The use of the Vagal Nerve Stimulator (VNS) to help control seizures in people with some forms of epilepsy may also affect some patients' breathing during sleep, according to results of a small pilot study at the University of Michigan.
Published in the medical journal Neurology, the study focuses on four epilepsy patients who chose to have VNS devices implanted in their chests after medication failed to control their seizures. They were taking part in a trial to assess how VNS affects daytime sleepiness - a study that found a positive effect from low to medium VNS intensities. But the night-time breathing problem was spotted during overnight monitoring of their airflow, blood oxygen and effort.
All four had far more breathing decreases or interruptions when the device was on than when it was off. All four had an overall lower airflow and breathing effort after getting their devices than before, an effect seen during almost every "on" cycle of their devices. Most of the breathing events, though, were not severe or long-lasting enough to be classed as apneas (an 80 per cent or greater decrease in breathing for more than 10 seconds), or hypopneas (a breathing interruption for more than 10 seconds together with a partial awakening, or a decrease in blood oxygen).
One male patient, who had a pre-existing but undiagnosed case of obstructive sleep apnea, experienced 11 apneas or hypopneas per total hour of sleep, including the time when the device was sending signals and the time when it wasn't. He had had only four such events per hour in the sleep test before he got his device.
The other patients, who had not had signs of sleep apnea in their baseline tests, also experienced an increase in apneas and hypopneas, although these remained below five times an hour, within the normal range. When Malow and her colleagues calculated the rate of interruptions during just the time when the device was stimulating their vagus nerve, it came out to 19 per hour.
Malow adds that follow-up studies of two patients showed that fine-tuning of their VNS devices may help reduce breathing disruptions. Both the patient with existing apnea and one of the others showed a decrease in apneas and hypopneas when the stimulus frequency of the electrical signal was decreased.
"In those with mild sleep apnea, decreasing the stimulus frequency or lengthening the time between stimulations may keep the problem from worsening,"
Malow explains. "And, as in the initial studies on VNS in depression, lower stimulus parameters may actually increase effectiveness. But more study is needed to tell for sure." Malow notes that VNS patients who experience sleep apnea may also be helped by the conventional treatments for the condition, including adjustments to sleeping position, weight loss, and use of a device known as CPAP that keeps the throat open during sleep using increased air pressure.