The lumbar plexus lies within the posterior aspect of the psoas major muscle and is formed by the first four lumbar ventral rami or the anterior divisions of spinal nerves L1-4. There also may be a contribution from the ventral ramus of the 12th 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 upper and larger bifurcation then divides again to form two nerves. One of these nerves, a final branch of the lumbar plexus, is the ilioinguinal nerve. This article will describe the anatomical course and function of the ilioinguinal nerve, followed by any clinical pathology.
The ilioinguinal nerve exits the psoas major at its upper lateral border inferior to the iliohypogastric nerve, the other nerve that originates from the first lumbar ventral ramus. It crosses the quadratus lumborum and iliacus muscles in an oblique direction before entering the transversus abdominus muscle near to the anterior aspect of the iliac crest. It then enters the internal oblique muscle before crossing the inguinal canal posterior to the spermatic cord. The ilioinguinal nerve then emerges with the spermatic cord from the superficial inguinal ring to supply skin in the groin region.
The ilioinguinal nerve is smaller than the iliohypogastric nerve and is sometimes so small that it joins with the iliohypogastric nerve where it enters the quadratus lumborum. In this instance, a branch of the iliohypogastric nerve will then take its place. The ilioinguinal nerve can sometimes be absent and in this case, the iliohypogastric nerve will then supply its territory.
The ilioinguinal nerve has both a sensory and motor role. The nerve supplies sensory fibres to the transversus abdominus and internal oblique muscles. It also supplies the anteromedial aspect of the skin of the thigh, the skin covering the upper scrotum and the skin over the root of the penis in males or the skin over the mons pubis and labium majus in females. The ilioinguinal nerve also has motor fibres that innervate the transversus abdominus and internal oblique muscles.
The ilioinguinal nerve is susceptible to damage during abdominal surgery or from trauma to the abdominal wall. Injury to the ilioinguinal nerve can lead to weakening of the transversus abdominus and internal oblique muscles, which can lead to the development of an inguinal hernia. Moreover, inguinal incisions carried out to repair inguinal hernias are a high-risk incision and may injure the ilioinguinal nerve. The nerve may also be damaged if it is included in the suture during incision closure causing nerve entrapment. Injury to the nerve can result in pain in the sensory distribution described above.
Both ilioinguinal nerve damage and entrapment during an inguinal hernia repair can lead to Post-Herniorraphy Pain Syndrome or inguinodynia, which is chronic pain in the groin region lasting for greater than 3 months following inguinal hernia repair surgery, otherwise known as a herniorraphy. This can be treated with analgesia or physical therapy but these have been shown to have little effect. Ilioinguinal nerve block has been shown to be effective in reducing groin pain following herniorraphy. Nerve blocks can temporarily and reversibly block nerve transmission, leading to pain relief.
Local anaesthesia administration between the layers of the transversus abdominus and internal oblique muscles, as well as between the internal oblique and external oblique muscles, can lead to temporary analgesia. The injection is placed 2cm medial and 2cm superior to the anterior superior iliac spine and can involve various chemical agents such as anaesthetics, steroid, glycerol or alcohol.