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* From the Departments of Medicine (Drs. Ip, Lam, Tsang, and Lam, and Ms. Mok), Statistics and Actuarial Science (Dr. Lauder), and Psychiatry (Dr. Chung), The University of Hong Kong, Hong Kong, SAR, China.
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; e-mail: msmip{at}hkucc.hku.hk
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Abstract |
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 TOP Abstract IntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Methods: Sleep questionnaires were
distributed to 1,542 men (age range, 30 to 60 years), and 784
questionnaires were returned. Subsequently, full polysomnographic (PSG)
examinations were conducted in 153 questionnaire respondents. Subjects
with an apnea-hypopnea index (AHI) 
5 were recalled for clinical
assessment.
Results: Questionnaire respondents were
similar in age and body mass index (BMI) to the general community in
the target age range and gender. Habitual snoring was reported by 23%
of this cohort and was associated with excessive daytime sleepiness
(EDS), hypertension, witnessed abnormal breathing pattern, BMI, and leg
movements during sleep. Allowing for subject bias in undergoing PSG,
the estimated prevalence of SDB and obstructive sleep apnea syndrome
(OSAS) (defined as SDB in the presence of EDS) at various AHI cutoff
threshold values was 8.8% and 4.1% (AHI 5), 6.3% and 3.2% (AHI
10), and 5.3% and 3.1% (AHI 15). Multiple stepwise logistic
regression analysis identified BMI, habitual snoring, time taken to
fall asleep, and age as predictors of SDB at AHI 5. Analysis of
anthropometric parameters indicated that the relative risk of OSAS
attributable to obesity was less than in white subjects.
Conclusion: This community-based study of sleep apnea among
middle-aged men in Hong Kong using full PSG demonstrated an estimated
prevalence of OSAS (AHI 5 and EDS) at 4.1%. Increasing BMI and age
were associated with SDB, although factors other than adiposity may
also have an important pathogenic role in OSA in Chinese
subjects.
Key Words: middle-aged Chinese men • polysomnography • sleep apnea
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Introduction |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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To date, there has been scanty documentation of the prevalence of SDB in Asians. Reported data were based on symptoms,15 portable sleep monitoring systems,16 17 or cohorts of subjects with specific medical problems.18 19 The main objective of this study was to obtain an estimate of the prevalence of SDB documented with polysomnography (PSG) in middle-aged Chinese men, using a cohort of male office-based workers in Hong Kong as a model. The secondary objective was to determine the clinical and anthropometric features that were predictive of SDB in this cohort.
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Materials and Methods |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Sample
The target population included male office-based workers in
three public institutions in Hong Kong. We distributed a questionnaire
(modified from NAPS Document No. 05017)3
to men
aged 30 to 60 years, to be filled out by the subjects. It consisted of
questions concerning demographic data, sleep habits, snoring history,
breathing irregularities and leg movements during sleep, daytime
sleepiness, smoking and drinking history, medical history, family
history of sleep apnea, and job nature. All subjects were invited at
the same setting to undergo sleep study, and those who volunteered were
contacted for subsequent PSG. The study was approved by the Ethics
Committee of The University of Hong Kong.
Collection of PSG Data
Studies were conducted at the sleep laboratory at Queen Mary
Hospital. Measures of body habitus were recorded by standard
anthropometric methods.20
BP was checked in the morning
after PSG. PSG (Alice 3 System; Healthdyne; Atlanta, GA)
consisted of continuous polygraphic recording from surface leads for
EEG, electro-oculography, electromyography, 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 leg and sleep position.
Interpretation of PSG Data
PSG records were scored manually. Sleep data were scored
according to standard criteria.21
An abnormal breathing
event during objectively measured sleep was defined according to the
commonly used clinical 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 (the apnea-hypopnea index [AHI]) was calculated as the
summary measurement of SDB. Arousals were identified according to
established criteria.22
Assessment of Daytime Sleepiness
Daytime sleepiness was assessed with four subjective questions.
Using a 5-point scale (0 to 4), the subjects rated how often they (1)
felt excessively sleepy during the daytime; (2) felt unrefreshed or
tired during the day, regardless of how long they had slept; (3) fell
asleep or dozed off momentarily while watching TV, reading, or at
meetings/church; and (4) 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 OSAS
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 OSAS was
AHI 5 in the presence of EDS (as defined by the questions above),
and data for three cutoff AHI values are presented .
Clinical Assessment of Subjects
All subjects who had an AHI 5 on PSG (n = 64) were
recalled for assessment. In particular, symptoms of OSAS and other
vascular risks were reassessed. Subjects were given management advice
along the usual code of practice in our medical center, which was
adapted from current opinion.23
24
Calculation of Prevalence
Questionnaire respondents were classified into two groups based
on the self-reported presence of snoring: group 1, habitual snorers
(snoring frequency 3 days per week; group 2, nonhabitual snorers
(snoring frequency < 3 days per week). Those who were not sure were
classified as nonhabitual snorers.
About 20% of the questionnaire respondents had PSG performed, and the PSG subjects were considered sample subjects for their corresponding groups (snorers or nonsnorers) who only responded to questionnaire. The mean age and body mass index (BMI) of sample subjects were compared with those who did not have PSG in the respective snoring category. If no significant difference was found, the prevalence rate of SDB in the PSG group of snorers or nonsnorers would be considered representative of the corresponding questionnaire respondents group.25 If a significant difference was found between the PSG group and no-PSG group, a conservative estimate was adopted, treating the SDB subjects documented by PSG 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: the estimated number of subjects with SDB among snorers and nonsnorers/total number of questionnaire respondents x 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, and 50 to 60 years) and whether their BMI was < 23 or 23,
where 23 is the recently proposed threshold BMI value for overweight in
Asians.26
Statistical Analysis
Descriptive statistics were used to summarize subject
characteristics and questionnaire data. Comparison between groups was
done with the Student’s t test for continuous variables and
Pearson’s 
2 test for discrete variables. All
significance tests were two sided, and a value of p < 0.05 was
considered statistically significant. To adjust the effect of various
factors on the likelihood of developing snoring and SDB, a multiple
logistic regression analysis was employed. This analysis included all
variables that were found to be significant in the respective
between-group comparisons (Table 1
).
Stepwise logistic regression was used to determine the principal
covariates affecting SDB.
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Results |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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The questionnaire respondents had a mean age of 41 years and mean BMI of 23.9, which were similar to those of the general community for the target age group (30 to 60 years) and gender (men) at approximately 42 years of age and 24 BMI, respectively.27 28 29 The percentage of self-reported habitual snorers was 23%, and the mean age, BMI, and features suggestive of sleep apnea were significantly higher among habitual snorers (Table 1) . Both snorers and nonsnorers showed significant difference in BMI (p < 0.001) between subjects who came for PSG and those who did not. Stepwise logistic regression identified EDS, history of hypertension, witnessed breathing abnormality, BMI, and leg movements during sleep as significant correlates of habitual snoring.
PSG Data
One hundred fifty-three questionnaire respondents came for PSG.
Among those who did not come, 90 respondents were contacted by
telephone; the reasons given for refusal were the inconvenience of the
test, and their subjective assessment that they had no sleep problem.
Of 153 polysomnograms, 3 were rejected: 2 due to suboptimal technical
recording and 1 due to insufficient sleep time (< 4 h). The
distributions of sleep stages of the PSG-positive group (AHI 5) and
PSG-negative group were similar, but those with AHI 5 spent more
time sleeping on their backs, had more snoring, and had higher arousal
indexes (Table 2
).
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5, 10, and
15, respectively. Since in both snorers and nonsnorers the PSG
subjects had significantly higher BMIs than the corresponding no-PSG
subjects, the projection of the prevalence of SDB was based on the
conservative estimate (ie, assuming that no subject in the
no-PSG group would have SDB). Including the four subjects with OSAS
already diagnosed, the minimum prevalence of SDB (AHI 5) and OSAS
(AHI 5 and EDS) in the original study population of 1,542 men would
be 4.4% and 2.1%, respectively. Based on the observation that the age
and BMI of the questionnaire respondents were similar to those of the
general male population in the corresponding age strata in Hong Kong,
it would be acceptable to use the questionnaire respondents as the
denominator of evaluation. This would yield an estimated prevalence of
SDB and OSAS using various cutoff points of AHI, respectively, at 8.8%
and 4.1% (AHI 5), 6.3% and 3.2% (AHI > 10), and 5.3% and
3.1% (AHI > 15; Fig 1 ).
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23.
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Clinical Reassessment of the Group With AHI 5
Sixty of the 64 subjects with SDB returned on recall. Four
subjects (all with AHI < 15) claimed no symptoms and turned down the
appointment. Of the 64 subjects, 28 subjects were initially considered
to have symptomatic SDB from questionnaire data. At clinical
reassessment, two subjects denied significant daytime sleepiness (one
of them had significant weight reduction), while the remaining 26
subjects were symptomatic and were offered nCPAP. Of these, 7 subjects
declined any treatment, 18 subjects received nCPAP, and all reported
significant symptomatic improvement, while 1 subject received oral
appliance with modest improvement. The other asymptomatic subjects all
opted for weight reduction, except two subjects who had vascular risk
factors (hypertension and diabetes mellitus) and received nCPAP and
oral appliance, respectively.
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Discussion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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5, and 4.1% were symptomatic with
daytime sleepiness.
Limitations of Study
An ideal study of prevalence involves assessment of each subject
in a large population with a "gold standard" diagnostic test
(ie, overnight PSG in this context). However, this is
extremely difficult in the assessment of SDB because the diagnostic
test is time consuming for the subject and expensive for the
investigator. Most studies have therefore elected to study a portion of
the population of interest and then estimate the prevalence of the
entire population, using either a combination of questionnaire and a
full/portable diagnostic system, or even questionnaires only. There
is a potential for bias when patients are sampled from a large
population and the results extrapolated back to the total group. In the
present study, only half of the questionnaires were returned. However,
the questionnaire respondents had mean age and BMI that were similar to
those of men in the target age strata in the local community; hence, we
believe that they were a nonbiased sample in terms of risks for SDB.
The limited participation rate for PSG was expected due to substantial
participant burden. Since the subjects came on a voluntary basis,
self-selection bias may be present and indeed some features associated
with sleep apnea were more prevalent in those who came for study.
Hence, we adopted the conservative estimate, assuming that all those
who did not undergo PSG did not have SDB, which may potentially
underestimate the prevalence of SDB.
Only a single-night study was done, and some mild cases might have been missed due to night-to-night variation; therefore, the true prevalence of SDB would be underestimated. However, several studies had shown that the mean AHI between the initial and second laboratory studies did not differ significantly.3 30 31
Our assessment of EDS was based on four questions relating to sleepiness. Although the sleepiness score thus obtained has not been validated against objective tests of sleepiness in this study, the questions were adapted from those validated in other studies.3 32 Furthermore, the four questions used in this study have been validated regarding their internal consistency, and the score thus derived correlated well with the Epworth Sleepiness Scale score,32 which was also assessed in the 150 subjects who came for PSG, as well as in another 120 subjects referred to our sleep clinic for clinical evaluation (data not shown).
Strengths of the Study
We used recommended in-laboratory full PSG for documentation of
SDB. We studied healthy office workers who were similar to the local
male population in two parameters that are important determinants of
SDB, BMI, and age; therefore, the results of the study may be
applicable to other men in the target age range.
Different from other population studies of SDB, all subjects with documented SDB have been recalled for clinical evaluation by a respiratory physician. This would allow confirmation of those subjects who suffer from clinically important SDB.
Prevalence of Snoring, SDB, and OSAS
The estimated prevalence of SDB (AHI 5) at 8.8% in Chinese
men from 30 to 60 years old is much lower than that of studies in white
subjects utilizing similar diagnostic criteria,3
4
which
reported a prevalence of about 25%. Our results are nearer to those of
a study of Chinese subjects in Singapore utilizing a diagnostic triad
of self-reported snoring, sleep symptoms of apnea/hypopnea, and/or
hypertension/wide neck circumference, which arrived at an estimated SDB
prevalence of 5.8% in the age range of 40 to 59 years.15
This would suggest that the prevalence of SDB in Chinese subjects is
really lower than that in white subjects. Despite this difference in
the prevalence of SDB, the estimated prevalence of symptomatic SDB in
our community at about 4% is very comparable to that reported in white
populations. It is unlikely that the estimation of OSAS was
factitiously high, since we have reassessed the majority of
subjects with SDB and fully evaluated their symptoms again. One
possible reason for this difference in the prevalence of SDB, compared
to white series, may be self-selection bias that favored recruitment of
PSG subjects with EDS, the main distinguishing criterion between SDB
and OSAS. This would lead to most of the OSAS subjects being identified
and only a minority of SDB subjects being identified, since those
without symptoms have not undergone PSG. The prevalence of OSAS we
found is much higher then that recently reported,17
at 0.1
to 2.3% in Chinese male and female students aged 19 years based on
limited PSG (MESAM IV; Madaus Medicin-Elektronik; Freiburg,
Germany) in a random sample of 88 of 1,901 questionnaire
respondents. It is recognized that the prevalence of sleep apnea is
higher in middle-aged subjects and higher in men then women; therefore,
the substantial difference in prevalence of OSAS reported from the two
studies is not unexpected
Correlates of SDB
Obesity is a significant risk factor for SDB in white
populations.3
4
5
6
In this study, a higher BMI was a risk
factor for SDB in Chinese subjects as well. Obesity
defined33
as BMI > 30 is not a common problem in Chinese
subjects, as evidenced by only 5% of the study subjects having BMI
> 30. However, recently, a new consensus for definition of obesity in
Asians has been proposed, with BMI 23 as the threshold at risk
value for obesity.26
The mean BMI of the present cohort at
24 is similar to that reported by other studies of random community
male subjects in the same age strata in Hong Kong.28
29
This BMI value is lower than that reported in white
communities,4
25
33
but more comparable to that of other
Chinese ethnic populations.15
Therefore, our subjects with
SDB whose mean BMI was 27 were obese by peer comparison. To further
assess the impact of obesity on SDB in our population, the relative
risks of having SDB in relation to measures of body habitus were
calculated. On comparing the odds ratios in our subjects with those
reported in the Wisconsin study,3
the risks of having SDB
due to an increase in any index of adiposity were much lower in our
population. Since the prevalence of OSAS in Chinese subjects is
comparable to that of white subjects, these findings suggest that
adiposity, although a definite risk factor for SDB in Chinese subjects,
carries less weight than it did in white subjects (Table 5)
. The other
risk factors for SDB, such as pharyngeal narrowing, retrognathia or
micrognathia, and pharyngeal collapsibility might assume greater
pathogenic significance in Chinese subjects.
The correlation between age and SDB has been studied by
different investigators with dissimilar results, with suggestion of a
rise in prevalence of SDB with age.3
4
We found a rising
trend in the prevalence of SDB as age increased, which was evident in
both BMI < 23 and 23 groups (Table 4)
.
Although SDB is a relatively common problem, there is currently no reliable screening test for the disorder. In this cohort, EDS did not correlate with SDB. Since sleepiness, regardless of the cause, probably favored self-selection for PSG, its discriminating power within this cohort would be diminished. As indicated by logistic regression analysis, parameters including higher BMI, habitual snoring, ease of falling asleep at night, and older age were predictive of the presence of SDB. The application of these easily identifiable predictive factors would assist in appropriate referral and prioritization for PSG. With the high prevalence of SDB and limited resources, this might be useful for better utilization of resources.
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Conclusion |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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Acknowledgements |
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Footnotes |
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This study was supported by the Competitive Earmarked Research Grant HKU457/96M from the Hong Kong Research Grants Council, Hong Kong.
Received for publication April 10, 2000. Accepted for publication August 9, 2000.
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References |
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TOPAbstractIntroductionMaterials and MethodsResultsDiscussionConclusionReferences |
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