This study investigated the anatomic variants of the upper airway (lateral cephalometry) related to sex and central obesity in Asian patients with OSA. The different phenotypes of OSA determined by sex and central obesity may permit us to make appropriate therapeutic plans for OSA patients. The key findings of this study were as follows: (1) Soft palate was found to be the main target of OSA and its anatomic changes show significant correlations with AHI. (2) Phenotypes of soft palate were different between sexes: increased length for male and increased thickness for female. (3) Only male OSA patients showed clear associations between central obesity and the upper airway anatomy. (4) MPH was a male-specific parameter especially for moderate and severe OSA and showed a significant correlation with both AHI and central obesity.
Male with OSA showed typical characteristics of central obesity with increased NC and WHR. There have been many studies demonstrating the altered neck anatomy in male with OSA including increased neck circumference (anthropometry) and parapharyngeal fat deposition (CT or MRI) [16,17,18]. Therefore, it is reasonable to state that central obesity (thick neck) in male with OSA may cause the altered anatomy of the upper airway. To illustrate OSA-related upper airway anatomy, a total of eight well-known parameters on lateral cephalometry were used for craniofacial anatomy (SNA, SNB, and ANB), upper airway (SPL, SPT, and RPS), and lower airway (RLS and MPH). Lateral cephalometry does not show that the volume of airway and cephalometric analysis was affected by the head position. Nevertheless, it has been used in maxillofacial deformity analysis and orthodontic diagnosis. The lateral cephalometry has been used for its simplicity and economical method for measuring anatomical structure related to the airway. In this study, we standardized the data by correcting the cranio-cervical inclination in order to obtain accurate measurements of parameters of the airway. This study clearly demonstrated that the upper airway anatomy measured with lateral cephalometry had different characteristics and associations with AHI and central obesity, depending on sex.
SNA, SNB, and ANB were used to evaluate the anterior-posterior position of maxilla and mandible to cranial base on lateral cephalometry. Retrognathia is well-known anatomic phenotype affecting the pathogenesis of OSA [19,20,21,22]. Lowe et al. [21] showed that patients with OSA had the smaller and posteriorly positioned mandible which decreased the overall airway space on lateral cephalometry. In contrast, others reported that there was no difference in upper airway volume based on the intermaxillary relationship using cone beam computed tomography (CBCT) [23]. However, our study demonstrated that mandible was positioned posteriorly in female with OSA (increased ANB) which was independent with obesity and had significant correlations with AHI. Thus, retrognathia is an important anatomic phenotype affecting the presence and severity of OSA in female.
There are many reports using lateral cephalometry to investigate the influence of soft tissues on OSA. Previous studies reported that increased soft palate length and thickness are associated with OSA on lateral cephalometry [21, 24,25,26,27]. In the study targeting 62 males, Yu et al. [28] also reported that longer SPL was associated with higher prevalence of OSA. In contrast, Cillo et al. [29] reported that there were no significant changes in SPL in OSA. However, in this study, we found that male with OSA had a different phenotype of the soft palate compared with female with OSA. Male with OSA had significantly increased SPL which showed significant association with AHI and NC. Alternatively, female with OSA had significantly increased SPT which showed significant association with AHI but not with central obesity (NC and WHR). Therefore, the soft palate may become elongated in conjunction with the weight gain which can aggravate the upper airway narrowing in male. However, female may have an intrinsic variance of SPT which is independent with central obesity. Female with thick palate may have increased risk for the development of OSA (Fig. 3).
Retro-palatal and retro-lingual spaces are the most common sites of upper airway narrowing in OSA [30, 31]. Dynamic studies such as drug-induced sleep endoscopy (DISE) also reported that there are isolated or multiple sites of upper airway collapse in OSA [32]. However, in our cephalometric study, there was no difference in RPS and RLS between OSA and controls regardless of the sex. Moreover, they did not show any correlation with AHI and central obesity. Thus, we concluded that 2-dimensional analysis of the upper airway space may have limitations to represent OSA. To investigate the space anatomy, upper airway endoscopy may be advantageous to show 3-dimensional views and dynamics in OSA.
The position of the hyoid bone is related to the retro-lingual space, and lower hyoid indicates the narrowing of RLS and also is associated with weak tonicity of the genioglossus muscle. De berry et al. [33] reported that the hyoid bone in the lower position could displace the base of the tongue further downward leading to the airway obstruction of hypopharynx more easily. However, in our study, there was no difference in MPH between OSA and controls regardless of sex. By subgroup analysis, MPH in male showed significant associations with AHI and central obesity (NC and WHR). Moreover, ROC curves showed that MPH is an anatomic factor representing the moderate-to-severe OSA in male. Whittle et al. [16] reported that male had larger soft tissue volume around the airway compared with female which is consistent with our study. Thus, both intrinsic soft tissue volumes of the upper airway and obesity-related central fat deposition (NC) may affect the MPH in male. By comparison, MPH in female did not show any relationship with AHI and obesity, and therefore, the narrowing of RLS may be not involved in the pathogenesis of female OSA.
The authors acknowledge the weaknesses and limitations of our study. The lateral cephalometry was taken in a standing position and awake state which does not represent accurate measurements of the upper airway in the sleeping state. Also, the number of female was relatively fewer than male. Despite the above-mentioned limitations, our study clearly demonstrated the role of gender on the upper airway anatomic variation and its association with central obesity.