Sleep apnea linked to smaller brain volume, study finds

By Sandee LaMotte, CNN

(CNN) — A part of the brain connected to memory shrank in people with severe sleep-disordered breathing — which can include heavy snoring and sleep apnea — according to a new study. People with sleep apnea stop breathing for 10 seconds or more at a time multiple times a night.

“It is more and more acknowledged that this sleep disorder, if untreated, will increase the risk of dementia. Here, we provide new original evidence focusing on the medial temporal lobe, a brain region early affected in Alzheimer’s disease,” said lead study author Géraldine Rauchs, a postdoctoral research officer at the National Institute of Health and Medical Research in Caen, France, in an email.

The medial temporal lobe is an area of the brain essential for memory and recall of facts and events. Buried deep in that area is the hippocampus, a complex structure critical for learning, memory encoding and consolidation, and spatial navigation.

However, the loss of volume in the brain only occurred when a person also had signs of beta-amyloid plaques, a hallmark sign of Alzheimer’s disease, the study found.

“We found that greater severity of sleep apnea was associated with lower volumes in the hippocampus and in various subregions of the medial temporal lobe,” Rauchs said. “The people who did not have amyloid plaques did not have this lower brain volume, even if they had severe sleep apneas.”

Amyloid buildup in the brain can start decades before any clinical signs of cognitive decline — as early as the 30s and 40s, experts say. After age 70, one-third of cognitively normal people have amyloid in their brain tissue, according to a February 2022 study. Interestingly, the deposits do not necessarily result in Alzheimer’s disease — many older people have evidence of extensive amyloid in their brains and do not go on to develop Alzheimer’s.

However, it’s during sleep, especially deep, slow-wave sleep, that the brain clears away much of the amyloid buildup in the brain, said Rudy Tanzi, a professor of neurology at Harvard Medical School in Boston, who was not involved in the study.

In order to get enough deep sleep for the brain to rejuvenate, adults need at least seven hours of uninterrupted sleep. However, people who snore heavily or who have sleep apnea can experience hundreds of mini-arousals per night — even though they don’t realize it.

“It makes sense that sleep apnea was associated with reduced brain volume in older individuals, particularly those with amyloid plaque,” Tanzi said in an email.

“If the amyloid is not properly removed, it will initiate the cascade of Alzheimer’s disease pathology ultimately leading to dementia,” said Tanzi, who is also the director of the genetics and aging research unit at Massachusetts General Hospital in Boston.

Sleep apnea plus plaques

The study, published Wednesday in the journal Neurology, examined data collected on 128 people older than age 65 participating in Age Well, an ongoing randomized clinical trial in Caen, France. The trial is designed to test interventions on cognitive aging.

All participants were free of any cognitive, psychiatric or chronic diseases at the start of the study. None had sleep apnea at the start of the study, and those who developed sleep-disordered breathing and were treated with continuous positive airway pressure, also known as CPAP, were excluded from the study results.

The study group underwent brain scans, memory tests that were repeated at about 21 months, and an overnight sleep study done at their homes. Sleep testing showed 91 of the 122 study participants had moderate to severe sleep apnea, but most didn’t have many obvious symptoms — only 11 people experienced excessive daytime sleepiness.

Twenty-six of the participants had amyloid plaques in their brain tissue. In those people, severe sleep-disordered breathing was linked to less volume in the medial temporal lobe. The more severe the apnea, the greater the loss of volume, the study found.

In addition, “lower volumes in some regions of the medial temporal lobe were associated with worse memory performance assessed 18 months later,” Rauchs said.

However, no shrinkage was found in the brains of people with sleep apnea who had no amyloid plaques. This finding suggests certain individuals may be more vulnerable to the adverse effects of sleep disorders that impact breathing and interrupt sleep, Rauchs said.

“People who are in the very early stages of the Alzheimer’s continuum showed a specific vulnerability to sleep apneas,” Rauchs said. “Further studies should look at whether treating sleep-disordered breathing could potentially improve cognition and prevent or delay neurodegeneration.”

Sleep apnea and the brain

It’s estimated 936 million adults worldwide between the ages of 30 and 69 may suffer from sleep apnea, with many more people undiagnosed. If the sleep apnea is severe and untreated, people have three times the risk of dying from any cause, according to research.

A study published in May found people with severe sleep apnea who spent less time in slow-wave, deep sleep had more damage to the white matter of the brain than people who had more slow-wave sleep. For every 10% decrease in time spent in deep sleep, there was an increase of white matter hyperintensities (lesions) in the brain similar to the effect of being 2.3 years older, the study found.

Severe sleep apnea was associated with worrisome changes in the brains of middle-aged and older adults, a July 2018 study found. People who had low levels of oxygen in their blood from sleep apnea were more likely to have thinner brain matter in certain regions linked to dementia, the study showed.

When tested, those people had less ability to remember and recall new information. At the same time, they were more likely to have thickness in other regions of the brain, which the researchers said could be indications of signs of swelling and inflammation.

Rauchs said in a prior study she found amyloid burden in the brain was “predicted by the severity of hypoxia associated to sleep apneas. Hypoxia is a lack of oxygen to the brain that occurs in sleep apnea when a person stops breathing multiple times each hour.

“We surmise that hypoxia may also have a deleterious effect on the volume of the regions of the medial temporal lobe. Indeed, some regions of the medial temporal lobe such as the hippocampus are particularly vulnerable to hypoxia,” she said.

Studies such as these reinforce the need to be assessed by a sleep specialist and treated for sleep apnea. Work with a physician “to keep your brain healthy and clear of amyloid deposits which initiates the cascade of Alzheimer’s pathology eventually leading to nerve cell loss and cognitive decline,” Tanzi said.

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