New research finds that women with a severe form of sleep apnea are more likely to receive a cancer diagnosis.
The recent study, which features in the European Respiratory Journal, analyzed data on 20,000 adults with sleep apnea.
About 2% of the participants also had a diagnosis of cancer in their medical history.
The data came from the European Sleep Apnea Database (ESADA), which has medical and visit records on adults registered at 33 centers across Europe.
The international research team points out that while the findings do not prove that sleep apnea causes cancer, there appears to be a clear link in females.
"It's reasonable to assume that sleep apnea is a risk factor for cancer or that both conditions have common risk factors, such as [being] overweight," says Ludger Grote, an adjunct professor and chief physician in sleep medicine at Gothenburg University in Sweden.
"On the other hand," he adds, "it is less likely that cancer leads to sleep apnea."
Obstructive sleep apnea
Obstructive sleep apnea (OSA), or sleep apnea, is a condition in which a person experiences short and repeated interruptions of breathing while asleep. The condition results from the inability of the throat muscles to keep the airway open.
There is another form of apnea called central sleep apnea, which occurs due to a failure in brain signaling. Of the two types of apnea, OSA is by far the most common.
OSA can cause disturbed, fragmented sleep and lead to an insufficiency of oxygen.
According to the National Sleep Foundation (NSF), OSA affects 5–20% of adults in the United States.
The estimate comes from studies that used formal tests and assessments. The NSF suggest that the actual percentage of people in the U.S. with sleep apnea is likely to be greater.
"The condition of sleep apnea is well-known to the general public and associated with snoring, daytime fatigue, and elevated risk of cardiovascular disease, especially in men," Dr. Grote explains.
He and his colleagues note that while there is growing evidence for a link between OSA and cancer, it remains under debate. The main reason appears to be the small numbers of study participants and "poor characterization" of types of OSA and cancer.
Theories about cancer and sleep apnea
Scientists have proposed several theories for a link between OSA and cancer. One theory emphasizes that the two conditions share common risk factors, such as age and obesity.
Another theory is that intermittent hypoxia — a condition in which the body does not get enough oxygen — and sleep fragmentation, both of which occur in OSA, may also explain the link to cancer.
This type of link could involve changes in the development of blood vessels, immune function, and the tissue environment of tumors.
However, until the recent research, very few studies had examined whether biological sex made a difference to the link between OSA and cancer.
In their results, Dr. Grote and his colleagues found, as expected, that there was an association between older age and a higher risk of cancer.
However, when they adjusted the results of the analysis to take out the potential influence of age, sex, body mass index (BMI), alcohol intake, and smoking, they found a possible association between intermittent nocturnal hypoxia and higher rates of cancer.
Stronger link in females
In addition, the analysis revealed that the link between nocturnal hypoxia and higher rates of cancer was stronger in females and weaker in males.
The results suggest that the risk of cancer is two to three times higher in females with severe symptoms of sleep apnea.
Dr. Grote says that previous research has tended to focus on the link between OSA and malignant melanoma. In the light of his team's findings, "Cancer of the breast or womb may now become a new area," he suggests.
Perhaps a combination of female sex hormones and stress arising from nocturnal hypoxia in OSA triggers the start of cancer or reduces the body's immune defenses, he muses.
"It's impossible to say for sure what causes underlie the association between sleep apnea and cancer, but the indication means we need to study it in more depth."
Dr. Ludger Grote