF, quadratus femoris muscle; BF, biceps femoris muscle.

Previous studies have focused mainly on origin and relationship of PFCN to PM [2, 3]. In the present case, the PFCN was observed to originate and follow its regular course except for the above communication. Jiamjunyasiri et al. [2] established the origin, course, distribution of PFCN as well as elucidated spatial relationship among its branches. Tubbs et al. [5] in their study studied the detailed course of the perineal branch of PFCN in 20 cadavers. PFCN was found to travel along with IGN through split sciatic nerve (SN) and pierce the PM. High splitting branching pattern of PFCN has also been observed by Kachniarz and Dellon (2021) [7] where it gave medial and lateral branches distal to PM. If PFCN pierces PM then features of nerve entrapment are evidenced when IGN pierces PM. The medial and lateral branches are spared in proximal splitting of PFCN.

Many plausible causes have been cited for PFCN neuropathy including compression by PM, prolonged use of thigh tourniquet, gluteal injections, nerve trauma due to prolonged exercise, biking, pressure on the gluteal maximus due to prolonged sitting on hard surfaces, pelvic tumours, and venous malformations [8]. PFCN entrapment leads to pain in buttocks, medial side of thigh and in the leg as well as sensory disturbances in gluteal region and back of thigh [8].

Cluneal neuralgia, caused due to the compression of the inferior cluneal nerves, the branches of the PFCN coursing beneath the inferior border of GM, a distinct cause of perineal pain and needs to be distinguished from the pudendal pain. The persistent pain after pudendal nerve surgeries suggested the fact that there is an entrapment of additional structures apart from pudendal nerve as a cause of apparent failure of the surgery [3].

There are limited studies in literature regarding the course and anatomic relationships of the IGN. It is primarily described as a solely motor nerve. IGN is very prone to iatrogenic injuries during posterior and posterolateral approaches to hip causing nerve compression and entrapments such as piriformis syndrome, intramuscular gluteal injections, hip surgeries and nerve compression due to pelvic or colorectal masses [9]. The gluteal region is also predisposed to the development of pressure ulcers, and ulcerative changes in case of incumbent loss of sensitivity [9]. The cutaneous branches of IGN can be severed during the posterior approaches of hip leading to sensory loss or neuropathies. The dysfunction in IGN needs to be clearly differentiated from other gluteal pathologies viz. entrapment of SN, PFCN, piriformis syndrome, sacroiliac or L5 facet joint pathologies etc [10]. Patients present with gluteal pain, atrophy of GM and weakness in hip extension and are managed with treating the cause, local anaesthetic or corticosteroid injections under fluoroscopic or ultrasound guidance. Botulinum injections are also tried for refractory patients [8]. The evidence of the origin of cutaneous branch from IGN was first described by Iwanaga et al. [10], in 2018. Our study also reports a similar cutaneous branch emerging from IGN and passing under the lower border of GM.

The diagnostic assessment PFCN of involvement is primarily clinical and also reported by but electrophysiological techniques. The use of diagnostic nerve blocks of PFCN can assist in defining a possible lesion of the nerve as the cause of the pain. This has been attempted using anatomical landmarks or computed tomography (CT) guidance or high resolution magnetic resonance imaging (MRI) [10]. In case of failure of conservative management, the injection of PFCN is attempted by fluoroscopic-, CT/MRI-guided procedures and even in certain cases, surgery is also necessitated for nerve entrapment [11].

PFCN block is required for surgical procedures in the vicinity of the thigh, knee, and the postero-superior part of the leg, e.g., arthroplasty, menisectomy, or meniscal repair. Combined femoral nerve and SN block is well suited for at/below knee surgeries and provides anaesthesia in anteromedial and anterolateral thigh but posterior thigh is unaffected [12]. A pneumatic thigh tourniquet is often used to provide a bloodless operating field in such surgeries because a below knee tourniquet can interfere with the operative field and can lead to common peroneal nerve trauma [12]. Block of PFCN is desirable in such cases to increase the tolerance of long duration thigh tourniquets. Labat’s posterior approach of blocking SN providing additional PFCN block is thus the preferred procedure to avoid tourniquet pain and provides more advantage than anterior/lateral approaches [13].

The communications between PFCN and IGN could prove helpful in above situations. The function of IGN can be assessed on attempting PFCN stimulation and the grade of the elicited motor response could be a reliable determinant of ensuring adequate PFCN block for avoiding the tourniquet pain. This is also helpful during posterior or poster-lateral approach to hip joint in hip-replacement surgeries. The incidence of damage to peripheral nerves during the insertion of prosthetic hip has been estimated to be 0.5%–0.8% [11]. This avoids any iatrogenic injury to IGN opposed to deep surface of GM. Preservation of PFCN or its branches is also desirable while harvesting the free inferior gluteal myocutaneous flap where sensory loss in distribution of PFCN is a frequent sequela [6].

The embryological basis of the variations in IGN and PFCN course may be due to molecular incoordination in axonal guidance in the development of peripheral nervous system. The formation of growth cones is heralded by expression of specific receptor molecules and correct axonal routing is dependant on a host of local attractive/repulsive and chemoattractant cues [14]. The axon pathfinding is governed by a complex orchestration of attractive and repulsive guidance molecules like netrins, slits, semaphorins, ephrins, etc. [15]. Differential expression of these molecules also can lead to abnormal pathway of the nerve. Thus, any perturbation in the molecular regulation in the axonal guidance affecting major guiding forces, viz. chemoattraction, chemorepulsion, contact attraction, and repulsion may lead to variations of nerves as observed in the current study.

In conclusion, the communication between PFCN and IGN holds importance in diverse diagnostic and reconstructive procedures. Nevertheless, the characterization of specific nerve fibres within the communicating branch by immunohistochemical methods remains the basis for in-depth understanding of the functional importance in such communications.

The authors would like to sincerely acknowledge the people who donated the bodies of their loved ones for the advancements in science.

Conceptualization: V Dhawan, SSS. Data acquisition: V Dhawan, SSS, S Seth. Data analysis or interpretation: V Dhawan, S Seth, V Deshmukh, MB. Drafting of the manuscript: V Dhawan, SSS, V Deshmukh, S Singh. Critical revision of the manuscript: V Dhawan, S Singh. Approval of the final version of the manuscript: all authors.

No potential conflict of interest relevant to this article was reported.

None.