Functional anatomy of the accessory nerve studied through intraoperative electrophysiological mapping

Abstract

OBJECTIVE Classically the 11th cranial nerve (CN XI, or accessory nerve) is described as having a cranial and a spinal root, the latter arising from the upper segments of the spinal cord through a number of very fine rootlets. According to classical knowledge, the cranial root gives motor innervation to the vocal cords, whereas the spinal root provides the motor innervation of the sternocleidomastoid muscle (SCM) and of the upper portions of the trapezius muscle (TZ). The specific function of each of the rootlets of the spinal component is not well known. Therefore the authors aimed to map, using intraoperative direct electrical stimulation and electromyographic (EMG) recordings, the innervation territory of these rootlets in relation to their exit level from the CNS. METHODS Forty-nine patients undergoing surgery with intradural exposure at the craniocervical junction were enrolled in the study. The EMG recordings included the sternal and clavicular parts of the SCM (SCM-S and SCM-C), the superior and middle parts of the TZ (TZ-S and TZ-M), and whenever possible the vocal cords. The main trunk of CN XI, its roots (both cranial and spinal), and when possible the fine cervical rootlets, were stimulated at predetermined locations, from the jugular foramen down to the lowest cervical level exposed. The EMG responses were collected, and a map of the responses was drawn up. RESULTS Monitoring and stimulation of the spinal root were performed in all cases, whereas for the cranial root this was possible in only 19 cases. A total of 262 stimulation sites were explored: 70 at the common trunk of the nerve, 19 at the cranial root, 136 at various levels on the spinal root, and 37 at the cervical rootlets. A vocal cord response was obtained by stimulation of the cranial root in 84.2% (16/19); absence of response was considered to have a technical origin. In no case did the vocal cords respond to the stimulation of the spinal root or rootlets. Stimulation of the cervical rootlets yielded responses that differed according to the level of stimulation: at C-1 the SCM-S responded 95.8% of the time (23/24); at C-2 the SCM-C responded 90.0% of the time (9/10); at C-3 the TZ-S responded 66.6% of the time (2/3); and below that level only the TZ-M responded. The spinal root stimulated at its various levels responded accordingly. CONCLUSIONS The function of each of the rootlets of CN XI appears to be specific. The cranial root contributes, independently of the spinal root, to the innervation of the vocal cords, which makes it a specific entity. The spinal root innervates the SCM and TZ with a cranio-caudal motor organization of its cervical rootlets.