Hypoglossal Nerve Stimulation Therapy

Introduction

The prevalence of obstructive sleep apnea (OSA), its complex pathophysiology, and the suboptimal adherence rates with positive pressure therapy require alternative treatment strategies in many OSA patients. Hypoglossal nerve stimulation (HNS) represents a novel and unique therapeutic approach that combines a surgical implant procedure with a titratable medical device to provide multilevel upper airway improvement via neuromodulation of the hypoglossal nerve. A recent multicenter prospective trial of HNS reported significant reduction in polysomnographic measures of disease severity and significant improvement in patient-reported quality of life measures that were maintained at 3 years follow-up, with overall low morbidity and good patient acceptance and adherence. HNS therapy has established itself as a key component of the OSA treatment armamentarium, although further work is needed to advance the technology, determine the most appropriate patient phenotypes, refine the implant procedure, and optimize stimulation parameters and titration protocols.

Traditional surgical procedures for sleep apnea exclusively target the anatomic structural vulnerability of the upper airway. There is increasing evidence that the neuromuscular control of breathing during sleep also plays a key role in the pathophysiology of sleep-disordered breathing for many OSA patients. A simplified version of the upper airway neuromuscular feedback loop has been described with afferents from the pharynx, through the superior laryngeal nerve, to the nucleus solitarius and efferents from the hypoglossal nerve motor nucleus to the genioglossus muscle. The genioglossus muscle is the primary upper airway dilator muscle and, in normal healthy subjects, responds to negative intraluminal pressure with a corresponding increase in genioglossus electromyographic (EMG) activity. This negative pressure reflex appears to be dysfunctional in many patients with OSA. Augmenting the efferent limb of the neuromuscular feedback loop by stimulating the hypoglossal nerve, or the genioglossus muscle directly, may provide a therapeutic mechanism for some OSA patients.

Two key factors play an important role in the potential success of the HNS therapy: (1) the ability of the therapy to provide multilevel upper airway improvement and (2) the ability to adjust and titrate the therapy. Imaging, fluoroscopy, and drug-induced sedated endoscopy (DISE) evaluation of HNS therapy responders demonstrated that HNS resulted in enlargement of the retropalatal space as well as the retrolingual portion of the airway. Anterior displacement of the hyoid bone also occurred in the majority of participants. This multilevel improvement may be due to a number of factors but is primarily thought to occur from the mechanical coupling of the tongue and palate via the palatoglossal and palatopharyngeus muscles. During therapy titration, prior studies have shown a graded increase in airflow and airway measurements with increasing stimulation amplitudes. Unlike traditional sleep apnea surgery, HNS therapy can be titrated in the clinical or sleep laboratory setting to optimize both effectiveness and comfort across a longitudinal care model.

The role of anatomic phenotyping of the upper airway has been increasingly recognized as essential to successful patient selection and improved outcomes. For example, feasibility study data on the currently available HNS system suggest that outcomes are better in patients without a complete circumferential pattern of collapse at the soft palate during DISE. HNS differs from traditional OSA airway surgery in that the procedure does not alter the anatomy of the upper airway and is technically reversible. Unlike volumetric reduction of the base of the tongue or other surgeries of the pharynx, the HNS surgical procedure is completely external to the pharynx, thus substantially reducing postoperative discomfort and recovery time, and minimizing or even eliminating the traditional risks of hemorrhage, dysphagia, change in taste, or other untoward pharyngeal side effects of anatomy-altering procedures.

Key Operative Learning Points

Successful HNS therapy requires proper patient selection, anatomic phenotyping, meticulous procedure execution, and precise titration of stimulation parameters postoperatively.
Intraoperative hypoglossal nerve monitoring is used to identify and selectively capture the distal tongue protrusor branches supplying the genioglossus muscle and to exclude lateral branches innervating the tongue retractor muscles.

Preoperative Period

As discussed in Advanced Palatal Surgery, Chapter 54 , a comprehensive sleep medicine history remains the cornerstone of preoperative evaluation. Once a patient is determined to be intolerant of or unable to achieve adequate benefits with standard medical treatments, surgical options may be considered. The optimal anatomic and physiologic phenotypes for HNS remain unclear and controversial. Physical examination, upper airway endoscopy, and imaging techniques are available to describe the upper airway anatomy and tailor a comprehensive and multilevel surgical treatment plan appropriately. The current US Food and Drug Administration (FDA)-approved HNS system has been studied in and approved for the treatment of moderate-severe OSA as second-line therapy after failure of positive pressure therapy. Several preoperative considerations are specific to HNS therapy and are also highlighted in the following list ( ^∗∗ ).

History

1.
History of present illness
- a.
  Sleep-related symptoms and impact on quality of life, including snoring, witnessed apnea, gasping, choking, nocturnal awakenings, nocturia, morning headaches, daytime sleepiness, and cognitive dysfunction
- b.
  Timing, duration, and onset of symptoms. Previously untreated severe OSA may be further complicated by associated cardiovascular comorbidities. Specific events may coincide with OSA onset such as an injury to the nose, surgery of the mandible, weight gain, cerebrovascular events, and change in sleep position due to musculoskeletal problems. All may have therapeutic implications.
- c.
  The presence of other sleep disorders such as insomnia and restless legs syndrome may confound the assessment of OSA outcome measures. Successful management of the patient will often require treatment of both the OSA and the additional comorbid sleep pathology. A comprehensive sleep medicine history is essential.
- d.
  Prior OSA treatment history, both medical and surgical, including detailed information on side effects, treatment response, and adherence with PAP and other medical devices. Excellent positive airway pressure (PAP) adherence in the setting of persistent daytime sleepiness, for example, is suggestive of a non-OSA etiology of sleepiness, and surgery may not be indicated.
2.
Sleep laboratory testing
- a.
  Type of study: Home sleep apnea test (HST) versus in-lab polysomnography (PSG)
- b.
  Sleep efficiency: Poor sleep efficiency may raise concern for unreliable sleep study results or the possibility of comorbid insomnia.
- c.
  Sleep architecture: Abnormal sleep architecture may be caused by OSA as well as by medications (e.g., antidepressants, steroids) or other sleep pathology.
- d.
  Sleep continuity: An elevated arousal index or abnormal sleep continuity can also be caused by OSA or by chronic pain, medications, or other sleep or medical pathology.
- e.
  Respiratory analysis: Snoring, apnea-hypopnea index (AHI), respiratory disturbance index (RDI), and oxygen saturation should be assessed to determine the severity of OSA. The presence of central sleep apnea, Cheyne–Stokes respiration, or other sleep-related hypoventilation/hypoxemia conditions (e.g., due to cardiopulmonary disease) should be ruled out, as surgery may be contraindicated in such conditions.
- f.
  Signs of other sleep pathology: PSG should evaluate for periodic limb movement disorder (PLMD), nocturnal seizure activity, REM-behavior disorder (RBD), or other nonrespiratory sleep disorders.
3.
Past medical history
- a.
  ^∗∗ Medical conditions requiring the regular use of magnetic resonance imaging (MRI) such as multiple sclerosis or central nervous system neoplasm. MRI is contraindicated in patients with the current HNS implantable device.
- b.
  ^∗∗ History of allergy to metal or other synthetic materials
- c.
  Comorbidities associated with OSA (hypertension, diabetes, myocardial infarction, atrial fibrillation, cerebrovascular event, depression, gastroesophageal reflux disease, erectile dysfunction)
- d.
  Chronic back or neck pain, fibromyalgia, or other pain syndromes that may negatively impact sleep
- e.
  Cardiopulmonary disease (e.g., chronic obstructive pulmonary disease, valvular heart disease, or congestive heart failure) that may complicate control of breathing at night and introduce additional respiratory pathology beyond OSA
- f.
  History of dysphagia
- g.
  History of neuromuscular disease
4.
Past surgical history
- a.
  ^∗∗ Prior surgery or injury to the tissues of the neck or upper chest (e.g., radiation of the head and neck, breast cancer surgery, pectoralis muscle flap, neck dissection) may be relative contraindications to HNS implant surgery and need to be evaluated on an individual basis.
- b.
  Prior sinonasal surgery
- c.
  Prior pharyngeal or skeletal OSA surgery
- d.
  Prior bariatric surgery
5.
Medications
- a.
  Anticoagulants.
- b.
  Opiate pain medication, benzodiazepines, or other medications that can alter nocturnal control of breathing.
- c.
  Selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants (TCAs), lithium, or other neuropsychiatric medication that can independently alter sleep architecture and continuity.
- d.
  Stimulant therapy (e.g., modafinil or methamphetamines) that may confound the assessment of daytime sleepiness.
6.
Family history
- a.
  A strong family history of OSA may provide insight into the anatomic/structural vulnerability.
7.
Social history
- a.
  Occupation
  - i.
    ^∗∗ Occupations that include repeated mechanical trauma to the neck and chest (e.g., contact sports, martial arts) or the use of heavy firearms (e.g., hunting, law enforcement, military) may be relative contraindications to implantable HNS therapy due to possible damage to the device or patient discomfort. The pulse generator placement may be adjusted, however, to avoid contact with the stock of a rifle or shotgun.
  - ii.
    OSA in truck drivers, airline pilots, railroad engineers, and other occupations involving public safety risk has additional implications on treatment and reporting of outcomes.
  - iii.
    Shift workers may have residual sleep-related symptoms due to their circadian rhythm disturbance, even after otherwise successful OSA treatment.
- b.
  Alcohol: Evening alcohol use has been correlated with increased obstructive respiratory events.
- c.
  Tobacco: Smoking has been shown to independently increase the risk of moderate to severe OSA as well as increase sinonasal and pulmonary disease.
- d.
  Total sleep duration: Behaviorally induced insufficient sleep is one of the most common causes of daytime sleepiness and may confound the assessment of OSA treatment outcomes. Sleep deprivation has also been shown to exacerbate OSA.

Physical Examination

1.
Anterior rhinoscopy
- a.
  Acquired nasal deformity, septal deviation, turbinate hypertrophy, rhinitis, nasal polyps, and other nasal valve pathology may increase upper airway resistance and directly contribute to sleep-disordered breathing.
- b.
  Increased nasal resistance may also increase negative intraluminal pressure in the pharynx and increase the fraction of maladaptive mouth breathing, further contributing to OSA pathophysiology.
- c.
  Nasal surgery designed to lower nasal resistance should be considered either in conjunction with or prior to pharyngeal surgery in most cases.
2.
Facial skeleton
- a.
  Significant maxillary or mandibular hypoplasia may require orthodontic or orthognathic surgical correction prior to consideration of pharyngeal or HNS surgery.
- b.
  Occlusion:
  - i.
    Angle’s classification
  - ii.
    Degree of overjet and overbite
3.
Oral cavity
- a.
  Tongue ridging or scalloping suggests relative macroglossia in relation to the size of the mandible.
- b.
  Quality and quantity of dentition has implications on the availability of adjunctive custom mandibular repositioning devices (oral appliance) in the treatment plan.
- c.
  Height and width of the hard palate. A narrow high-arched soft palate may increase the difficulty of soft tissue work on the soft palate and may negatively affect treatment outcomes.
- d.
  The presence of large palatal tori may reduce operative exposure and make palatal surgery more technically challenging.
4.
Oropharynx
- a.
  Sagittal configuration of the soft palate
- b.
  Length of the soft palate
- c.
  Modified Mallampati (MM) or Friedman Tongue Position (FTP)
- d.
  Size of the tonsils
- e.
  Size and structure of the lateral oropharyngeal wall
- f.
  Size and configuration of the uvula
5.
Neck
- a.
  Circumference of the neck
- b.
  Position of the hyoid bone: A low or inferiorly positioned hyoid bone suggests a longer pharyngeal airway.
6.
Chest
- a.
  To evaluate for chest wall deformity, scars, or other signs of prior surgery or trauma
7.
Cranial nerves
- a.
  Examination with specific attention to the functional status of the hypoglossal nerves and the facial nerve
8.
General health
- a.
  Blood pressure
- b.
  Cardiovascular
- c.
  Respiratory
- d.
  Mental
9.
Flexible fiberoptic laryngoscopy
- a.
  Awake
- b.
  DISE

Imaging

1.
Chest radiograph
- a.
  Preoperative screening
- b.
  Evaluate for cardiopulmonary disease.
- c.
  Rule out immobility of the hemidiaphragm.
2.
Imaging of the facial skeleton may be indicated in select cases preoperatively, particularly in cases of prior mandibular surgery, suspected craniofacial abnormalities, or signs of maxillomandibular deficiency. Imaging studies may include:
- a.
  Lateral cephalometric radiograph
- b.
  Orthopantomogram
- c.
  Computed tomography (CT) three-dimensional imaging
3.
Sinonasal imaging may be indicated in patients with suspected chronic sinonasal disease when simultaneous or staged nasal surgery is being considered with HNS surgery.
4.
Modified barium swallow study may be indicated prior to expansion sphincter pharyngoplasty (ESP) or transpalatal advancement pharyngoplasty (TPA) in patients with baseline dysphagia or velopharyngeal insufficiency symptoms, particularly in those with swallowing problems after prior pharyngeal surgery.

Indications

1.
Moderate to severe OSA patients who are not significantly obese and who are intolerant or have inadequate adherence with PAP therapy or other first-line medical device options
2.
Drug-induced sedated endoscopy (DISE) is currently recommended to evaluate the anatomic location and pattern of pharyngeal collapse prior to proceeding with HNS therapy. Particular attention is paid to the pattern of palatal collapse and the degree of multilevel coupling between the protrusion of the tongue and enlargement of the retropalatal space. Early feasibility studies have demonstrated poorer results with a complete concentric pattern of palatal collapse but higher success rates with an anterior-posterior pattern of palatal collapse ( Fig. 56.1 ).
3.
Patients who are willing to undergo implantation of a neurostimulation device should not have anticipated MRI requirements and should also have the ability and dexterity to operate a basic remote control.

Relative Contraindications

1.
Patient factors
- a.
  High perioperative risk
- b.
  Anticoagulation
- c.
  Poor nutritional status
- d.
  Allergy to metal
- e.
  Occupation associated with repetitive trauma to the neck or chest
2.
Anatomic factors
- a.
  Complete concentric pattern of palatal collapse on DISE
- b.
  Body mass index (BMI) greater than 32 kg/m

Preoperative Preparation

Anesthesiology consultation if there is a concern regarding a difficult airway or intubation
Hold or bridge anticoagulation.
Have patient bring continuous positive airway pressure (CPAP) or oral appliance (if still available and in use) the day of surgery in case airway support is needed in the hospital.

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