The twelfth cranial nerve or hypoglossal nerve is an entire motor nerve, controlling the intrinsic and extrinsic muscles of the tongue, as well as the infrahyoid strap muscles through an anastomosis with the cervical plexus, called the ansa cervicalis.

Damage to the hypoglossal nerve produces characteristic clinical and radiological manifestations of which unilateral tongue atrophy is the most common. Dysfunction of the hypoglossal nerve may be a consequence of supranuclear, nuclear or infranuclear disease. The nuclear and infranuclear hypoglossal nerve can be divided into four segments: the medullary, cisternal, skull base and extracranial segments. Because each segment is usually affected by different disorders, localizing a lesion to a particular segment allows the radiologist to narrow the differential diagnosis.

ANATOMY:

The cortical centre for lingual movement is located within the precentral gyrus, which sends fibers to the hypoglossus nucleus. The paired hypoglossus nuclei are located on the floor of the fourth ventricle, within the medulla oblongata. This is the medullary segment. The fibers course anteriorly, lateral to the medial lemniscus, to exit the medulla in the preolivary sulcus. The rootlets lie posterolateral to the vertebral artery within the premedullary cistern. This is the cisternal segment. Then the rootlets merge to form the hypoglossal nerve within the hypoglossal canal of the occipital bone. This is the skull base segment (Fig. 1). Emerging from the hypoglossal canal, the hypoglossal nerve begins its extracranial course as it enters the nasopharyngeal carotid space. As the nerve passes inferiorly, it comes to lie between the internal jugular vein and the carotid artery, superficial to the vagus nerve (Fig. 2). At the level of the angle of the mandible it then takes an anterior course, lying at the inferior border of the posterior belly of the digastric muscle. After exiting the carotid space the nerve continues anteriorly and inferiorly towards the hyoid bone, crossing the lingual artery to run along the surface of the hyoglossal muscle. This segment of the hypoglossal nerve lies within the sublingual space (Fig. 3). The nerve then continues forward to supply the genioglossus muscle to the tip of the tongue.

As already mentioned, the hypoglossal nerve not only supplies motor innervation to the intrinsic and extrinsic muscles of the tongue but also contributes motor innervation to the infrahyoid strap muscles (sternohyoid, sternothyroid and omohyoid muscle) through the ansa cervicalis. The ansa cervicalis is made up of fibers from the first to third cervical nerves (C1-C3) that course through the carotid space together with the hypoglossal nerve (Fig. 4).