## 4 Discussion  
Rapid palatal expansion (RPE) is a validated and recommended therapeutic approach for treating a constricted maxillary upper jaw, particularly in growing patients. However, there has long been controversy surrounding the potential role of RPE in alleviating nasal obstruction and improving physiological nasal breathing. This controversy stems from the belief that restricted airways may contribute to the pathophysiology of obstructive sleep apnea syndrome (OSAS) in growing patients [27]. The scientific evidence shows that there is a direct correlation between nasal resistance, AHI, snoring time, and time spent with SpO2 below 90% in the pediatric population and a significant inverse correlation between total sleep time, sleep quality, and mean arterial oxygen saturation during sleep [17]. In order to assess any RPE-induced improvements on respiratory dynamics, it is necessary to quantify nasal aerodynamics, demonstrating changes between pre- and post-expansion. In recent times, rhinomanometry has made it possible to investigate nasal airway physiology with quantifiable and comparable parameters; in fact, numerous studies in the literature have evaluated the functional effects of RPE on nasal breathing with this instrumental method. [PERSON] et al. [28] evaluated a sample of 27 children aged 5 to 13 years with maxillary contraction using AAR, acoustic rhinometry, and postero-anterior teleradiography before  
Figure 4: Questionnaire scales for sleep-related breathing disorders (SRBDs). Figure 5: Pediatric Sleep Questionnaire (PSQ) in the reduced 22-item form (PSQ-SRDB), translated and validated in Italian. and 12 months after RPE, comparing it to an untreated control group. The AAR reports a remarkable reduction in nasal resistance after orthodontic treatment only under decongestion. [PERSON] et al. [29] found a statistically significant reduction in NAR immediately after expansion (25.5%), stable over the following 9-12 months, similar to what was achieved by [PERSON] et al. [30] and [PERSON] et al. [31]. Nasal airway impairment and the presence of obstruction are believed to be related to high nasal resistance and, in some cases, an oral breathing pattern. In order to determine the average nasal cross-sectional size and the effect of age on nasal breathing, [PERSON] et al. [32] analyzed 102 patients aged 6 to 15 years under resting breathing conditions. The study showed that from 6 to 14 years, nasal airway size increases by an average of 0.032 cm\({}^{2}\) per year: this explains the physiological reduction in nasal resistance. At the same time, the nasal breathing mode seems to increase with age: after the age of 8, most children have a nasal breathing mode. Thus, nasal breathing seems to be subject to physical and behavioral aspects in the context of an individual's development. Beyond the known influence of nasal size on nasal breathing ability, there is evidence, even with adequate airways, of a certain percentage of children breathing orally. The literature considers that RPE has a local effect on the upper airway, which is more evident at the bony level than at the mucosal level; it is probably related to compensatory post-expansion hypertrophy of the nasal mucosa and soft tissue adaptation [22; 33]. Although most studies agree that the RPE induces a reduction in nasal resistance to airflow, the promoted improvements on nasal resistances show a weak correlation with dental expansion [34]. In addition, the influence of RPE on nasal resistances does not appear to be stable: a return to values close to baseline is expected about 30 months after treatment [22; 23; 35]. Therefore, it is not possible to demonstrate a long-lasting effect of RME on nasal respiratory function. In most of the above studies, the study group was selected by placing the presence of obstructive diseases of the first airway as an exclusion criterion, performing the evaluation on an otolaryngologically healthy population. Only [PERSON] et al. [30] and [PERSON] et al. [35] included subjects with obstructive problems.