A detailed review of the spinal accessory nerve and its anatomical variations with cadaveric illustration

Abstract

The spinal accessory nerve, considered part of the eleventh cranial nerve, provides motor innervation to sternocleidomastoid and trapezius. A comprehensive literature review and two cadaveric dissections were undertaken. The spinal accessory nerve originates from the spinal accessory nucleus. Its rootlets unite and ascend between the denticulate ligament and dorsal spinal rootlets. Thereafter, it can anastomose with spinal roots, such as the McKenzie branch, and/or cranial roots. The spinal accessory nerve courses intracranially via foramen magnum and exits via jugular foramen, within which it usually lies anteriorly. Extracranially, it usually crosses anterior to the internal jugular vein and lies lateral to internal jugular vein deep to posterior belly of digastric. The spinal accessory nerve innervates sternocleidomastoid, receives numerous contributions in the posterior triangle and terminates within trapezius. Its posterior triangle course approximates a perpendicular bisection of the mastoid-mandibular angle line. The spinal accessory nerve contains sensory nociceptive fibres. Its cranial nerve classification is debated due to occasional non-fusion with the cranial root. Surgeons should familiarize themselves with the variable course of the spinal accessory nerve to minimize risk of injury. Patients with spinal accessory nerve injuries might require specialist pain management.

Keywords: Accessory; Classification; Course; Nerve; Variation.

Fig. 1

Dorsal spinal cord showing path of spinal accessory nerve (SAN) from its rootlets to the jugular foramen. Posterior inferior cerebellar artery (PICA) emerges between SAN and vagus nerve. Cere cerebellum, CNVII/VIII facial-vestibulocochlear nerve complex, CNIX glossopharyngeal nerve, CNX vagus nerve, CRs cranial roots of accessory nerve, DL denticulate ligament, DSR dorsal spinal rootlets, Dura and Arach dura and arachnoid mater, JF jugular foramen, PICA posterior inferior cerebellar artery, PSA posterior spinal artery. SAN spinal accessory nerve, SANr spinal accessory nerve rootlets, VA vertebral artery, IVth fourth ventricle

Fig. 2

Photograph and Schematic of posterior inferior cerebellar artery penetrating spinal accessory nerve rootlets (A and B) and of posterior inferior cerebellar artery passing between glossopharyngeal and vagus nerves (C and D) (Saylam et al. 2007). B medulla oblongata (bulbous), BA basilar artery, C cerebellum, O olive, P posterior inferior cerebellar artery, VA vertebral artery, VII facial nerve, VIII vestibulocochlear nerve, IX glossopharyngeal nerve, X vagus nerve, XI spinal accessory nerve, Arrow anterior inferior cerebellar artery, Arrowheads rootlets of hypoglossal nerve

Fig. 3

Accessory nerve cranial filaments (arrowheads), rootlets (asterisks) and roots (Liu et al. 2014a) (permission received from publisher for redistribution). CR cranial root, Ob obex, Ov olive, SR spinal root, IX glossopharyngeal nerve, X vagus nerve, XI accessory nerve, XII hypoglossal nerve, Uppermost and next rootlets are indicated by black and white colours respectively. Associated filaments are labelled with the same respective colours

Fig. 4

McKenzie branch linking the spinal accessory nerve to a ventral C1 rootlet. a shows accessory nerve in situ, b and c show accessory nerve reflected. Saylam et al. (2009a, b) (permission received from publisher for redistribution). C2 dorsal C2 root, dl denticulate ligament, SC spinal cord, va vertebral artery, XI pinal accessory. Asterisk anterior spinal medullary artery, nerve, Thin arrow McKenzie branch, Thick arrowheads ventral C1 rootlets

Fig. 5

Dorsal root anastomoses occurring medial and lateral to the spinal accessory nerve (SAN) trunk. DC Dorsal Collumn, DSR dorsal spinal root, Lat Anast lateral anastomosis, Med Anast.  medial anastomosis, SAN spinal accessory nerve, SN spinal nerve, VSR ventral spinal root

Fig. 6

Relationship between spinal accessory nerve (SAN) and internal jugular vein (IJV) within the jugular foramen according to (A) Yigit et al. (2018) and (B) Saman et al. (2011) (exterior skull base views), CA carotid artery, IJV internal jugular vein

Fig. 7

Relationship between the spinal accessory nerve and the internal jugular vein. A and B SAN crossing the IJV anteriorly and posteriorly; C and D SAN passing through/around IJV duplication; E SAN passing through IJV fenestration. IJV internal jugular vein, SAN spinal accessory nerve

Fig. 8

Cadaveric dissection showing anterior crossing of spinal accessory nerve (SAN) over internal jugular vein (IJV) and lateral positioning relative to IJV at the posterior belly of digastric muscle. AC_IR inferior root of ansa cervicalis, AC_SR superior root of ansa cervicalis, CNVII(st)  facial nerve stump, CNIX glossopharyngeal nerve, CNIXStPh glossopharyngeal nerve branch to stylopharyngeus muscle, CNX vagus nerve, FA facial artery, ICA internal carotid artery, IJV internal jugular vein, LON lesser occipital nerve, PBoD posterior belly of digastric muscle, SAN spinal accessory nerve, SCM sternocleidomastoid muscle, SCMbr sternocleidomastoid branch of occipital artery, SMG submandibular gland, StH stylohyoid muscle, StPh stylopharyngeus muscle, StP(r)  styloid process (reflected), TB temporal bone

Fig. 9

Extradural course and termination of the spinal accessory nerve (SAN). AC_SR superior root of the ansa cervicalis, Br.Pl brachial plexus, CbTrap cervical branch to trapezius muscle, CNVII facial nerve stump, CNIX glossopharyngeal nerve, CNX vagus nerve, CP cervical plexus, DSN dorsal scapular nerve, EJV(st)  external jugular vein stump, ICA internal carotid artery, IJV internal jugular vein, LON lesser occipital nerve, LS levator scapulae muscle, LTN long thoracic nerve, OmH omohyoid, PBoD posteror belly of digastric muscle, PN phrenic nerve, SAN spinal accessory nerve, ScAnt scalenus anterior muscle, SCM sternocleidomastoid muscle, ScMed scalenus medius muscle, ScPost scalenus posterior muscle, SMG submandibular gland, SSA suprascapular artery, SSV suprascapular vein, St.(r)  reflected styloid process, StH stylohyoid, StPh stylopharyngeus, Trap trapezius muscle

Fig. 10

A Line joining C2 transverse process with tip of shoulder. B Perpendicular bisection of mastoid-mandibular angle line (modified from Chandawarkar et al. (2003), permission received from publisher for modification and redistribution)

Fig. 11

Spinal accessory nerve (SAN) exiting the posterior border of sternocleidomastoid superior to Erb’s point. CbTrap cervical branch to trapezius, EJV external jugular vein, GAN greater auricular nerve, LON lesser occipital nerve, PN phrenic nerve, PrVF prevertebral fascia, SAN spinal accessory nerve, SCM sternocleidomastoid muscle, SCNs supraclavicular nerves, Superf. V.  superficial veins, TCN transverse cervical nerve, Trap trapezius muscle, asterisk Erb’s point

Fig. 12

Commonest distribution of the spinal accessory nerve (SAN) cranial root Liu et al. (2014b) (permission received from publisher for redistribution). Blue vagus nerve. Green spinal root of accessory nerve, Red cranial root of accessory nerve