Mandibular advancement splint treatment outcome is related to the interaction between airway enlargement and dilatory muscle function with mandibular advancement

Rationale. Mandibular advancement splint (MAS) therapy for obstructive sleep apnea (OSA) enlarges the airway by holding the mandible forward, but efficacy is variable and difficult to predict. Using tagged magnetic resonance imaging (MRI), we have observed that genioglossus inspiratory dilatory movement differs with OSA status and is related to upper airway crosssectional area. We have also observed that airway enlargement with MAS differs between people as the posterior tongue does not always advance. We hypothesized that the way in which mandibular advancement alters tongue inspiratory dilatory movement and airway enlargement will relate to treatment outcome. Methods. 49 untreated OSA participants (11 women, apnoea hypopnoea index (AHI) 10-61 events/hr, body mass index (BMI) 18-41 kg/m² , aged 19-76 years) underwent a MRI scan wearing a MAS. Mid-sagittal tagged MRI images were collected to quantify tongue advancement during mandibular advancement and inspiratory dilatory patterns with the jaw in neutral position and advanced to 70% of the maximum during wakefulness. Treatment outcome was determined with polysomnography after approximately 12 weeks of therapy. Results. Twenty two subjects were responders (AHI≤10 events/h +50%), 8 were partial responders (>50% AHI reduction), and 19 were non-responders (change in AHI <50%). MAS altered the inspiratory dilatory pattern for 35% of participants. Fewer obese participants hadtongue advancement (>1 mm) during mandibular advancement than in those with BMI in the normal range (36% vs 92%, Fisher’s exact test, p= 0.05). Participants who relied on active airway dilation to maintain airway patency with the jaw in neutral position were more likely to change their inspiratory movement pattern to minimal dilation with mandibular advancement (Fisher’sexact test, P= 0.008). They were also less likely to be MAS treatment responders when the tongue advancement during mandibular advancement was larger than 1 mm (Fisher’s exact test, p= 0.03, n= 33). Conclusions. We observed heterogeneity in how mandibular advancement affected airway size and active dilation of the airway during inspiration between OSA patients, and this interaction differed between MAS treatment outcome groups. These findings provide insight into the potential mechanisms of MAS efficacy vs. failure and these concepts may be helpful to inform strategies to predict and improve MAS treatment outcomes.