An Overview of the Lumbar Plexus

The lumbar plexus forms within the psoas major muscle, and is derived from contributions arising from the first three lumbar ventral rami, along with additional contributions from parts of the 4th lumbar ventral ramus and 12th thoracic ventral ramus (▶ Fig. 1.1, ▶ Fig. 1.2, ▶ Fig. 1.3).

Fig. 1.1 Schematic drawing of the lumbar plexus. Note that the right side has had a section of the psoas major and minor muscles removed to show the course of the underlying lumbar plexus branches.

Fig. 1.2 Schematic drawing of the lumbar plexus following removal of the vertebral bodies and after opening of the anterior dura mater. The psoas muscles on the left side have been removed.

Fig. 1.3 Lateral view of the branches of the left lumbar plexus following removal of the psoas muscles and partial removal of the iliac crest.

1.2 Iliohypogastric Nerve (T12, L1)

The iliohypogastric nerve forms within the lumbar plexus, and arises from contributions from the 12th thoracic ventral ramus and 1st lumbar ventral ramus. It courses past the psoas major and exits from the upper lateral border of psoas major, at a point between the anterior surface of quadratus lumborum and the posterior aspect of the kidney. The iliohypogastric nerve then traverses past the posterior part of the transversus abdominis, traveling superior to the iliac crest. Near the iliac crest, it divides into anterior and lateral branches between the transversus abdominis and internal oblique. The anterior branch of the iliohypogastric nerve supplies surrounding muscles, such as the internal oblique and transversus abdominis, while also supplying sensory innervation of the suprapubic skin. The lateral cutaneous branch supplies the posterolateral gluteal skin. ¹^, ² In the presence of a pyramidalis muscle, it is likely to be innervated by the iliohypogastric nerve.

1.3 Ilioinguinal Nerve (L1)

The ilioinguinal nerve exists as a collateral branch of the first lumbar ventral ramus, and it emerges from along the lateral border of the psoas major alongside or just below the iliohypogastric nerve. It then proceeds down the anterior surface of the quadratus lumborum toward the upper portion of iliacus. It travels through transversus abdominis near the iliac crest, and then pierces the internal oblique to supply it. During its descent, it passes 3 cm medial and 3.5 cm inferior to the anterior superior iliac spine. At this point, the ilioinguinal nerve is located in the plane between the internal and external abdominal obliques and travels through the inguinal canal. It then exits the superficial inguinal ring on the anterior aspect of the round ligament/spermatic cord. The ilioinguinal nerve goes on to supply sensory innervation of the medial thigh, along with the root of the penis and anterior scrotum in males or the mons pubis and labium majus in females. ¹

1.4 Genitofemoral Nerve (L1, L2)

The genitofemoral nerve forms within the psoas major from contributions arising from the first and second lumbar ventral rami. It is first seen at the anterior surface of psoas major. It then proceeds downward along the psoas major, traveling within the fascia iliaca and posteriorly to the ureter and peritoneum. The genitofemoral nerve then travels along the lateral border of the common and external iliac arteries, and divides into genital and femoral branches at a point above the inguinal ligament. The genital branch enters the deep inguinal ring by traveling through the transverse and spermatic fasciae. It then descends down the inguinal canal deep to the round ligament/spermatic cord and provides innervation of the cremaster muscle. Finally, it exits through the superficial inguinal ring and gives off sensory branches to the lateral aspect of the scrotum in males or the mons pubis and labium majus in females along with surrounding parts of the thigh. The femoral branch of the genitofemoral nerve travels alongside the femoral artery underneath the inguinal ligament and travels through the femoral sheath at a point superficial and lateral to the femoral artery and distal to the inguinal ligament. It then emerges from the sheath and fascia lata to provide sensory innervation of the skin of the upper anterior part of the femoral triangle. ³

1.4.1 Variation

The iliohypogastric nerve may be absent from the lumbar plexus in up to 20% of cases. The ilioinguinal nerve is not seen in less than 3% of the population, and in 25% of cases, it may emerge with the iliohypogastric nerve. Within the inguinal canal, the ilioinguinal nerve is usually seen anterior to the spermatic cord but it may also lie within or posterior to the cord, or outside the inguinal canal entirely. The genitofemoral nerve arises from two separate branches in 20% of cases, and in some instances either of its branches or the entire nerve may be absent.

1.4.2 General Mechanisms of Injury

The iliohypogastric, ilioinguinal, and genitofemoral nerves may be injured along their course through the anterior and posterior abdominal wall either directly by trauma (e.g., surgical injury or during suture ligation) or indirectly by ischemic damage secondary to reduced perfusion or stretch. These nerves may be most prone to injury during certain portions of their course through the abdomen and surrounding regions. The anterior branch of the iliohypogastric nerve may be most vulnerable to injury during its course through the internal and external obliques. The ilioinguinal nerve appears to be prone to injury during its descent along the anterior abdominal wall, and the genitofemoral nerve is vulnerable to injury throughout its entire course but this risk is most pronounced in the retroperitoneum and within the inguinal canal.

1.4.3 Context of Injury

These nerves may undergo iatrogenic injury during abdominal and pelvic surgery (e.g., appendectomy, repair of inguinal hernia, gynecologic procedures through transverse incision, femoral artery catheterization, fascial closure, and procedures for urinary incontinence such as needle suspension and tension-free vaginal tape). Surgery to repair an inguinal hernia commonly results in injury to the ilioinguinal nerve. Laparoscopic inguinal hernia repair also may cause injury to the femoral branch of the genitofemoral nerve, whereas open inguinal hernia repair may damage the genital branch. The femoral branch may also be injured during femoral artery catheterization procedures.

1.4.4 Effects of Injury

Clinical manifestations of injury to the iliohypogastric nerve appear to differ based on whether injury occurs above or below the anterior superior iliac spine. In injuries occurring below the anterior superior iliac spine, some loss of sensory innervation of suprapubic skin may be observed; however, this is rarely seen due to collateral sensory innervation of this region. Injuries occurring above the anterior superior iliac spine may, however, weaken muscles such as the internal oblique, and transversus abdominis. A direct hernia may form secondary to this loss of muscle innervation. Injury to the ilioinguinal nerve may lead to numbness and paresthesia over skin of the genitalia, and entrapment injuries during surgery may lead to the development of recurrent pain along its area of distribution. Injury to the genitofemoral nerve will also produce paresthesia and anesthesia along its area of distribution.

1.5 Lateral Cutaneous Nerve of the Thigh (L2, L3)

The lateral cutaneous nerve of the thigh is derived from contributions arising at the second and third ventral rami of the lumbar nerve, and is seen emerging from the lateral margin of psoas major. It then proceeds downward and deep to the fascia that covers the iliacus, and passes underneath or less commonly through the inguinal ligament at a point medial to the anterior superior iliac spine. In its course through the thigh, the lateral cutaneous nerve of the thigh initially runs deep to the fascia lata and over the surface of the sartorius muscle. It then pierces the fascia lata to supply sensory innervation to the lateral aspect of the thigh. At this point, it divides into anterior and posterior branches. The anterior branch serves to provide sensory innervation of skin along the anterolateral thigh and goes on to contribute to the patellar plexus. The patellar plexus is formed by the anterior branch of the lateral cutaneous nerve of the thigh, anterior cutaneous branches of the femoral nerve, and infrapatellar branches of the saphenous nerve. The posterior branch of the lateral cutaneous nerve of the thigh provides sensory innervation of skin from the greater trochanter to the middle of the thigh. ⁴^, ⁵

1.5.1 Variation

Anatomical variations of the lateral cutaneous nerve of the thigh are observed in around 25% of the population, and the nerve may be completely absent in around 10% of individuals.