February 20, 2024 | Researchers in Spain are advocating the use of sweat in metabolomics studies to measure the severity of sleep apnea. It’s an ideal alternative to blood and urine because the sampling is noninvasive and accessible, and the biofluid doesn’t require much manipulation by the test operator that can introduce variability, according to Laura Castillo, researcher and Ph.D. student at the University of Córdoba.
In a proof-of-concept study that was published recently in the Journal of Sleep Research (DOI: 10.1111/jsr.14075), metabolic overnight changes in sweat collected from sleep apnea patients were evaluated by Castillo and her team and classified based on their oxygen desaturation index (ODI). The intent was to show that ODI can help clinicians better discern which patients are in most urgent need of therapy among those in the queue.
Sleep apnea is currently diagnosed based on the apnea-hypopnea index (AHI), or episodes of shortness of breath suffered per hour, with 30 or more constituting severe disease. The problem with the AHI, says Castillo, is that it only considers the number of apnea-hypopnea events while ODI reflects the number of these episodes that are concerning.
Here, researchers used the ODI to show the seriousness of sleep apnea based on the number of events where oxygen saturation decreased by more than 3%. It was used to augment data provided by the AHI, with which the ODI was demonstrated to have a linear relationship.
She and her team identified a total of 78 metabolites, about 10% of which were useful for characterizing disease severity. Most, unsurprisingly, were related to energy production and oxidative stress. The metabolic profile of individuals reveals if they have intermittent hypoxemia that heightens their risk for permanent metabolic changes caused by the disease—and possible further repercussions due to those changes, notably cardiovascular complications.
Metabolites related to amino acids metabolism were “significantly different overnight depending on the severity group” of patients (severe or non-severe), Castillo reports. These include tyramine, which helps regulate blood pressure, and alanine, a source of energy for muscles and the central nervous system. Both are related to many other metabolic processes and contribute to multiple bodily functions and comorbidities.
Blood samples, which are typically used for sleep apnea diagnosis and severity characterization, are complex biofluids that contain far more proteins and metabolites than sweat and can interfere and belabor metabolic analysis, says Castillo. Urine samples allow for a less invasive sample collection process, but as a practical matter can’t always be obtained due to illness or the patient’s inability to cooperate.
For these reasons, sweat is a great sampling option, says Castillo. An added advantage is that a relatively low volume of sweat is required to obtain good test results.
There is not yet a standard protocol for collecting sweat, Castillo says, but after some experimentation she and her colleagues opted to use a system (Macroduct Sweat Analysis) that involves passively stimulating sweat via stainless steel electrodes attached to the arm. The sweat then gets collected in a disposable plastic device for analysis.
Other sweat collection methods require individuals to exercise, which is not necessarily ideal for sleep apnea patients, or patches, where the disadvantages are both low volume of biofluid that can be absorbed onto a gauze or cloth pad and the contaminants from plastics or additives that can interfere with test results, she says.
During sleep apnea events, the body is denied oxygen and that in turn provokes fatigue and slows down energy production, says Castillo in talking about the identified metabolites. Oxidative stress is a secondary effect of all diseases—even the normal aging process—and disrupts proper cellular functioning and irreversibly weakens the body overall.
Castillo’s research group started studying sleep apnea due to a collaboration with the respiratory unit at Reina Sofía University Hospital in Córdoba, which was investigating noninvasive alternatives to exhaled breath collectors that patients found uncomfortable. Discussions ensued about how the disease wasn’t being studied through the night, but rather using a home testing device measuring point-in-time breathing and oxygen levels.
It was then that the idea emerged to track the evolution of those measurements with the latest study, she says. In addition to an initial test to classify patients as having either severe or non-severe sleep apnea, the same procedure could be repeated every two or three years to track patient improvement or decline. The timetable would depend on symptoms and how quickly the disease is advancing.
Immediate next steps include convincing physicians to adopt the untargeted metabolomic strategy, Castillo says. Limiting factors include the expense and availability of gas chromatography–mass spectrometry instruments and the expertise to operate them.
Globally, almost one billion people are affected by obstructive sleep apnea with prevalence exceeding 50% in some countries (The Lancet Respiratory Medicine, DOI: 10.1016/S2213-2600(19)30198-5). Awareness and knowledge of sleep apnea remains concernedly low, says Castillo, which is responsible for large numbers of people suffering from the severe form of the disease with its accompanying disabilities and high treatment costs. Right now, she adds, people with sleep apnea are “like cats who hide their disease until they are in really bad condition” in that they suffer for years, undiagnosed and therefore untreated.