Sleep apnea linked to changes in the brain's wiring that may raise risk of dementia, stroke

Sleep apnea may be linked to certain indicators of worse brain health, according to a new study.

The study, published Wednesday (May 10) in the journal Neurology, looked at people with obstructive sleep apnea (OSA), a condition where the upper throat muscles relax during sleep and block the airway. The researchers used magnetic resonance imaging (MRI) and sleep studies to examine if sleep apnea and altered sleep were associated with two kinds of unusual patterns in the brain's white matter — the insulated wires that extend from brain cells.

Both severe sleep apnea and reduced time spent in the deep stages of sleep were associated with these two biomarkers, which have each previously been linked to an increased risk of developing Alzheimer's disease or general cognitive decline, as well as having a stroke.

The researchers found "a pretty significant association between slow-wave sleep duration, or deep sleep time, and these white matter measures," said Bryce Mander, an assistant professor of psychiatry and human behavior at the University of California, Irvine, who was not involved in the study.

Related: Human brain looks years 'older' after just one night without sleep, small study shows 

The study included 140 people with OSA whose average age was about 73 years old. All the participants were screened for one of the two biomarkers, but only 103 were screened for the second, as well. The overall group was relatively evenly split between people with mild, moderate and severe OSA, and most people had no cognitive impairment during the study.

One of the biomarkers that the researchers looked for in brain scans were white matter hyperintensities, "bright spots commonly seen in the white matter of the brain on MRI, which appear with aging and vascular disease," Dr. Diego Carvalho, an assistant professor of neurology at the Mayo Clinic Center for Sleep Medicine and the senior author of the study, wrote in an email to Live Science. "They represent areas of damage to the white matter."

The second biomarker, called "fractional anisotropy of the genu of the corpus callosum," is related to how water flows through brain cells' wires, or axons. Changes in this water flow, wrote Carvalho, are related to axon damage and can be an early sign of vascular disease. The corpus callosum is a bundle of nerve fibers that connect the two hemispheres of the brain and contains the organ's densest white matter. 

Using polysomnography, meaning the variety of data collected during a sleep study, the researchers found that participants who spent less time in slow-wave sleep appeared more affected by both types of biomarkers in their MRIs. Separately, people with severe OSA also appeared more impacted by white matter abnormalities than people with mild or moderate OSA. 

The research reveals associations between sleep apnea, deep sleep, and white matter abnormalities, but it can't say whether these sleep differences caused the abnormalities, or if differences in white matter could be interfering with sleep. It also could be that sleep and white matter abnormalities impact one another, said Mander, contributing to a vicious cycle of poor sleep quality and worse brain health.

"To me, I think that's the most likely outcome," he said.

One limitation of the study is that polysomnography data was only collected for the first few hours participants slept; people's sleep patterns might have differed later in the night. After their sleep study, anyone who met criteria for sleep apnea was given treatment in the form of a continuous positive airway pressure (CPAP) machine, which can help keep the airway open. Carvalho said that data from this "treatment phase" wasn't included because differences in how people responded to the CPAP would have muddied the data.

MRIs and polysomnography were taken, on average, about 1.7 years apart, but researchers only collected each type of data at one point in time, offering only snapshots of patients' brains and sleep patterns and not how they might change over time. The study also did not follow participants to see who might go on to develop dementia.  

Mander said that future research should examine how sleep apnea's impact on rapid eye movement (REM) sleep, the stage of sleep when most dreaming happens, might affect the brain. (Slow-wave sleep occurs in certain stages of non-REM sleep.) However, the new study introduces important ideas about "how sleep apnea and sleep disturbance might be associated with cognitive decline," he said.