Sonoanatomy of the ilioinguinal, iliohypogastric, genitofemoral, obturator, and pudendal nerves: a practical guide for US-guided injections

The iliohypogastric (IHN), ilioinguinal (IIN), and genito-femoral (GFN) nerves are branches of the lumbar plexus, deriving from the T12–L1 nerves, the L1 and the L1–L2, respectively^{(4, 6, 7)}.

The IHN arises at the lateral border of the proximal psoas muscle 5–9 cm laterally and cranially to the posterior superior iliac spine, then crosses obliquely in front of the quadratus lumborum muscle, and enters the transversus abdominis muscle 2.0–12.3 cm dorsally to the anterior superior iliac spine (ASIS)^{(1, 4)}. It then divides between that muscle and the obliqus internus abdominis into the lateral cutaneous, anterior, and genital branches, supplying the skin over the lower part of the rectus abdominis or mons pubis. The area where the nerve is found to have the least anatomical variability is on the quadratus lumborum muscle⁽¹⁾.

The IIN arises from the lateral border of the psoas major just below the IH within 4.4–8.6 cm cranially to the posterior superior iliac spine, and enters the transversus abdominis at the area of the anterior iliac crest with a very variable course^{(1, 4)}. In relation to the ASIS, the IIN penetrates the abdominal wall within 0.5–13.0 cm dorsally, and supplies the skin of the superomedial thigh, the skin over the root of the penis and anterior scrotum in men and the mons pubis and labium majus in women⁽¹⁾. The area where the nerve is found to have the least anatomical variability is on the quadratus lumborum muscle⁽¹⁾.

The GFN courses anterior to the psoas muscle and the common and external iliac arteries. Above the inguinal ligament, it divides into the genital (external spermatic) and the femoral (lumboinguinal) nerves^{(1, 4)}. Both branches are located medially to the IHN and IIN, and centrally on the psoas muscle. The genital branch of the GFN runs retroperitoneally, and enters the abdominal wall at the area of the internal inguinal ring, forming a circle around the ring to reach the scrotum or the labia majora. The femoral branch of the GFN passes underneath the inguinal ligament to follow the external iliac artery, and then enters the sheath of the femoral vessels, lying superficial and lateral to the femoral artery^{(1, 4, 7)}. The genital GFN branch supplies the skin of the anterolateral scrotum or labium majus, and contains a motor branch to the cremaster muscle. The femoral GFN branch innervates the skin of the anterior upper thigh below the inguinal ligament or the femoral triangle⁽⁴⁾.

The distance between the GF, IH and II is the shortest centrally on the psoas muscle⁽¹⁾. There are many variations and communications between the sensory branches of these nerves, implying the need for examining these three nerves as a whole in the proper clinical setting^{(7, 8)}.

Entrapment or injury of the IIN, IHN, and GFN, known as border nerve pain syndrome, leads to chronic burning neuropathic pain and/or paresthesia spreading from the lower abdomen to the inguinal region, the superomedial aspect of the thigh, the scrotum, and the labia majora, resulting in severe adverse impact on normal daily activities or work^{(9, 10)}. The distinction between IIN/IHN and GFN entrapment is challenging due to their overlapping sensory innervation⁽¹¹⁾. These nerves are at risk to injury from lower abdomen incision leading to post-operative groin pain known as inguinodynia or post-herniorrhaphy pain syndrome⁽⁵⁾. IIN/IHN entrapment or injury may present after herniorrhaphy (54%), appendectomy, hysterectomy, abdominoplasty, and orchiectomy^{(5, 9, 12)}. Postoperative GF entrapment is rare (0.3%), mainly occurring after vasectomy, nephrectomy, urethral sling, Pfannenstiel incision, appendectomy, and retroperitoneal operations including aortic aneurysm repair⁽¹³⁾. Other less common causes of border nerve pain syndrome include trauma, compression by a pelvic lesion or endometriosis, bicycle riding or tight clothing⁽¹⁰⁾.

Originating from the L2–4 nerve roots, the obturator nerve (OBN) pierces the psoas muscle, runs through the obturator foramen, and extends to the anterior thigh dividing into the anterior and posterior branches. The area of bifurcation is usually located within the obturator canal, less commonly in the pelvis or in the thigh. The branches are separated by the adductor brevis muscle at the level of the thigh⁽¹⁴⁾. The OBN supplies the skin of the medial thigh and above the medial side of the knee, and provides motor innervation of the adductor muscles. No cutaneous innervation is provided in more than half of individuals⁽¹⁴⁾.

Entrapment of the OBN, also known as “obturator tunnel syndrome” includes symptoms such as groin and medial thigh pain (73%), weakness of the adductor muscle, and paresthesia along the lateral peroneal region⁽¹⁵⁾. Patients may also present with deep pain or sensory loss in the medial thigh from the pubic area to the medial knee, which can be stimulated by forced extension, abduction, and internal rotation of the hip (Howship–Romberg sign)⁽¹⁶⁾.

The OBN is prone to compression after prolonged lithotomy position during gynecological or urological operations⁽¹⁷⁾ and after total hip arthroplasty (0.7–1%), mainly caused by cement extrusion⁽¹⁸⁾. It partially innervates the knee joint capsule and the gracilis muscle, and may be blocked in total knee arthroplasty or during harvesting of the gracilis tendon in anterior cruciate ligament reconstruction⁽¹⁴⁾. It also gives sensory innervation to the hip joint; so obturator nerve blockade may be used to control acute pain after hip surgery⁽¹⁴⁾.

Originating from the S2–S4 nerve roots, the PDN courses distally and posteriorly between two muscles, the piriformis and coccygeus muscles, to exit the pelvis through the greater sciatic foramen^{(19, 20)}. It then runs between the sacrotuberous (ST) and sacrospinous (SS) ligaments at the level of the inferior posterior ischial spine (IPIS) on the medial side of the internal pudendal artery (IPA)^{(19, 20)}. Then, it passes through the lesser sciatic foramen to enter into Alcock’s canal⁽¹⁹⁾, finally dividing into three terminal branches; the inferior rectal branch, the perineal branch, and dorsal sensory nerve of the penis or clitoris^{(19, 20)}. It supplies the skin of the penis or clitoris, the perineal area, and the posterior scrotum or labia majora. It carries sensory, motor, and autonomic fibers; however, injury causes sensory rather than motor deficits⁽²⁰⁾.

PDN entrapment causes pudendal neuralgia, a chronic severe disabling neuropathic condition⁽²⁰⁾. Symptoms are highly variable depending on the level of nerve damage and the presence of anatomic variations, including hyperalgesia, paresthesia, allodynia and burning sensation at the perineum and genital area, increased urinary urgency, painful sexual intercourse, and foreign body sensation in the rectum⁽¹⁹⁾. Pain may worsen with sitting, and may radiate to the lower limbs⁽¹⁹⁾. Damage to the nerve may lead to fecal incontinence⁽²⁰⁾. Pudendal neuralgia may result from mechanical injury, including compression or stretching of the nerve, and from non-mechanical causes, such as viral infections, multiple sclerosis, and diabetes mellitus⁽²⁰⁾. It is a common complication after pelvic surgery, but it can also be caused by direct trauma to the buttocks, vaginal delivery, chronic constipation, excessive cycling, or prolonged sitting⁽²⁰⁾. There are four types of pudendal nerve entrapment syndrome depending on the level of compression, including proximally below the piriformis muscle, as the pudendal nerve exits the greater sciatic notch (type 1), entrapment between sacrospinous and sacrotuberous ligaments (type 2), entrapment in Alcock’s canal due to obturator internus muscle spasm (type 3), and entrapment of terminal branches (type 4)⁽²¹⁾. The most common site of entrapment is Alcock’s canal and between the sacrospinous and sacrotuberous ligaments^{(21, 22)}. The diagnosis of pudendal neuralgia remains clinical, and US-guided diagnostic injections using anesthetic may aid in confirming the clinical suspicion^{(21, 22)}. US-guided PDN block at the level of the inferior posterior ischial spine (IPIS) is as accurate as fluoroscopically-guided injections⁽²³⁾.