What is the link between sleep apnea and neurodegenerative conditions? Researchers investigate.
Obstructive sleep apnea is characterized by the occasional inability to breathe while asleep, due to a collapse of the airway. This may cause the sleeper to wake up repeatedly during the night whenever breathing becomes difficult, resulting in disturbed sleep patterns.
Recent data suggest that in the United States, the prevalence of this disorder is around 34 percent for men and 17 percent for women, which makes it a fairly common health issue. However, specialists say that in up to 80 percent of cases, the condition remains undiagnosed.
Tied with a large number of negative health outcomes — including heightened risk of heart attack and type 2 diabetes — obstructive sleep apnea may also be linked to an increased risk of Alzheimer's disease in the older population, new research suggests.
The new study, which was published yesterday in the American Journal of Respiratory and Critical Care Medicine, shows that older people affected by obstructive sleep apnea have higher levels of amyloid beta, the peptides involved in the brain plaque buildup that is characteristic of Alzheimer's disease.
"Several studies have suggested that sleep disturbances might contribute to amyloid deposits and accelerate cognitive decline in those at risk for [Alzheimer's disease]," explains senior author Dr. Ricardo S. Osorio, from the New York University School of Medicine in New York City.
"However," he continues, "so far it has been challenging to verify causality for these associations because [obstructive sleep apnea] and [Alzheimer's disease] share risk factors and commonly coexist."
Severity of sleep apnea and amyloid levels
For the study, the researchers recruited 208 participants aged between 55 and 90, none of whom had any current cognitive impairments or depression. Also, none of them used continuous positive airway pressure, which is a common treatment for sleep apnea.
In their tests, Dr. Osorio and his research team performed a medical procedure known as a "lumbar puncture" to collect cerebrospinal fluid — or the fluid contained by the brain and spinal cord — as well as positron emission tomography (PET), in view of measuring each participant's levels of amyloid beta.
The team found that over 50 percent of the study participants had obstructive sleep apnea. Of these, 36.5 percent had a mild form of the disorder, and 16.8 percent had a severe form.
Moreover, 104 of the original participants also took part in a longitudinal study that lasted for a period of 2 years. This study revealed a link between the severity of sleep apnea and the concentration of amyloid beta detectable in cerebrospinal fluid in the long-term.
In other words, individuals who experienced more apneas throughout their normal sleep period had increased levels of amyloid beta. The PET scans confirmed this correlation.
'Nonlinear link' at play
However, there was no direct relationship between the severity of sleep apnea and the onset of cognitive impairment in the study participants.
Study co-author Dr. Andrew Varga, from the Icahn School of Medicine at Mount Sinai in New York City, NY, suggests that one explanation may be that any changes in the amyloid levels happen in the preclinical or early stages of Alzheimer's.
"The relationship between amyloid burden and cognition is probably nonlinear and dependent on additional factors."
Dr. Andrew Varga
In the published paper, the researchers also admit that their study faced a number of limitations, such as "the relative short duration and the lack of longitudinal sleep data." Another somewhat limiting factor, they add, is that only some of the participants opted to be involved in the 2-year follow-up study.
Nevertheless, the researchers argue that their findings point to a link between cognitive decline and sleep apnea, and that physical aging significantly influences this link.
"Results from this study, and the growing literature suggesting that [obstructive sleep apnea], cognitive decline, and [Alzheimer's disease] are related, may mean that age tips the known consequences of [obstructive sleep apnea] from sleepiness, cardiovascular, and metabolic dysfunction to brain impairment," explains Dr. Osorio.
"If this is the case," he adds, "then the potential benefit of developing better screening tools to diagnose [obstructive sleep apnea] in the elderly who are often asymptomatic is enormous."