The aim of the present study was to compare the anatomical balance of the upper airway between Dutch and Chinese patients with OSA. The results indicated that the Dutch group had a significantly larger tongue area and a larger tongue length compared to the Chinese group, while the Chinese group had a smaller maxilla length compared to the Dutch group. However, the anatomical balance of the upper airway of both groups was not significantly different.
Comparisons of the upper airway morphology
For the primary outcome variable, we did not find a significant difference in the anatomical balance of the upper airway between Dutch and Chinese patients with OSA. Further, the observed effect size d for the difference in the anatomical balance of the upper airway was 0.4, which is between small and medium. With this effect size, the difference in the anatomical balance between both groups may be not clinically relevant either [
23,
24]. In contrast to our results, a study of Schorr et al. [
11] has suggested that Caucasians with OSA have a larger anatomical imbalance compared with the Japanese-Brazilians with OSA. However, for calculating the anatomical balance, they used the volume of the bony tissue rather than the volume of the bony enclosure as the denominator, which may cause bias and explain the different results as compared to our results. A study of Lee et al. [
10] has suggested a similar anatomical balance between Caucasian and Chinese OSA groups, which is similar to our results. However, they used a simplified definition of the anatomical balance, defined as ratios of BMI to mandibular and maxillary bony dimensions, which may be less accurate. The definition used in the present study has been used widely in the literature to investigate the role of the anatomical balance of the upper airway in the pathogenesis and treatment of OSA [
13,
14,
25]. Thus, by using a more generalized and accurate definition, the present study confirms that the anatomical balance of the upper airway is similar in Dutch and Chinese OSA groups.
For the secondary outcome variables, the Dutch group had a significantly larger tongue size and larger tongue length compared to the Chinese group. These results are similar to those of previous studies [
9-
12], which indicates that when the OSA severity is similar, Caucasian patients are more overweight, while Asian patients tend to have a smaller maxilla and mandible.
Previous studies have suggested that the craniofacial skeletal difference between Asians and Caucasians, such as restricted bony structures in Asians, is an important reason for a greater tendency of OSA development in Asians [
9,
26]. However, both bony structures and soft tissues can influence the upper airway morphology. By taking into account both factors, the present study indicates that the anatomical imbalance may be similar for both groups. However, in addition to the anatomical factor, several non-anatomical factors are also crucial determinants for upper airway collapse, such as impaired upper airway dilator muscle activity, ventilatory control stability (i.e., high loop gain), and low arousal threshold [
27]. The study of Lee et al. [
28] has suggested that a low arousal threshold is a less common mechanism in the pathogenesis of the Chinese OSA group compared to the Caucasian OSA group. Further, the study of O’Driscoll et al. [
29] has suggested that the loop gain is significantly higher in the Caucasian group than in the Chinese group. However, both studies included moderate-to-severe OSA patients, which may represent a different study sample as compared to our study. To the best of our knowledge, the difference in the non-anatomical factors between both races in patients with mild-to-moderate OSA is not clear yet. Understanding the individual pathogenesis can help in a personalized treatment approach in OSA [
27]. Therefore, future research is needed to investigate the roles of anatomical and non-anatomical factors in the pathogenesis of OSA.
Demographic characteristics
Lee et al. [
10] have suggested that the referral approach for the clinical assessment of OSA may be influenced by the differences in socioeconomic status, cultural, and environmental factors between the Caucasian and Chinese groups. This is consistent with the phenomenon that we discovered during the recruitment process. There were more patients with severe OSA referred to the sleep laboratory in China than in the Netherlands. To minimize the selection bias, we only recruited patients with mild to moderate OSA in both groups. Besides, both groups were similar in BMI, age, and sex. Therefore, the comparisons of the upper airway morphology between both groups were not biased by these factors.
Clinical relevance
Based on the non-significant results of the present study, it is possible that the anatomical balance of the upper airway plays a similar role in the pathogenesis of OSA in both races. Therefore, it may be speculated that treatment, which mainly targets the anatomical factors, might result in similar treatment results in both groups. Further studies will be performed in our lab to evaluate the treatment effects of therapy in both races.