What is the pathophysiology of spinal accessory nerve (SAN) injury?

Updated: Mar 04, 2020
  • Author: Rohan R Walvekar, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Rarely, the spinal accessory nerve (SAN) is inadvertently transected during exploration of the posterior cervical triangle. Alternatively, a small branch of the SAN that innervates the upper trapezius muscle may be transected if it is not clearly identified during neck dissection. In nerve-sparing procedures, the proposed mechanism of SAN injury is traction, skeletonization, and devascularization of the nerve during neck dissection. The result of these insults is proposed to be segmental demyelinization due to local ischemia, leading to diminished or complete loss of nerve function. [25] SAN injury results in axonal degeneration, as evidenced by needle EMG and scar formation that are responsible for muscle atrophy and contractures. [24]

Stretch or traction on the SAN compromises intraneural microvascular flow, leading to ischemia and consequently axonal rupture and degeneration. In general, when stress on the SAN due to traction is distributed over longer periods of time, it is tolerated better than rapid and large stretches of the nerve (eg, as seen in sudden acceleration-deceleration injuries). [20]

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