Hypoglossal nerve stimulation (HNS) plays an increasingly important role in managing patients with obstructive sleep apnea (OSA) who do not tolerate CPAP therapy and are not eligible for other alternative treatment options, such as mandibular advancement devices or positional therapy. The posterior upper airway space dimensions are crucial in managing patients with HNS in the patient selection process and therapy control. The lateral collapse of the upper airway is of crucial importance. Lateral collapse at the palatal level and of the oropharyngeal walls is a well-established negative predictive factor for therapeutic success. Patients with complete concentric collapse at the palatal level (pCCC) in drug-induced sedation endoscopy (DISE) must be excluded from the implantation of HNS, which is cumbersome and invasive. Endoscopy has the inherent limitation that only one level can be observed at a given time, and assessment is possibly hampered by phlegm.

During activation and titration of HNS, tongue protrusion is observed in the awake patient. However, this method does not allow for assessing the opening of the retroglossal (RG) and retropalatal (RP) airway space, which is the ultimate therapeutic goal. Insufficient opening of the airway is the reason for non-responders with HNS. Insufficient upper airway opening can be either at the retropalatal or retroglossal level. The study aims to identify insufficient airway openings better using sub-mental ultrasonography. Sub-mental standardized and orientated ultrasonography offers a quantitative, reproducible way of assessing transverse upper airway dimensions and anatomic features of the upper airway in a rapid and non-invasive manner. In addition, anatomic characteristics of the airway's adjacent tissue, such as the size and shape of the tongue, may also have an impact on the effectiveness of HNS. Tongue morphology and posterior airway space assessment could be used in preoperative evaluation and during therapeutic titration of HNS. The clinical routine could be included tongue morphology and posterior airway space assessment without additional patient risks. However, the clinical value of assessing posterior airway space and tongue morphology in patients with HNS is yet unknown.

Inclusion Criteria:

• Patients older than 18 year
• patients with a hypoglossal nerve implantation
• control group: 15 subjects without OSA (AHI<10/h)
• written informed consent
• sufficient knowledge of German or French to understand informed consent

Exclusion Criteria:

• Patients younger than 18 years
• unwillingness to give informed consent
• incapable of performing Müller's maneuver
• history of head and neck surgery other than HNS, tonsillectomy, and maxillomandibular advancement
• diagnosis of congestive heart failure or chronic pulmonary disease
• diagnosis of co-morbid sleep disorders, including central sleep apnea
• pregnancy

39 obstructive sleep apnea patients with hypoglossal nerve stimulation and 15 subjects in the control group