Sleep stage dynamics differ between children with and without obstructive sleep apnea.

DURING POLYSOMNOGRAPHY, DURATIONS OF CONTIGUOUS NOCTURNAL SLEEP BETWEEN AWAKENINGS WERE RECENTLY REPORTED TO FOLLOW EXPONENTIAL DISTRIBUTIONS.1 Durations of specific sleep stages, between awakenings or other sleep stages, also seem to follow exponential distributions.2 The investigators who made these observations in small numbers of normal subjects and patients with obstructive sleep apnea raised the possibility that these analyses of sleep dynamics could have clinical utility: the parameters of the distributions might well distinguish individuals with abnormally fragmented sleep from those with normal sleep. An exponential distribution, whose probability density function is expressed as p(t|τ) = exp(−t/τ) / τ, is characterized by a single parameter, the time scale parameter τ. The time t is the time between transitions, and the time scale parameter τ reflects the mean time between transitions. 1/τ reflects the mean frequency of transitions, and thus, characterizes the variability of the data. For exponentially distributed data that vary over a continuous range of values, τ is estimated as the mean of the data observations. If the distribution of sleep stage durations (the time between sleep stage transitions) follows an exponential distribution, then the distribution of sleep stage durations can be characterized by the time scale parameter, which can be estimated from sleep stage data routinely generated in sleep laboratories. As this parameter characterizes the mean time between sleep stage transitions, it also reflects inversely the amount of sleep fragmentation. To test the possible clinical utility of sleep dynamics analysis, we recently compared polysomnograms of 15 adult women with fibromyalgia syndrome to those of 15 age- and gender-matched volunteers.3 Subjects showed exponential distributions of sleep stage durations. Women with fibromyalgia showed notably shorter (by 27%) mean durations of stage 2 sleep, to an extent that correlated with subjective pain measures. In contrast, among standard measures of sleep, the sleep stage proportions and sleep efficiency failed to distinguish the 2 groups; only the number of sleep stage changes did so, to a marginally significant extent. To date, analysis of sleep dynamics have not to our knowledge been applied to more than small numbers of adults with sleep disordered breathing (SDB), or to any children. Although pediatric SDB is believed to cause substantial neurobehavioral morbidity,4–6 at least in part through sleep fragmentation,7,8 previous comparisons of routinely scored sleep time, sleep efficiency, and sleep stages among children with and without SDB have revealed no consistent differences.9 Similarly, studies of children with SDB, before and after surgical treatment, have revealed no differences in 7 children10; no differences in 14 children11; and only a minor postoperative increase in total sleep time and stage 1 sleep in 26 children.12 In contrast, a recent study of 110 children aged 1 to 16 years, before adenotonsillectomy for obstructive sleep apnea, and again 1 to 16 months after, showed postoperative improvement in sleep efficiency, fewer awakenings, decreased stage 1 sleep, increased slow wave sleep, and decreased REM sleep.13 However, these changes were small enough on average (and counterintuitive in the case of REM sleep) as to carry little obvious clinical significance. Furthermore, most of the participants were obese, and three-quarters did not respond completely to treatment. Therefore, whether standard sleep stage assessments distinguish children with and without SDB in a meaningful way remains unclear. Demonstration that SDB effects are reflected in readily available polysomnographic features that have yet to be fully explored, such as sleep dynamics, could improve insight into SDB pathophysiology and clinical care. The aim of this study, therefore, was to examine sleep dynamics and compare results to those obtained with standard sleep staging of polysomnograms that had been recorded from children before and one year after adenotonsillectomy (for SDB) or unrelated surgical care. We took advantage of a subset of data available from the prospective, well-characterized Washtenaw County Adenotonsillectomy Cohort.14–20 Our hypotheses were that children with and without SDB would, like adults, show exponentially distributed sleep stage durations; standard sleep stage measures would show only few differences of small magnitude between SDB patients and controls; sleep dynamics would distinguish the two groups; and sleep dynamics could provide a useful new framework in which to understand changes in sleep stages after treatment for pediatric SDB.