Iliohypogastric Nerve

The iliohypogastric nerve (▶ Fig. 3.1, ▶ Fig. 3.2, ▶ Fig. 3.3) measures approximately 210 mm in length ¹ and is a mixed nerve that provides both motor and sensory innervation to numerous structures in the lower abdominopelvic regions. The motor component contributes to the innervation of both the transversus abdominis and internal oblique muscles that function to maintain abdominal tone, increase intra-abdominal pressure, and, particularly for the internal oblique, cause lateral flexion. ² The sensory component supplies the skin over the hypogastric region, as well as the upper lateral aspect of the thigh and gluteal region. ³^, ⁴^, ⁵^, ⁶ The clinical relevance of this nerve lies in its vulnerability to injury, most commonly intraoperatively during attempts to gain access to the peritoneal cavity, pelvis, or perineum. This can result in motor dysfunction of the lower abdominal wall and sensory disturbances in the aforementioned regions.

Fig. 3.1 Drawing of the course of the iliohypogastric nerve on the posterior abdominal wall (upper arrow) and on the anterior abdominal wall (lower arrow). This nerve’s iliac branch is also seen crossing over the iliac crest.

Fig. 3.2 The course of the iliohypogastric nerve shown on the left side with the anterolateral abdominal muscles removed.

Fig. 3.3 The intramuscular course of the iliohypogastric nerve on the right side. Also note its lateral cutaneous branch coursing over the iliac crest.

(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.)

3.2 Anatomy

The origin of the iliohypogastric nerve is the ventral ramus of the L1 spinal nerve, ⁵ measuring approximately 4 mm in diameter. ¹ After leaving the L1–L2 intervertebral foramen, the L1 spinal nerve root bifurcates into the iliohypogastric and ilioinguinal nerve posterior to the psoas major. ²^, ⁴ Then, emerging from the superior lateral margin of the psoas major and posterior to the medial arcuate ligament at the level of L1–L2, ⁶ the iliohypogastric nerve, measuring 2.2 mm in diameter, enters the abdominal cavity and descends laterally between the anterior surface of the quadratus lumborum and the posterior surface of the inferior pole of the kidney to continue onto the transversus abdominis. ²^, ⁴^, ⁷^, ⁸^, ⁹^, ¹⁰^, ¹¹^, ¹²^, ¹³ The iliohypogastric nerve subsequently pierces the surface of the posterior aspect of the transversus abdominis and travels parallel to the iliac crest between the transversus abdominis and the internal oblique. The nerve then divides into two branches—the lateral cutaneous (iliac) nerve and the hypogastric (anterior) nerve. ³^, ⁴ The iliac branch pierces the internal oblique muscles and travels across the tubercle superior to the iliac crest, where, just above this, it pierces the external oblique aponeurosis and splits into two cutaneous branches—one traveling and providing sensory innervation of the skin over the tensor fasciae latae on the lateral aspect of the upper thigh, while the second cutaneous branch provides sensory information of the integument over the gluteus medius and lateral third of gluteus maximus muscles at the level of the greater trochanter. The hypogastric branch continues to travel between the transversus abdominis and internal oblique muscles, providing motor innervation to both muscles, until reaching the level of the anterior superior iliac spine, where it pierces the internal oblique muscle to run parallel to the inguinal ligament ventromedially between the internal and external oblique muscles. Along a vertical line drawn from the midpoint of the outer margin of the superficial inguinal ring, the hypogastric nerve pierces the external oblique aponeurosis, providing sensory innervation to the suprapubic integument. ³

The location in which the iliohypogastric nerve pierces the internal oblique is 1.5 to 8 cm medial to the anterior superior iliac spine on the left and 2.3 to 3.6 cm medial to the anterior superior iliac spine on the right. ¹⁴ The iliohypogastric nerve pierced the abdominal wall 2.8 ± 1.3 cm (range, 1.1–5.5 cm) medial and 1.4 ± 1.2 cm (range, 0.6–5.1 cm) inferior to the anterior superior iliac spine, and then coursed on a straight path to terminate 4.0 ± 1.3 cm (range, 2.0–12.6 cm) lateral to the linea alba. ⁵

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

3.3 Variations

The iliohypogastric nerve can exhibit variability in several ways. First, the iliohypogastric nerve may be absent in up to 20% of lumbar plexuses. Interestingly, this absence of the iliohypogastric leads to minimal sensory deficits as the sensory regions of the genitofemoral and ilioinguinal nerves largely overlap with that of the iliohypogastric. ⁴ Secondly, the iliohypogastric nerve can vary in its point of origin and can be classified into morphological types I–IV. In 7%, the iliohypogastric nerve arose at T12 and was classed as type I. Type II, noted in 14%, arose conjointly from T12 and L1. Type III, which occurred in 10%, had its origin from L1, while type IV (6%) started from both T11 and T12. ⁵ Thirdly, the iliohypogastric nerve can communicate with other nerves in the lumbar plexus including the lateral femoral cutaneous and ilioinguinal via small accessory nerve branches over the surface of the transverus abdominis muscle in 5 and 55%, respectively. ⁵^, ¹⁵^, ¹⁶^, ¹⁷^, ¹⁸^, ¹⁹^, ²⁰^, ²¹^, ²² Additionally, the subcostal nerve can directly contribute to the iliohypogastric nerve. ⁴^, ⁵^, ¹⁷^, ²³^, ²⁴ Fourthly, morphological variability of the nerve has also been observed. For example, the ilioinguinal nerve can be found in place of the hypogastric branch of the iliohypogastric nerve prior to exiting the superficial inguinal ring. ⁴