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Sleep, 20(9):705-706

© 1997 American Sleep Disorders Association and Sleep Research Society

Fast Track Publication

Estimation of the Clinically Diagnosed Proportion of Sleep

Apnea Syndrome in Middle-aged Men and Women

Terry Young, Linda Evans, Laurel Finn and Mari Palta

Sleep and Respiration Research Group, Department of Preventive Medicine, University of Wisconsin-Madison, Madison,

Wisconsin, US.A.

Summary: The proportion of sleep apnea syndrome (SAS) in the general adult population that goes undiagnosed

was estimated from a sample of 4,925 employed adults. Questionnaire data on doctor-diagnosed sleep apnea were

followed up to ascertain the prevalence of diagnosed sleep apnea. In-laboratory polysomnography on a subset of

1,090 participants was used to estimate screen-detected sleep apnea. In this population, without obvious barriers to

health care for sleep disorders, we estimate that 93% of women and 82% of men with moderate to severe SAS

have not been clinically diagnosed. These findings provide a baseline for assessing health care resource needs for

sleep apnea. Key Words: Sleep apnea-Epidemiology-Sleep disorders-Prevalence.

The disparity between the high prevalence of occult

sleep apnea in the general population and the pre-

sumed low level of its clinical recognition has prompt-

ed concern that clinically significant disease is being

missed (1-3). The diagnosed prevalence, however, has

never been measured, so the magnitude of the disparity

is unknown. To address this, we determined the prev-

alences of clinically diagnosed and screen-detected

sleep apena syndrome (SAS) in the Wisconsin sleep

cohort study, an ongoing population-based study.

METHODS

The sample comprised state employees, aged 30-60

years, who resided in south-central Wisconsin, an area

served by a university-based sleep clinic. All employ-

ees had comprehensive health insurance as part of em-

ployment benefits. Data for this analysis are from a

mailed questionnaire administered to the total sample

(n = 4,925) and in-laboratory polysornnography con-

ducted, according to clinical diagnostic guidelines, on

a subsample (n = 1,090). The sampling and overnight

study protocol have been previously reported (4).

Clinically diagnosed cases of sleep apnea were as-

Accepted for publication July 1997.

Address correspondence and reprint requests to: Terry Young,

Ph.D., Department of Preventive Medicine, University of Wiscon-

sin-Madison, 504 Walnut Street, Madison, WI 53705, U.S.A.

certained by mail and telephone follow-up of partici-

pants who indicated on the questionnaire that they

were told by a doctor they had sleep apnea. Of the 49

positive respondents, 16 reported that they had been

tested or examined and were told they had sleep apnea

syndrome; of these, all but three had received treat-

ment (surgery or nasal continuous positive airway

pressure). These 16 were considered to have diagnosed

sleep apnea syndrome. Of the remaining 33 positive

J

respondents, 30 admitted they had not been examined

or diagnosed but suspected they had sleep apnea; one

was deceased, one could not be located, and one failed

to respond.

Screen-detected SAS was defined by a combination

of self-reported sleepiness ("often" or "almost al-

ways" experience excessive daytime sleepiness or do

not feel rested regardless of the hours of sleep) and

the occurrence of apneas (no airflow for at least 10

seconds) and hypopneas (40% decrease in respiratory

effort accompanied by 4% oxygen desaturation) during

sleep. Prevalences for mild to severe sleep apnea (five

or more apneas and hypopneas per hour of sleep and

daytime hypersornnolence) and moderate to severe

sleep apnea (15 or more apneas and hypopneas per

hour of sleep and daytime hypersomnolence) were es-

timated for the subsample and extrapolated to the total

sample using software for stratified samples (5).

705

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706

T. YOUNG ET AL.

TABLE 1. Comparison of adults with clinically diagnosed

and screen-detected SAS in a sample of employed men and

women aged 30-60 years (n = 4,925)

Variable

Number (prevalence)

Women

Men

Total

% Men

% Ever married

Educational attain-

ment: % with high

school or less

% History of doctor-

diagno~ed hyperten-

sion

% History of doctor-

diagnosed cardio-

Clinically

diagnosed

SAS

2 (0.08%)

14 (0.62%)

16 (0.32%)

88

94

18.8

60

vascular disease

56

Age (mean years. SD) 49 (8)

Income (mean dollars/

hour, SD)

Body mass index

[weight (kg)/height2

15.9(5.8)

Screen-

Screen-

detected

detected

muuerate

mild to

to severe

severe SAS

SAS

93 (3.5%) 27 (1.0%)

155 (6.8%) 77 (3.4%)

248 (5.0%) 104 (2.1%)

60***

70**

88

86

39

35

31***

44

34**

49

44 (8)*** 43 (9)***

12.6(4.1)** 13.1(4.3)**

(cm)]

32.3(7.1)

32.7(7.6)

34.7(7.9)

% Nonwhite ethnicity

0

7.3

5.4

SAS, sleep apnea syndrome; SD, standard deviation. Screen-de-

tected SAS was measured in a subsample of 1,090 participants and

extrapolated to the total sample that provided clinically diagnosed

prevalence; mild to severe SAS is defined as daytime hypersomnol-

ence often or almost always and five or more apneas and hypopneas

per hour of sleep; moderate to severe SAS is defined as daytime

hypersomnolence often or almost always and 15 or more apneas and

hypopneas per hour of sleep.

** p < 0.1 for comparison with value for clinically diagnosed

group.

*** p < 0.05 for comparison with value for clinically diagnosed

group.

RESULTS

On the basis of the clinically diagnosed and screen-

detected prevalences (Table 1), we calculated the per-

centage of "undiagnosed" screen-detected cases

[100% minus percentage of screen-diagnosed cases

that had been clinically diagnosed (1)]. We estimate

that 93% of women and 82% of men with moderate

to severe SAS were missed; for the less severe defi-

nition of SAS, 98% of women and 90% of men were

missed. The estimates are based on a nearly complete

ascertainment of clinical diagnosis in the sample (94%

follow-up). If the three men lost to follow-up had all

been clinically diagnosed, the proportion of missed

cases would drop by only 2 and 4% for moderate to

Sleep, Vol. 20, No.9, 1997

severe and mild to severe categories, respectively. Di-

agnosis status was based on what the participant's phy-

sician had concluded after evaluation, regardless of

whether the participant was evaluated in a sleep lab-

oratory. Requiring polysomnographic evidence for di-

agnosis may have eliminated false positives but could

have also eliminated true positives. The latter conse-

quence would seriously underestimate the true propor-

tion of clinically recognized SAS.

Comparison of clinically diagnosed and screen-de-

tected cases on sociodemographic characteristics sug-

gested that diagnosed cases were more likely to be

men, have a history of cardiovascular disease, be older,

be Caucasian, and have higher income and educational

attainment. The higher proportion of cardiovascular

disease in clinically diagnosed cases may reflect sleep

clinic referral as a result of medical care for comorbid

conditions (see Table 1).

DISCUSSION

Our findings indicate that even for a popUlation with

access to a sleep disorders clinic, at least 80% of all

moderate to severe SAS in middle-aged men and

women is likely missed. This estimate provides a start-

ing point for health policy debate on the appropriate

response to the high population prevalence of sleep

apnea. The results, however, cannot be extrapolated to

older adults, for whom sleep apnea may be even less

likely to be diagnosed. Although only male gender and

age were statistically significant correlates of clinically

diagnosed versus undiagnosed SAS, our findings sug-

gest that there is a selection bias that may lead to in-

equitable care: people with lower socioeconomic sta-

tus, nonwhites, and women may be the most under-

served.

REFERENCES

I. Strollo PJ, Rogers RM. Obstructive sleep apnea. N Engl J Med

1996;334:99-104.

2. National Commission on Sleep Disorders Research. Wake up

America: a national sleep alert. Washington, DC: Government

Printing Office, 1993.

3. Strohl KP, Redline S. Recognition of obstructive sleep apnea.

Am J Respir Crit Care Med 1996;154:279-89.

4. Young T, Palta M, Dempsey J, Skatrud J, Weber S, Badr S. The

occurrence of sleep-disordered breathing among middle-aged

adults. N Engl J Med 1993;328:1230-5.

5. SAS Inc. SUDAAN user's manual, release 6.0. Cary, NC: Sta-

tistical Analysis Systems Institute, Inc., 1992.

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