# The Heritability of Upper Airway Dimensions Using MRI Scans in Twins
## 4 Discussion
found a strong genetic determination regarding tongue AP diameter in mice; meanwhile, the other dimensions of the tongue were mainly affected by environmental factors [45]. In OSA patients, the soft palate tends to appear thicker and has an increased anteroposterior diameter [46; 47]. In our study, the dimensions of the soft palate and the uvula were similarly determined by heritable components and environmental factors. [PERSON] et al. considered the retropalatal airway as the most important structure contributing to OSA. They examined 117 male subjects and found that the dimensions of the nasopharynx, the cross-sectional area at the hard palate level, and pharyngeal length were associated with the RP patterns and OSA severity as well [48]. Our study correlates with these findings as we found a strong environmental background behind the narrowest point of the PAS, which is usually in the retropalatal region. The studies investigating tongue anatomy in OSA patients mainly focused on tongue volume, fat distribution, and muscle activity. They found that muscle activity and the muscle fiber distribution of the genioglossus muscle and increased fat tissue in the tongue can affect OSA severity [29; 49; 50]. Our study indicates that the tongue dimensions may be primarily determined by genetic factors and could be the reason for the high heritability of OSA [29; 49; 50]. Fat deposition around the tissues of the upper airway could also lead to space reduction [49]. [PERSON] et al. aimed to validate the hypothesis that fat tissue accumulation contributes to OSA on 28 subjects and found significantly more fat tissue around the pharyngeal cavity in OSA patients compared to healthy controls [1]. [PERSON] et al. studied the importance of the lateral parapharyngeal wall in OSA. In their study, the airway narrowing in OSA patients was predominant in the lateral dimension, which was explained by larger pharyngeal walls rather than the enlargement of the parapharyngeal fat tissue [51]. [PERSON] et al. found that the lateral pharyngeal wall and supraglottic airway collapse were associated with OSA surgical failure [52]. In a study on 33 adults by [PERSON] et al., the parapharyngeal fat pad increased with the age and BMI of their study group; they also found that larger fat tissue volume led to the narrowing of the retropalatinal space [53]. Other studies found that the thickening of the soft tissue by fat deposition in the submandibular neck region could also lead to narrowed airways [53; 54]. These studies have already indicated the importance of pharyngeal tissue thickness and fat area, and our results suggest that these traits are environmentally determined. This could mean that with appropriate interventions (i.e., diet, pharyngeal muscle training, medications), these factors can be eliminated. On the other hand, we found a strong heritable component regarding the thickness of the subcutaneous neck fat tissue. This could be due to the different anatomical distribution between brown adipose tissue and white adipose tissue. In adults, subcutaneous brown adipose tissue was reported to be found in the anterior neck region but not the parapharyngeal region [55; 56]. In addition, studies have reported different gene expressions in the regulation of fat distribution at different anatomical regions, particularly in subcutaneous vs visceral adipose tissue [57; 58]. It could suggest that brown adipose tissue and white adipose tissue have different heritable backgrounds. There are very few family and twin studies currently available regarding OSA markers. [PERSON] et al. examined 272 subjects from 29 families, and their data suggested a significant familial aggregation of symptoms associated with sleep-disordered breathing [59]. Twin and family studies suggest that obesity and craniomorphology are highly determined by genetic contribution [15; 60]. [PERSON] et al. found a significant familial aggregation with a high heritability of insomnia disorder in a study based on 75 adults and their 180 relatives [61].