From the Guidelines
The hypoglossal nerve (CN XII) courses anteriorly inferior to the hyoid bone as it travels to provide somatic motor innervation to the intrinsic and extrinsic muscles of the tongue. This anatomical relationship is crucial 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 the segment where it passes inferior to the hyoid bone 1. The extracranial hypoglossal nerve travels caudally within the carotid space and then courses anteriorly, distributing terminal branches to the muscles of the tongue, except the palatoglossus muscle, and the infrahyoid strap muscles via the ansa cervicalis.
Key points to consider regarding the relationship between the hypoglossal nerve and the hyoid bone include:
- The hypoglossal nerve's position inferior to the hyoid bone makes it susceptible to injury during surgical procedures in the submandibular region 1.
- Hypoglossal nerve palsy can present clinically with dysarthria and deviation of the tongue to the side of the lesion upon tongue protrusion, and can be caused by a multitude of factors, including brainstem infarct, demyelinating disease, tumors, vascular lesions, trauma, inflammatory, and infectious processes 1.
- The most common causes of isolated hypoglossal nerve palsy are malignant tumors along the course of CN XII in the carotid space and in the sublingual space, and dissection of the internal carotid artery can result in isolated acute CN XII palsy or multiple variable patterns of CN palsies 1.
From the Research
Relationship between the Hypoglossal Nerve and the Hyoid Bone
- The hypoglossal nerve has a specific relationship with the hyoid bone, as it passes near this structure during its course in the neck 2.
- The point where the hypoglossal nerve crosses the internal carotid artery (ICA) is approximately 12.24 mm superior to the greater horn of the hyoid bone 2.
- The hyoid bone can be used as a landmark to identify the branches of the hypoglossal nerve, particularly when combined with the morphology of the extrinsic muscles of the tongue 3.
- The geniohyoid branch of the hypoglossal nerve arises near the hyoglossus muscle, which is related to the hyoid bone 3.
- In surgical procedures, the hyoid bone is used as a reference point for the reconstruction of the mobile tongue and suprahyoid muscles, including the insertion of muscles between the mandible and the hyoid bone 4.
Anatomical Considerations
- The hypoglossal nerve is divided into several segments, including the cisternal, intracanalar, descending, horizontal, and ascending segments, each with specific relationships to surrounding structures 5.
- The nerve supplies the intrinsic and extrinsic muscles of the tongue, as well as the geniohyoid muscle, and its branching pattern is characterized by positional relationships to the components of the extrinsic muscles 3.
- The hypoglossal nerve exits the skull through the hypoglossal canal and is a purely motor nerve, responsible for the innervation of the tongue muscles and the geniohyoid muscle 6.