This is the first study to analyze polysomnographic features, cephalometric parameters and contributors on both positional and REM dependency in OSA patients. The results showed that positional and REM-related OSA patients had more mild traits of OSA compared to non-positional and not-REM-related OSA patients, respectively. Anatomical factors were more closely related to REM dependency than positional dependency.
Previous studies have reported that positional OSA patients had less severe overall AHI, higher oxygen saturation, lower percentage of time below 90% oxygen saturation than non-positional OSA patients.9,15 REM-related OSA was usually common in younger age and less severe cases than not-REM-related OSA.11,16 The results of this study were consistent with previous literature. It is generally accepted that supine position during sleep has detrimental effects on sleep breathing disorder symptoms. Most previous studies on the effect of body position on sleep apnea have shown that sleeping in a supine position increases the severity of sleep apnea.15,17 Some researchers suggested that positional OSA patients appear to have a milder form of OSA because such patient spend less sleep time in the supine position, implying the possibility of successfully applying positional therapy, but its efficacy remains controversial.18,19 There was no significant difference in time spent in supine position between positional and non-positional OSA patients of our study. The underlying mechanism may rather be related to obesity. In this study, positional OSA patients had a significantly lower BMI compared to non-positional OSA patients, and logistic regression analysis results showed that BMI was the only significant risk factor for positional dependency. Weight gain can result in an increase in the thickness of the lateral pharyngeal walls of the upper airway which are already narrow in OSA patients. It can also result in further narrowing of the lumen and increase in collapsibility of pharyngeal space even in a lateral sleeping position.20 Another relevant hypothesis is that positional OSA is an intermediate state in the progression from snoring to OSA.18 Recent researches on the interaction between unfavorable upper airway geometry, reduced lung volume, instability of upper airway dilator muscles, arousal threshold, and ventilatory control instability have improved our understanding on the effect of positional dependency on upper airway collapsibility.21
In accordance with previous studies, REM-related OSA was more commonly observed in younger age, women, and had less severe apnea symptoms compared to not-REM-related OSA patients in our study.11,22 The results of our study also showed that REM-related OSA patients were younger, and had higher sleep efficiency and lower overall arousal index than non-REM-related OSA patients. Moreover, not-REM-related OSA patients showed significantly higher supine AHI, overall AHI, non-supine AHI, NREM AHI, and lower mean oxygen saturation and NREM oxygen saturation compared to REM-related OSA patients. Several possible mechanisms for REM dependency have been proposed until now. Muscle tone of the tongue and pharyngeal dilator muscles decrease and the upper respiratory resistance increases in REM sleep, so that OSA appears more easily and in a more severe level in REM sleep than in NREM sleep.23 Meanwhile, NREM predominant OSA was suggested to be associated with ventilatory instability, which is a cause of greater dynamic reduction in ventilation before and after wakefulness.24
The results of this study showed that the prevalence of OSA was higher in men (85.0%) than in women, and the male-to-female ratio was lower in positional and REM-related OSA patients than non-positional and not-REM-related OSA patients, respectively, but the difference was not statistically significant. Such findings are consistent with previous studies reporting male predominance, especially a higher male-to female ratio in more severe OSA groups.25 The prevalence of positional OSA was 83.5% in our study. This value is somewhat higher than that reported in previous studies which report a prevalence of 53 to 72% in OSA patients and higher in the Asian population compared to Caucasians.10,26
Previous studies found that positional OSA patients with higher non-supine AHI tend to more easily transform into non-positional OSA patients within a few years.20,26 This implies the importance of early diagnosis and intervention of positional OSA patients to prevent progression into a more severe OSA type. Identification of positional dependency could be important for diagnosis as well as in the evaluation of treatment efficacy. Such factors might be relevant when choosing treatment modalities including positional therapy, continuous airway positive pressure (CPAP), and mandibular advancement devices (MAD), since the latter was found to be effective in positional OSA patients. Furthermore, it has been reported that the combination of CPAP, MAD, and positional therapy is more effective than applying any one treatment modality alone.27
Interestingly, logistic regression analysis revealed age, severe OSA, and several anatomical variables as risk factors for REM dependency, but not gender and BMI. The results on the association with age, severe OSA, and BMI on REM dependency generally are in line with those from prior studies.11,28 However, the role of gender on REM dependency is less clear. Several studies explained female predominance in REM-related OSA patients which could be explained by gender differences in upper airway stability and hormonal factors. It is known that men show greater upper airway resistance and collapsibility in NREM sleep than women, and women have greater genioglossus activity in the waking state than men.29,30 In contrast, others showed that there were no associations between gender and REM dependency.16
Although certain craniofacial structures are generally known as risk factors for OSA, there were few studies about cephalometric analysis results in REM-related OSA patients.13 Therefore, the results of this study may provide clinical evidences on the pathophysiological role of craniofacial characteristics in REM-related OSA. An inferiorly displaced hyoid bone as measured by AH⊥MP, large soft plate measured by SPT and inferior oral airway space were shown as risk factors for REM dependency.
REM dependency may be affected by unfavorable conditions of the hard and soft tissue surrounding oral and upper airway space, and reflect unique features that are different from those related to positional dependency.
There are several limitations in our study. First, this study was limited by its retrospective design. Confounding factors that may affect positional and REM dependency may have been neglected due to bias in subject and data selection. Nevertheless, we collected and analyzed all clinical, polysomnographic, and cephalometric data from consecutive patients following strict selection criteria to lessen the possibility of bias. Second, lateral cephalometric radiographs were taken in upright position under wakefulness. Although this is due to an inherent limitation of the way in which standardized lateral cephalometric radiographs are taken, it cannot truly reflect the positional characteristics of the hard and soft tissue during sleep. Magnetic resonance imaging under sedation could be considered as an alternative, but it is difficult to implement such a method to all patients due to high cost and time related issues. Further extensive studies based on various clinical variables and treatment outcomes are needed to more comprehensively understand the exact pathophysiology of positional and REM-related OSA.