Prevalence of obstructive sleep apnea in Asian adults: a systematic review of the literature

Obstructive sleep apnea (OSA) is a common medical condition and a form of sleep disordered breathing (SDB), which is characterized by repetitive complete and/or partial collapses (apnea and hypopnea respectively) of the upper airways. The disease is classified as mild, moderate and severe based on the number of apneas and/or hypopneas per hour of sleep, known as the apnea-hypopnea index (AHI). This is assessed by polysomnography (PSG) or other forms of sleep monitoring [1]. Obesity, aging, male sex, smoking and alcohol intake are the risk factors for OSA.

Subjects with OSA may complain of excessive daytime sleepiness (EDS) or insomnia, nocturia and morning headaches, but some patients with OSA may be asymptomatic. Several clinically helpful questionnaires are available for detecting patients at high risk for OSA such as the Berlin Questionnaire [2] and Epworth Sleepiness Scale (ESS) [3], but the diagnosis is made by PSG or by home sleep monitoring, and these questionnaires are only of ancillary use. For more general information on OSA, please refer to a well- written review article on this topic [4].

Asia is the most heavily populated continent, with some groups living in an underdeveloped environment. To date there are no published scientific reports on the general prevalence of OSA in Asia.

The rationale for this manuscript is to systemize the current data on the prevalence of OSA and patients at risk for OSA obtained from the Asian population and to highlight potential shortcomings, which should be addressed in future epidemiologic works.

A total 732 articles were found on the prevalence of OSA in Asia and the prevalence of patients at risk for OSA based on questionnaire and/or symptom assessment. Of these 676 articles were excluded after abstract screening for the following reasons: not focused on the studied outcomes (n-416), studies in pediatric population (n-90), reviews, comments/editorials (n-67), articles not published in English (n-39), small sample (n-33), studies performed in patients with certain comorbidities (n-20), studies performed outside Asia (n-9),  animal studies (n-7).

Fifty one full text articles were evaluated, with 28 articles excluded for the following reasons: hospital- based studies (n-18), not focused on studied outcomes (n-7), duplicate publications (n-2). Twenty four articles covering 47,957 patients (26,042 men and 21,915 women) were eligible based on the aforementioned criteria. Many studies were excluded for more than one reason.

Below, we review the data on the prevalence of patients at risk for OSA first, and then the data on the prevalence of OSA based on the instrumental assessment. The diagram of the literature search is shown in Figure 1.

OSA is a particularly common and underrecognized medical disorder. It is associated with increased morbidity and mortality from cardiovascular causes, and traumatic accidents due to EDS. OSAS, which is characterized by abnormal AHI and symptoms of EDS, is present in 2% of women and 4% of men living in Western communities.

The Asian continent is heavily populated, and many groups live in an underdeveloped environment. These factors pose some difficulties in assessing the disease burden in this area. As expected most studies came from developed countries such as Japan, China, Singapore, Turkey and others. Interestingly, no epidemiologic studies have been conducted in Israel, despite the many works performed there in the area of OSA pathophysiology.

The retrieved studies were divided into two main groups: studies using sleep questionnaires and those studies using various sleep studies including PSG. Community studies are more likely to portray epidemiology with better accuracy than single center hospital studies. Second, hospital studies usually enroll patients with a high pre-test probability of diagnosis, which is true for studies using questionnaires/symptomatology as well as PSG studies. Given the latter concern, hospital based studies were excluded from this review.

It is well known that sleep questionnaires despite being useful in assessing risk for OSA, are not interchangeable with instrumental sleep studies and cannot quantify the severity of disease. Thus, the prevalence of people at high risk for OSA based on questionnaires cannot be simply converted into the prevalence of OSA. Another flaw is that questionnaires such as the ESS cannot rule out other sleep disorders; in fact this scale was primarily invented to detect EDS [3], and not OSA or patients at high risk for OSA.

Ten community studies that used sleep questionnaires and assessed OSA associated symptoms were found. Most of these studies used modified versions of sleep questionnaires with questions regarding snoring, nocturnal apneas, EDS, daytime fatigue etc. The smallest sample was 527 in the Iranian study [15] and the largest sample was 8,483 in the Japanese study [16]. The reported prevalence of risk for OSA prevalence ranged from 4.98% to 27.3%, both in the Iranian studies [14, 15]. Male gender, older age, higher BMI, greater waist to hip ratio and neck circumference, illiteracy, alcohol intake and smoking were associated with at high risk for OSA.

Fourteen community studies using sleep monitoring were found. Some of these were two phase studies that used a sleep questionnaire and sleep monitoring. The smallest sample was 106 in the study from Singapore [33] and the largest sample was 5,020 in the Korean study [31]. The prevalence of OSA ranged from 3.7% in the Japanese study [28] to 88.81% in the Chinese study [23]. Male gender, older age, a higher BMI and waist to hip ratio, greater neck circumference, arterial hypertension, smoking, snoring and a higher ESS score were related to OSA. The striking difference in prevalence can be attributed to variations in sample size and different populations studied, since some predominantly assessed patients with OSA related symptoms such as snoring, witnessed apnea etc.

Since the studies were of different methodological quality, tested different populations, used various types of sleep monitoring to assess OSA and many countries lack any epidemiologic data, it is particularly difficult to extrapolate the data to the global OSA/OSAS prevalence in Asia. However, the studies performed by Ip et al. [20, 21], Hui et al. [22] and Reddy et al. [30] are likely to be representative of their studied populations. Based on these results it is likely that average prevalence of OSA is around 7%, and OSAS prevalence is around 3.5% in Hong- Kong and 13.74% for OSA and 3.57% for OSAS in India.

Several abstracts from the databases mentioned in the “Search strategy and selection criteria” were found relevant for discussion. We used the same sample size cut off for abstract reports as for regular articles. However, it is necessary to note that it is difficult to analyze the abstracts in a thorough fashion due to limited word count. Pablo et al. screened 458 Philippine medical students with Berlin questionnaire and ESS [34]. These researchers showed that approximately 75.9% of studied participants had symptoms of EDS, but no data was provided on the Berlin questionnaire scores. It is essential to mention that their sample of medical students may not represent a general Philippine population. Liu et al. interviewed 666 patients undergoing anesthesia with Berlin questionnaire at West China Hospital of Sichuan University, China [35]. These investigators found that 11.7% were found to be at high risk for OSA. The individuals at high risk for OSA had a greater prevalence of high blood pressure, snoring and EDS. Ardic et al. screened 5,021 adults (2,598 women) with Berlin questionnaire in Ankara, Turkey [36], abstract 0418. These researchers showed that 13.7% of the screened population was at high risk for OSA. Li et al. retrospectively analyzed the PSG data of 2,335 individuals (1,960 men) who were suspected to have OSA [36], abstract 0466 in West China Hospital of Sichuan University, China. They showed a greater prevalence of OSA among men. Since, the above study was assessing the PSG of the patients who likely were initially at high risk for OSA, it is impossible to extrapolate this data to general Chinese population. Chang et al. studied 284 subjects with snoring problems with home sleep monitoring [36], abstract 0467. These investigators found that 61.3% of individuals had SDB. Since, these individuals had complaints of snoring before the actual sleep testing; they were at high risk for OSA, which make this finding inapplicable to general Chinese population.

Overall, based on the present studies it is difficult to estimate the potential prevalence of OSA/OSAS in the general population in these countries, because of the heterogeneity of the subjects and methods used to assess patients at risk and different types of sleep monitoring devices used.

Moreover, there are still no prevalence data for most Asian countries, including Bangladesh, Mongolia, Syria, former Soviet Union countries located in Asia and others. Future studies should recruit patients, who are generally considered to be at low risk for OSA to give a better understanding of the OSA burden on the Asian continent. This will give a better insight into the prevalence of OSA in the general Asian population. Therefore, more studies are needed to provide a better knowledge on the OSA burden in the Asian continent.

Many published studies assessed people with high pre-test probability of OSA; thus their results may overestimate the true burden of disease. Therefore, more studies are needed to improve our knowledge on the OSA burden in Asia.