What are the biomechanics of spinal accessory nerve (SAN) injury?

Updated: Dec 08, 2020
  • Author: Rohan R Walvekar, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Anatomic study of the SAN has long maintained a debate as to the exact contributions of this nerve and other cervical motor nerves to the innervation of the trapezius muscle. In 1933, Bardeen suggested that the origin of motor input to the trapezius muscle was purely from the cervical nerves. Subsequent anatomic study reported a possible plexus composed of both cervical nerves and contributions from the SAN that collectively provided trapezial motor innervation. The classic and much-used Gray’s Anatomy assigned cervical nerves to a proprioceptive sensory role, with only the SAN providing motor innervation to the trapezius. [6, 7]

Current evidence suggests a variable contribution of both cervical and SAN motor innervation to the trapezius. This explains the unpredictable motor and sensory deficits that arise from transection of the nerves to this muscle. For example, significant preservation of trapezial function has been reported even in radical neck dissections where the SAN was intentionally sacrificed. [5] SAN damage that results from radical neck dissection was first described by Ewing and Martin (1952), although Nahum (1961) coined the term "shoulder syndrome," describing the clinical syndrome of pain and shoulder dysfunction that is associated with SAN injury. [8, 9]

Interest in methods for evaluating SAN function and changes after surgical neck dissection have led to the use of various diagnostic modalities, such as electromyography (EMG) and ultrasonography. [9, 10, 11] Likewise, methods of nerve repair following surgical injury have been also been investigated. Harris and Dickey first described a cable grafting technique to restore SAN function. [12] Gou et al have described an alternative technique for SAN reconstruction that used a sternocleidomastoid muscle—greater auricular nerve (GAN) flap, with the advantage of having an interpositioned graft vascularized by fascia and muscle with no donor site morbidity. [13] Diagnostic methods and nerve-salvaging strategies are discussed in further detail in the Workup section.

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