The study of 1,475 people found that those with moderate to severe sleep apnea at the beginning of the study were 3 to 4 times more likely to have a stroke than a comparable group of patients without sleep apnea during the next four years.
The study did not find any significant increase in the odds of having a stroke for people with mild sleep apnea compared with people without sleep apnea.
"The stroke risk we found for people with moderate to severe sleep apnea is quite significant-double the risk of other well-known risk factors for stroke, such as hypertension or diabetes," said senior researcher Douglas Bradley, M.D., Professor of Medicine and Director of the Centre for Sleep Medicine and Circadian Biology at the University of Toronto. He and colleagues Michael Arzt, M.D., and Terry Young, Ph.D., found that the risk of stroke was significant even after other risk factors for stroke, such as high blood pressure and obesity, were taken into account.
In obstructive sleep apnea, a person's airways narrows, or totally collapses, during sleep. As a result, a person stops breathing briefly multiple times throughout the night. The person's sleep is interrupted often, which may cause excessive daytime sleepiness or even high blood pressure.
The patients in the study were defined as having moderate to severe obstructive sleep apnea if their breathing stopped or slowed at least 20 times per hour of sleep. People whose breathing stopped or slowed between 5 and 20 times per hour of sleep were considered to have mild sleep apnea.
One reason obstructive sleep apnea may increase stroke risk is that it has been shown to cause high blood pressure, which is the most common risk factor for stroke, Dr. Bradley said. Another possible reason is that when a person stops breathing, the lack of oxygen kicks in the body's "fight or flight" response. Part of that reaction is to make the blood more clottable, and blood clots in the brain can cause a stroke.
"This study provides yet another reason why it's important to treat sleep apnea," Dr. Bradley said. "It also raises the question of whether people with sleep apnea should be put on aspirin therapy or given other anti-clotting drugs, which is what is done for other people with stroke risk. This is an issue that needs further research."