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TORONTO, Dec. 1 - Sleeping poorly? If it is the result of moderate to severe sleep-disordered breathing, it may mean a heightened risk of a stroke, reported researchers here.

In the four years after diagnosis people with moderate to severe sleep-disordered breathing have nearly 4.5 times the risk of stroke as do people without the condition, according to Douglas Bradley, M.D., director of Toronto General Hospital's Sleep Research Laboratory here.

It's the first time such breathing disorders -- sleep apnea and hypopnea -- have been shown to increase the risk of stroke, said in an interview. In the long run, he added, the finding implies that treating the disorders -- a standard therapy is a continuous positive airway pressure (CPAP) machine -- might reduce the incidence of stroke.

"That's what we hope," he said, although the current study, reported in the Dec. 1 issue of the American Journal of Respiratory and Critical Care Medicine, only reports on the prevalence and incidence of stroke as they relate to breathing disorders.

Dr. Bradley and colleagues conducted a two-part study -- one cross-sectional and one prospective -- on people recruited in 1988 to the Wisconsin Sleep Cohort Study, a stratified random sample of state employees in Wisconsin between ages 30 and 60.

For the study, participants were stratified according to the Apnea-Hypopnea Index, which counts the number of occurrences of either apnea or hypopnea in an hour. People with fewer than five episodes were regarded as having normal sleep and were used as a reference group, Dr. Bradley said.

People with between five and 20 episodes an hour were regarded as having a mild sleep disorder, while those with more than 20 were defined as having severe to moderate disease, he said.

Of the study's 1,475 participants, 76% were normal, 17% had a mild disorder, and 7% had moderate to severe sleep-disordered breathing.

Cross-sectional analysis at baseline found 14 of the normal participants, two of those with a mild disorder, and six with moderate to severe disorder had previously had a stroke.

After adjusting for age, sex, body mass index, alcohol and smoking, people with sleep-disordered breathing had an odds ratio for stroke of 4.33 (with a 95% confidence interval from 1.32 to 14.24) compared to the reference group.

Even when the analysis was adjusted to take hypertension and diabetes into account, the odds ratio remained high -- 3.83, with the 95% confidence interval from 1.17 to 12.56. Both results were statistically significant at p=0.02 and p=0.03 respectively.

"After we controlled for everything," he said, "it's still 3.8 times greater than for people who don't have sleep apnea. That's very high."

For the prospective study, those who had previously had a stroke were excluded, Dr. Bradley said. Participants were examined at baseline and then re-examined at four-year intervals. Over the 12-year study period, 14 participants suffered a first-ever stroke.

The researchers found:

• Without adjustment for other risk factors, the odds ratio for stroke among those with the worst disorder -- compared to the reference group -- was 4.31 (with a 95% confidence interval from 1.31 to 14.15) over any four-year period.
• When age and sex were accounted for, the odds ratio became 4.48 (with the 95% confidence interval from 1.31 to 15.33.)
• Both results were statistically significant at p=0.02.

Because the number of strokes was small, Dr. Bradley said, the researchers were unable to adjust for all possible risk factors. Indeed, when body mass index was taken into account, the odds ratio -- though still high at 3.08 -- lost statistical significance.

One of the key findings, Dr. Bradley said, is that in all cases, the sleep disorder preceded the stroke.

"That's a very important new piece of information," he said, because it rules out the possibility that strokes cause sleep apnea.

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