Previous research has linked sleep apnea to cancer progression. However, these studies included only a small number of participants, and there may have been some bias in their measurements.
Obstructive sleep apnea is a breathing disorder in which the airways repeatedly close while the patient is sleeping, causing fragmented sleep that increases risk for a variety of other health problems. About 18 million Americans have sleep apnea.
The new study looked at 10,149 sleep apnea patients who took part in a sleep study between 1994 and 2010, and researchers cross-referenced that information with health administrative databases from 1991-2013.
At the start of the study, 5.1% of participants had been diagnosed with cancer. Study participants were followed for an average of 7.8 years, and during this follow-up period, 6.5% of the participants who did not have cancer at the start of the study developed cancer. The most common cancers in the study were prostate, breast, colorectal and lung cancers.
However, after adjusting their results to take into account various cancer risk factors, the researchers were unable to find a causal link between sleep apnea and cancer.
Although the researchers could not confirm the findings of previous studies that had suggested this link, when analyzing subgroups of data they did find an association between oxygen desaturation - a decrease of oxygen in the blood caused by sleep apnea - and the development of smoking-related cancers.
"The mechanisms are still unclear," author Dr. Tetyana Kendzerska - of the Institute for Clinical Evaluative Sciences and Women's College Hospital, University of Toronto in Canada - told Medical News Today, "and those results obtained from subgroup analyses should be interpreted with caution."
She explained, however, that some research has shown low levels of oxygen can increase cancer progression:
"In experimental studies on animals, hypoxia (low level of oxygen) has been shown to cause tumors to grow more quickly by triggering the growth of blood vessels and preventing timely death of cancer cells, thus resulting in metastasis."
Why did this study not confirm earlier research?
The researchers note that in some previous studies, the participants were older than the participants in this study, with higher body mass index (BMI) and more severe obstructive sleep apnea - so oxygen desaturation might have been common within this group, which could have influenced cancer development.
Other studies, meanwhile, had too few participants to provide robust evidence, or used unreliable measures and classification systems.
This was a large study, which incorporated a long follow-up time. However, the study was unable to gather data on the participants' cancer stage and the management of their cancers, which could potentially have affected the results.
Also, because there were relatively few patients in the study who developed cancer, it was not possible for the researchers to fully examine how different types of cancer may be associated with sleep apnea.
MNT asked Dr. Kendzerska whether her team was surprised by the findings.
"The longitudinal evidence on this relationship is very limited, and one of four studies published before ours also reported a lack of association. The mechanism of an association is unclear, and only chronic intermittent hypoxemia was postulated as a potential link. As such, we were not surprised at the lack of association."
Earlier this year, we reported on another study published in CMAJ that found people with sleep apnea have a higher risk of pneumonia.
And in 2013, MNT looked at a study conducted by researchers from the University of California-Los Angeles, which found that sleep apnea may increase the risk of various health problems for women in particular.