Background Obesity is known to be an important risk factor for OSA; however, OSA can also be seen in nonobese patients with a small maxilla and/or mandible as well as in all obese patients with such features. Thus, we hypothesized that regional factors, oropharyngeal crowding associated with fat deposition, and maxillomandibular enclosure size closely related to the severity of OSA. Methods A total of 703 male Japanese subjects were enrolled; theywere classified into obese (BMI ≥ 30 kg/m2; n = 158) and nonobese (BMI < 30 kg/m2; n = 545) groups. Using lateral cephalometric analysis, we measured the tongue size (TG), lower face cage (LFC), and TG/LFC ratio (ie, oropharyngeal crowding) to evaluate the state of upper airway crowding. The correlations between these cephalometric measurements and BMI, age, and the apnea-hypopnea index (AHI) were evaluated. Results In obese subjects, the TG/LFC ratio, BMI, and TG positively correlated with AHI, whereas, in nonobese subjects, age, BMI, and TG/LFC significantly correlated with AHI. Subsequent stepwise multiple linear regression analysis revealed that the variables associated with AHI differed between obese and nonobese OSA subjects, although BMI and TG/LFC were significantly associated with AHI in both groups. In particular, the contribution of TG/LFC to AHI was larger than that of BMI in the obese group. Conclusions Oropharyngeal crowding is a local anatomic factor that independently relates to the severity of OSA in both obese and nonobese patients; the more crowded the upper airway, the more severe the OSA.
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