Introduction: Injury of the Spinal Accessory Nerve (SAN) during neck dissection can lead to paralysis, weakness and pain of the trapezius and sternocleidomastoid (SCM) muscles, termed Accessory Nerve Disorder (AND). This condition is characterized by shoulder pain, weakness, and decreased range of motion. It is estimated that inadvertent injury to the SAN occurs in 70% of neck dissections. We performed a cadaver study to characterize anatomic variations of the trapezius branch of the SAN to minimize the risk of iatrogenic AND during neck dissections.
Methods: Fifteen cadavers were dissected resulting in a total of 24 recorded SANs. The SAN was traced from the skull base to the SCM. The trapezius branch was identified and traced to its insertion site in the trapezius muscle. Three aspects of the SAN were analyzed and recorded: the course of the trapezius branch in relationship to the SCM, the number of trapezius branches, and the number of cervical rootlet contributions to the SAN.
Results: Twenty three (96%) of the 24 trapezius branches were protected by the SCM, either running posterior to the muscle or branching off the SAN main trunk within the SCM. Only one trapezius branch (4%) was found to have an anteromedial branching pattern from the SAN main trunk. This nerve coursed medial to the SCM before wrapping posteriorly around the SCM to reach the trapezius. Nine of the trapezius branches (37.5%) had no contributions from cervical rootlets, whereas 6 (25%) had one, 8 (33%) had two, and one (4%) had three. Eighteen of the trapezius branches (75%) entered the muscle as one nerve, five (21%) as two, and one (4%) could not be commented on. There was variability between the right and left sides for all three of the recorded characteristics.
Conclusions: In the majority of cases, the trapezius branch of the SAN is protected by running posterior the SCM. There is a high degree of variability of the cervical rootlet contributions to the trapezius branch. Most of the trapezius branches enter the muscle as a single nerve trunk. Finally, these anatomic characteristics frequently differ between the right and the left sides in the same cadaver.