## 4 Discussion
In this study, differently from AR and previous experiences conducted on children/adolescents affected by lower airway obstruction [14; 32], the percentage of subjects with asthma did not differ in the two groups. This result could be explained by the fact that the children affected by asthma enrolled in the study showed higher adherence to the therapy (50%) than those affected by AR (26%), or, more probably, by the fact that although a large population was considered, there was only a very low percentage (about 5%) of subjects with asthma in the two groups. More studies examining larger groups of asthmatic children are warranted. When all the variables that could have influenced the crossbite or ogival palate were analyzed, not only AR but also age were found to be important factors. When a backward stepwise regression model was used, in fact, we observed that AR and age did influence crossbite/ogival palate, while all the other variables including asthma did not. In particular, it became apparent that those with AR had almost double the risk of having a crossbite/ogival palate (194%). On the other hand, with each additional year of age, the risk of having crossbite/ogival palate was reduced to 90% of that of the previous year (Table 3). These results not only confirm that by causing upper airway obstruction, AR is a significant risk factor for developing crossbite/ogival palate, but they also highlight the fact that orofacial dysfunction may begin very early in terms of age and may become evident at the time crossbite/ogival palate presents. Independently of oral habits, AR is implicated in malocclusion as it is able early on to produce nasal obstruction leading to craniofacial growth changes in children with mixed dentition [33]. As the children grew older, we observed a reduced probability of crossbite and/or ogival palate, probably because the craniofacial growth that had taken place was able to reduce the effect/s of nasal obstruction and its consequences on the malocclusion. Data regarding breathing habits and the presence of AR/asthma were analyzed in the 108 children in the study group who had completed the treatment cycle at the time the study was carried out. This was conducted in an effort to evaluate if the presence of AR and/or asthma affected the treatment's outcome. Interestingly, although AR was found to have an effect on crossbite/ogival palate, neither AR nor asthma were correlated with treatment relapse. This result could be explained by the hypothesis that the significant improvement in the nasal airflow and patency produced by the transverse expansion treatment [28; 34] exceeds the effect/s of allergy and asthma on the upper airways. Nevertheless, it is possible that this result is linked to the retrospective nature of the study. Moreover, as we did not have information about the type of allergy (perennial or seasonal) affecting the patients, we were unable to study if the lack of correlation between AR and the treatment relapse could also be due to a higher prevalence of seasonal than perennial AR. The main limitations of this study are its retrospective nature, the relatively short follow-up, and the lack of data regarding the immunologic status of the study and control populations and the genetic and epigenetic variables that could influence facial growth patterns or cause allergic rhinitis/asthma [35].