From the Guidelines
The hypoglossal nerve is located in the head and neck region, originating in the dorsal medulla, traversing the premedullary cistern, and entering the hypoglossal canal, before traveling caudally within the carotid space and distributing terminal branches to the tongue and infrahyoid strap muscles. The hypoglossal nerve (CN XII) arises from the hypoglossal nucleus in the dorsal medulla, as described in the study by 1.
Key Points
- The nerve traverses the premedullary cistern dorsal to the vertebral artery and then forms the nerve, which enters the hypoglossal canal where it is surrounded by a venous plexus.
- The extracranial hypoglossal nerve travels caudally within the carotid space after which it courses anteriorly inferior to the hyoid where it distributes terminal branches providing somatic motor innervation to the intrinsic and extrinsic muscles of the tongue (except the palatoglossus muscle) and the infrahyoid strap muscles via the ansa cervicalis, as noted in the study by 1.
- The location of the hypoglossal nerve is critical in understanding the potential causes of hypoglossal nerve palsy, which can occur due to injury of the nerve at any point along its course, including brainstem infarct, demyelinating disease, tumors, vascular lesions, trauma, inflammatory, and infectious processes, as discussed in the study by 1.
Clinical Implications
- Hypoglossal nerve palsy typically presents clinically when a nuclear or infranuclear lesion is present and is characterized by dysarthria and deviation of the tongue to the side of the lesion upon tongue protrusion.
- The most common causes of isolated hypoglossal nerve palsy are malignant tumors both along the course of CN XII in the carotid space and in the sublingual space, as well as dissection of the internal carotid artery, which can result in isolated acute CN XII palsy or multiple variable patterns of CN palsies including involvement of CN IX through CN XII, as noted in the study by 1.
From the Research
Location of the Hypoglossal Nerve
The hypoglossal nerve is located in the following areas:
- It exits the brainstem in the preolivary sulcus 2
- It passes through the premedullary cistern 2
- It exits the skull through the hypoglossal canal 2, 3
- It is divided into 5 segments: cisternal, intracanalar, descending, horizontal, and ascending 3
- It has relationships with surrounding structures such as the carotid bifurcation, external carotid artery, internal carotid artery, occipital artery, and sternocleidomastoid artery 4, 5
Relationship with Surrounding Structures
The hypoglossal nerve has variable relationships with surrounding structures, including:
- The distance from the carotid bifurcation to the point at which the hypoglossal nerve crosses over the internal carotid artery ranges from 3.89 to 37.03 mm 4
- The distance from the bifurcation to the point at which the hypoglossal nerve crosses over the external carotid artery ranges from 2.63 to 29.43 mm 4
- The sternocleidomastoid artery has a close relationship with the hypoglossal nerve and can be used as a landmark to identify the nerve 4
- The occipital artery can make a loop around the hypoglossal nerve and exert pressure on it in some cases 5