This clinical trial will evaluate the effect of treatment with hypoglossal nerve stimulation on the underlying mechanisms of obstructive sleep apnea. Several disease mechanism parameters are known to be associated with obstructive sleep apnea. However, currently, only the location of upper airway collapse is routinely examined in clinical practice using sleep endoscopy. Among other parameters, airway collapsibility is a widely studied mechanism. This parameter indicates how easily a patient's upper airway tends to collapse and can be assessed with additional measurements during sleep endoscopy.

The aim of this trial is to investigate the effect of hypoglossal nerve stimulation on collapsibility during sleep endoscopy. This information will provide a better understanding of the physiological mechanisms of hypoglossal nerve stimulation. In the long term, the investigators hope this knowledge will allow for more personalized care by tailoring treatment to the specific needs of each patient.

Obstructive sleep apnea (OSA) is one of the most prevalent respiratory disorders, characterized by recurrent pharyngeal collapses during sleep. This disturbance results in fragmented, nonrestorative sleep. Furthermore, intermittent hypoxemia can lead to both acute and chronic elevation of blood pressure and serves as a significant risk factor for all-cause mortality. OSA symptoms include snoring, unrefreshing sleep, fatigue, excessive sleepiness and nocturnal gasping or choking.

OSA is diagnosed using polysomnography (PSG), during which several parameters are measured throughout the night, including airflow, electroencephalography, electromyography, oxygen desaturation and heart rate. Using these measures, OSA severity is quantified by the apnea-hypopnea index (AHI), capturing the number of apneas and hypopneas per hour of sleep.

The standard treatment for OSA is continuous positive airway pressure (CPAP), which opens the upper airway by creating a pneumatic splint. Alternative treatments include mandibular advancement device (MAD) treatment, which (re)opens the upper airway by protruding the mandible, positional therapy to avoid supine position, drug treatments, hypoglossal nerve stimulation treatment and other surgical treatments. While CPAP is characterized by an overall greater efficacy, adherence might be limited. Non-CPAP treatments are characterized by a higher adherence, yet their efficacy is patient dependent.

Respiration-synchronized hypoglossal nerve stimulation (HNS) is an innovative technique in which the hypoglossal nerve is stimulated to protrude the tongue during inspiration. While HNS has demonstrated clinical efficacy, its impact on the underlying pathophysiological mechanisms of OSA remains insufficiently understood. Five pathophysiological parameters are known to be associated with OSA treatment outcome: site of collapse, upper airway collapsibility, ventilatory control instability (loop gain), muscle responsiveness and arousal threshold. These key pathophysiological traits have also been shown to be associated with HNS treatment outcome.

Currently, only the site of collapse is routinely assessed in clinical practice using drug-induced sleep endoscopy (DISE). The remaining traits, particularly collapsibility, usually require complex overnight pressure-drop studies that are not feasible for routine clinical use. Collapsibility is commonly assessed in research using the critical closing pressure (Pcrit), where a higher Pcrit indicates a more collapsible airway.

A recent technique developed by our research group allows for the assessment of the critical closing pressure (Pcrit) during DISE using a modified nasal mask and CPAP device. While the clinical effectiveness of HNS is proven, its specific effect on upper airway collapsibility is unknown.

This study aims to quantify the effect of HNS on upper airway collapsibility by measuring Pcrit during DISE, both with and without active stimulation. This research is vital for understanding the mechanical effects of HNS therapy and may ultimately improve patient selection and the delivery of personalized medicine for OSA.

Inclusion Criteria:

• 18 years or older.
• Treated with HNS-therapy for OSA (AHI ≥15/hour sleep)
• Capable of giving informed consent
• Baseline polysomnography performed at Antwerp University Hospital

Exclusion Criteria:

• Patients did not receive HNS-therapy at the Antwerp University Hospital
• Central apneas accounting for ≥25% of total apneas during baseline polysomnography
• Known medical history of intellectual disability, memory disorders or current psychiatric disorders (psychotic illness, major depression, or acute anxiety attacks as mentioned by the participant).
• Simultaneous use of other treatment modalities to treat OSA (outside of HNS-therapy)
• Esophageal ulceration, tumors, diverticulitis, bleeding varices, sinusitis, epistaxis, recent nasopharyngeal surgery
• Pregnancy or willing to become pregnant
• Excessive alcohol or drug use (> 20 alcohol units/week or any use of hard drugs)