Surgical anatomy of the spinal accessory nerve: review of the literature and case report of a rare anatomical variant.

OBJECTIVE: To evaluate the prevalence of variations in the anatomical route of the spinal accessory nerve from the base of the skull to the point where it enters the trapezius muscle. A case report is used to demonstrate an example of a rare but clinically important anatomical variant of this nerve.

METHODS: An independent review of the literature using Medline, PubMed and Q Read databases was performed using combinations of terms including 'spinal accessory nerve', 'anatomy', 'surgical anatomy', 'anatomical variant', 'cranial nerve XI' and 'shoulder syndrome'.

RESULTS: Our report demonstrates marked variation in spinal accessory nerve anatomy. At the point of crossing over the internal jugular vein, the spinal accessory nerve passes most commonly laterally (anterior) to the internal jugular vein. The reported incidence of this lateral relationship varies from 67 to 96 per cent. The nerve can also pierce the internal jugular vein, as demonstrated in our case study, with incidence ranging from 0.48 to 3.3 per cent.

CONCLUSION: Anatomical variations of the spinal accessory nerve are not uncommon, and it is important for the surgeon to be aware of such variations when undertaking surgery in both the anterior and posterior triangles of the neck.