Accessory nerve: summarized anatomical topology in a narrative review

Introduction The spinal accessory nerve (cranial nerve XI) is a motor and proprioceptive nerve that supplies sternocleidomastoid and trapezius muscles, thus it is essential for proper shoulder function. Given its variable anatomy and clinical implications, this study aimed to review the currently knowledge of its topography, branching and clinical implications. Materials and methods We followed SANRA guidelines and performed literature search using PubMed, Web of Science, and Embase databases up to November 2024. Selected studies included written in English, original, peer-reviewed cadaveric, radiologic, and surgical articles with clearly defined objectives, methods, and results regarding accessory nerve anatomy. Two reviewers independently screened the studies, with consensus reached through a third reviewer when necessary. Results Extracted data included morphometric parameters, nerve course, branching, exit and ending points, and clinical considerations from 71 studies. Results highlighted marked variability in exit points, with the nerve commonly emerging at or near the sternocleidomastoid muscle, leaving the cranium through jugular foramen. Its relationship to the internal jugular vein and cervical plexus was inconsistent, presenting frequent anatomical variants that we described. Branching patterns to both sternocleidomastoid and trapezius muscles varied widely, with up to five branches. Discussion The clinical analysis led us to the conclusion that the nerve’s superficial course through the posterior cervical triangle renders it especially vulnerable during lymph node dissection, biopsies, tumor removals and other surgical procedures. Injury leads to “shoulder syndrome”, characterized by pain, weakness, and scapular winging, that often requires reconstructive intervention. Understanding the accessory nerve’s anatomical variability is critical for surgical planning, intraoperative nerve preservation and improving patient outcomes.