In this article, the author discusses the anatomy of the spinal accessory nerve and injury of the nerve. The spinal accessory nerve is most commonly injured as the result of surgical procedures in the neck. This review discusses physical examination, electrophysiologic testing, and management of injury to the spinal accessory nerve.
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• Spinal accessory nerve palsy is usually iatrogenic, related to surgery in the posterior triangle of the neck.
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• Shoulder shrug is a poor method for testing the function of the trapezius muscle; arm abduction over the head is much better.
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• Spinal accessory nerve lesions cause marked disability and pain.
Historical note and terminology
The spinal accessory nerve, cranial nerve XI, is a purely motor nerve that has both cranial and spinal components. The motor fibers originate from 2 separate motor nuclei in the brainstem and spinal cord. The cranial portion arises from cells adjacent to the nucleus ambiguus and involves fine fibers that join the vagus nerve to supply the muscles of the pharynx via pharyngeal and recurrent laryngeal branches (66; 17); however, a cadaveric study failed to find evidence for a cranial portion of the spinal accessory nerve in 15 specimens (27). The spinal nucleus typically originates in the first to fifth cervical segments. Motor fibers from several cervical levels join to form the spinal portion of the accessory nerve. The nerve then courses upward through the foramen magnum and joins the cranial portion briefly before exiting the skull. Cranial nerve XI exits the skull via the jugular foramen along with the glossopharyngeal (IX) and vagus (X) nerves. The accessory nerve then courses downward and posteriorly in 1 of the 3 compartments of the parapharyngeal space (55). It lies anterior to the internal jugular vein in 55% to 70% of cases and posterior to the internal jugular vein in 30% to 45% of cases (17; 22). However, the relationship of the spinal accessory nerve to the internal jugular vein has been disputed depending on whether the determination is made in cadavers or in surgical series and depending on what level in the neck one is evaluating. In a surgical series, the nerve was found lateral to the internal jugular vein in 95% of cases (59). There are rare cases where the nerve appears to pierce the jugular vein (44). It then either pierces or passes posterior to, and gives off, branches to the sternocleidomastoid muscle. The spinal accessory nerve then runs obliquely across the posterior triangle of the neck to the undersurface of the trapezius muscle that it also supplies. There is considerable anatomic variability (61).
Iatrogenic injury of the nerve is most common in the posterior triangle of the neck, where it assumes a coiled appearance in most people (62). Symes and Ellis dissected the nerve bilaterally in 25 cadavers in order to determine whether there was a consistent anatomical arrangement of nerve fibers that might allow surgeons to avoid injuring the nerve in this region (57). They found that there was considerable variability of the anatomic relationships of the nerve, making the general surgical approach to the posterior triangle valueless. Branches from the second, third, and fourth cervical segments unite with the spinal accessory nerve either in the posterior triangle or on the undersurface of the trapezius muscle and contribute to the nerve supply of that muscle.