Moderate to severe obstructive sleep apnea (OSA) has been shown to increase the risk of ischemic stroke, by as much as three times in men. Although sleep apnea frequently goes undiagnosed, population-based studies indicate that as many as 1 in 15 adults has moderate to severe OSA. Researchers at Mayo Clinic in Rochester, Minn., are uncovering the mechanisms by which OSA increases ischemic stroke risk, as well as strategies for managing that risk in patients.
A recent Mayo study found that cardioembolic stroke is far more common in patients with OSA than in patients without OSA. The retrospective case-control study examined the records of 53 patients who had polysomnography at Mayo Clinic between 2000 and 2011, and who had an ischemic stroke within one year after the sleep study. Thirty-two of the patients met the criteria for OSA and were classified as cases; 21 did not meet criteria for OSA and were classified as controls.
Among the OSA cases, 71.9 percent had cardioembolic strokes, compared with 33.3 percent in the control group. Large artery atherosclerosis and small vessel occlusion were found to be more common in patients without OSA. In addition, the frequency of cardioembolic stroke rose with the severity of OSA. Among patients with OSA, 84 percent had at least one cardioembolic risk factor, such as dilated cardiomyopathy, compared with 52 percent of controls.
"It may be that OSA leads to structural and physiologic changes in the heart that can predispose patients to cardioembolic stroke via mechanisms other than atrial fibrillation," says Melissa C. Lipford, M.D., a neurologist at the Center for Sleep Medicine at Mayo Clinic in Minnesota who led the study. "The higher rates of cardioembolic strokes in OSA patients may also be due to a greater proportion of undiagnosed paroxysmal atrial fibrillation in this group."
An important strength of the study is that patients were all diagnosed with OSA prior to the stroke. Had the study included patients diagnosed with OSA in the post-stroke period, it would be unclear whether OSA was a risk factor leading to the stroke or if the stroke itself caused the OSA, via oropharyngeal weakness or other mechanisms.
"These results suggest that a high level of suspicion for cardioembolism is warranted when a patient with OSA suffers a stroke," Dr. Lipford says. "When OSA patients present with a cryptogenic stroke, further cardiac work-up may be considered, such as transesophageal echocardiography to identify cardioembolic risk factors, Holter monitoring or even extended cardiac monitoring to identify paroxysmal atrial fibrillation. But we must still employ a multifaceted risk-reduction strategy because OSA also increases risk of large artery atherosclerotic and small vessel occlusion strokes."
Further research is needed to determine precisely how OSA increases stroke risk. But Dr. Lipford notes that OSA patients generally don't experience the 10 to 15 percent drop in systolic blood pressure that typically occurs during sleep. "During an apneic episode, the body asserts an amazing amount of effort to try to open the airway and get a breath in," she says. "Oxygen levels go down, and carbon dioxide levels go up." These repetitive apneic episodes are associated with sympathetic nervous system surges that increase blood pressure and cause heart rates to fluctuate, leading over time to hypertension and atrial fibrillation, which are prime risk factors for ischemic stroke.
Whether or not sleep apnea was present before stroke, most patients experience OSA after stroke. OSA in stroke patients is associated with early neurologic worsening, decreased functional recovery and increased mortality.
Yet continuous positive airway pressure (CPAP), the typical therapy for sleep apnea, poses significant challenges for stroke patients:
"If somebody puts something over your face, you don't understand why, and if you don't have the dexterity to remove it, it can feel claustrophobic," Dr. Lipford says.
At Mayo, neurologists who are sleep specialists take additional steps to treat OSA in stroke patients. Time is spent determining the most appropriate mode of communication for explaining to the patient the rationale behind CPAP therapy. Nurses and respiratory technicians work with patients and their families to determine the most comfortable CPAP interface, including a nasal-only interface if possible. CPAP may be started gradually, such as during daytime naps, before use at night.
For patients who can't tolerate CPAP, Mayo sleep specialists suggest alternatives. "In some cases it may be as simple as sleeping on your side instead of your back," Dr. Lipford says. "Even a reduction in OSA severity can be helpful." Other alternatives include weight loss, nasal expiratory positive pressure devices and mandibular repositioning appliances that shift the lower jaw forward to help keep soft tissues from blocking airways.
"At Mayo Clinic, we understand the challenges faced by stroke patients that make OSA treatment difficult," Dr. Lipford says. "But OSA treatment is crucial. If sleep improves, patients are more alert during the day and more apt to participate in rehabilitation therapies. Treated patients have improved neurologic recovery and their ongoing risk of stroke is reduced."