ANN ARBOR, Mich. – Patients who snore or have other symptoms of sleep apnea often undergo testing in a sleep laboratory to measure the number of breathing pauses and arousals that occur while they slumber. But doctors find these tests do not effectively predict daytime consequences suspected to arise from sleep apnea, such as sleepiness in adults or hyperactivity in children.
Now, neurologists at the University of Michigan Health System and engineers at Altarum Institute in Ann Arbor, Mich., have discovered evidence that the disruption of sleep in sleep apnea may be much more frequent than the breathing pauses, or apneas, themselves.
In two research papers published in the February issue of the journal Sleep, the researchers describe for the first time evidence that on average, brain waves change with each breath, not just the short periods of the night when apneas occur. Although the data are preliminary, they suggest a whole new thinking in sleep research that eventually might help doctors predict who will suffer consequences of sleep apnea, and who will respond to treatment.
"Complicated studies that require time, money, and technical expertise are often performed in sleep laboratories," says Ronald Chervin, M.D., director of the Sleep Disorders Center and Michael S. Aldrich Sleep Disorders Laboratory at UMHS. "The most common reason is to gauge the severity of sleep apnea. A frustrating problem has been that results of these studies have not predicted the behavioral outcomes of sleep apnea very well. That makes us think that maybe we don't have the best laboratory measures; maybe we are not recording some of the most important features of sleep apnea."
Millions of people experience sleep apnea, a condition in which repeated pauses in breathing during sleep cause many arousals during the night. The nocturnal arousals, in turn, are suspected to be an important cause of daytime symptoms: sleepiness in adults, and attention problems or hyperactivity in children.
Typical measures for assessing sleep apnea involve counting the number of times a patient's breathing is obstructed or partly obstructed. But sleep specialists also know that people with sleep apnea often work harder than normal to breathe even in between these episodes.
"Our idea was that maybe every snore or every difficult breath is actually arousing the brain to some small extent," Chervin explains. "The problem is that much of what happens in brain waves cannot be seen by the human eye, even though highly trained sleep technicians and physicians carefully review the sleep study."
And this is where Altarum Institute engineers were able to help. Working with Chervin, they applied their long experience in signal pattern detection to develop a novel computer program. The computer measures the extent to which brain wave activity varies, on average, with the breathing cycle during sleep.
To test the new algorithm, the researchers used sleep studies that had been recorded from children scheduled to have tonsillectomy surgery to correct sleep apnea. The first paper details results from one child, and shows for the first time that on average brain wave activity as reflected in the electroencephalogram (EEG) did change with the child's breathing cycle, even when no pauses in breathing occurred. This brain wave link to the breathing cycle was less prominent when the child was retested one year after his tonsils were removed.
For the second paper, the researchers looked at results from 10 children, most suspected to have sleep apnea, to determine whether the new link between brain activity and breathing predicted daytime sleepiness or decreased attention.
"We looked at the relationship between one EEG signal and the breathing cycle, and in most of the children we found significant correlation between the amount of energy in the EEG signal and different phases in the breathing cycle," says Joseph Burns, Ph.D., a senior scientist who led the effort at Altarum.
Despite the small number of children initially tested, the strength of that correlation did show some promise as a predictor of sleepiness and reduced attention.
The researchers are now studying more children. Confirmation of their initial findings could help to answer questions about how sleep apnea leads to daytime sleepiness.
"This could give us insight into the physiology of how sleep apnea causes sleep disruption, daytime sleepiness, attention deficit and behavioral problems," Chervin says. "Currently, we think sleepiness arises because apneas cause arousals that we can easily see in brain wave patterns. Maybe these obvious, full arousals are less important than thousands of briefer arousals, or microarousals, that can only be detected by computers. If we could prove this, we might improve our ability to identify who has a serious sleep and breathing problem and who might benefit from treatment."
In addition to Chervin and Burns, researchers included Deborah L. Ruzicka, RN, Ph.D., of the UMHS Sleep Disorders Center, and Nikolas S. Subotic, Ph.D., Christopher Roussi, MS., and Brian Thelen, Ph.D., from Emerging Technologies Group at Altarum Institute.
The University of Michigan and Altarum Institute have filed a provisional patent application on the signal-analysis algorithm used to track changes in the brain during the respiratory cycle. Funding for the study came from the National Institutes of Health, the University of Michigan General Clinical Research Center, and the nonprofit Altarum Institute.
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