Subcostal Nerve

Anatomy of the Subcostal Nerve

The subcostal nerve has been acknowledged as an anatomical feature since the 1800s (▶ Fig. 2.1 and ▶ Fig. 2.2). ¹ There are 12 pairs of thoracic spinal nerves originating from the spinal cord. The first 11 thoracic nerves (T1–T11) lie between the ribs and are called intercostal nerves, whereas the 12th thoracic nerve (T12) lies below the last rib and is called the subcostal nerve. ² Generally, there are four main branches of intercostal nerves: collateral, lateral cutaneous, anterior cutaneous, and muscular. ² Each thoracic spinal nerve is formed from the dorsal and ventral roots from each segment of the spinal cord, and each spinal nerve then gives rise to a dorsal and ventral ramus. The ventral ramus receives postganglionic sympathetic fibers from the sympathetic trunk via the gray rami. The ventral rami of T6 to L1 emit branches at each segment of the vertebral column. ²^, ³ The ventral ramus of T12 (subcostal nerve) is larger than the other ventral rami, and gives off a communicating branch to the first lumbar ventral ramus. This communicating branch is also called the dorsolumbar nerve. Shortly after, it gives off a collateral branch, which helps to supply the intercostal muscles and the parietal pleura, ²^, ⁴ and then continues to accompany the subcostal vessels along the inferior border of the 12th rib. From superior to inferior, the subcostal vein, artery, and nerve run alongside one another at the lower border of the 12th rib. The subcostal vein and artery are hidden within the costal groove, leaving the subcostal nerve vulnerable to risk for iatrogenic injury during surgical procedures. The subcostal nerve then courses posterior to the lateral arcuate ligament and kidney, and anterior to the upper region of the quadratus lumborum. ² It then perforates the aponeurosis of the origin of the transversus abdominis and continues to course between the transversus abdominis and internal oblique ² and is then distributed in the same manner as the lower intercostal nerves. ⁴ It imparts muscular branches to the muscles of the anterior abdominal wall. ¹^, ⁵ Its innervation to the most inferior part of the external oblique, the rectus abdominis, and the transversus abdominis ⁶^, ⁷^, ⁸ attests to its role in respiration. The subcostal nerve then joins with the first spinal nerve of the lumbar plexus, the iliohypogastric nerve, and sends a branch to the pyramidalis (a muscle of the anterior wall that attaches to the pubic symphysis and crest). Innervation by the subcostal nerve allows this muscle to tense the linea alba. ²^, ⁹ As with each intercostal nerve, the subcostal nerve has a branch called the lateral cutaneous branch, which pierces the internal and external oblique muscles. It then usually descends over the iliac crest approximately 5 cm posterior to the anterior superior iliac spine (ASIS) and innervates the anterior gluteal skin, but some filaments can descend as low as the greater trochanter of the femur. ²^, ⁴^, ⁸

Fig. 2.1 Right lateral dissection of the subcostal nerve. The right 12th rib is colored in blue and the lumbar spine is seen at the top of the image. Also note the 11th intercostal nerve just superior (right) to the 12th rib. The subcostal in this specimen shows three large branches that, here, are traveling superficial to the transversus abdominis muscle. Inferior to the subcostal nerve (left), the L1 ventral ramus is shown dividing into a superior iliohypogastric nerve and inferior ilioinguinal nerve.

Fig. 2.2 Another right-sided lateral dissection of the subcostal nerve (arrow). The midline spine is shown at the top of the image. Also note the 11th intercostal nerve just superior (right) to the subcostal nerve. Inferior to the subcostal nerve (left), the L1 ventral ramus is shown dividing into a superior iliohypogastric nerve and inferior ilioinguinal nerve. Below (left) the L1 nerve, the L2 ventral ramus is seen.

2.2 Variations of the Subcostal Nerve

It has been shown in the literature that the subcostal nerve does not always contribute to the lumbar plexus. ⁵^, ¹⁰ In addition, the literature highlights variations in the innervation of the pyramidalis muscle. ⁸^, ¹¹ Tokita reported that the anterior rami of T12, L1, and L2 can all give branches to the pyramidalis muscle. ⁸ D’Souza et al. also found variations in the thickness of the lateral cutaneous branch of the subcostal nerve. ⁷