Around 10% of adults contend with obstructive sleep apnea, a condition characterized by symptoms of brain dysfunction such as extreme daytime sleepiness, depression, anxiety and memory problems. Now, a new study published in PLOS One suggests the damage done in the brains of sleep apnea sufferers is down to weaker brain blood flow.
The study was led by Paul Macey, of the University of California-Los Angeles (UCLA) School of Nursing, and funded by the National Institute of Nursing Research.
People with obstructive sleep apnea (OSA) typically make gasping or snorting noises periodically during sleep, which momentarily interrupts their sleep hundreds of times a night. Every time their breathing stops, their blood oxygen level drops, damaging cells in the body.
According to the Centers for Disease Control and Prevention (CDC), if other medical problems such as congestive heart failure or nasal obstruction are present, the condition may resolve with treatment of these problems.
However, gentle air pressure delivered during sleep - usually in the form of a nasal continuous positive airway pressure (CPAP) device - can also serve as an effective treatment.
'Weaker brain blood flow response' in OSA patients
For their latest study, Macey and colleagues measured brain blood flow in sleep apnea patients using a non-invasive MRI procedure called the global blood volume and oxygen dependent (BOLD) signal.
They explain that this method is typically used to examine brain activity, and since previous research showed that sleep apnea sufferers often have poor regulation of blood in the brain, they used the whole-brain BOLD signal to observe blood flow in participants with and without OSA.
"We know there is injury to the brain from sleep apnea," says Macey, "and we also know that the heart has problems pumping blood to the body, and potentially also to the brain." He explains that by using the BOLD method, they were able to observe changes in oxygenated blood amounts throughout the whole brain.
Participants from the study, which included both men and women with and without OSA, had their BOLD signals measured while they were awake during three physical tasks:
- The Valsala maneuver, in which they breathed out forcefully through a small tube that raises the pressure in the chest
- A hand-grip challenge, in which the participants squeezed hard with their hand
- A cold-pressor challenge, in which the participants' right foot was placed in icy water for 1 minute.
Macey says the Valsala maneuver did not yield significant differences between the participants with sleep apnea and without; however, with the hand-grip and cold-pressor challenges, the individuals with OSA had a "much weaker brain blood flow response."
The investigators hypothesize that these differences were due to the signals from the nerves in the arms and legs needing to be processed through high brain areas that control sensation and muscle movement - which was slower because of the brain injury.
They add that because changes from the Valsala maneuver are mainly driven by blood pressure signaling in the chest, it does not require the muscle-controlling parts of the brain.
Another finding from the study reveals that this problem is greater in women with OSA than men, which the researchers say could explain why women have worse apnea-related outcomes. Additionally, other studies from UCLA have previously shown that brain injury from the condition is significantly worse in women than in men.
Though their findings are significant, the researchers note several limitations to their study.
One is that the hand-grip was a subjective, non-isometric challenge; the team says ideally, the test would involve holding a grip pressure at a predetermined maximum, rather than a perceived maximum. As such, this challenge "may not be considered a strictly equivalent challenge across subjects or groups," they say.
Additionally, because the female OSA group only had six subjects, the generalizability of their sex-specific finding is somewhat limited. And at least four of the OSA patients had diabetes and potentially undiagnosed hypertension, which are associated with neural deficits and altered cerebral blood flow, which could be confounding factors.
Still, the researchers conclude their study by noting:
"These findings, indicative of reduced cerebral blood flow changes to autonomic challenges in [obstructive sleep apnea], complement earlier reports of altered resting blood flow and reduced cerebral artery responsiveness. Females are more affected than males, an outcome which may contribute to the sex-specific brain injury in the syndrome."
They add that they are currently investigating whether OSA treatment can reverse the damaging effects of the brain damage.