## 4 Discussion  
This retrospective study compared the general, polygraphic, and endoscopic characteristics between responders and non-responders to mandibular advancement during DISE. Particularly, the sites (velopharynx, orophnx, tongue, epiglottis), degrees (none, partial, complete), and patterns (latero-lateral, anteroposterior, circular) of upper airway collapse were described at baseline and with TMA simulator placed intraorally in order to detect characteristics that, if confirmed in further prospective studies, will help in identifying beneficial and/or adverse phenotypes related to MAD treatment outcome. A previous prospective study investigated the locations and degree of upper airway collapse to different mandibular advancements during DISE using a selector advance mandibular device but failed to evaluate the collapse pattern which is an important variable for phenotypes identification and included both snoring and OSA patients [22]. Our study is the first to be conducted on a large sample undergoing DISE with a TMA simulator and including only patients with OSA, thus enhancing the generalizability of the results. A further strength of this study is the ability to evaluate the changes in upper airway collapsibility during DISE using a TMA simulator that is customized to the individual patient. In the present study the average age of the two groups, responders (50.76 \(\pm\) 9.83 years) and non-responders (48.77 \(\pm\) 9.86 years) were comparable. Similar to the results of the prospective study by [PERSON] et al. [24], in which age did not prove to be a success predictor for mandibular advancement device therapy, despite numerous retrospective studies indicating how younger patients are more likely to respond positively to MAD [8; 9; 25]. The discrepancy between the results of the different studies can likely be related to the variability of the samples analyzed. Concerning age, in our study patients should have good general health with few controlled comorbidities and a low risk for general anesthesia to justify a diagnostic DISE. Data from BMI evaluation showed results in agreement with previous findings [26], in fact, the BMI was significantly lower in the responder group (28.09 \(\pm\) 3.21 kg/m\({}^{2}\)) compared to the non-responder group (29.14 \(\pm\) 3.81 kg/m\({}^{2}\)). The analysis of the polygraphic characteristics showed significant differences between the two groups, except for the average oxygen saturation (meanSat %). The baseline AHI was higher in the non-responder group, although the trend was toward a severe disease in both groups (mean AHI \(\geq\) 30 ev/h). In accordance with the statement by the American Academy of Sleep Medicine in 2015 [13], our study indicates how there could be a positive response to MAD even in patients with severe OSA. Likewise, the AHIsup and AHInonsup also were significantly higher in the non-responder group than in the responder group. The evaluation of the respiratory parameters showed significant differences between the two groups regarding ODI: Satmin and CT90: the responder group showed lower values of the ODI and CT90 indices, while higher values of minimum saturation. Mean saturation was higher in the responder group, although not significantly. The findings of our study are consistent with those of [PERSON] et al. [27], although their study is a prospective analysis based on a sample of only 41 patients. The percentage of the obstructive sites without a TMA simulator showed significant differences only for the orophnx, where the non-responders group presented a greater