What are important landmarks and variations of the spinal accessory nerve (SAN) that make it vulnerable to injury?

Updated: Feb 09, 2018
  • Author: Rohan R Walvekar, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

In the posterior neck, the SAN has a superficial and unpredictable course beneath the superficial cervical fascia that makes it vulnerable to injury. It is embedded in fibrofatty tissue and is found in relation to a group of 5-10 superficial nodes. [6] Several anatomic landmarks and important variations in its course require careful consideration while identifying the nerve.

Important anatomic landmarks and variations of the SAN (see the image below) are as follows:

Shoulder orthosis for scapulohumeral alignment. Shoulder orthosis for scapulohumeral alignment.

See the list below:

  • Surface anatomy: Draw a line from the angle of the mandible to the tip of the mastoid process. The course of the SAN is indicated by bisecting this line at a right angle and extending the second line downward across the posterior triangle. [11]

  • The greater auricular nerve has been used as a landmark to identify the SAN as it emerges from the posterior margin of the SCM muscle. The SAN is always found above the greater auricular nerve within a distance of 10.7 mm, with a standard deviation of 6.3 mm. [31]

  • In the upper neck, 2 arrangements of the SAN have been described in relation to the internal jugular vein (IJV). In the more common (75-90%) anterior variant, the SAN nerve crosses in front of the IJV; in the less common (10-25%) variant, the nerve crosses behind the vein (see the first image below). Rarely, the SAN may traverse a divided IJV and appear to travel through the lumen of the vein (see the second image below). [32, 33] The SAN may also be palpable as a cord as it runs through the upper neck beneath the SCM muscle, dividing zone II into levels IIA and IIB. This often serves as a guide to the surgeon to begin nerve identification.

    Relationship of internal jugular vein to the spina Relationship of internal jugular vein to the spinal accessory nerve (SAN).
    Spinal accessory nerve (SAN) posterior to the inte Spinal accessory nerve (SAN) posterior to the internal jugular vein.
  • As the nerve approaches the sternocleidomastoid muscle (SCM), it may perforate the cleidomastoid portion of the SCM (80%) or run posterior to it (20%). [34] The following 3 types of SAN innervation of the SCM have been described:

    • Type A, the nonpenetrating type

    • Type B, the partially penetrating type

    • Type C, the completely penetrating type

  • The SAN is known to form a plexus prior to its insertion into the trapezius. Shiozaki et al described 5 types of innervation of the trapezius by the main trunk of the SAN and its branches, in which the number of branches that innervate the muscle ranged from 0-4. [27] More importantly, greater branching in the nerves seems to be associated with a thinner main trunk.

  • Another important landmark for the surgeon is the relationship of the clavicle to the point of insertion of the SAN into the trapezius muscle. The SAN can be identified approximately 51.3 mm (standard deviation 17 mm) above the clavicle as it enters the anterior border of the trapezius. This is a constant landmark and is often helpful in identifying the distal end of the nerve. [31, 35]

  • The superficial cervical vein, a branch of the external jugular vein, vascularizes the anterior margin of the trapezius muscle close to the site at which the main trunk of the SAN innervates the trapezius muscle. The superficial cervical vein is therefore a useful anatomic landmark that runs slightly inferior to the SAN. [27]


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