A rare anatomical star shaped branching pattern of spinal accessory nerve: A case report with review of literature

Abstract

The spinal accessory nerve provides motor innervation to the sternocleidomastoid and trapezius muscle. It is an extremely important structure to be preserved during neck dissection to avoid sequalae related to shoulder dysfunctions. The incidence of shoulder dysfunction and morbidity can be attributed to varied anatomy and branching pattern of the nerve or the contribution by the cervical plexus to the motor innervations of the trapezius muscle. Hence it is important to have knowledge of the varied anatomy and branching pattern of the spinal accessory nerve to avoid the possible shoulder morbidity and dysfunction following neck dissections. Lanisnik B etal’s study showed that there are three recognizable branching patterns of the spinal accessory nerve for innervation of the trapezius muscle. In type 1, the SAN enters the Sternocleidomastoid muscle and a single trapezius muscle branch exits from the posterior border of the SCM after receiving communications from the cervical nerves, especially C2 and C3. In type 2, the motor branch for trapezius muscle separates from the main trunk at level II, before the nerve enters the sternocleido-mastoid muscle. In the type 3 pattern, CN XI enters the SCM in the same way as described in type 1, and the motor branch for the trapezius muscle exits from the SCM muscle behind its posterior border; however, it does not immediately travel to level V and the trapezius muscle, but instead takes a more medial course and mixes with the cervical nerves, predominantly C2 and C3. In this case report, we will discuss an unusual branching pattern of spinal accessory nerve similar to the type 3 variant as explained by Lanisnik that we have encountered during a modified radical neck dissection, in a case of Squamous cell carcinoma of right buccal mucosa.