# Hypoglossal Nerve Stimulation for OSA

## Introduction

**Hypoglossal nerve stimulation (HGNS)** represents a paradigm shift in the surgical management of obstructive sleep apnea. The **Inspire Upper Airway Stimulation (UAS) system** is the first FDA-approved implantable neurostimulation device for moderate-to-severe OSA in patients who have failed or cannot tolerate CPAP therapy. By stimulating the hypoglossal nerve synchronously with respiration, the device activates the genioglossus and other tongue protrusor muscles to maintain airway patency during sleep.

## Mechanism of Action

### Hypoglossal Nerve Anatomy

**Motor nerve (CN XII)** arising from the medulla, exiting through the hypoglossal canal. Innervates all intrinsic and most extrinsic tongue muscles. Key distinction between **protrusor fibers** (genioglossus, geniohyoid) and **retrusor fibers** (styloglossus, hyoglossus). The device selectively stimulates the **medial branches** that innervate tongue protrusors. Retrusor branches are excluded from the stimulation cuff to prevent paradoxical airway narrowing.

### Device Components

**Stimulation lead and cuff electrode**: Placed around the medial hypoglossal nerve branches. **Sensing lead**: Placed between the internal and external intercostal muscles to detect respiratory effort. **Implantable pulse generator (IPG)**: Placed in a subcutaneous pocket in the right infraclavicular region. **Patient remote control**: Activates the device at bedtime and deactivates upon waking. Stimulation is delivered during inspiration, synchronized by the respiratory sensing lead.

## Patient Selection

### Inclusion Criteria (Based on STAR Trial)

| Criterion | Requirement |
|-----------|-------------|
| Age | >= 18 years |
| AHI | 15-65 events/hour |
| CPAP | Failed or intolerant |
| BMI | <= 35 kg/m2 |
| Central apnea index | <25% of total AHI |
| DISE | Absence of complete concentric collapse at velopharynx |

Age >= 18 years (typically >= 22 in practice). Moderate-to-severe OSA: **AHI 15-65** on polysomnography. Failure of or inability to tolerate CPAP. **BMI <= 35 kg/m2** (higher BMI associated with poorer outcomes). No significant central or mixed apneas (central apnea index <25% of total AHI).

### DISE Requirements

**Drug-induced sleep endoscopy** is mandatory prior to implantation. Favorable anatomy: **Absence of complete concentric collapse (CCC)** at the retropalatal level. CCC is a contraindication because HGNS does not address circumferential palatal obstruction. Anterior-posterior or lateral collapse patterns at the velum are acceptable.

### Contraindications

Complete concentric collapse at the velopharynx on DISE. BMI > 35 (relative; emerging data suggest efficacy up to BMI 40 in select patients). Central sleep apnea or Cheyne-Stokes respiration, Neuromuscular disease affecting the tongue. Patients requiring MRI of certain body regions (conditional MRI compatibility varies by device generation).

![Inspire Upper Airway Stimulation system components and implant positions](images/hgns-inspire-components.jpg)

## Surgical Technique

### Implantation Procedure

Performed under **general anesthesia** (typically oral intubation). Three small incisions: Submandibular, right infraclavicular, and right lateral chest wall. **Step 1**: Submandibular dissection to identify the hypoglossal nerve. Dissect between the submandibular gland and digastric tendon. Identify the nerve on the hyoglossus muscle deep to the mylohyoid. **Selective nerve dissection**: Identify and exclude retrusor branches (styloglossus, hyoglossus). Place the stimulation cuff around the protrusor-selective nerve branches. Confirm tongue protrusion with intraoperative nerve stimulation. **Step 2**: Place the sensing lead between internal and external intercostal muscles via the lateral chest incision. **Step 3**: Create the IPG pocket in the right infraclavicular area. **Step 4**: Tunnel the leads subcutaneously to the IPG and connect. **Intraoperative testing**: Confirm appropriate tongue protrusion and respiratory synchronization. Procedure duration: Approximately **2-3 hours**.

### Postoperative Management

Outpatient or overnight observation. Device remains **off for 1 month** to allow tissue healing and lead stabilization. Activation performed in the sleep clinic at 4-6 weeks postoperatively. Gradual titration of stimulation parameters over several weeks. **In-laboratory titration PSG** performed after initial acclimation to optimize voltage settings.

## Evidence and Outcomes

### STAR Trial (Stimulation Therapy for Apnea Reduction)

Multicenter prospective study; 126 patients implanted. **12-month results**: Median AHI decreased from 29.3 to 9.0 (68% reduction). **Surgical success (Sher criteria)**: 66% of patients achieved AHI <20 with >50% reduction. ODI (oxygen desaturation index) decreased from 25.4 to 7.4. ESS decreased from 11.6 to 7.0. **5-year follow-up**: Sustained efficacy with median AHI of 6.2; 75% response rate.

### ADHERE Registry

Real-world outcomes from over 2,000 patients, Consistent with STAR trial findings, Median AHI reduction from 36 to 7. Higher BMI (32-35) showed somewhat reduced but still meaningful response.

### Quality of Life

Significant improvement in Epworth Sleepiness Scale scores. Improved Functional Outcomes of Sleep Questionnaire (FOSQ) scores. High patient satisfaction and adherence rates (>80% nightly use). Bed partner satisfaction also significantly improved.

![STAR Trial outcomes demonstrating AHI reduction at 12 months and 5-year follow-up](images/star-trial-outcomes.jpg)

## Complications

**Tongue weakness**: Transient in most cases; rarely persistent. **Tongue abrasion**: From repetitive protrusion against the teeth; managed with dental guard. **Discomfort at stimulation site**: Resolved with parameter adjustment. **Device malfunction or lead migration**: May require revision surgery. **Infection**: Rare (<2%); may require explantation. **Stimulation of non-target muscles**: Platysma or neck musculature; corrected by reprogramming.

## Expanding Indications

Investigation of efficacy in patients with **BMI 35-40**. Bilateral hypoglossal nerve stimulation under study. Potential role in patients with **complete concentric collapse** using combined therapies. Pediatric applications under investigation for children with Down syndrome and residual OSA after tonsillectomy. MRI-conditional device generations expanding imaging compatibility.

![Intraoperative view of hypoglossal nerve dissection and selective cuff placement](images/hgns-intraoperative-nerve.jpg)

## Key Clinical Pearls

DISE is mandatory before implantation; complete concentric collapse at the velum is a contraindication for HGNS. Selective stimulation of protrusor branches while excluding retrusor fibers is critical for efficacy and to avoid paradoxical airway narrowing. The STAR trial demonstrated sustained 5-year outcomes with median AHI reduction to 6.2. The device is activated 1 month postoperatively and requires titration polysomnography for optimization. HGNS fills an important gap between CPAP failure and more invasive surgical options like maxillomandibular advancement.

## References

1. Strollo PJ Jr, Soose RJ, Maurer JT, et al. Upper-airway stimulation for obstructive sleep apnea. *N Engl J Med*. 2014;370(2):139-149.
2. Woodson BT, Soose RJ, Gillespie MB, et al. Three-year outcomes of cranial nerve stimulation for obstructive sleep apnea: the STAR trial. *Otolaryngol Head Neck Surg*. 2016;154(1):181-188.
3. Heiser C, Knopf A, Bas M, et al. Selective upper airway stimulation for obstructive sleep apnea: a single center clinical experience. *Eur Arch Otorhinolaryngol*. 2017;274(3):1727-1734.
4. Kent DT, Lee JJ, Strollo PJ Jr, Soose RJ. Upper airway stimulation for OSA: early adherence and outcome results of one center. *Otolaryngol Head Neck Surg*. 2016;155(1):188-193.