Anatomical investigation of the accessory nerve

Abstract

The eleventh cranial nerve, known as the accessory nerve (CN XI), consists of a cranial and a spinal component. The spinal root is responsible for the motor innervation of the sternocleidomastoid (SCM) muscle and the trapezius (TZ) muscle. During neck dissection, shoulder syndrome presents in patients due to the injury of the CN XI, which inadvertently leads to the paralysis of the SCM muscle and TZ muscle. The specific function of the CN XI is not well-known as the anatomy described in the literature is inconstant, and variation is found with regard to the anastomotic connections with other nerves. The purpose of this research was to investigate the anatomical description of the CN XI in the posterior triangle, as well as its possible connections to the posterior root of the first cervical nerve (C1). Ten cadaveric spinal cord specimens were used in the examination of the CN XI's possible connections to other nerves, and 38 adult embalmed cadavers were used in the investigation of the CN XI's course through the posterior triangle of the neck. The results obtained indicated that the spinal root of the CN XI arises mainly from rootlets of C1 to C5 spinal nerves. The most common variation found was the posterior root of C1 spinal nerve joining the CN XI and ending at the point of anastomosis (Type IV). This type of anastomosis was seen in four cadavers (40%) on both sides and this connection type was found more in males than in females. Using the average lengths of the SCM muscle and TZ muscle to divide the posterior triangle into a superior, middle and inferior third it was found that nerve mainly coursed from the middle third of the SCM muscle to the inferior third of the TZ muscle in 32/76 (42.11%) specimens and this was seen more in males than females. From the data collected, the nerve's position, course, and branching were not statistically significant (p>0.05) when compared to sex (male and female) and side (left or right) of the cadaver. However, it was found that using the superior borders of the sternal and acromial ends of the clavicle can be used to narrow down the entry and exit points of the CN XI. There was also a high intra/inter-rater reliability for the measurements using the clavicle as a bony vi landmark. However, when using the soft tissue landmarks, the measurements varied significantly between the observers. Based on the results from the present study there is great variability of interconnections with the CN XI found in the posterior triangle and spinal cord. These anatomical findings can attempt to explain why function loss may also vary in different patients when the CN XI is injured. Due to the great variation found within the CN XI's origin, course and function this study concludes that when conducting surgical neck dissection, one should avoid removing any nerve, including the cervical plexus nerves. Keywords: Accessory nerve, Clavicle, Erb’s point, First cervical spinal nerve, Nerve of McKenzie, Spinal accessory nerve palsy, Sternocleidomastoid, Trapezius