The Hypoglossal Nerve: Anatomical Study of Its Entire Course

doi:10.1016/j.wneu.2017.10.006

World Neurosurgery

Volume 109, January 2018, Pages e486-e492

https://doi.org/10.1016/j.wneu.2017.10.006 Get rights and content

Objective

Only a few anatomic studies of the entire course of the hypoglossal nerve (cranial nerve XII) have been reported. We analyzed all relationships of the 12th nerve with surrounding structures from the brainstem to the tongue through a microscopic perspective. A comprehensive anatomically and clinically oriented classification of its different segments is proposed.

Methods

Ten formalin-fixed adult human cadaveric heads (20 sides) were dissected with the aim to explore the entire course of cranial nerve XII via lateral suboccipital, far lateral partial, or total transcondylar routes. Different segments of the nerve were identified based on the hypoglossal course and its relationship with surrounding structures. Measurements of every portion of the nerve were taken in all specimens during dissection.

Results

The hypoglossal nerve was divided into 5 segments: cisternal, intracanalar, descending, horizontal, and ascending. Detailed and comprehensive examination of basic anatomic relationships through the view of different transcranial and endoscope-assisted approaches was performed. A new perspective of the hypoglossal canal is proposed, and the venous plexus surrounding the intracanalar segment of the nerve is described in detail.

Conclusions

Classification of 5 segments for the hypoglossal nerve seems anatomically valid, and it is surgically oriented with respect to all surgical approaches. Precise knowledge of the relationships with the surrounding structures may help to prevent some complications during surgery, and it is useful to explain, segment by segment, the pathogenic mechanisms for nerve injuries that are evidenced by lesions that exist along the entire intracranial and extracranial course.

Introduction

During the past 2 decades, advances in the field of surgical microanatomy have allowed important progress in the development of surgical techniques to improve excision of skull base lesions. Knowledge of the detailed anatomy and pathway of the hypoglossal nerve is critical for the management of lesions located in the posterior cranial fossa. Several pathologic processes, such as infections, inflammatory diseases, traumas, and skull base tumors (e.g., neurinomas, metastasis, glomus jugulare masses), may damage the 12th cranial nerve, which is the motor supply of the tongue, leading to paralysis of both extrinsic and intrinsic muscles. Furthermore, this nerve has an important role in respiration and swallowing; a unilateral lesion1, 2 does not cause severe restriction in function, but a bilateral lesion may cause severe swallowing disturbances, dysarthria, and respiratory difficulties owing to airway obstruction.3, 4

To understand pathologies of the hypoglossal nerve and relative treatments, knowledge of the entire course of the nerve is extremely important. Knowledge of the anatomic relationships with the surrounding neurovascular structures and muscles and having some constant landmarks to follow are essential in surgical procedures when preservation of the nerve is required. The nerve can be divided into 2 main parts: intracranial and extracranial. Neurosurgeons, otorhinolaryngologists, and maxillofacial surgeons deal with the pathology of the different segments of the nerve during surgical treatments to posterior fossa, neck, or mouth. To our knowledge, very few articles are reported in the pertinent literature regarding the microsurgical anatomy of the hypoglossal nerve in its entire course. In the present anatomical study, we describe our observations of the relationships of the nerve with the surrounding structures, from its exit zone on the medulla oblongata to its ending on the muscles of the tongue, describing all landmarks we have encountered while performing the dissections.

Section snippets

Materials and Methods

Anatomic dissections were performed at the Center of Biotechnology of the “Antonio Cardarelli Hospital” in Napoli on 10 formalin-fixed adult human cadaveric heads (20 sides), in which the arterial and venous systems had been injected with red and blue latex, respectively. The specimens were from 7 men and 3 women with mean age at the time of death of 72 years (range, 59–85 years).

Cadaveric dissections and nerve measurements were performed under surgical microscope (Carl Zeiss; Oberkochen,

Results

We divided the hypoglossal nerve into 5 segments according to its course and considering the relationships with surrounding structures: cisternal, intracanalar, descending, horizontal, and ascending.

Discussion

In the last 2 decades, several authors have described the microsurgical anatomy of the hypoglossal nerve.11, 18, 19, 20 Nevertheless, so far, no one has systematically followed and described in detail the entire course of the nerve from its exit zone in the medulla oblongata to the tongue.

The lower cranial nerves, being very close to each other at the skull base, may be involved in syndromes related to meningiomas, glomus tumors, schwannomas, and other tumors. Collet syndrome is due to large

Conclusions

We propose a new 5-segment classification of the hypoglossal nerve. This exhaustive anatomic analysis, which considers the entire intracranial and extracranial course of the nerve along with its relationships with surrounding structures, is valid and clinically and surgically oriented when considering the classic microscopic approaches. The present study could be useful to explain, segment by segment along the intracranial and extracranial course of the nerve, the pathogenic mechanisms of nerve

Acknowledgments

The authors thank Dr. Santolo Cozzolino, Director of Center of Biotechnologies, “A. Cardarelli” Hospital, Napoli, Italy, for his highly qualified cooperation in the anatomic dissection.

References (25)

C. Loh et al.
Cranial nerve XII: the hypoglossal nerve
Semin Ultrasound CT MR
(2002)
G. Bademci et al.
Microsurgical anatomy of the hypoglossal nerve
J Clin Neurosci
(2006)
Z. Shiozawa et al.
Unilateral tongue atrophy due to an enlarged emissary vein in the hypoglossal canal
Surg Neurol
(1996)
I. Toldo et al.
Unilateral hypoglossal nerve palsy due to neurovascular conflict in a child
Brain Dev
(2009)
V.S. Cheng et al.
Unilateral hypoglossal nerve atrophy as a late complication of radiation therapy of head and neck carcinoma: a report of four cases and a review of the literature on peripheral and cranial nerve damages after radiation therapy
Cancer
(1975)
K. Nagai et al.
Unilateral hypoglossal nerve paralysis following the use of the laryngeal mask airway
Anaesthesia
(1994)
T.F. Macedo et al.
Bilateral hypoglossal nerve palsy due to vertical subluxation of the odontoid process in rheumatoid arthritis
Br J Rheumatol
(1988)
A. Stewart et al.
Bilateral hypoglossal nerve injury following the use of the laryngeal mask airway
Anaesthesia
(2002)
L.L. Osburn et al.
Hemilingual spasm: defining a new entity, its electrophysiological correlates and surgical treatment through microvascular decompression
Neurosurgery
(2010)
A. Perneczky et al.
Keyhole Approaches in Neurosurgery
(2008)

A. Leblanc

The Cranial Nerves Anatomy Imaging Vascularisation

(1995)

J.P. Mattos et al.

Dissection technique for the study of the cerebral sulci, gyri and ventricles

Arq Neuropsiquiatr

(2008)

Cited by (22)

Three-Dimensional Anatomy of the Hypoglossal Canal: A Plastinated Histologic Study
2023, World Neurosurgery
To provide a precise description of the morphology and morphometry of the hypoglossal canal (HC) and its relationship with surrounding structures by using the epoxy sheet plastination technique.
Thirty human cadaveric heads were plastinated into 5 sets of gross transparent plastination slices and 43 sets of ultrathin plastination sections. The HC were examined at both macro- and micro levels in these plastination sections and the reconstructed 3-dimensional visualization model.
The HC was an upward arched bony canal with a dumbbell-shaped lumen. According to the arched trajectory of its bottom wall, the HC could be divided into a medial ascending segment and a lateral descending segment. The thickness of the compact bone in the middle part of the HC was thinner than that at the intracranial and extracranial orifices. In 14 of 43 sides (32.6%), the posterior wall or the roof of the HC were disturbed by passing venous channels which communicated the posterior condylar emissary vein and the inferior petroclival vein. The trajectory of hypoglossal nerve in HC is mainly from anterosuperior to posteroinferior. The meningeal dura and the arachnoid extended into the HC along the hypoglossal nerve to form the dural and arachnoid sleeves and then fused with the nerve near the extracranial orifice of the HC.
Knowledge of the detailed anatomy of the HC can be helpful in avoiding surgical complications when performing surgery for lesions and the occipital condylar screw placement in this complex area.
Intraoperative Neuromonitoring of Hypoglossal Nerves Using Transcranial and Direct Electrical Stimulation During Extracranial Internal Carotid Artery Surgery
2023, World Neurosurgery
We explored whether the electromyogram (EMG) and the motor evoked potential (MEP) are useful for monitoring the function of the hypoglossal nerve during surgery targeting the cervical segment of the internal carotid artery.
The present study included 6 patients with internal carotid arterial stenosis (1 patient underwent bilateral surgeries) and 1 patient with a cervical carotid artery aneurysm. In 5 of the 8 procedures, the EMGs were recorded. We examined whether changes in the MEP and/or EMG were capable of predicting postoperative hypoglossal nerve deficits.
None of the 6 patients who underwent a total of 7 carotid endarterectomy (CEA) procedures experienced postoperative hypoglossal nerve morbidity. In 2 of the 7 procedures, the MEP disappeared or decreased significantly during CEA. In all 4 cases in which the hypoglossal nerve was directly stimulated during CEA, stable and reproducible EMGs were obtained throughout the manipulation of the internal carotid artery. Hypoglossal nerve morbidity was observed in the one case that underwent aneurysm removal and end-to-end anastomosis of the internal carotid artery. In this case, while the MEP decreased significantly during the operation, the EMG showed true-positive results and false-negative results, depending on the stimulation site.
The monitoring of hypoglossal nerve function using EMG appears to be accurate if an appropriate stimulation site is selected. Hypoglossal nerve monitoring using MEP can produce false-positive results. Combined monitoring using both MEP and EMG is recommended in cases where exposure of the hypoglossal nerve is expected to be technically difficult.
The Hypoglossal Nerve
2023, Seminars in Ultrasound, CT and MRI
Citation Excerpt :
These trunks unite in the hypoglossal canal in an anterolateral course. The hypoglossal canal is located in the occipital bone, caudal to the jugular foramen, connecting the foramen magnum to the parapharyngeal space, having as components the hypoglossal nerve, the venous plexus, and meningeal branches of the ascending pharyngeal artery.7-9 After exiting the canal, the hypoglossal nerve emits dural branches.
The hypoglossal nerve is the 12th cranial nerve, exiting the brainstem in the preolivary sulcus, passing through the premedullary cistern, and exiting the skull through the hypoglossal canal. This is a purely motor nerve, responsible for the innervation of all the intrinsic tongue muscles (superior longitudinal muscle, inferior longitudinal muscle, transverse muscle, and vertical muscle), 3 extrinsic tongue muscles (styloglossus, hyoglossus, and genioglossus), and the geniohyoid muscle. Magnetic resonance imaging (MRI) is the best imaging exam to evaluate patients with clinical signs of hypoglossal nerve palsy, and computed tomography may have a complementary role in the evaluation of bone lesions affecting the hypoglossal canal. A heavily T2-weighted sequence, such as fast imaging employing steady-state acquisition (FIESTA) or constructive interference steady state (CISS) is important to evaluate this nerve on MRI. There are multiple causes of hypoglossal nerve palsy, being neoplasia the most common cause, but vascular lesions, inflammatory diseases, infections, and trauma can also affect this nerve. The purpose of this article is to review the hypoglossal nerve anatomy, discuss the best imaging techniques to evaluate this nerve and demonstrate the imaging aspect of the main diseases that affect it.
Microanatomic analyses of extratemporal facial nerve and its branches, hypoglossal nerve, sural nerve, and great auricular nerve
2023, Brazilian Journal of Otorhinolaryngology
Citation Excerpt :
Above the carotid triangle, the submandibular gland was dissected upward. In the superior of the hyoid bone, the hypoglossal nerve was found to cross these two arteries during dissection of the internal and external carotid arteries.12 Sampling was made from the portion of the hypoglossal nerve before branching.12
To investigate microanatomic organizations of the extratemporal facial nerve and its branches, hypoglossal nerve, sural nerve, and great auricular nerve.
Nerve samples were dissected in 12 postmortem autopsies, and histomorphometric analyses were conducted.
There was no significant difference between the right and left sides of the nerve samples for the nerve area, fascicle area, number of fascicles and average number of axons. The lowest mean fascicle number was found in the hypoglossal nerve (4.9 ± 1.4) while the highest was in great auricular nerve (11.4 ± 6.8). The highest nerve area (3,182,788 ± 838,430 μm²), fascicle area (1,573,181 ± 457,331 μm²) and axon number (14,772 ± 4402) were in hypoglossal nerve (p < 0.05). The number of axons per unit nerve area was higher in the facial nerve, truncus temporofacialis, truncus cervicofacialis and hypoglossal nerve, which are motor nerves, compared to the sural nerve and great auricular nerve, which are sensory nerves (p < 0.05). The number of axons per unit fascicle area was also higher in motor nerves than in sensory nerves (p < 0.05).
In the present study, it was observed that each nerve contained a different number of fascicles and these fascicles were different both in size and in the number of axons they contained. All these variables could be the reason why the desired outcomes cannot always be achieved in nerve reconstruction.
The oral tongue and floor of mouth in three dimensions (3D): A digital anatomical model derived from radiology, peer-reviewed literature, and medical illustration
2022, Operative Techniques in Otolaryngology - Head and Neck Surgery
Citation Excerpt :
The HN, along with CN IX, X, and XI, all descend into the neck in the space between the styloid process and transverse process of C1. From here, it travels between the internal carotid artery on the medial surface of the internal jugular vein, descending inferiorly70,71. The HN continues its inferior course, traveling laterally to the external carotid artery, until a branch of the occipital artery to the sternocleidomastoid muscle takes off from the ECA (Fig. 4).
Combining published literature, radiographic imaging, and medical illustration, this study aimed to develop an anatomically accurate 3-dimensional (3D) digital model of the human tongue, and to make this model publicly available for education and training purposes. The intrinsic and extrinsic muscles of the tongue were manually segmented from a head and neck CT angiogram of a healthy 29-year-old female. Data derived from published anatomical studies were compiled to provide additional anatomical detail to each tongue muscle. These anatomical details were then incorporated into a model by a 3D medical illustrator. A total of nine muscles were segmented for this study, along with the mandible and the hyoid bone. 3D meshes of the extrinsic muscles of the tongue, intrinsic muscles of the tongue, and the mylohyoid were created from CT angiography segmentation data. These meshes were refined in digital 3D space based off data from peer-reviewed anatomical studies. The final model was then uploaded to a publicly available 3D model repository. The tongue is a complex 3D organ with important roles in swallowing, speaking, and airway protection. By combining radiographic data with published anatomical descriptions, and through the collaboration between neuroradiology, head and neck surgery, and medical illustration, we developed an anatomically accurate, detailed 3D model of the oral tongue, which may serve as a powerful anatomy teaching aid.
Hypoglossal Nerve (Cranial Nerve XII) Stimulation
2020, Otolaryngologic Clinics of North America
Citation Excerpt :
Once deep to the mylohyoid muscle, the nerve branches into lateral and medical branches to innervate its target musculature (Fig. 1). The lateral branches innervate the styloglossus and hyoglossus muscles, while the medial branch innervates the genioglossus and the intrinsic tongue musculature (transverse and vertical muscles).1,2 The tongue consists of intrinsic and extrinsic musculature, which is innervated primarily by the hypoglossal nerve save the palatoglossus, which is innervated by the vagus nerve.

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: Conflict of interest statement: The authors declare that the article content was composed in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Original ArticleThe Hypoglossal Nerve: Anatomical Study of Its Entire Course

Objective

Methods

Results

Conclusions

Introduction

Section snippets

Materials and Methods

Results

Discussion

Conclusions

Acknowledgments

Semin Ultrasound CT MR

J Clin Neurosci

Surg Neurol

Brain Dev

Unilateral hypoglossal nerve atrophy as a late complication of radiation therapy of head and neck carcinoma: a report of four cases and a review of the literature on peripheral and cranial nerve damages after radiation therapy

Cancer

Unilateral hypoglossal nerve paralysis following the use of the laryngeal mask airway

Anaesthesia

Bilateral hypoglossal nerve palsy due to vertical subluxation of the odontoid process in rheumatoid arthritis

Br J Rheumatol

Bilateral hypoglossal nerve injury following the use of the laryngeal mask airway

Anaesthesia

Hemilingual spasm: defining a new entity, its electrophysiological correlates and surgical treatment through microvascular decompression

Neurosurgery

Keyhole Approaches in Neurosurgery

The Cranial Nerves Anatomy Imaging Vascularisation

Dissection technique for the study of the cerebral sulci, gyri and ventricles

Arq Neuropsiquiatr

Original Article
The Hypoglossal Nerve: Anatomical Study of Its Entire Course