Ilioinguinal Nerve

Anatomy and Variations

The ilioinguinal nerve (▶ Fig. 4.1, ▶ Fig. 4.2, ▶ Fig. 4.3) originates from the anterior rami of the L1 spinal nerve but can often receive contributions from T12, L2, and L3. ¹^, ²^, ³^, ⁴^, ⁵^, ⁶ Its diameter is 2.2 mm with a range from 1.3 to 3.3 mm, inversely proportional to that of the iliohypogastric nerve to which it runs lateral and inferior. ³^, ⁷ In a minority of cases, 20% according to Klaassen et al, the ilioinguinal nerve forms a common trunk with the iliohypogastric nerve but separates shortly after exiting the intervertebral foramen. ³^, ⁸ Communicating branches with the subcostal, and lateral femoral cutaneous nerves have been reported. ¹

Fig. 4.1 Intramuscular course of the ilioinguinal nerve. (Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012. Illustration by Karl Wesker.)

Fig. 4.2 Course of the left ilioinguinal nerve with anterolateral muscles removed.

Fig. 4.3 Termination of the ilioinguinal nerve as the anterior scrotal nerve. (Reproduced with permission from Gilroy AM, MacPherson BR, Ross LM, Schuenke M, Schulte E, Schumacher U. Atlas of Anatomy. 2nd ed. New York, NY: Thieme Medical Publishers; 2012. Illustration by Karl Wesker.)

The ilioinguinal nerve continues laterally as it courses past the proximal and lateral border of the psoas major muscle approximately 4.4 to 8.6 cm cranial to the posterior superior iliac spine, and more generally passing posterior to the inferior pole of the kidney. ⁶^, ⁸^, ⁹ Continuing to descend inferolaterally, ¹^, ² it travels anterior to the quadratus lumborum and transversus abdominis, passing inferiorly to the lower pole of the kidney and posterior to the ascending colon if describing the right orientation of ilioinguinal nerve or descending colon if describing the left orientation. It then pierces the transversus abdominis cranially to the midpoint between the anterior superior iliac spine and the iliac crests, up to 3.0 cm, and in 13% of cases, according to Reinpold et al, it passes slightly inferior to the midpoint of the superior iliac spines. ⁵^, ⁹ In this area between the transversus abdominis and internal oblique known as the transversus abdominis plane, the ilioinguinal nerve may communicate with the hypogastric branch of the iliohypogastric nerve through small accessory fibers. ¹^, ² After traveling for a short distance between the transversus abdominis and internal oblique muscles and supplying the inferior fibers of the transversus abdominis with motor innervation, it penetrates the internal oblique and supplies it. ⁶^, ⁸^, ¹⁰^, ¹¹ According to Avsar et al, this penetration occurs 4.85 cm inferomedially (range, 3–6.4 cm) from the anterior superior iliac spine on the right and 3.37 cm inferomedially (range, 2–5 cm) on the left, 2.99 cm (range, 0.2–6.1 cm) from McBurney’s point on the right and 3.74 cm (range, 1.8–7.5 cm) on the left. ¹² Whiteside et al found a similar proximity to the anterior superior iliac spine ¹³; in pediatric patients, this proximity is much closer. ¹⁴ On a line connecting the anterior superior iliac spine to the umbilicus, the ilioinguinal nerve is 1.9 mm from the former (range, 0.61–4.01 mm) on the left and 2.0 mm (range, 0.49–3.44 mm) on the right. ¹⁴

Between the external and internal oblique muscles, the ilioinguinal nerve is concealed by the fascia of the external oblique until it reaches the round ligament or spermatic cord. ³^, ⁷ According to Ndiaye et al, it perforates the external oblique muscle fascia before reaching the spermatic cord or round ligament in 28.72% of cases, so it is extra-aponeurotic. As it passes over the superficial aspect of the internal oblique muscle, it is on average about 1.015 cm (range, 0–4 cm) from the inguinal ligament, but the distance is less than 1 cm in 66% of cases. ⁷

As it passes between the external and internal oblique muscles, it does not enter the inguinal canal through the deep inguinal ring but rather by piercing the canal wall. ¹⁵ The ilioinguinal nerve and its sensory aspect course in the inguinal canal with two distinct patterns according to Rab et al. ⁵ In type A (43.7%), the cutaneous aspect of the nerve joins the genitofemoral nerve either between the internal and external oblique muscles lateral to the deep inguinal ring, within the deep inguinal ring, or near the lateral aspect of the inguinal canal; the genital branch of the genitofemoral nerve runs ventrally on the spermatic cord or round ligament. ⁵ In types B (28.1%), C (20.3%), and D (7.8%), the genital branch of the genitofemoral nerve and the cutaneous portion of the ilioinguinal nerve enter the deep inguinal canal, and the ilioinguinal nerve courses on the ventral aspect of the round ligament or spermatic cord. ⁵ Types B, C, and D differ mainly in how the genital branch of the genitofemoral nerve and genitofemoral branch to the cremaster muscle course compared to the round ligament or spermatic cord, but the placement of the ilioinguinal nerve ventral to the spermatic cord is consistent. ⁵^, ¹⁰^, ¹⁶ The classification by Rab et al is supported by earlier work by Moosman and Oelrich. In the “aberrant” course described by Moosman and Oelrich, the sensory component of the ilioinguinal nerve was incorporated into the genital branch of the genitofemoral nerve in 35% of cases, posteriorly located within the spermatic cord (or posterior to the round ligament) and coursing downward deep to the cremasteric layer. ¹⁵^, ¹⁶ This “aberrant” path matches the type A classification by Rab et al, types B–D mirroring the “classical” course. ⁵^, ¹⁵^, ¹⁶ However, other literature has reported fewer instances of the “aberrant” course. ¹⁷

As it exits the inguinal canal through the superficial inguinal ring, it lies superior to the spermatic cord or round ligament. ¹⁷ After emerging from the superficial inguinal ring with the spermatic cord, the cutaneous aspect of the ilioinguinal nerve supplies the proximal medial thigh, external genitalia, skin over the inguinal canal, anterior hemiscrotum, and the root of the penis in males or the mons pubis and lateral aspect of the labia majora in females. ¹^, ²^, ⁴^, ⁶^, ⁸^, ¹⁰^, ¹¹^, ¹⁶ The distribution of the genital branch of the genitofemoral nerve overlaps with this area and also provides cutaneous innervation to the anteromedial thigh, anterior hemiscrotum, or mons pubis and labia majora. ⁵^, ¹⁰ The overlapping cutaneous distributions of these two nerves were also classified by Rab et al: type A (43.7%) having contributions only from the genital branch of the genitofemoral nerve, type B (28.1%) only from the cutaneous aspect of the ilioinguinal nerve, and the remainder from both nerves. Type C (20.3%) also has cutaneous branches of the ilioinguinal nerve to the mons pubis, inguinal crease, and root of the penis or labia majora, while the genital branch of the genitofemoral nerve innervates the inferior aspects of the inguinal and anteromedial thigh regions. In type D (7.8%), both nerves contribute to these regional distributions. ⁵

It should be noted that Ndiaye et al, Salama et al, Wijsmuller et al, and Al-dabbagh all found the ilioinguinal nerve to be absent, often unilaterally, in about 7% of inguinal dissections. ⁷^, ¹⁵^, ¹⁷^, ¹⁸ Distal anastomotic terminal branches with the iliohypogastric nerve have also been reported. ⁵

The pubic symphysis is innervated by branches from the iliohypogastric, ilioinguinal, and pudendal nerves.

4.2 Pathology

Injury to the ilioinguinal nerve is commonly iatrogenic in origin, incurred during lower abdominal surgery or by nerve entrapment owing to scarring following surgery and pathological impingement. ⁶^, ¹⁹^, ²⁰^, ²¹ The surgeries that frequently injure the ilioinguinal nerve are herniorrhaphy (most common), laparoscopic procedures in abdomen, treatment of stress incontinence in women, and surgeries using a Pfannenstiel incision such as cesarean section births, appendectomy, prostatectomy, inguinal hernioplasty, and abdominal hysterectomy. ⁶^, ¹³^, ²²^, ²³^, ²⁴^, ²⁵

The nerve can be directly injured during surgery if the path of the incision crosses the nerve or stretches it while the surgical field is being manipulated, as in the Pfannenstiel incision and open hernia repairs. ¹⁵^, ²⁰^, ²²^, ²⁶^, ²⁷ A Pfannenstiel incision is made 2 to 3 cm superior to the pubic symphysis, followed by stretching 8 to 15 cm and cutting through the skin, subcutaneous fat, rectus sheath, and laterally through the fasciae of the transversus abdominis and internal and external oblique muscles. The anterior fascia and linear alba are separated from the rectus and pyramidalis muscles to allow for their separation at the midline and to gain access to the peritoneum. ²⁰^, ²¹^, ²² This incision and surgical field passes through the area occupied by the ilioinguinal nerve, which can therefore be injured directly or through scarring. Open hernia repair can cause ilioinguinal neuralgia specifically due to stretching of the abdominal wall, accidental incising of the nerve, or the use of a mesh causing a neuroma to form. ⁷^, ²⁶^, ²⁸^, ²⁹ Direct iatrogenic injury to the nerve is most likely when splitting the internal oblique aponeurosis, manipulating the spermatic cord, or manipulating the surgical field, as the ilioinguinal nerve runs parallel to the inguinal canal. ⁷^, ³⁰ Caution should also be observed when approaching or operating in the triangle of doom, between the ductus deferens and the spermatic vessels, and the triangle of pain, the space created by the overlapping courses of the femoral, genitofemoral, and lateral femoral cutaneous nerves, owing to the high chance of ilioinguinal nerve injury. ⁹^, ³¹^,