Relationship Between Maxillary Transverse Deficiency and Respiratory Problems: A Systematic Review of the Effectiveness of Devices over the Past Decade

Background/Objectives: Maxillary transverse deficiency is linked to impaired nasal breathing and pediatric sleep-disordered breathing. This systematic review evaluated the effects of maxillary expansion (ME) on upper-airway morphology and breathing function in growing patients. Methods: The search was conducted on PubMed/MEDLINE, Scopus, ScienceDirect, Cochrane CENTRAL, and gray literature (January 2015–April 2025). Eligible RCTs, controlled trials, and cohort/observational studies assessed airway morphology and/or respiratory outcomes after ME in pediatric/adolescent patients. Risk of bias was evaluated with RoB 2 (RCTs) and ROBINS-I (non-randomized studies). The findings were synthesized qualitatively and certainty graded with GRADE. Results: Forty-one studies were included. Imaging consistently showed enlargement of the nasal cavity and nasopharynx after expansion, whereas the effects in the oropharynx and hypopharynx, as well as in the maxillary sinuses, were smaller or variable. Objective patency improved in several studies (higher peak nasal inspiratory flow, reduced nasopharyngeal obstruction, and nasal resistance), whereas computational fluid dynamics generally showed non-significant trends toward lower resistance. Spirometry improved, particularly in oral breathers (gains in FEV1, FVC, FEF25–75%). Polysomnography indicated reductions in AHI and improved oxygenation in some pediatric OSA cohorts, although other RCTs reported null PSG effects. Caregiver-reported sleep and quality-of-life outcomes were consistently enhanced. Device design modestly influenced regional widening, but overall respiratory effects were similar across expanders. By GRADE, certainty was low for airway morphology and very low for breathing function. Conclusions: In growing patients, ME reliably enlarges upper-airway compartments, especially the nasal cavity and nasopharynx, yet functional improvements are heterogeneous. Standardized outcomes and integrated morphological–functional assessments are needed to strengthen the evidence base