Microsurgical anatomy of the hypoglossal nerve.

The aim of the present study was to review the surgical anatomy of the hypoglossal nerve (HN), to reveal its relationships on its course and to provide some landmarks to its identification. Ten cadaveric head dissections (20 sides) were performed using microsurgical techniques. The anatomical relationships between the HN and other nerves, muscles, arteries and veins were carefully recorded, and some measurements were made between the HN and related structures. Thus, various landmarks were determined for the easy identification of the HN. In addition, the hypoglossal triangle, which containes major vascular structures, is described. The HN is divided into three main parts: cisternal, intracanalicular and extracranial. The HN arises from the medulla as a line of rootlets situated along the anterior margin of the lower two-thirds of the olive in the preolivary sulcus. It is the newly described 'hypoglossal' triangle in the anterior neck that is bordered by the descending hypoglossus laterally, transverse hypoglossus inferiorly and inferior border of the stylohyoid muscle superiorly. In our specimens, we determined that the HN was 3-7 mm (mean 5 mm) inferior to the digastric tendon, as well as mostly superficial. The occipital artery arose from the posterior surface of the external carotid artery (ECA) 6-9 mm (mean 7 mm) above the carotid bifurcation. There is also an important 'cross' between the occipital artery and the HN. In all cadavers, this crossing point was 7-9 mm (mean 8 mm) superior to the emergence of the occipital artery from the ECA. In conclusion, understanding the detailed anatomy of the HN and using landmarks to identify the nerve are crucial for surgery in the region.