Cranial Nerve XI: The Spinal Accessory Nerve

Clinical Vignette

A 23-year-old medical student noted swollen lymph nodes in the left posterior triangle of his neck. He was otherwise asymptomatic. The student health service told him that he was overly concerned and there was nothing wrong. Within a period, he became quite fatigued with bouts of fever and sought another opinion from a respected internist. Except for the presence of abnormal left posterior cervical and axillary lymphadenopathy, his clinical examination was normal. Results of a mononucleosis spot test were normal but liver function studies revealed elevated transaminases. Excision of cervical lymph nodes was performed.

During the procedure, the surgeon queried the student about the risks of this minor operation. The student replied that it is important to exercise caution so as not to cut the spinal accessory nerve (CN-XI) because paralysis of the trapezius muscle would result. When he returned to his rotation on the chief of surgery’s service, the student was informed by the intern that the chief, on hearing about his missing student, commented that there was a 50% 5-year mortality rate among his patients undergoing this procedure.

The professor of pathology could not arrive at a diagnosis and sent the node to the Armed Forces Institute of Pathology and to the Mayo Clinic for more definitive opinions. They did not believe it represented a lymphoma; 6 weeks later, the test for infectious mononucleosis was positive. The senior editor of this text notes that it is now 49 years since he had this biopsy.

The lymph node revealed cellular hyperplasia and atypia but general preservation of lymph node architecture. It was felt that the pathology likely represented a reactive process without evidence of a definitive malignancy or lymphoma. A repeat monospot now returned positive and he was instructed to convalesce, with little physical activity, over 2 weeks. A similar case was encountered by the author soon after. However, in this case, upon returning to exercise, sagging of the left shoulder, restricted shoulder movements, and pain (especially when elevating the arm) were noted by the patient. An evaluation revealed atrophy and weakness of the trapezius on the left with downward and outward winging of the scapula upon arm extension. An electromyography (EMG) confirmed an accessory nerve lesion with denervation potentials isolated to the upper trapezius.

Comment: This vignette exemplifies that physicians always need to give careful consideration to every patient complaint, especially those in the medical profession. The patient was labeled with “medical studentitis,” and a peer created further emotional turmoil for him by suggesting he might die within 5 years. Fortunately, a benign mechanism was established.

Occasionally, patients undergoing similar procedures experience iatrogenic laceration to the spinal accessory nerve. Such leads to significant shoulder pain and atrophy of the unilateral trapezius muscle and residual scapular winging. The more proximally innervated sternocleidomastoid muscle (SCM) is spared. Cervical lymph node biopsies may be the most common cause of CN-XI palsy.

Cranial nerve CN-XI, or the spinal accessory nerve (SAN), serves primarily a motor function for the neck and shoulder. It has an intriguing functional array with one of the two major muscles it innervates, the SCM, inserting on the ipsilateral occiput. When one side contracts, it turns the head in the opposite direction; for example, a right SCM contraction turns the head to the left and vice versa. Both SCM muscles contracting simultaneously results in neck flexion.

The seemingly paradoxical function of the SCM is also of interest and used in rare circumstance of a hysterical pseudo-hemiparesis or functional somatization with secondary gain. Patients feigning a right hemiparesis will give way when asked to turn their head against resistance to the right, not realizing that it is the left SCM that turns the head contralaterally. Thus when asked to turn their head to the asymptomatic left, they use the right SCM without difficulty.

The SAN is primarily a motor nerve innervating the SCM and trapezius muscles in the neck and back (Fig. 10-1). In contrast to the other cranial nerves, its lower motor neuron cell bodies are located primarily within the spinal cord. The accessory nucleus is a cell column within the lateral anterior gray column of the upper five or six cervical spinal cord segments. Proximally it lies nearly in line with the nucleus ambiguus and caudally within the dorsolateral ventral horn. Originating from the accessory nucleus, the rootlets emerge from the cord and unite to form the trunk of CN-XI. This extends rostrally through the foramen magnum into the posterior cranial fossa. Intracranially, it accompanies the caudal fibers of the vagus nerve (CN-X) exiting the skull through the jugular foramen. The SAN then descends in close proximity to the internal carotid artery and internal jugular vein (Fig. 10-2).

Figure 10-1 Accessory Nerve (XI): Schema.

Figure 10-2 Cervical Plexu in Situ.

Once the spinal accessory nerve is extracranial, it is joined by fibers derived from the third and fourth upper cervical ventral rami. Some of these cervical fibers may innervate the caudal trapezius, whereas the proximal trapezius and the entire SCM muscle are primarily innervated by CN-XI. The spinal accessory nerve then emerges from the midpoint of the posterior border of the SCM, to cross the posterior triangle of the neck superficial to the levator scapulae. It is here that this cranial nerve is in close proximity to the superficial cervical lymph nodes. Further caudally, approximately 5 cm above the clavicle, it passes into the anterior border of the trapezius muscle, which it also innervates.

There is a minor afferent component to the SAN that carries primary proprioceptive function for the two muscles it innervates. Also, a minor cranial root contribution to the spinal accessory consists of a few fibers originating in the caudal portion of the nucleus ambiguus. These fibers traverse with the intracranial spinal accessory nerve and exit through the jugular foramen.

The supranuclear innervation of the CN-XI nuclei is still a matter of debate. Although the trapezius muscle is innervated from the opposite hemisphere, there is some question as to whether the supranuclear innervation of the SCM is also contralateral. One standard neuroanatomy text, Brodal 1998, states that with clinical corticobulbar lesions there is paresis of the contralateral SCM as well as the trapezius. Others note, based on intracarotid Amytal injections, that the SCM is innervated predominantly from the ipsilateral hemisphere. Suffice it to say that the most proximal and midline musculature can be activated bilaterally. Therefore, one needs to be circumspect when attempting to lateralize the source of unilateral SCM weakness.