DALLAS, Jan. 9 -- Sleep disorders associated with heavy snoring pose the greater stroke risk, researchers reported today in Stroke: Journal of the American Heart Association.
The study by U.S. and German researchers is the first to indicate a possible mechanism for stroke risk that could explain why some sleep disorders are more dangerous than others.
Earlier epidemiological studies had found an increased risk of stroke in people with all apneas and hypopneas. Obstructive apnea causes breathing to stop temporarily; obstructive hypopnea causes dangerously reduced ventilation.
Based on the epidemiological studies, scientists have speculated that the strokes could be caused by high blood pressure or a lack of oxygen, says Nikolaus Netzer, M.D., a visiting professor of pulmonary medicine at Case Western Reserve University and a researcher at the University Hospital at Ulm, Germany. Netzer is co-author of the journal report that looked at physiological changes in the body during individual episodes.
He and his colleagues studied 11 men and one woman, all heavy snorers, treated at the University Hospital in Freiburg, Germany. Most people with these sleep disorders tend to have predominantly obstructive syndrome with just a few central apneas. However, these 12 subjects were chosen because they had a high likelihood of having all the types of apneas and hypopneas, the researchers say.
Five of the 12 subjects also had high blood pressure. The scientists monitored the patients during sleep testing to determine their oxygen levels and the amount of blood flowing to the middle cerebral artery, the brain's major blood source.
They found a significant decline in blood flow to the brain in 76 percent of the obstructive hypopnea episodes (heavy snorers) and 80 percent of the obstructive apneas, but only 14 percent of central apneas.
In the obstructive syndromes, the reduced oxygen flow combined with the obstructions in the airways to create negative pressure in the chest, Netzer says. When the sleepers tried to breathe, the obstructions caused the negative pressure in the chest to increase and monitors indicated a reduced blood flow to the brain. Because of the obstruction, the chest moves to try to breathe in, but the obstruction causes the negative pressure to increase, and this decreases blood flow out of the heart. "It's almost like sucking on a straw," he says. The negative pressure leads to a fall in blood pressure and a fall in blood flow to the brain.
In central apnea, the nervous system does not tell the body to take a breath, so the chest remains still until the end of the episode, the researchers explained.
When the person does breathe, the unobstructed airway creates no negative pressure and, therefore, is less likely to affect blood flow to the brain. "The results were consistent with our hypothesis," Netzer says.
"Earlier papers did not refer to any kind of stroke mechanism; they just reported a close relationship between stroke and sleep apnea syndrome," Netzer says. "Our hypothesis is that the negative thoracic (chest) pressure causes a reduction in blood flow in the middle cerebral artery that may lead to stroke." A vibration of the soft palate and uvula causes all snoring. Mild snoring, or social snoring, poses no health problems, says Kingman P. Strohl, M.D., co-author of the study and director of the Sleep Research Center at Case Western Reserve University in Cleveland. However, in obstructive sleep apnea-hypopnea syndrome, the soft palate and uvula completely (apnea) or partially (hypopnea) block the throat during sleep and cause heavy snoring.
"Heavy snoring with apnea is characterized by loud, resuscitative snores -- or snorts -- after a pause in breathing. It can be heard in the next room," Strohl says. Sufferers wake themselves hundreds of times each night in an effort to get enough oxygen. They get up exhausted and tend to fall asleep at work or while driving. Obstructive sleep disorders affect men more than women and occur most often in moderately or severely overweight people who sleep flat on their backs, although individuals of normal weight also can be affected.
Another sleep disorder, central apnea, occurs when the nervous system's breathing center temporarily stops sending the body instructions to breathe. Heavy snoring is less common in that condition, he says.
Their co-researchers include Peter Werner, M.D., and Isabel Jochums, M.D., both in the division of pulmonary medicine at the University Hospital, Freiberg, Germany; and Manfred Lehmann, M.D., director of Sports Medicine, University Hospital, Ulm, Germany.
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