Anatomic variation of the spinal accessory innervation pattern of the sternocleidomastoid muscle: A cadaveric study

Several studies have described the anatomical course of the spinal accessory nerve (SAN) and its associated landmarks. In particular, the SAN course through the sternocleidomastoid (SCM) has been reported as either intramuscular or branching without penetration of the SCM. However, it remains unclear what the occurrence of either anatomical course is, as various studies have reported inconsistent findings. For example, Lee SH et al. (South Korea) and Kierner SA et al. (Austria) reported that the SAN more commonly penetrates the SCM (54.1% and 63%, respectively) rather than branching in a clinical and cadaveric studies. Yet, Anehosur V. et al. reported that the SAN more commonly branches without penetration (54%) in a clinical study of Indian patients undergoing neck dissection. We performed a cadaveric study on 31 subjects using an anterior triangle approach to visualize the anatomy of the SAN as it courses towards the SCM. All the subjects were White and included 35% male (N=11) and 65% female (N=20). Our study shows that the SAN courses through SCM intramuscularly 65% of the time, while giving off branches without penetration 35% of the time. The observed difference from the previously reported numbers suggests a potential ethnicity-associated variation in the course of the SAN in relation to the SCM. Understanding the SAN anatomy and its variations is critical when performing neck dissection to avoid iatrogenic injuries to the nerve. Taken together, our study demonstrates a preponderance for the SAN to penetrate the SCM prior to coursing toward the posterior triangle of the neck.