Microsurgical Anatomy of the Terminal Hypoglossal Nerve Relevant for Neurostimulation in Obstructive Sleep Apnea

Defining the Shape of the Mylohyoid Muscle: A Morphometric Imaging Study

OBJECTIVES

Implantable hypoglossal nerve stimulation (HNS) therapy is an evolving therapeutic alternative for patients with refractory obstructive sleep apnea (OSA). The muscular anatomy of this region has implications for surgical access through this zone as well as positioning and anchoring of hardware in this area. The purpose of this study was to radiologically describe the topography of the mylohyoid muscle and adjacent structures across a wide age spectrum.

METHODS

We retrospectively evaluated computed tomography scans of the neck in 102 patients who were imaged for reasons unrelated to the floor of mouth or submental space. Patients with prior surgery or pathology in the area of interest were excluded. Fourteen relevant muscle measurements were made on a midline sagittal image and a coronal image positioned at the midpoint between the hyoid bone and the mandible.

RESULTS

We included 49 men and 53 women with an average age of 44 years (range 19-70). The average mylohyoid length was 42 mm; the average distance between the anterior digastric bellies was 17 mm. The average angle of the central mylohyoid was 174° in the sagittal plane and 164° in the coronal plane. Several measurements were significantly correlated with patient age, including the angle measurements and the distance between the digastric muscles. Aberrant digastric anatomy was common.

CONCLUSIONS

The mylohyoid muscle has multiple radiologically distinct segments with predictable curvatures. An understanding of submental muscular anatomy, along with its variability between patients, may be beneficial to the development of bilateral implantable neurostimulation technology for the treatment of refractory OSA.

LEVEL OF EVIDENCE

N/A Laryngoscope, 134:2970-2975, 2024.

Ultrasound Shear Wave Elastography of the Tongue during Selective Hypoglossal Nerve Stimulation in Patients with Obstructive Sleep Apnea Syndrome

Currently, there is no established technique to directly measure extrinsic tongue muscle activation during selective hypoglossal stimulation therapy (sHNS) in patients with obstructive sleep apnea syndrome (OSAS) in a simple, non-invasive clinical setting. Ultrasound shear-wave elastography (US-SWE) enables quantitative measurement of tissue stiffness. We investigated whether US-SWE is able to detect changes in muscle stiffness of the tongue during sHNS. Patients with OSAS treated with sHNS were prospectively enrolled. A standardized US-SWE protocol was used to selectively measure tissue stiffness of the geniohyoid muscle (GH) and genioglossus (GG) muscles on the side of stimulator implantation (sGH, sGG) and on the contralateral side (nGH, nGG) without and with sHNS. Eighteen patients were included (median age = 62 years, interquartile range: 56-65, 83.3% male). Median shear-wave velocity (SWV) increased during contraction with each patient's clinically prescribed therapeutic regimen in the sGH (+19%, p = 0.020) and sGG (+81%, p < 0.001) and decreased during contraction in the nGH (-8%, p = 0.107) and nGG (-8%, p = 0.396). Differences in SWV during contraction were significant only on the side of stimulation (sGG +81%, sGH +19%). SWE measurements had excellent reliability as reflected by a Cronbach α value ≥0.9 for all target muscles pre- and post-contraction and an item-total correlation ≥0.5. US-SWE allows reliable measurement of SWV as an indicator of muscle stiffness of extrinsic tongue muscles. This non-invasive method provides new possibilities to distinguish and characterize responders from non-responders in hypoglossal stimulation therapy. Compared with the regular visual assessment of tongue movement, US-SWE of individual muscle groups provides a new non-invasive imaging tool in patients with OSAS.

Improving outcomes of hypoglossal nerve stimulation therapy: current practice, future directions and research gaps. Proceedings of the 2019 International Sleep Surgery Society Research Forum

Hypoglossal nerve stimulation (HGNS) has evolved as a novel and effective therapy for patients with moderate-to-severe obstructive sleep apnea (OSA). Despite positive published outcomes of HGNS, there exist uncertainties regarding proper patient selection, surgical technique, and the reporting of outcomes and individual factors that impact therapy effectiveness. According to current guidelines, this therapy is indicated for select patients, and recommendations are based on the Stimulation Therapy for Apnea Reduction (STAR) trial. Ongoing research and physician experiences continuously improve methods to optimize the therapy. An understanding of the way in which airway anatomy, OSA phenotypes, individual health status, psychological conditions and comorbid sleep disorders influence the effectiveness of HGNS is essential to improve outcomes and expand therapy indications. This manuscript presents discussions on current evidence, future directions, and research gaps for HGNS therapy from the 10th International Surgical Sleep Society expert research panel.

Ultrasound Localization and Percutaneous Electrical Stimulation of the Hypoglossal Nerve and Ansa Cervicalis

OBJECTIVE

Hypoglossal nerve stimulation for obstructive sleep apnea (OSA) can be effective for appropriately selected patients, but current patient selection criteria are complex and still result in a proportion of nonresponders. Ansa cervicalis stimulation of the infrahyoid cervical strap muscles has recently been proposed as a new form of respiratory neurostimulation (RNS) therapy for OSA treatment. We hypothesized that percutaneous stimulation of both nerves in humans with temporary electrodes would make testing of the physiologic response to different RNS strategies possible.

STUDY DESIGN

Nonrandomized acute physiology study.

SETTING

Tertiary care hospital.

METHODS

Fifteen participants with OSA underwent ultrasonography and placement of percutaneous electrodes proximal to the medial division of the hypoglossal nerve and the branch of the ansa cervicalis innervating the sternothyroid muscle (ACST). Procedural success was documented in each participant, as were any failures or procedural complication.

RESULTS

The hypoglossal nerve was successfully localized in 15 of 15 (100%) participants and successfully stimulated in 13 of 15 (86.7%). The ACST was successfully localized in 15 of 15 (100%) participants and successfully stimulated in 14 of 15 (93.3%). Stimulation failure of the hypoglossal nerve was due to suboptimal electrode placement in 1 participant and electrode displacement in the other 2 cases. No complications occurred.

CONCLUSIONS

The hypoglossal nerve and ACST can be safely stimulated via percutaneous electrode placement. Larger trials of percutaneous stimulation may help to identify responders to different RNS therapies for OSA with temporary or permanent percutaneous electrodes. Techniques for electrode design, nerve localization, and electrode placement are described.

Real-Time, Intraoperative, Ultrasound-Assisted Transoral Robotic Surgery for Obstructive Sleep Apnea

OBJECTIVES/HYPOTHESIS

To investigate the lingual artery (LA) position in the tongue base through intraoperative ultrasound (IOU) imaging during transoral robotic surgery (TORS) and evaluate bleeding complications with or without the assistance of IOU.

STUDY DESIGN

Cohort study with historical control.

METHODS

Patients with obstructive sleep apnea (OSA) who underwent TORS for tongue base resection were recruited since 2016. During surgery, ultrasound imaging was employed to identify anatomic parameters of the LA in the tongue base, including distance to the midline and arterial depth and diameter.

RESULTS

Ninety-three OSA patients (82 men, 88.2%) were analyzed. Mean age was 42.2 ± 10.0 years and body mass index was 29.2 ± 4.5 kg/m² . Average apnea-hypopnea index (AHI) was 58.1 ± 21.4 events/hour. Overall, 70 patients who underwent TORS with IOU had a shorter operation time (191.7 ± 3.8 vs. 220.1 ± 6.6 minutes), lower total blood loss (11.3 ± 10.8 vs. 19.6 ± 26.7 mL), and higher tongue base reduction volume (7.1 ± 2.5 vs. 3.9 ± 1.6 mL) than 23 patients who underwent TORS without IOU. Significant predictors of arterial depth included higher AHI level during the rapid eye movement sleep (P = .038), larger tonsil size (P = .034), and more elevated Friedman tongue position (P = .012). Postoperative complications associated with LA injury were not found in patients subjected to IOU.

CONCLUSIONS

With the assistance of IOU, surgeons can confidently determine LA position. The use of IOU can maximize efficiency and minimize catastrophic bleeding complications when OSA patients undergo TORS for tongue base resection.

LEVEL OF EVIDENCE

4 Laryngoscope, 131:E1383-E1390, 2021.

Implantable Neurostimulation for Treatment of Sleep Apnea: Present and Future

Hypoglossal nerve stimulation (HNS) therapy represents a novel approach and a paradigm shift in the evolution of obstructive sleep apnea (OSA) treatment as a hybrid surgically implanted, medically titratable device. Unlike traditional sleep apnea surgical procedures, HNS augments the neuromuscular activity of the pharynx, preserves upper airway structure and function, and has the potential to provide multilevel upper airway improvement with one procedure. The early success of HNS sets the stage for new technology development, further investigation into optimal patient selection and therapy titration, and application to other subsets of the OSA population.

Current landscape in motoneuron regeneration and reconstruction for motor cranial nerve injuries

The intricate anatomy and physiology of cranial nerves have inspired clinicians and scientists to study their roles in the nervous system. Damage to motor cranial nerves may result from a variety of organic or iatrogenic insults and causes devastating functional impairment and disfigurement. Surgical innovations directed towards restoring function to injured motor cranial nerves and their associated organs have evolved to include nerve repair, grafting, substitution, and muscle transposition. In parallel with this progress, research on tissue-engineered constructs, development of bioelectrical interfaces, and modulation of the regenerative milieu through cellular, immunomodulatory, or neurotrophic mechanisms has proliferated to enhance the available repertoire of clinically applicable reconstructive options. Despite these advances, patients continue to suffer from functional limitations relating to inadequate cranial nerve regeneration, aberrant reinnervation, or incomplete recovery of neuromuscular function. These shortfalls have profound quality of life ramifications and provide an impetus to further elucidate mechanisms underlying cranial nerve denervation and to improve repair. In this review, we summarize the literature on reconstruction and regeneration of motor cranial nerves following various injury patterns. We focus on seven cranial nerves with predominantly efferent functions and highlight shared patterns of injuries and clinical manifestations. We also present an overview of the existing reconstructive approaches, from facial reanimation, laryngeal reinnervation, to variations of interposition nerve grafts for reconstruction. We discuss ongoing endeavors to promote nerve regeneration and to suppress aberrant reinnervation and the development of synkinesis. Insights from these studies will shed light on recent progress and new horizons in understanding the biomechanics of peripheral nerve neurobiology, with emphasis on promising strategies for optimizing neural regeneration and identifying future directions in the field of motor cranial neuron research.

The specific branches leading to the genioglossus muscle: three-dimensional localisation using skin reference points

INTRODUCTION

Supra-selective stimulation of the branches destined for the horizontal part of genioglossus muscle (GGh) could be a target of choice in the treatment of mild-to-severe obstructive sleep apnea syndrome. The main aim of our study was to assess a percutaneous method for the three-dimensional localisation of the terminal branches destined to GGh.

MATERIALS AND METHODS

Twenty cadaveric hypoglossal nerves were dissected and included in the injection protocol. The distance between the posterior edge of the mandibular symphysis and the hyoid bone on the sagittal midline as the approximated distance of the geniohyoid muscle (dGH) was measured before any dissection. Methylene blue mixed with a thickening agent, was injected. The injection point was defined in relation to dGH, in an orthonormal coordinate system. For each dissection, we recorded the theoretical and the real (X, Y, Z) coordinates of GGh motor points and measured their distance to each other.

RESULTS

X was accurately estimated. Y and Z were overestimated by + 5.34 ± 5.21 mm ([Formula: see text]) and + 4.79 ± 3.99 mm ([Formula: see text]) on average, respectively. We found a more significant difference between the theoretical and real Y and Z coordinates in the subgroup BMI < 25 kg/m² (8.6 ± 4.5 mm and 6.9 ± 2.5 mm, respectively, p = 0.0009), and of Z in subgroup with dGH ≥ 50 mm (6.89 ± 3.26 mm, p = 0.0494).

CONCLUSIONS

X can be estimated accurately using the relationship [Formula: see text]. Y seems to be related to BMI and Z may be estimated with the relationship [Formula: see text]. This three-dimensional localisation could be very helpful to facilitate placement of cuff electrodes to manage refractory sleep apnea.

Implantation of the nyxoah bilateral hypoglossal nerve stimulator for obstructive sleep apnea

Objectives

This report describes the surgical implantation of a novel bilateral hypoglossal nerve stimulator (Genio system®, Nyxoah S.A., Belgium) and the successful treatment of a patient with moderate obstructive sleep apnea (OSA).

Study Design

Surgical technique description and case study report.

Methods

The bilateral implantable stimulator (IS) simultaneously stimulates both genioglossus (GG) muscles to reduce airway obstruction. At night, patients wear an activation chip under their chin that wirelessly transmits energy to the implant and enables the nerve stimulation. Surgical implantation of the IS is performed under general anesthesia by making a small incision above the hyoid bone and dissecting through the platysma, mylohyoid, and geniohyoid muscles to the GG muscle. The hypoglossal nerve branches are then identified, followed by suturing the IS in place. The system was evaluated in an otherwise healthy, 60-year-old woman with moderate OSA (apnea hypopnea index (AHI): 25/hr, nadir O₂ saturation: 78%). Appropriate stimulation settings were determined at 2-, 3-, and 6-months post implantation during polysomnography (PSG) and changes in apnea and hypopnea events and oxygen desaturation recorded.

Results

The surgery was well tolerated by the patient with an uneventful recovery. The PSG at 6 months showed that AHI per hour, obstructive apnea events per hour, hypopnea events per hour, and oxygen desaturation index have been reduced by 88%, 92%, 88%, and 97%, respectively, and nadir O₂ saturation improved to 91%.

Conclusions

The absence of complications and considerable reduction of apnea and hypopnea events in this case study help demonstrate the potential safety and efficacy of the bilateral hypoglossal nerve stimulator.

Level of Evidence

4.

Clinical neurophysiology of apnea

Understanding the clinical neurophysiology of apnea generation encompasses discussion of the neuroanatomic aspects of central respiratory rhythm and pattern generation, including the central respiratory control networks, central and peripheral chemoreceptors, mechanisms of respiratory muscles, and sleep state dependent differences. Anatomical and functional links to apnea also involve central respiratory motor output recruited from the hypoglossal nerve, which has led to novel treatments for obstructive sleep apnea. Autonomic fluctuations occur in relation to sleep-wake and sleep states (i.e., REM vs NREM sleep), with both parasympathetic and sympathetic contributions. Finally, our understanding of the pathophysiology of obstructive sleep apnea now includes concepts of critical closing pressure of the upper airway, increased loop gain as reflected by high responsiveness to external perturbations, inadequate responsiveness of upper airway muscle recruitment, and reductions in arousal threshold leading to ventilatory instability. In turn, these concepts have led to the development of novel therapies such as hypoglossal nerve stimulation and targeting key culprit physiologic mechanisms specific to the individual.

Updates of operative techniques for upper airway stimulation

Selective upper airway stimulation has been established as an additional treatment for obstructive sleep apnea (OSA). Essential for the treatment is the precise placement of the cuff electrode for select branches of the hypoglossal nerve, which innervate the protrusors and stiffeners of the tongue. A direct approach to the distal hypoglossal nerve has been established to achieve this goal. For surgeons, detailed knowledge of this anatomy is vital. Another decisive step is the placement of the sensing lead between the intercostal muscles. Also, the complexity of follow-up care postoperatively should be kept in mind. The aim of this article is to provide the latest knowledge on the neuroanatomy of the hypoglossal nerve and to give surgeons a step-by-step guide on the current operative technique. Laryngoscope, 126:S12-S16, 2016.

Microsurgical Anatomy of the Hypoglossal and C1 Nerves: Description of a Previously Undescribed Branch to the Atlanto-Occipital Joint

OBJECTIVE

Distal branches of the C1 nerve that travel with the hypoglossal nerve have been well investigated but relationships of C1 and the hypoglossal nerve near the skull base have not been described in detail. Therefore, the aim of this study was to investigate these small branches of the hypoglossal and first cervical nerves by anatomic dissection.

METHODS

Twelve sides from 6 cadaveric specimens were used in this study. To elucidate the relationship among the hypoglossal, vagus, and first and cervical nerve, the mandible was removed and these nerves were dissected under the surgical microscope.

RESULTS

A small branch was found to always arise from the dorsal aspect of the hypoglossal nerve at the level of the transverse process of the atlas and joined small branches from the first and second cervical nerves. The hypoglossal and C1 nerves formed a nerve plexus, which gave rise to branches to the rectus capitis anterior and rectus capitis lateralis muscles and the atlanto-occipital joint.

CONCLUSIONS

Improved knowledge of such articular branches might aid in the diagnosis and treatment of patients with pain derived from the atlanto-occipital joint. We believe this to be the first description of a branch of the hypoglossal nerve being involved in the innervation of this joint.

Submental Ultrasonographic Parameters among Patients with Obstructive Sleep Apnea

Objective

This study aimed to determine the value of submental ultrasonography (US) parameters for diagnostic workup among patients with obstructive sleep apnea (OSA) and to determine whether there is a correlation between US findings and the severity of OSA.

Study Design

Cross-sectional analysis.

Setting

Tertiary education hospital.

Subjects and Methods

The study included 147 patients with suspected OSA who underwent submental US to evaluate various parameters following overnight polysomnography. US findings were compared with the apnea-hypopnea index and other parameters.

Results

All US parameters, except for subcutaneous tissue thickness, were significantly different among patients with OSA. Of note, distance between lingual arteries (DLA), geniohyoid muscle thickness (GMT), and lateral parapharyngeal wall thickness (LPWT) were significantly greater in the patients with severe OSA than those with mild and moderate OSA ( P < .001). GMT had the strongest correlation with OSA ( r = 0.419, P < .001); LPWT and DLA also had high correlation coefficient values ( r = 0.343, P < .001, and r = 0.342, P < .001, respectively). Stepwise regression analysis showed that GMT (beta = 0.243, P = 0.004), LPWT (beta = 0.236, P = 0.004), and DLA (beta = 0.204, P = 0.008) were the most significant factors for predicting the severity of OSA according to the apnea-hypopnea index.

Conclusion

Submental US can be used to determine whether there is a correlation between US findings and severity of OSA. GMT could be considered a novel parameter for determining the severity of OSA.

Sonographic evaluation of tongue motions during upper airway stimulation for obstructive sleep apnea-a pilot study

INTRODUCTION

The objective was to evaluate the feasibility of sonographic evaluation of functional tongue motion as a tool to evaluate postoperative outcomes in human subjects using breathing-synchronized stimulation of the hypoglossal nerve-a novel therapy option for patients with obstructive sleep apnea (OSA).

MATERIAL AND METHODS

Sixteen patients with OSA (n = 16, age 60.4 ± 10.2, BMI 28.7 ± 2.4, AHI 35.0 ± 11.8) underwent sonographic evaluation of tongue motion after initiation of therapy with the Inspire II Upper Airway Stimulation system. Sonographic examination was performed in four different planes (A = floor of the mouth frontal, B = base of the tongue horizontal, C = floor of the mouth parallel to mandible, and D = floor of the mouth median sagittal) in an attempt to visualize tongue surface, tongue and hyoid motion, and the distance of protrusion.

RESULTS

Identification of the tongue surface was achieved in all cases in planes B, C, and D and 81 % of patients in plane A. Tongue motion was evident on the right (implant) side in 63 % in plane A and 75 % in plane B. Distance of protrusion was measured in plane B at 1.04 cm (±0.51), in plane C at 1.08 cm (±0.47), and in plane D at 0.96 cm (±0.45). Hyoid protrusion was measured in plane C or D and was 0.57 cm (±0.39). Significant correlations among the three planes were observed, but there was no correlation to the reduction of apnea-hypopnea index.

CONCLUSION

The results indicate feasibility of sonography to identify tongue and hyoid motions during upper airway stimulation. Useful sonographic planes and landmarks, which allow visualization of dynamic effects of upper airway stimulation, could be established. The evaluation of the tongue in a horizontal (B) and in a sagittal plane (D) appears to be superior to the other investigated planes. The approximate tongue protrusion needed to generate a significant reduction of AHI and ODI was 1 cm.