Anatomic relationship between the spinal accessory nerve and the jugular vein: a cadaveric study

Previous studies of the course of the Spinal Accessory Nerve (SAN) and its relationship to the Internal Jugular Vein (IJV) have yielded conflicting results because of the small number of anatomic specimens and anatomic variability. Classic teaching in Head and Neck Surgery is that the SAN almost always crosses the IJV anteriorly in the upper neck. However, because of the morbidity associated with the injury to the IJV during nerve dissection, it is imperative that the surgeon is wary of the posteriorly crossing nerve. In order to further elucidate the anatomy of the SAN in relation to its surrounding structures, we have studied its anatomy at various points. Specifically, we have aimed to: (1) characterize the anatomic relationship of the SAN to the IJV at three major points: (a) within jugular foramen (JF), (b) at base of skull (BoS), and (c) at the posterior belly of the digastric muscle, (2) record the distance travelled by the SAN from the BoS to its medial to lateral crossing of the IJV, and (3) characterize the anatomy of the JF by with respect to greatest length, width, and partitioning. Sixty-one cadavers, 27 male, and 34 female (84 necks) were dissected and the course of the SAN was followed from the BoS to the crossing the IJV. Data recorded included the relationship of the SAN to the IJV (a) within the JF from an intracranial view, (b) exiting the JF at BoS, and (c) in the neck at the level of the posterior belly of the digastric muscle where anterior versus posterior positioning of the crossing nerve with respect to the IJV was noted. The distance travelled by the SAN from BoS until crossing the IJV, the length and width of the JF within the cranial fossa, and JF partitioning were also recorded. Within the JF, the SAN travelled anteromedial to the IJV in 73/84 (87%) necks. While exiting the JF, the SAN was found lateral to the IJV in 56/84 (67%) of necks. In the anterior triangle of the neck the SAN crossed the IJV anteriorly in 67/84 (80%) necks, posteriorly in 16/84 (19%) and in the one case of IJV bifurcation, the nerve pierced the vein. The average distance travelled by SAN from BoS to crossing the IJV was 2.38 cm. The average length and width of the JF were, respectively, 1.42 and 0.78 cm, and the IJV was partitioned in 36/84 necks, with 3 of the partitions being bony and the remainder fibrinous. No relationship was found between JF dimensions/partitioning and the anatomic relationship of the structures exiting it. In this study, the dimensions and relationship of the IJV and SAN are described in detail. This relationship is specifically noted at three major points, namely within the cranium, at the BoS, and in the anterior neck triangle. In its medial to lateral path in the anterior neck triangle, the SAN crossed the IJV anteriorly in a majority of the cases. However, a posteriorly crossing nerve was not uncommon. These findings support results in previous literature in that the SAN is located anterior to the IJV in the majority of the cases, however, it is imperative for the surgeon to be mindful to the anatomic variability and possible posterior crossing of the IJV by the SAN in the neck to avoid injury to the IJV during the dissection of the nerve. The distance travelled by the nerve prior to crossing the IJV was measured and can be used as a helpful tool for the surgeon in finding the nerve during dissections. We were not able to demonstrate a correlation between the relationship of the SAN and IJV at other recorded points and their crossing relationship. Similarly, no correlation was found between the anatomy of JF and the relationship of the SAN and IJV at any point.