Sleep apnea may hasten memory and thinking declines, leading to earlier diagnoses of mild cognitive impairment and Alzheimer's disease, suggests a study released Thursday.
Patients with sleep apnea were, on average, diagnosed with mild cognitive impairment (MCI) nearly 10 years earlier than those who didn't suffer from breathing problems during their slumber, according to New York University researchers.
The timespan for developing Alzheimer's also seemed to speed up: Those with sleep apnea were diagnosed, on average, five years sooner than sound sleepers, the authors reported.
"This study is adding to the emerging story that sleep apnea may be contributing in some way to the acceleration of cognitive decline as you age," said study coauthor Dr. Andrew Varga, an instructor in medicine at the New York University Sleep Disorders Center. "And that is potentially another good reason to get evaluated and treated."
The study was published in the journal Neurology.
Research has shown that sleep apnea is quite common in older adults, affecting as many as 53 percent of men and 26 percent of women. And many go undiagnosed, experts say.
Varga and his colleagues reviewed the medical histories of 2,470 people aged 55 to 90 who had participated in an earlier study designed to look for markers of Alzheimer's disease.
At the outset, study volunteers were categorized as being free of memory and thinking problems, or in the early stages of mild cognitive impairment, or with Alzheimer's disease.
The researchers found that sleep apnea was associated with a much quicker decline in cognitive function. But there was some good news from the study: Treatment for breathing problems during sleep appears to be protective, delaying the onset of MCI by approximately 10 years.
Essentially, this meant that people who got treated declined at the same speed as people who didn't have apnea at all, Varga said. The treatment can include machines that help people breath better as they sleep.
It's not clear yet exactly how sleep apnea is damaging the brain.
Studies have shown that "sleep apnea is associated with repetitive drops in the blood oxygen level, which can affect various organs in the body differently," said Dr. Charles Atwood, a sleep specialist and an associate professor of medicine at the University of Pittsburgh School of Medicine.
Oxygen levels drop because people with sleep apnea have periods during the night when their throats close up and they briefly stop breathing, said Atwood, who was unaffiliated with the study. A common cause is obesity, he said.
The weight of the flab around the neck pushes down on the throat at a time when its muscles have relaxed with sleep. That pressure can briefly close the throat until the person partially awakens, gasping for air. And that can happen as many as 300 to 400 times a night, Atwood said.
"They often don't wake up enough that they are conscious of being awake," Atwood said. "Then they wake up in the morning feeling really tired an unrefreshed."
Sleep is the time for the brain's janitors to sweep up all the waste. If sleep is disturbed, those janitors might not get the job done, leaving an accumulation of the proteins that gunk up nerve cells.
And, as it turns out, low oxygen levels may hurt some parts of the brain more than others.
"It's known that certain neurons in the hippocampus — where much of Alzheimer's is thought to start — are exquisitely sensitive to drops in oxygen," Varga said. "Sleep apnea may just stress those neurons out."
Another possibility is that disrupted sleep messes with the brain's housecleaning. Sleep is the time for the brain's janitors to sweep up all the waste. If sleep is disturbed, those janitors might not get the job done, leaving an accumulation of the proteins that gunk up nerve cells, said Dr. Mark Wu, a sleep medicine specialist and an associate professor of neurology at Johns Hopkins Medicine.
The important message here "is that Alzheimer's is untreatable and it's a big, big problem," said Wu, also unaffiliated with the new study. "It costs the United States billions of dollars. If we could reduce the speed at which it develops, people might then die of cancer or a heart attack and not Alzheimer's."
"The message has to be loud and clear to family practitioners, primary care physicians and ob-gyns that sleep apnea should be screened for," agreed Dr. Alon Avidan, a professor of neurology at the University of California, Los Angeles, and director of the UCLA Sleep Disorders Center.
"Unfortunately, sleep apnea doesn't hurt like chest pain. It doesn't bring up a specific complaint. The patient may be a little sleepy or confused. People often fail to make the correlation that the daytime sleepiness may be related to disrupted sleep at night," Avidan added.
In fact, people often aren't diagnosed until there is a dire consequence, Avidan said. "Either they are in a car accident because they were sleepy or they develop high blood pressure or have a stroke."