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The authors cite conflicting estimates of the frequency of sleep-disordered breathing in the general population, stemming partly from different criteria in different studies. The most common measure for diagnosing sleep-disordered breathing has been the polysomnographic variable of the Respiratory Distress Index or RDI (equivalent to the Apnea/Hypopnea Index or AHI often used), which means the number of apneas plus hypopneas per hour of sleep. The most common threshold of RDI/AHI for diagnosing sleep disordered breathing has been 5, though some studies have used 10, 15, 20, 30, or even 40. Another measure used in making the diagnosis has been blood oxygen saturation, sometimes using a drop of 4% or more as a threshold. [NOTE: The 1991 edition of the International Classification of Sleep Disorders requires more than 5 events per hour, each of which must be longer than 10 seconds in duration--plus other features.]
One study found 21% of industrial workers to have RDIs of 5 or more; another found this in 26% of men ages 30 to 69; another found this in one-quarter of an elderly population; yet another found 9% of female and 24% of male government employees ages 30-60 to meet this criteria. Especially high frequencies have been found in snorers (52%) and Hispanics living at high altitude (62% of men ages 40-64 and 48% of women. But bear in mind that RDI/AHI is only one component of the sleep apnea syndrome--others being symptoms, such as excessive daytime sleepiness, and associated features on the polysomnogram such as arousals occuring with the apneas or hypopneas, or oxygen desaturation. Taking these features into consideration, the aforementioned researchers gave much lower estimates for the actual prevalence of sleep apnea in their populations, ranging from 1.3% to 5%, the higher estimates generally applying to men only. This research group tried to define more accurately the prevalence of medically significant sleep-disordered breathing using home-recording of sleep for three nights.
Over the course of four years, they first made a telephone survey of homes in four areas of San Diego, selected to represent different incomes/class/ethnic compositions. In this manner, they interviewed 1,085 people aged 40-64, which represented 74% of those who could be contacted by phone. The initial interview was mainly to get basic demographic characteristics of the person and ask to visit for a home interview and sleep recording, to which 34% agreed. The home interview included sleep-related questions, questionnaires designed to measure depression, and measurement of blood pressure, height, weight, neck skinfold, and circumference of neck, waist, and hip. The interviewer left the subject with a wrist-activity monitor, a sleep log, and a sleep monitoring system that measured oxygen saturation, pulse rate, and two measures of snoring, for three consecutive nights, for which the subjects were paid. The wrist activity monitor was used to estimate times of sleep and the oxygen desaturations to estimate frequency of apneas/hypopneas. Those subjects with substantial desaturations were followed up later at one-year intervals. They arbitrarily chose a threshold of 20 desaturations per hour to make a "conservative" estimate of pathological sleep-disordered breathing.
Ultimately 355 subjects completed the study, including 190 women and 165 men, of whom 80% were white, 13% Hispanic, 3% black, and 4% other, mainly Asian. This was similar to the 1990 census figures of racial distribution in San Diego.
Of these 355, 63% reported snoring. Almost all subjects showed repeated oxygen desaturations, the medians being 4.3 for women and 6.7 for men, the averages being 7.3 and 10.5 respectively--the difference between medians and means reflecting the fact that the distribution of this variable was "skewed" meaning that there was a group with much higher frequencies (whereas the group with low frequencies of necessity stopped at 0). Overall, 7.9% had frequencies above the "20" threshold, including 5.3% of women and 10.9% of men. This "pathological" level was reached more often by older subjects, though the frequency peaked at ages 50-54 (16%) and dropped above that age. Non-whites reached this level three times as often as whites (about 16% vs. 6%). Although the group as a whole was not obese, the 21% who were obese (body mass index or BMI of 30 or above) had about 10 times the frequency of pathologic levels as the non-obese group, reaching a frequency of 50% in the most obese (BMIs=40-51). From their data, adjusting for age and ethnicity in the U.S. population as a whole, the authors estimated a U.S. populaton prevalence of pathological sleep disordered breathing as 7.2%, 5.2% for women and 9.3% for men, within the 40-64 year age group.
Obesity appeared to be the strongest predictor of this disorder, especially neck and waist circumference and weight-to-height ratio. Snoring was also a significant predictor. Higher blood pressures were also associated with the disorder, though not diagnosable hypertension. Also associated was dissatisfaction with sleep and amount of waking during sleep.
In follow-ups of 22 cases at variable intervals averaging about two years later, the frequency of desaturations increased slightly (by 1/hr) but not significantly.
The authors noted that their estimates of the prevalence of sleep apnea syndrome in the general population were higher than those of previous researchers, which they attributed to including more non-whites (who had higher rates of illness). They noted also the disparity betwen their own estimates of the male:female ratio of apneics (about 2:1) and the overwhelmong predominance of men at many sleep clinics. They were surprised by the rather weak associations of the disorder with various medical illnesses and symptoms commonly connected with sleep apnea, and suggested that clinic samples were biased for people with symptoms, failing to represent that portion of the sleep-disordered population who are relatively free of symptoms. They did note the association of sleep apnea with blood pressure elevations and disturbed sleep, but suggested that symptom-free cases with modest degrees of sleep apnea might not need treatment.
Since the criteria used by these authors for diagnosing sleep-disordered breathing are substantially different from those used by sleep labs to diagnose sleep apnea, I would expect most people with sleep apnea to question the relevance of their figures to themselves. In fact, however, the authors wre critical of current diagnostic criteria, suggesting that sleep-disordered breathing in itself represented an entity which is only partly represented by the subgroup of patients with that condition who go for treatment. The authors did study a small group of 18 of their subjects with a single night in the sleep lab. These showed RDIs very close to the frequency of desaturations by their criteria; though they didn't mention the correspondence of wrist activity monitor data with actual sleep time, other studies have shown these to be similar, though not as close as the apnea index and the desaturation index.
One might infer from this that the authors set a fairly high threshold, equivalent to an RDI of 20, for diagnosing sleep-disordered breathing, possibly leading to an underestimation of its population prevalence, at least if one considered a lower level of RDI pathological. They (deliberately) didn't use the other criteria set forth in the Diagnostic Manual to make their diagnoses, which would inflate their figures if one tried to compare them with the group of diagnosed sleep apneics, which the authors specifically declined to do. The authors might argue that the symptomatic criteria, while applicable to symptomatic patients attending sleep clinics, aren't relevant to asymptomatic patients with sleep disordered breathing in the general population. Consequently, they did not attempt to combine their desaturation estimate of apneas/hypopneas with one of the two other objective sleep variables they had available (arousals and bradycardia) to make their diagnoses, nor did they exclude the medical illnesses. One like myself who is concerned with symptomatic sleep apnea under treatment, is left in perplexity with their approach to diagnosis, even given the constraints of their limited home monitoring.
Setting this reservation aside, it is still impressive that minority groups showed substantially higher rates of sleep disordered breathing. Other researchers have also found something like this with respect to sleep apnea. It remains to be explained, though other researchers have found anatomical differences in the upper airway structures of blacks that would seem to predispose them to sleep apnea, possibly related to the higher frequency of hypertension in this racial group. The finding of a "peak" of sleep disordered breathing in the early 50's, while the authors explain it away on a statistical basis, has been found by others studying sleep apnea, and the suggestion has been made that the more severely ill sleep apneics die earlier of its complications, reducing the prevalence of the disease in the older population. The association of sleep disordered breathing, like sleep apnea, with obesity is nothing new, nor is the higher frequency in men than women--or the fact that this higher rate in men does not approach their overwhelming predominance in sleep clinics. One possible inference is the women are not being adequately identified and diagnosed.
The authors did innumerable statistical analyses, which raises the likelihood that they will come across "significant" findings that occur by chance and really signify nothing. They attempted to adjust for this with other statistics. However, one unfortunate consequence of such an approach is that it ignores the fact that many prior hypotheses exist, and that the status of a hypothesis (like the association of sleep disordered breathing with elevated blood pressure) confirmed by data is different (i.e., more "significant") than the status of a finding not predictable from prior hypotheses (such as a negative correlation of sleep disordered breathing with heart trouble other than heart attacks, which they also found).
Finally, I find their concept of a large reservoir of asymptomatic people with sleep disordered breathing in the general populaton interesting but arguable. On any lab test, there are "outliers" for whom the lab results are real, but there is no "illness" associated with the laboratory abnormality. Illness cannot be wholly defined by a lab measure; it requires other evidence of bodily malfunction related to that lab result. People in the general population with white blood cell counts higher than the normal range but no related symptoms might be grouped together as "hypergranulocytic" but few doctors would want to diagnose them with an illness of any type. Indeed, sporadic elevations of white blood cells counts are commonplace and commonly disregarded. The authors themselves are explicit in distinguishing their term "sleep disordered breathing" from diagnosable "sleep apnea" and take issue with the usual criteria for the latter diagnosis.
Sleep is undoubtedly subject to multifactorial determination. That is to say, sleep disordered breathing is only one of many factors which can disturb sleep. It may well be that, unless the severity of sleep disordered breathing is extreme, other factors must combine with the underlying apneic "predisposition" to produce the actual "syndrome" of sleep apnea, which is after all what concerns not only clinicians but those responsible for issues of public health.
WHAT DO YOU THINK ABOUT THIS ARTICLE AND MY COMMENTS? CAN YOU RELATE THIS MATERIAL TO ANY PERSONAL EXPERIENCES?
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