The Subcostal Nerve during Lateral Approaches to the Lumbar Spine

There has been a recent increase in minimally invasive spine surgery. One approach utilized is the lateral transpsoas approach, first described by Ozgur et al in 2006. ¹ Intraoperative monitoring is used in accessing the space between the 12th rib and iliac crest. Lumbar lateral interbody fusion procedures have gained popularity due to several advantages: low blood loss, preservation of the posterior musculature and ligamentous chain, the ability to perform an extensive discectomy, and placement of a large intervertebral graft across the apophyseal ring leading to indirect decompression. ² As the use of lateral transpsoas approaches continues to increase, it is important for surgeons to remain aware of procedural complications such as postoperative nerve palsies. ³

The corridor used to gain access to the retroperitoneal space during the lateral transpsoas approach lies between the 12th rib and the iliac crest, which is primarily supplied by the 11th intercostal and subcostal nerves with lesser contributions from the 10th intercostal nerve and L1. ⁴ Minimal access lateral spine exposure involves dissection through the abdominal musculature in addition to possible resection of the rib. However, details of the innervation pattern of the abdominal oblique muscles with the initial dissection for this procedure have not been well studied, particularly in relation to lateral abdominal hernia. Most literature on iatrogenic abdominal wall hernias resulting from lateral approaches has been derived from renal and thoracic approaches. ⁵^, ⁶^, ⁷ Such deformities significantly affect quality of life, especially in patients younger than 60 years, and may be a result for up to 50% of patients who undergo a flank incision. ⁶

In an earlier anatomical study of the subcostal nerve in relation to the lateral approach, we found that it was the predominant nerve in regard to abdominal oblique muscle innervation ( Fig. 16.1). ⁸ Regarding size, the subcostal nerve was the largest (average diameter 6 mm) nerve in this region and had a wider field of distribution with more branches (average eight) compared to the L1 (average four) and 11th intercostal nerves (average two). L2 only contributed small branches to the lateral abdominal wall musculature on two sides. The average diameter of the L1 and L2 nerves was 4.2 (range, 3–4.8 mm) and 3.8 mm (range, 2.8–4.4 mm), respectively. The proximal 6 to 10 cm of each of these nerves had few, if any, branches. Additionally, the subcostal nerve was often found (75%) up to 5 cm inferior to the 12th rib in its initial course. The area of least concentration (i.e., “safe” zone) was located in the direct lateral position at an approximate midpoint between the lower edge of the 12th rib and the superior-most aspect of the iliac crest. At the direct lateral approach region, the subcostal nerve and its branches comprised, on average, approximately two-thirds of the superior nerves located between the iliac crest and 12th rib. Additionally, a previously undescribed branch of the subcostal nerve was found to travel posterior to the quadratus lumborum and anastamose with the remaining normally placed subcostal nerve at or near the direct lateral position on 15% (two right sides and one left side) of studied sides. Laterally, extensive neural interconnections were found between the subcostal, intercostal, and L1 nerve fibers.

Fig. 16.1 Right-sided dissection of a cadaveric specimen between the right rib and iliac crest noting the nerves of this region. The subcostal (yellow), iliohypogastric (white), and ilioinguinal (black) nerves are seen.

The intercostal nerves are composed of four branches: muscular, collateral, lateral cutaneous, and anterior cutaneous. The collateral branch arises at the angle of the ribs and supplies the intercostal muscles and parietal pleura as it courses closely with the subcostal vessels. ⁹ The subcostal nerve, which was found in our study to be the dominant nerve in the incision for a lateral transpsoas approach, travels with the subcostal vein and the subcostal artery in the inferior aspect of the 12th rib. While the subcostal vein and artery travel superiorly to the nerve in the grove of the 12th rib, the nerve is often exposed along its inferior margin. Knowledge of this anatomical pattern can help prevent aggressive dissection inferior to the rib, with subsequent damage, during surgical approaches. The subcostal nerve then courses posterior to the lateral arcuate ligament and kidney and anterior to the quadratus lumborum. In 15% of cadaver sides, however, the following unusual, previously undescribed anatomical nerve pattern was observed: a large branch of the subcostal nerve travels posterior to the quadratus lumborum muscle, and then joins the main part of the subcostal nerve more laterally. The subcostal nerve continues between the transversus abdominis and the internal oblique muscles and assists in innervating the lateral abdominal wall muscles. ¹⁰ The subcostal nerve assists with respiration by innervating the rectus abdominus, the transversus abdominis, and the inferior portion of the external oblique. ¹⁰^, ¹¹^, ¹²

Naturally, one might be inclined to assume that because the subcostal nerve gives muscular branches to the muscles of the anterior wall, damage to the subcostal nerve might also reduce the function of these muscles. However, Standring ¹⁰ stated that the anterolateral wall of the abdomen is innervated by several branches of segmented spinal nerves. Therefore, damage to one spinal nerve is very unlikely to produce any notable loss in muscle tone. Effective reduction in anterolateral wall muscle function would require damage to multiple spinal nerves or their branches. The lateral cutaneous branch innervates the skin of the anterolateral abdomen at its respective thoracic level. The subcostal nerve travels over the anterior superior iliac spine and pierces the rectus sheath, giving branches to the skin over the anterior region inferior to the umbilicus. ⁹^, ¹⁰ A few fibers from the subcostal nerve may travel inferior to the greater trochanter of the femur. It also has smaller contributions to the parietal peritoneum and diaphragm. ¹⁰ The iliohypogastric nerve often receives a branch from the subcostal nerve.

Intercostal neuralgia, pain over the corresponding dermatome, may be caused by tumor, nerve entrapment, thoracotomy, or herpes zoster virus. ⁸^, ¹³ In a study of 68 patients who experienced postoperative abdominal pain and upper thigh numbness ipsilateral to the site of percutaneous nephrolithotomy, it was found that the most commonly affected area was innervated by the subcostal nerve. ¹⁴ A neuroma of the 12th nerve may occur after a nephrectomy. ¹⁵ Rib fracture leading to pseudohernia as a result of subcostal nerve damage has been reported. ¹⁶

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