Obstructive sleep apnea has long been observed. A Community Study of Sleep- Disordered Breathing in Middle-Aged Chinese Women in Hong Kong*

Transcription

1 A Community Study of Sleep- Disordered Breathing in Middle-Aged Chinese Women in Hong Kong* Prevalence and Gender Differences Mary S. M. Ip, MD, FCCP; Bing Lam, MRCP, FCCP; Lawrence C. H. Tang, MD; Ian J. Lauder, PhD; Toi Yan Ip, MPhil; and Wah Kit Lam, MD, FCCP Study objectives: To investigate the prevalence of sleep-disordered breathing (SDB) and obstructive sleep apnea syndrome (OSAS) in community-based, middle-aged Chinese women, and to compare the differences between gender with a similar study in men. Design: A cross-sectional study conducted in Hong Kong from 1998 to Setting: Sleep questionnaires were distributed to women (30 to 60 years old) in three offices and two community centers. All were invited to undergo full polysomnography in a sleep laboratory. Participants: Questionnaires were distributed to 1,532 women, and 854 questionnaires were returned. Polysomnography was conducted in 106 respondents. Measurements and results: Conservative estimated prevalence of SDB (apnea-hypopnea index [AHI] > 5) and OSAS (AHI > 5 plus excessive daytime sleepiness [EDS]) were 3.7% and 2.1%, respectively. Age-specific prevalence of OSAS was 0.5%, 2.2%, and 6.1% in the 30- to 39-year-old, 40- to 49-year-old, and 50- to 60-year-old age groups, respectively. Stepwise multiple logistic regression analysis identified body mass index (BMI) and age as predictors of SDB. Compared to Chinese men, the prevalence of SDB and OSAS in women was lower, but the gender difference decreased with age. The AHI of affected women was also significantly lower despite comparable BMI. Compared to men, women with SDB had same degree of self-reported snoring and a similar degree of EDS despite the lower AHI. Conclusions: This study demonstrated an estimated prevalence of OSAS at 2.1% among middleaged Chinese women in Hong Kong, with a 12-fold rise from the fourth to the sixth decade of life. BMI and age were significant independent predictors of SDB. Compared to men, women with SDB had lower AHIs, despite similar BMIs. (CHEST 2004; 125: ) Key words: middle-aged Chinese women; polysomnography; sleep apnea Abbreviations: AHI apnea-hypopnea index; BMI body mass index; CI confidence interval; EDS excessive daytime sleepiness; OR odds ratio; OSAS obstructive sleep apnea syndrome; REM rapid eye movement; SDB sleep-disordered breathing *From the Departments of Medicine (Drs. M.S.M. Ip, B. Lam, W.K. Lam and Ms. T.Y. Ip) and Statistics and Actuarial Science (Dr. Lauder), The University of Hong Kong; and Department of Obstetrics and Gynecology (Dr. Tang), Kwong Wah Hospital, Hong Kong, SAR, China. This study was supported by the Competitive Earmarked Research Grant No. HKU457/96M from the Hong Kong Research Grants Council, Hong Kong. Manuscript received March 19, 2003; revision accepted August 20, Obstructive sleep apnea has long been observed in clinical practice to be more common in men, 1 3 although it has been documented in population studies 4 6 that the male dominance of sleepdisordered breathing (SDB) may not be as high, suggesting an underpresentation and/or underrecognition of this problem in women in the clinic population. SDB may also have different prevalence and risk factors in different communities and ethnic groups. We have previously reported that 4.1% of middle-aged Chinese men in a community cohort of office workers in Hong Kong had symptomatic obstructive sleep apnea. 7 In this article, we report the prevalence of SDB in a community-based population Reproduction of this article is prohibited without written permission from the American College of Chest Physicians ( permissions@chestnet.org). Correspondence to: Mary S. M. Ip, MD, FCCP, Department of Medicine, The University of Hong Kong, 4/F, Professorial Block, Queen Mary Hospital, Pokfulam Rd, Hong Kong, SAR, China; msmip@hkucc.hku.hk CHEST / 125 / 1/ JANUARY,

2 of women aged 30 to 60 years, and compare their demographic, anthropometric, and clinical features with those of male counterparts. 7 Materials and Methods This study was performed in Hong Kong. Data were collected from July 1998 to November The study protocol has been previously described in the report on SDB prevalence in middleaged men. 7 Briefly, community-based subjects were approached with a questionnaire, and were also invited to undergo polysomnography. Anthropometric measurements and BP were measured, and blood for fasting lipids and glucose testing was obtained (results on analysis of these data will be separately presented). Interpretation of sleep studies and assessment of daytime sleepiness were all made similarly. The study was approved by the Ethics Committee of The University of Hong Kong. Sample The target population was women 30 to 60 years old in the community. Questionnaires (modified from National Auxillary Publications Services document No ), similar to that used in a study of men, with addition of questions on menopause and hormone replacement therapy, were distributed to the office staff of three public institutions, and to visitors of two community centers in Hong Kong. The public institutions were government departments for non health-related functions, while the community centers were operated by nongovernment organizations that provided a variety of welfare activities for the community. Collection of Polysomnographic Data Studies were conducted at the sleep laboratory at Queen Mary Hospital. Polysomnography (Alice 3 System; Respironics, Pittsburgh, PA), consisting of continuous polygraphic recording from surface leads for EEG, electrooculography, electromyography (chin and legs), ECG, thermistors for nasal and oral airflow, thoracic and abdominal impedance belts for respiratory effort, pulse oximetry for oxyhemoglobin level, tracheal microphone for snoring, and sensors for sleep position. Interpretation of Polysomnographic Data Polysomnographic records were scored manually. Sleep data were scored according to standard criteria. 8 An abnormal breathing event during objectively measured sleep was defined in the same way as the study in men 7 for the benefit of comparability, according to the criteria of either a complete cessation of airflow lasting 10 s (apnea) or a discernible reduction in airflow accompanied by a decrease of 4% in oxyhemoglobin saturation (hypopnea). The average number of episodes of apnea and hypopnea per hour of sleep (apnea-hypopnea index [AHI]) was calculated as the summary measurement of SDB. Arousals were identified according to established criteria. 9 Assessment of Daytime Sleepiness Daytime sleepiness was assessed with four subjective questions. Using a 5-point scale (0 to 4), the subjects rated the following: how often they felt excessively sleepy during the daytime; how often they felt unrefreshed or tired during the day, regardless of how long they had slept; how often they fell asleep or dozed off momentarily while watching television, reading, or at meeting/church; and how often they felt sleepy while driving. The answer was considered positive if the score was 2. Subjects were identified as having excessive daytime sleepiness (EDS) if they gave a positive response to three of the four questions. Definition of SDB and Obstructive Sleep Apnea Syndrome The minimum criterion for SDB was AHI 5, and data for three AHI cutoff threshold values at 5, 10, and 15, respectively, are presented. Similarly, the minimum criterion for diagnosis of obstructive sleep apnea syndrome (OSAS) was AHI 5inthe presence of EDS (as defined by the questions above), and data for three cutoff AHI values are presented. Calculation of Prevalence This was calculated as previously described. 7 Briefly, questionnaire respondents were classified as habitual snorers and nonhabitual snorers based on self-reported symptoms. Within each group, the age and body mass index (BMI) of those who underwent polysomnography and those who did not were compared. If significant differences were found, a conservative estimate was adopted, treating the subjects with SDB documented by polysomnography as the only subjects with SDB in the entire corresponding questionnaire group. The overall prevalence of SDB in the entire cohort would be calculated as follows: (estimated No. of subjects with SDB among snorers and nonsnorers)/(total No. of questionnaire respondents) 100%. Age-adjusted and BMI-adjusted prevalence rates were calculated by categorizing subjects into three age groups (30 to 39 years, 40 to 49 years, 50 to 60 years) and whether their BMIs were 23 and 23, where 23 is the recently proposed threshold BMI value for overweight Asians. 10 Statistical Analysis Descriptive statistics were used to summarize subject characteristics and questionnaire data. Comparison between groups was done with the Student t test (two sided) for continuous variables and 2 test for discrete variables; p 0.05 was considered statistically significant. To adjust the effect of various factors on the likelihood of snoring and SDB developing, multiple logistic regression analysis was employed. This analysis included all variables that were found to be significant in the respective between-group comparisons (Tables 1, 2). Stepwise logistic regression was used to determine the principal covariates affecting SDB. Comparison of anthropometric parameters, polysomnography data, and symptoms were compared between subjects in this study and their male counterparts in the previous reported study. 7 Data from both men and women were pooled for analysis of the association of SDB with obesity, reflected by body habitus parameters. Using multiple logistic regression, a separate model was fitted for each measure of body habitus because of multicollinearity. Age and sex were included in all the models. Furthermore, the risks of SDB associated with an increment of 1 SD in body habitus parameters were calculated, 4 and the odds ratios (ORs) and their 95% confidence intervals (CIs) were compared between this Chinese population and that reported in white subjects. 4 All analyses were done with the SPSS package (release 10.0 for Windows; SPSS; Chicago; IL). 128 Clinical Investigations

3 Table 1 Different Features Between Snorers and Nonsnorers* Variables Questionnaire Data Habitual Snorers Nonsnorers Total Subjects, No Age, yr BMI EDS 48 (39) 177 (25) 225 (27) Sleep problems Sleep choking 49 (40) 195 (27) 244 (29) Witnessed breathing 45 (36) 108 (15) 153 (18) irregularity Disruptive movement 58 (47) 202 (28) 260 (31) Wake up repeatedly 64 (52) 283 (40) 347 (41) Wake up too early in the 63 (51) 289 (40) 352 (42) morning and cannot get back to sleep Morning headaches 46 (37) 170 (24) 216 (26) Nasal congestion 48 (39) 142 (20) 190 (23) Leg movements 36 (29) 112 (16) 148 (18) Medical conditions Coronary heart disease 1 (0.8) 3 (0.4) 4 (0.5) Hypertension 23 (19) 35 (5) 58 (7) Diabetes mellitus 8 (6.5) 2 (0.2) 10 (1.2) Stroke 1 (0.8) 2 (0.2) 3 (0.4) *Data are presented as mean SD or No. (%) unless otherwise indicated. p 0.001, habitual snorers vs nonhabitual snorers by t test or 2 test where appropriate. p 0.05, habitual snorers vs nonhabitual snorers by t test or 2 test where appropriate. Results Questionnaires were distributed to 1,532 women, and 854 were returned (56%); 839 completed questionnaires were analyzed. The questionnaire respondents had age distribution and corresponding BMIs that were similar to that of the general community. 11,12 Their demographics, sleep-related symptoms, and medical history are shown in Table 1. Eighty-one percent were in the working population, of whom a vast majority were office workers with a secondary education level. The percentage of self-reported habitual snorers was 15%, and the mean age, BMI, and features suggestive of sleep apnea were significantly higher among habitual snorers (Table 1). Both habitual snorers and nonhabitual snorers showed significant difference in BMI between subjects who came for polysomnography and those who did not (snorers, p 0.046; nonsnorers, p 0.001). Stepwise logistic regression identified history of hypertension, witnessed breathing irregularity, nasal congestion at night, BMI, and leg movements during sleep as significant correlates of habitual snoring. Polysomnographic Data Table 2 Correlates of SDB* Variables AHI 5 AHI 5 Subjects, No Age, yr BMI Neck circumference, cm Waist, cm Hip, cm Waist/hip ratio Sum of skinfold thickness, mm Systolic BP, mm Hg Diastolic BP, mm Hg EDS 36 (49) 18 (58) Sleeping hours Habitual snorer 29 (39) 14 (45) Hypertension 8 (11) 6 (19) Witnessed apnea 23 (31) 7 (23) Partner moved due to snoring 4 (5) 4 (13) Time to fall asleep, min Epworth sleepiness scale Menopause 13 (18) 11 (35) *Data are presented as mean SD or No. (%). p by t test or 2 test. p 0.05 by t test or 2 test. One hundred six questionnaire respondents came for polysomnography. Ten subjects were receiving antihypertensive medications and/or antidiabetic medications, 3 subjects took sedatives/hypnotics more than five times a month, and 6 subjects were receiving miscellaneous medications, including nasal steroids, antihistamines, antiulcer drugs, and thyroxine replacement. Of the 106 polysomnographic evaluations, 1 was rejected due to a technical fault in the recording. The distribution of sleep stages of the polysomnography-positive (AHI 5) and polysomnographynegative groups were similar, but those with AHI 5 had more snoring (p 0.001). Prevalence of SDB and OSAS Among the 105 patients who underwent polysomnography, 31 subjects (30%), 16 subjects (15%), and 10 subjects (10%) had AHIs 5, 10, and 15, respectively. Due to the significant difference in BMI of the subjects who came for polysomnography and those who did not, the projection of the prevalence of SDB was based on the conservative estimate (ie, assuming no subject in the no-polysomnography group would have SDB). Since the age distribution and BMI of the questionnaire respondents were similar to those of the general middle-aged female population (30 to 60 years old) in Hong Kong, it would be acceptable to use the questionnaire respondents as the denominator of evaluation. Hence, CHEST / 125 / 1/ JANUARY,

4 the estimated prevalence of OSAS in the entire female cohort 30 to 60 years old would be 2.1% (Fig 1), with the age-specific prevalence of OSAS rising from 0.5 to 6.1% (Fig 2). At various cutoff points of AHI, estimated prevalence of SDB and OSAS were, respectively, 3.7% and 2.1% (AHI 5), 1.9% and 1.4% (AHI 10), and 1.2% and 0.8% (AHI 15) [Fig 1]. Factors Associated With SDB The significant correlates of SDB in the polysomnography subjects are shown in Table 2. Age, BMI, neck circumference, waist circumference, hip girth, BP measurements, and menopause status correlated with AHI, but not EDS. Stepwise logistic regression selected and retained BMI and age as the principal covariates. Figure 2 shows the age-specific prevalence of SDB and OSAS in the target population. An increasing trend for SDB and OSAS with age was present, and this was seen in subjects with BMI 23 and BMI 23 (Table 3). The prevalence of both SDB and OSAS in subjects 50 to 60 years old was markedly higher than in subjects 30 to 39 years old, in both the normal weight and overweight subjects (Table 3). Table 4 shows the ORs estimating the increased risk of SDB associated with an increment of 1 SD in the value of the specific measure of body habitus for obesity, using pooled data of men and women, compared to that of a previous study in white subjects. 4 The intervals overlapped for all covariates Figure 2. Age-specific prevalence rates of SDB (AHI 5) and OSAS (AHI 5 and EDS) in women. apart from waist-hip ratio. Thus the OR for this covariate was regarded as significantly different (p 0.05). Comparison of Men and Women Figure 3 shows the total and age-specific prevalence of OSAS of men and women. Women had a lower prevalence than men across all age groups, although the difference between the two sexes was much larger in the 30- to 39-year-old age group compared to the 50- to 60-year-old age group. The mean AHIs of women with SDB (Fig 4) were lower than those of the men as a total and in corresponding age groups, despite similar BMIs (Fig 5). Compared to men, women had relatively higher rapid eye movement (REM)-AHI in proportion to their total AHI: total AHI in women, (mean SD); REM-AHI, ; total AHI in men, ; REM-AHI, In terms of symptoms, women and men with SDB had the same degree of self-reported snoring (women, 45%; men, 61%; not significant) and similar degree of daytime sleepiness as represented by Epworth sleepiness scale (women, ; men, ). Discussion Figure 1. Prevalence rates using various cutoff points for AHI. SDB is defined as AHI 5, 10, or 15. OSAS is defined as EDS plus AHI 5, 10, or 15. In this community-based study of the prevalence of SDB among middle-aged women assessed by complete polysomnography, we conservatively esti- 130 Clinical Investigations

5 Age, yr Subjects, No. (%) Table 3 Prevalence of SDB and OSAS in Different Age and BMI Groups Prevalence of SDB, Mean (CI) Prevalence of OSAS, Mean (CI) Total BMI 23 BMI 23 Total BMI 23 BMI (44.8) 1.1 ( ) 0 ( ) 4.3 ( ) 0.5 ( ) 0 ( ) 2.2 ( ) (37.5) 3.8 ( ) 1.1 ( ) 7.8 ( ) 2.2 ( ) 1.1 ( ) 3.9 ( ) (17.6) 10.1 ( ) 4.0 ( ) 16.4 ( ) 6.1 ( ) 2.7 ( ) 9.6 ( ) Total 839 (100) 3.7 ( ) 0.9 ( ) 8.8 ( ) 2.1 ( ) 0.7 ( ) 4.7 ( ) mate that approximately 3.7% of this cohort had SDBatAHI 5, and 2.1% were symptomatic with daytime sleepiness. SDB among women has received growing recognition in recent years. Population studies 4,5 have demonstrated a prevalence of symptomatic obstructive sleep apnea of 1 to 2% among middle-aged white women. Our study presents the first data of SDB and OSAS in a Chinese community based on polysomnographic documentation. This study, very similar in design to that of the study 7 in men, has a similar limitation of potential bias introduced by the partial questionnaire response rate and restricted polysomnography participation rate. However, the questionnaire respondents had an age distribution within the target age strata and age-specific BMIs similar to those of the local population, allowing the results to be projected to the general population. The proportion of office workers in this study sample was higher than that of the general population in the same age strata. The effect of occupation or educational level on the prevalence of sleep apnea has not been well reported. These factors might affect the occurrence of SDB indirectly, through lifestyle and behavioral patterns that in turn influence body weight control. Since the average BMI of study subjects was similar to that of the general population, these differences in demographics probably would not affect the validity of generalization of the prevalence data to the community at large. Medications such as sedatives, tricyclic antidepressants, and stimulant drugs may alter sleep architecture and occurrence of SDB. A number of drugs, including sedatives, antidepressants, antihistamines, and lipophilic -blockers, may result in daytime sleepiness that confounds the symptoms of sleep apnea. Drug effect was unlikely to have affected the findings in this study, as there was no significant difference in the frequency of the use of these drugs between subjects with and without SDB/OSAS. Since the polysomnography participants had many more associated features of SDB than those who did not undergo polysomnography, we have adopted the conservative estimate of prevalence, assuming that all those who did not undergo polysomnography did not have SDB. This is likely to result in some underestimation of the prevalence. The estimated prevalence of OSAS at 2.1% in Chinese women 30 to 60 years old is similar to that of studies 4,6 of white subjects utilizing similar diag- Table 4 Relative Risks for SDB and Measures of Body Habitus* SD of Covariate OR for a 1-SD Increment in the Covariate 95% CI Measures Hong Kong United States Hong Kong United States Hong Kong United States BMI, kg/m Girth, cm Neck Waist Hip Waist/hip Skinfold thickness, mm Biceps Triceps Suprailiac Subscapular *Hong Kong pooled data of men and women in this study in Hong Kong; United States data of men and women in the Wisconsin study. 4 SDB was defined as AHI 5. The ORs are for the comparison with subjects in the same study who were not habitual snorers and whose AHIs were 5. All the logistic regression models included variables for age in years and gender. The CIs for waist/hip ratio for Hong Kong and US subjects do not overlap, and the ORs would be regarded as significantly different. CHEST / 125 / 1/ JANUARY,

6 Figure 3. Age-specific prevalence rates of OSAS (AHI 5 and EDS) in middle-aged Chinese men and women. nostic criteria. It is also comparable to that of a study 13 in Asian women in Singapore utilizing a diagnostic triad of self-reported snoring, sleep symptoms of apnea/hypopnea, and/or hypertension/wide neck circumference that arrived at an estimated SDB prevalence of 2.3% in female subjects 40 to 59 years old. As in men, higher BMI and advancing age were Figure 4. AHI in men and women with SDB in different age groups. Figure 5. BMI in men and women with SDB in different age groups. predictive of the presence of SDB in women. The prevalence of OSAS increased nearly 12-fold in the 50- to 60-year-old group, compared to the 30- to 39-year-old group. Although men also showed increasing prevalence with age, the escalation in the sixth decade was much less marked. It also appeared that within this middle-aged population, younger men and women with SDB were more obese than those who were in their fifties. This may reflect an increasing influence of advancing age on the development of SDB as found in other studies 14 that focus on elderly population. Since women have a distinct change in hormonal status relating to menopause, which corresponds temporally to age, it is tempting to speculate that this increase in prevalence of SDB, despite a lower BMI, may be related to menopause. However, there were not enough women above or below the age threshold of 50 years with different menopause status in our study population, and analysis of the prevalence of OSAS in relation to menopausal status independent of age was not possible. It has also been reported that prevalence of SDB in postmenopausal women receiving hormonal replacement therapy was similar to that of premenopausal women, 15 but again there were too few women receiving hormonal replacement therapy in this study population to make any meaningful analysis. Although the prevalence of SDB increased with age, the group mean AHI across the 3 decades was similar, which differed from the finding of higher AHI in postmenopausal women in a previous study Clinical Investigations

7 Obesity is a significant risk factor for SDB in white populations. 4,6,17,18 In this study in women and the previous study 7 in men, higher BMI is a major risk factor for SDB in Chinese subjects as well. Using the definition of BMI 23 in Asians as the overweight threshold, 10 our subjects with SDB, whose mean BMI was 27, were definitely obese by peer comparison. In comparison with white subjects in the Wisconsin study, 4 Chinese subjects had a lower risk attributable to central obesity, as indicated by the significant difference in the waist:hip ratios, and a pattern of lower risks relating to other obesity indexes such as BMI, neck circumference, and subcutaneous fat. This finding supports the notion that risk factors apart from obesity, such as craniofacial structure, 19 may be of greater pathogenic importance in Chinese compared to white populations. Apart from a lower prevalence of SDB in women, gender differences in features of OSA have been reported in studies of white subjects. Consistent with other data, our study showed that the severity of SDB as indicated by AHI in Chinese women was mostly in the mild range. The overall male:female ratio for OSAS was 2:1, but increased from 1.3:1 in the group with mild OSA (AHI 5 to 15) to 9:1 in the group with severe OSA (AHI 30) [data not shown]. It is also observed that women with SDB had lower AHIs compared to male counterparts despite a highly comparable BMIs between men and women. Similar to a previous Canadian study, 20 AHI during REM sleep relative to the total AHI was higher in women, although this may just reflect that women had less severe obstructive sleep apnea. The Wisconsin Sleep Cohort Study 23 reported that white women with SDB had similar symptoms as those of men. In our study, Chinese women with SDB had a similar degree of snoring and daytime sleepiness as men. The gender disparity of SDB in the clinical setting 24 and this community study suggests that SDB is underrecognized in women, despite similar symptoms as those in men. Conclusion To the best of our knowledge, this is the first report of prevalence of OSAS in middle-aged Chinese women using full overnight polysomnography. The study demonstrated that symptomatic OSAS affects at least 2.1% of this community-based population between 30 years and 60 years old, and prevalence increases markedly in the sixth decade of life. BMI and age were significant predictors of SDB. Compared to their male counterparts, the overall severity of SDB, as indicated by AHI in women, was significantly lower, despite comparable BMI. Women also had a similar degree of daytime sleepiness as male counterparts, despite lower AHI. Compared to white subjects, the relative risks of acquiring SDB in Chinese middle-aged subjects conferred by obesity is probably less. ACKNOWLEDGMENT: The authors thank Ms. Wendy Mok for assistance in statistical analysis. We also gratefully acknowledge the institutions and the community centers for allowing us to invite their staff/attendees to participate in the study. References 1 Guilleminault C, Quera-Salva MA, Partinen M, et al. Women and the obstructive sleep apnea syndrome. Chest 1993; 1: Vgontzas AN, Tan TL, Bixler EO, et al. Sleep apnea and sleep disruption in obese patients. Arch Intern Med 1994; 154: Block AJ, Boysen PG, Wynne JW, et al. Sleep, hypopnea and oxygen desaturation in normal subjects. N Engl J Med 1979; 300: Young T, Palta M, Dempsey J, et al. The occurrence of sleep-disordered breathing among middle-aged adults. N Engl J Med 1993; 328: Redline S, Kump K, Tishler PV, et al. Gender differences in sleep disordered breathing in a community-based sample. Am J Respir Crit Care Med 1994; 149: Olson LG, King MT, Hensley MJ, et al. A community study of snoring and sleep-disordered breathing: prevalence. Am J Respir Crit Care Med 1995; 152: Ip MSM, Lam B, Lauder IJ, et al. A community study of sleep disordered breathing in middle-aged Chinese men in Hong Kong. Chest 2001; 119: Rechtschaffen A, Kales AA, eds. A manual of standardized terminology, techniques and scoring system for sleep stages of human subjects. National Institutes of Health publication No Washington, DC: Government Printing Office, American Sleep Disorders Association. EEG arousals: scoring rules and examples: ASDA report. Sleep 1992; 15: International Diabetes Institute, World Health Organization. The Asia-Pacific perspective: redefining obesity and its treatment. Melbourne, Australia: Health Communications Australia Pty Ltd, 2000; Hospital authority statistical report, 1998/99. Hong Kong, China: Hong Kong Hospital Authority, Ko GT, Wu MM, Tang J, et al. Body mass index profile in Hong Kong Chinese adults. Ann Acad Med Singapore 2001; 30: Ng TP, Seow A, Tan WC. Prevalence of snoring and sleep breathing-related disorders in Chinese, Malay and Indian adults in Singapore. Eur Respir J 1998; 12: Young T, Peppard PE, Gottlieb DJ. Epidemiology of obstructive sleep apnea. Am J Respir Crit Care Med 2002; 165: Bixler EO, Vgontzas AN, Lin HM, et al. Prevalence of sleep-disordered breathing in women: effects of gender. Am J Respir Crit Care Med 2001; 163: Dancey DR, Hanly PJ, Soong C, et al. Impact of menopause on the prevalence and severity of sleep apnea. Chest 2001; 120: Bearpark H, Elliott L, Grunstin R, et al. Snoring and sleep apnea: a population study in Australian men. Am J Respir Crit Care Med 1995; 152: Ohayon M, Guillemenault C, Priest RG, et al. Snoring and breathing pauses during sleep: telephone interview survey of a United Kingdom population sample. BMJ 1997; 314: CHEST / 125 / 1/ JANUARY,

8 19 Ferguson KA, Takashi O, Lowe AA, et al. The relationship between obesity and craniofacial structure in obstructive sleep apnea. Chest 1995; 108: O Connor C, Thornley KS, Hanly PJ. Gender differences in the polysomnographic features of obstructive sleep apnea. Am J Respir Crit Care Med 2000; 161: Mohsenin V. Gender differences in the expression of sleepdisordered breathing: role of upper airway dimension. Chest 2001; 120: Walker RP, Durazo-Arvizu R, Wachter B, et al. Preoperative differences between male and female patients with sleep apnea. Laryngoscope 2001; 111: Young T, Hutton R, Finn L, et al. The gender bias in sleep apnea diagnosis: are women missed because they have different symptoms? Arch Intern Med 1996; 156: Ip MSM, Tsang KWT, Lam WK, et al. Obstructive sleep apnea syndrome: an experience in Chinese adults in Hong Kong. Chin Med J 1998; 111: Clinical Investigations