June 11, 2008 — People with obstructive sleep apnea (OSA), a common sleep disorder linked to heavy snoring, may lose tissue in brain regions that store short-term memories—the type used to recall a recent joke or recognize a person you met at a party—say researchers at the University of California at Los Angeles.
Daytime confusion and memory loss are known symptoms of sleep apnea; people with the condition often rouse hundreds of times a night (without fully waking) and are tired and forgetful the next day. But a study in the June 27, 2008, edition of the journal Neuroscience Letters suggests that there may be more to it than just that.
Why sleep apnea may affect memory
Memories are formed in the mammillary bodies, structures on the underside of the brain that resemble small breasts. When UCLA neuroscientists scanned the brains of 43 sleep apnea patients and 66 healthy volunteers using magnetic resonance imaging (MRI), they discovered that the sleep apnea patients' mammillary bodies were nearly 20% smaller than those of the untroubled sleepers.
"People always thought the memory deficit was just because their sleep was disturbed and they felt terrible," says study coauthor Ronald Harper, PhD, distinguished professor of neurobiology at the David Geffen School of Medicine at UCLA. "But it looks to be that the brain is actually injured, and the particular brain structure that's damaged is one of several that transfer recent memories into long-term memories."
Harper hypothesizes that repeated drops in oxygen lead to this brain injury and says that the more severe cases or those that go untreated may lead to greater damage in the mammillary bodies.
"You only have to watch a sleep apnea patient try to breathe at night to know the danger," Harper says. "It's strikingly clear that their brain is not getting enough oxygen." He explains that oxygen in the blood drops to dangerous levels when the throat collapses during sleep, a person rouses enough to gasp for breath (without fully waking), then the blood oxygen level shoots back up when breathing starts again. When the brain is flushed with oxygen over and over again after these drops, free oxygen radicals are produced and damage the brain.
How short-term memory works
In the movie 50 First Dates, Drew Barrymore's character wakes up every morning unable to remember the previous day. And so each day when she encounters Adam Sandler's character, it's as if she is meeting him for the first time. This is an exaggerated case, says Harper, but it's an example of what might happen if the mammillary bodies didn't function properly in the brain.
"It's a very particular kind of memory loss, associated with memories that you've incorporated very recently. After five minutes or so, these are turned into long-term memories, but for some reason folks with recent memory deficits can't turn those into long-term memories," says Harper. "So when they're introduced to someone or told a joke or explained a relationship between two things, after about five minutes they will no longer recall it. Everybody they meet will seemingly be for first time after five minutes; you can tell them a joke and it will always be funny to them because they're not able to store it as a long-term memory."
This type of recall problem, and the telltale shrunken mammillary bodies, have also been found in patients with chronic alcoholism and Alzheimer's disease.
Who is at risk
Sleep apnea affects more than 20 million Americans, although many people don't know they have the disorder because most symptoms occur at night. You may have sleep apnea if you snore or gasp for breath while sleeping at night, or if you wake frequently to use the bathroom. If you are a heavy drinker, overweight, or have a thick neck, your risk is also higher. (Read more risk factors for sleep apnea.)
Often spouses or bed partners are the people who first recognize nighttime signs of sleep apnea. Daytime symptoms, such as excessive sleepiness, trouble focusing, headaches, and sore throats, may also be warning signs. Without treatment, sleep apnea very rarely goes away on its own, so it's important to know when to seek medical attention.
What you can do to protect yourself
UCLA researchers plan to study whether vitamin B1 (also known as thiamin) supplements might help sleep apnea patients regain some of their lost brain tissue. The vitamin helps move glucose into the cells, preventing their death from oxygen starvation, and physicians currently use it to treat this type of memory loss in alcoholic patients.
It's too soon to say whether vitamin B1 can help sleep apnea patients, says Harper, but he notes that getting your daily recommended dose can't hurt. Foods rich in vitamin B1 include whole grains, lean meats, fish, and dried beans. (Talk to your doctor before taking any over-the-counter supplements.)
What is proven to help stimulate new brain growth, however, is regular exercise. "The evidence is pretty strong that if you exercise, you generate new cells in the hippocampus," says Harper. "And that's very encouraging because the hippocampus is also very important to memory."
Sleep apnea can be treated with dental devices, surgery, or a continuous positive airway pressure (CPAP) machine—a mask worn at night to help keep airways open.
Previous research has shown that sleep apnea can contribute to high blood pressure, heart attack, stroke, automobile accidents, and other health concerns. The UCLA findings highlight yet another reason the condition is so dangerous when left untreated, Elizabeth G. Nabel, director of the National Heart, Lung, and Blood Institute, said in a statement. "These results underscore the importance of early diagnosis and treatment of sleep-disordered breathing, which can have long-term effects on patients' health and well-being."