A?dangerous type of snoring known as sleep apnea can cause stroke by decreasing blood flow, raising blood pressure and harming the brain's ability to modulate these changes, researchers reported yesterday.

The study may help explain why people with sleep apnea are more likely to have strokes and to die in their sleep, the team led by Vahid Mohsenin of Yale University in Connecticut said.

An estimated 18 million Americans have sleep apnea, which is characterized by repeated episodes in which someone who is sleeping stops breathing.

"Three years ago we showed that patients with sleep apnea ... they die or have strokes three times more than people with a comparable age or risk factors without sleep apnea," Mohsenin said in a telephone interview.

"We asked the question of why they have a higher risk for stroke."

Writing in the Journal of Applied Physiology, Mohsenin and colleagues said they tested 48 middle-aged men and women, 22 of whom had sleep apnea but who were otherwise healthy. They checked blood pressure and used ultrasound to monitor blood flow in the brain.

They had the volunteers do a blood pressure test in which they squatted and then stood suddenly.

"We found that patients with sleep apnea had difficulty compensating for the change in blood pressure," Mohsenin said. "They actually had decreased blood flow to brain."

He said this showed the damage caused by sleep apnea continues throughout the day.

"When you are up and around and experience changes in your blood pressure, or during the night when you have fluctuations in blood pressure due to apnea, they have a hard time compensating for that," Mohsenin said.

"It is a carryover effect."

Now, Mohsenin said, his team will test whether statin drugs, known to reduce inflammation, can restore the lost brain function in sleep apnea sufferers.

The patients will also later be treated using airway pressurization masks, or CPAP.

"The important thing is to recognize sleep apnea early on so there won't be any significant damage to the brain," he said.

During sleep apnea episodes, the upper airway becomes blocked, hindering or stopping breathing and causing blood oxygen levels to drop and blood pressure to rise. The person eventually awakens and begins breathing, restoring normal blood oxygen and blood flow to the brain.

Symptoms of sleep apnea include feeling tired and needing a nap even after eight hours of sleep, loud snoring that disturbs others and snorting that indicates breathing has stopped.

Using an airway pressurization mask can help the brain restore normal function, Mohsenin said, although no study has shown it lowers the rate of strokes.

(Shanghai Daily via Agencies January 8, 2009)