IntroductionThe lumbar plexus lies within the posterior aspect of the psoas major muscle and is formed by the anterior divisions (ventral rami) of the first four lumbar spinal nerves, L1-4. There also may be a contribution of the ventral ramus of the 12th and final thoracic spinal nerve (T12). The lumbar plexus divides off to give 8 branches in total. The ventral ramus of L1 receives contributions from the ventral ramus of T12 before bifurcating. The larger superior bifurcation then branches again to form two nerves. The superior of these two is the iliohypogastric nerve, a final branch of the lumbar plexus. This article will describe the anatomical course and function of the iliohypogastric nerve, as well as its clinical pathology.
The iliohypogastric nerve emerges from the psoas major muscle at its upper lateral border before crossing posterior to the kidneys and obliquely in front of the quadratus lumborum muscle. It then enters the transversus abdominus muscle posteriorly at a level just above the iliac crest. Whilst the iliohypogastric nerve runs between the transversus abdominus and the internal oblique muscles, it divides into two branches:
- The anterior cutaneous branch
- The lateral cutaneous branch
The anterior cutaneous branch continues to run between the internal oblique and transversus abdominus muscles, innervating both. It runs through the internal oblique muscle about 2 cm medial to the anterior superior iliac spine, then passes through the aponeurosis of the external oblique muscle about 3 cm superior to the superficial inguinal ring before being distributed to the abdominal skin superior to the pubic area.
The lateral cutaneous branch runs through both the internal and external oblique muscles at a level superior to the iliac crest and posterior to the iliac branch of T12. It is then distributed to the posterior and lateral aspects of the gluteal skin.
The iliohypogastric nerve has both sensory and motor functions. The nerve supplies sensory fibres to the internal oblique, external oblique and transversus abdominus muscles as well as the suprapubic skin and posterolateral aspect of gluteal skin.
The iliohypogastric nerve also supplies motor fibres to the transversus abdominus and the internal oblique muscles. It also innervates the conjoint tendon, a tendon formed from the common aponeurosis of the transversus abdominus and internal oblique muscles.
Damage to the iliohypogastric nerve is rarely isolated and other nerves may be affected. Injury can occasionally occur during surgery with an oblique approach to the appendix. Damage to the iliohypogastric nerve can also occur in transverse incisions of the lower abdomen, during a hysterectomy, for example. Noticeable sensory loss is rare, as the suprapubic skin receives innervation from multiple nerves. Injuries to the nerve can occur due to a direct surgical trauma or due to entrapment of the nerve in scar tissue following the surgical procedure. Injuries in sport, such as muscle tearing or trauma, can also damage the nerve. Damage to the nerve can also rarely occur in pregnancy due to rapid expansion of the abdomen that occurs in the third trimester. This is referred to as idiopathic iliohypogastric syndrome.
Trauma or entrapment of the iliohypogastric nerve can cause a burning pain in the suprapubic and inguinal regions. Treatment is with a local anaesthetic injection, pain-relieving medications such as anti-inflammatories or with physical therapy such as cryotherapy.
Division of the iliohypogastric nerve above the level of the anterior superior iliac spine can cause weakening of the inguinal canal’s posterior wall. This can predispose to direct hernia formation.