Lateral costal artery: accessory thoracic vessel of clinical interest

The lateral costal artery (LCA), a supernumerary branch of the internal thoracic artery (ITA), occurs in several ethnic groups on one side of the thorax or on both, in 15-30% of cases. It has been considered responsible for the "steal-syndrome" of the coronary blood after coronary artery bypass grafting and it used occasionally for myocardial revascularization. To clarify its functional significance, an interpretation based on our findings and human and comparative anatomy and embryology has been attempted. We report on a case where a right LCA of about 2 mm in caliber, rising from the ITA 2.5 cm below the subclavian, coursed as far as the 4th intercostal space for a distance of 13 cm after the anterior axillary line. Anastomosing with the intercostal arteries, it can act as a blood derivative circuit of the thoracic wall. Embryologically, this artery, like the normal parietal arteries of the trunk, might form a longitudinal channel connecting the intersegmental arteries. In mammals having a thoracic cage transversely restricted (quadrupeds), the ITA is more lateral than in primates having a circular thorax, and gives off a ventral branch toward the sternum. It might be hypothesized that the sternal branch occurring in quadrupeds, undergoing adaptation to the thoracic shape of primates, may become the main trunk of the ITA, whereas the LCA may be the remnant of the ITA of quadrupeds. Because the LCA ran partly along the "milk line" of humans, it might be regarded as a supernumerary mammary artery.