Cranial Nerves XI and XII: Accessory and Hypoglossal

Anatomy

The SAN is primarily a motor nerve innervating the SCM and trapezius muscles in the neck and back ( Fig. 11.1 ). In contrast to the other CNs, 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. 11.2 ).

Once the SAN 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 SAN 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 CN 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. In addition, 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 SAN 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 ( ) states that with clinical corticobulbar lesions there is paresis of the contralateral SCM as well as the trapezius. Others note, based on intracarotid sodium 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.

Clinical Presentation and Diagnostic Approach

SAN lesions located intracranially or proximally to the innervation of the SCM cause weakness of both the SCM and the trapezius. If the SCM is weak, the patient experiences weakness when turning the head to the opposite side. Damage to the nerve within the posterior triangle of the neck spares the SCM and results in weakness of the trapezius only. Involvement of the trapezius manifests as drooping of the shoulder and mild scapular winging away from the chest wall with slight lateral displacement. Weakness in shoulder elevation and arm abduction above horizontal is typical. Winging is apparent with arms hanging along the trunk and becomes accentuated when patients abduct the arms. In contrast, scapular winging from serratus anterior weakness due to long thoracic nerve palsy is most prominent on forward elevation of the arms ( Fig. 11.3 ).

Most individuals with CN XI palsies present with shoulder or neck pain or both. The painful paresis can be sudden because of direct injury during procedures such as excision biopsies of lymph nodes in the posterior triangle of the neck, radical dissection of lymph nodes in the neck, or with trauma, or the paresis can be subacute such as with entrapment of the nerve within scar tissue or structural lesions such as tumors. As in all patients with neck and shoulder pain, careful exam and history are necessary to exclude lesions at the level of the cervical nerve roots or brachial plexus.

Electromyography is important for confirming that the lesion is confined to the distribution of CN XI. In addition, a gadolinium-enhanced magnetic resonance imaging (MRI) is appropriate if any question exists of a more widespread lesion other than a simple CN XI neuropathy.