The Hypoglossal Nerve: Anatomical Study of Its Entire Course

Hypoglossal Nerve Neuropathies-Analysis of Causes and Anatomical Background

The hypoglossal nerve is the last, and often neglected, cranial nerve. It is mainly responsible for motor innervation of the tongue and therefore the process of chewing and articulation. However, tumors, aneurysms, dissections, trauma, and various iatrogenic factors such as complications after surgeries, radiotherapy, or airway management can result in dysfunction. Correct differential diagnosis and suitable treatment require a thorough knowledge of the anatomical background of the region. This review presents the broad spectrum of hypoglossal neuropathies, paying particular attention to these with a compressive background. As many of these etiologies are not common and can be easily overlooked without prior preparation, it is important to have a comprehensive understanding of the special relations and characteristic traits of these medical conditions, as well as the most common concomitant disorders and morphological traits, influencing the clinical image. Due to the diverse etiology of hypoglossal neuropathies, specialists from many different medical branches might expect to encounter patients presenting such symptoms.

Three-Dimensional Anatomy of the Hypoglossal Canal: A Plastinated Histologic Study

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

The ptotic tongue-imaging appearance and pathology localization along the course of the hypoglossal nerve

CT and MRI findings of tongue ptosis and atrophy should alert radiologists to potential pathology along the course of the hypoglossal nerve (cranial nerve XII), a purely motor cranial nerve which supplies the intrinsic and extrinsic muscles of the tongue. While relatively specific for hypoglossal nerve pathology, these findings do not accurately localize the site or cause of denervation. A detailed understanding of the anatomic extent of the nerve, which crosses multiple anatomic spaces, is essential to identify possible underlying pathology, which ranges from benign postoperative changes to life-threatening medical emergencies. This review will describe key imaging findings of tongue denervation, segmental anatomy of the hypoglossal nerve, imaging optimization, and comprehensive imaging examples of diverse pathology which may affect the hypoglossal nerve. Armed with this knowledge, radiologists will increase their sensitivity for detection of pathology and provide clinically relevant differential diagnoses when faced with findings of tongue ptosis and denervation.

Clinical Neurology in Practice: The Tongue (Part 1)

BACKGROUND

The tongue is an essential organ for the development of certain crucial functions, such as swallowing and language. The examination of the tongue can be very useful in neurology, as the various types of lingual alterations can lead to certain specific diagnoses, the tongue being a kind of "mirror" of some neurological function.

REVIEW SUMMARY

In this study, we reviewed the literature on anatomy, physiology, and the various aspects of the examination of the tongue.

CONCLUSIONS

Examination of the tongue should be an integral part of the clinical examination of the cranial nerves. This study aimed to demonstrate the importance of tongue motor and non-motor functions in neurological practice.

Intraoperative Neuromonitoring of Hypoglossal Nerves Using Transcranial and Direct Electrical Stimulation During Extracranial Internal Carotid Artery Surgery

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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

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.

The Glossopharyngeal, Vagus and Accessory nerves: Anatomy and Pathology

The glossopharyngeal, vagus, and accessory nerves are discussed in this article, given their intimate anatomical and functional associations. Abnormalities of these lower cranial nerves may be intrinsic or extrinsic due to various disease processes. This article aims to review these nerves' anatomy and demonstrates the imaging aspect of the diseases which most commonly affect them.

Microsurgical Anatomy of the Hypoglossal Nerve in the Lateral Approaches to the Craniovertebral Junction: A Study on Fresh Non-Formalin-Fixed Human Specimens

The aim of this anatomical study is to describe the anatomy of the hypoglossal nerve (HN) from its origin to the extracranial portion as it appears by performing a combined posterolateral and anterolateral approach to the craniovertebral junction (CVJ). Twelve fresh, non-formalin-fixed adult cadaveric heads (24 sides) were analyzed for the simulation of the combined lateral approach to the CVJ. The HN is divided into three main parts: cisternal, intracanalicular, and extracranial The anatomical relationships between the HN and other nerves, muscles, arteries and veins were carefully recorded, and some measurements were made between the HN and related structures. Thus, various landmarks were determined for the easy identification of the HN. Understanding the detailed anatomy of the HN and its relationships with the surrounding structures is crucial to prevent some complications during CVJ surgery.

Meningoencephalitis Associated with SARS-Coronavirus-2

The aim of this work is to clarify the incidence of meningitis/encephalitis in SARS-CoV-2 patients. We conducted an initial search in PubMed using the Medical Subject Headings (MeSH) terms "meningitis," and "encephalitis,", and "COVID-19" to affirm the need for a review on the topic of the relationship between meningitis/encephalitis and SARS-CoV-2 infection. We included case series, case reports and review articles of COVID-19 patients with these neurological symptoms. Through PubMed database we identified 110 records. After removal of duplicates, we screened 70 record, and 43 were excluded because they focused on different SARS-CoV-2 neurological complications. For eligibility, we assessed 27 full-text articles which met inclusion criteria. Seven articles were excluded, and twenty studies were included in the narrative review, in which encephalitis and/or meningitis case reports/case series were reported. Neurological manifestations of COVID-19 are not rare, especially meningoencephalitis; the hypoxic/metabolic changes produced by the inflammatory response against the virus cytokine storm can lead to encephalopathy, and the presence of comorbidities and other neurological diseases, such as Alzheimer's disease, predispose to these metabolic changes. Further study are needed to investigate the biological mechanisms of neurological complications of COVID-19.

Microanatomic analyses of extratemporal facial nerve and its branches, hypoglossal nerve, sural nerve, and great auricular nerve

OBJECTIVE

To investigate microanatomic organizations of the extratemporal facial nerve and its branches, hypoglossal nerve, sural nerve, and great auricular nerve.

METHODS

Nerve samples were dissected in 12 postmortem autopsies, and histomorphometric analyses were conducted.

RESULTS

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).

CONCLUSION

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.

Application of hypoglossal nerve constraint in definitive radiotherapy for nasopharyngeal carcinoma: A dosimetric feasibility study

PURPOSE

Radiation-induced hypoglossal nerve palsy is an infrequent but debilitating late complication after definitive radiotherapy for head and neck cancers. D1cc < 74 Gy (equivalent dose in 2 Gy fractions, EQD2) has been proposed as a potential dose constraint that limits 8-year palsy risk to < 5%. This study sets to perform detailed dosimetric assessments on the applicability of this novel dose constraint in advanced nasopharyngeal carcinoma (NPC).

MATERIALS AND METHODS

This is a retrospective single-institution dosimetry study. NPC radiotherapy plans were identified from an institutional database, with an aim to select 10 eligible cases. Bilateral hypoglossal nerves were retrospectively contoured following a standard atlas. Cases with either one, or both, hypoglossal nerves D1cc exceeded 74 Gy EQD2 were included. Dosimetry of hypoglossal nerves, planning target volumes (PTV) and normal structures before and after application of the new hypoglossal nerve constraint were compared and analyzed.

RESULTS

Ten NPC cases were replanned. All hypoglossal nerve contours overlapped with high-dose PTV, predominantly at regions of gross nodal diseases. D1cc in 15 out of 20 hypoglossal nerves exceeded 74G y EQD2 at initial plans. All nerves fulfilled the pre-specified constraint of 74Gy EQD2 after re-plan. Median hypoglossal nerve D1cc reduced from 74.8Gy (range, 74.1 to 77.4Gy) to 73.5Gy (range, 72.4 to 74.0Gy) (p < 0.001), corresponded to a projected reduction in 8-year palsy risk from 5%-14% to 3%-5%. PTV V100 was maintained above 95% in all cases. Dose increments in near-maximum (D2) and decrements in near-minimum (D98) were < 1 Gy. Safety dosimetric parameters of standard head and neck organs-at-risk showed no significant changes.

CONCLUSIONS

Hypoglossal nerve D1cc < 74 Gy EQD2 is a dosimetrically feasible constraint in definitive radiotherapy for NPC. Tumor target coverage and normal organ dosimetry were not compromised with its usage. Its routine application should be considered in definitive radiotherapy for head and neck cancers.

The Five Diaphragms in Osteopathic Manipulative Medicine: Neurological Relationships, Part 1

In osteopathic manual medicine (OMM), there are several approaches for patient assessment and treatment. One of these is the five diaphragm model (tentorium cerebelli, tongue, thoracic outlet, diaphragm, and pelvic floor), whose foundations are part of another historical model: respiratory-circulatory. The myofascial continuity, anterior and posterior, supports the notion the human body cannot be divided into segments but is a continuum of matter, fluids, and emotions. In this first part, the neurological relationships of the tentorium cerebelli and the lingual muscle complex will be highlighted, underlining the complex interactions and anastomoses, through the most current scientific data and an accurate review of the topic. In the second part, I will describe the neurological relationships of the thoracic outlet, the respiratory diaphragm and the pelvic floor, with clinical reflections. In literature, to my knowledge, it is the first time that the different neurological relationships of these anatomical segments have been discussed, highlighting the constant neurological continuity of the five diaphragms.

Hybrid antero-lateral transcondylar approach to the clivus: a laboratory investigation and case illustration

BACKGROUND

Surgical treatment of lesions involving the ventral craniovertebral junction (CVJ) and the lower clivus, traditionally involved complex lateral or transoral approaches to the skull base. However, mid or upper clivus involvement requires more extensive lateral approaches. Recently, the endoscopic endonasal approach (EEA) has become the standard for upper CVJ lesions and medial clival, and a valuable alternative for those tumors extending in its upper third as well as laterally. However, the EEA is associated with an increased risk of post-operative CSF leakage and infection when the tumor is characterized by an intradural extension. Furthermore, whenever the tumor has significant lateral and/or inferior extension below the odontoid process, the chances for a complete resection decrease.

METHOD

To analyze the extent of exposure of a hybrid microscopic-endoscopic transcondylar antero-lateral approach to the CVJ and clival region, and to verify its effectiveness in terms of mid and upper clival access. Five silicone-injected cadaver heads were used. Following a standard antero-lateral approach, condylectomy and jugular tubercle drilling were performed, after which angled endoscopes were utilized to extend the bone resection to the clivus. A volumetric assessment of the amount of clival removal was carried out. A case of CVJ chordoma operated through this approach is presented.

RESULTS

The hybrid antero-lateral transcondylar approach provides adequate exposure of the ventral CVJ, up to the dorsum sellae and the sphenoid sinus, the contralateral petrous apex, and the contralateral paraclival internal carotid artery (ICA). Approximately 60% of the total clival volume can be removed with this approach. The main limitation is the limited visualization of the ipsilateral paraclival ICA and petrous apex.

CONCLUSION

The hybrid antero-lateral transcondylar approach is a valuable surgical option for CVJ tumor extending from C2 to the mid and upper clivus.

Glioblastoma of the cerebellopontine angle and internal auditory canal mimicking a peripheral nerve sheath tumor: case report

Glioblastoma (GBM) of the internal auditory canal (IAC) is exceedingly rare, with only 3 prior cases reported in the literature. The authors present the fourth case of cerebellopontine angle (CPA) and IAC GBM, and the first in which the lesion mimicked a vestibular schwannoma (VS) early in its natural history. A 55-year-old man presented with tinnitus, hearing loss, and imbalance. MRI identified a left IAC/CPA lesion measuring 8 mm, most consistent with a benign VS. Over the subsequent 4 months he developed facial weakness. The tumor grew remarkably to 24 mm and surgery was recommended; the main preoperative diagnosis was malignant peripheral nerve sheath tumor (MPNST). Resection proceeded via a translabyrinthine approach with resection of cranial nerves VII and VIII, followed by facial-hypoglossal nerve anastomosis. Intraoperative frozen section suggested malignant spindle cell neoplasm, but final histopathological and molecular testing confirmed the lesion to be a GBM. The authors report the first case in which absence of any brainstem interface effectively excluded a primary parenchymal tumor, in particular GBM, from the differential diagnosis. Given the dramatic differences in treatment and prognoses between malignant glioma and MPNST, this case emphasizes the importance of surgical intervention on an aggressively growing lesion, which provides both the best probability of local control and the critical tissue diagnosis.

The intracanalicular segment of the hypoglossal nerve: An anatomical study using magnetic resonance imaging

Few studies have documented the morphology of the intracanalicular segment of the hypoglossal nerve (CSHN). Therefore, the aim of this study was to characterize the CSHN using magnetic resonance imaging (MRI). In total, 95 patients underwent thin-sliced, contrast MRI. The axial and coronal images were used for analysis. The CSHNs were bilaterally identified in 97% and 94% of the 95 patients on the axial and serial coronal images, respectively. On axial images, length of the hypoglossal canal was measured as 8.2 ± 1.66 mm on the right and 8.4 ± 1.71 mm on the left. The CSHN was delineated as a slightly tortuous, linear structure with variable length. The CSHN course in the hypoglossal canal could be classified into the ventral, central, and ventrodorsal types, with the ventral type most predominant and found in 65% on the right side and 43% on the left. The angle formed by the CSHN and perpendicular line was highly variable. On serial coronal images, the CSHN course in the hypoglossal canal was also variable and could be found in the any part of the canal. The CSHN is a distinct structure characterized by morphological variability, which can influence the type of hypoglossal neuropathy arising from the CSHN.

Radiation-induced hypoglossal nerve palsy after definitive radiotherapy for nasopharyngeal carcinoma: Clinical predictors and dose-toxicity relationship

BACKGROUND AND PURPOSE

Radiation-induced hypoglossal nerve palsy is a debilitating and irreversible late complication after definitive radiotherapy for nasopharyngeal carcinoma (NPC) and other skull base tumors. This study sets to evaluate its incidence and clinical predictive factors, and to propose relevant dosimetric constraints for this structure to guide radiotherapy planning.

MATERIALS AND METHODS

We undertook a retrospective review of 797 NPC patients who underwent definitive intensity-modulated radiotherapy (IMRT) between 2003 and 2011. Cumulative incidence and clinical predictors for radiation-induced hypoglossal nerve palsy were evaluated. Archived radiotherapy plans were retrieved and 330 independent hypoglossal nerves were retrospectively contoured following standardized atlas. Optimal threshold analyses of dosimetric parameters (Dmax, D0.5cc, D1cc, D2cc, Dmean) were conducted using receiver operating characteristic curves. Normal tissue complication probability was generated with logistic regression modeling.

RESULTS

With a median follow-up of 8.1 years, sixty-nine (8.7%) patients developed radiation-induced hypoglossal nerve palsy. High radiotherapy dose, premorbid diabetes, advanced T-stage and radiological hypoglossal canal involvement were independent clinical risk factors. Maximum dose received by 1 cc volume (D1cc) was the best predictor for the development of radiation-induced nerve palsy (AUC = 0.826) at 8 years after IMRT. Hypoglossal nerves with D1cc of 74 Gy EQD2 had an estimated palsy risk of 4.7%. Nerves with D1cc <74 Gy EQD2 had significantly lower risk of palsy than those ≥74 Gy EQD2 (2.4% vs 20.8%, p <0.001).

CONCLUSION

Incidence of radiation-induced hypoglossal nerve palsy was high after definitive IMRT for NPC. D1cc <74 Gy EQD2 can serve as a useful dose constraint to adopt during radiotherapy planning to limit palsy risk to <5% at 8 years after IMRT.