OSA and OHS
“With sleep apnea we tend to focus on anatomic considerations, but there may be relationships or interactions between obesity and neuromuscular function and neuroventilatory control,” Susheel P. Patil, MD, PhD, director of the sleep medicine program for University Hospitals and assistant professor at Case Western Reserve University, Cleveland, said in an interview.
Dr. Susheel P. Patil
Some studies suggest, for instance, that TNF-alpha can increase obstructive sleep apnea (OSA) susceptibility and severity through its neuroventilatory modulating properties during sleep. And the potential for additional proinflammatory cytokines produced by adipose tissue to similarly affect upper airway neuroventilatory control is an “intriguing line” of inquiry for researchers in the sleep apnea space, he said.
Leptin is of interest particularly in obesity hypoventilation syndrome (OHS), which is characterized by chronic daytime hypercapnia. Best known as a satiety hormone, leptin is produced by adipose tissue and suppresses appetite at the central nervous system level. But it has long been known that leptin also affects ventilation and the control of breathing.
When transported across the blood-brain barrier, leptin increases the hypercapnic ventilatory response, Babak Mokhlesi, MD, MSc, codirector of the Rush Lung Center and chief of pulmonary, critical care, and sleep medicine at Rush University Medical Center in Chicago, said in an interview.
Research suggests that patients with OHS may have resistance to leptin at the central nervous system level – with leptin not reaching the sites of ventilatory control. This is a “prevailing theory” and could explain why these patients “do not augment their ventilation to maintain homeostasis, normal levels of CO2,” Dr. Mokhlesi said.
Dr. Babak Mokhlesi
“Why some patients with severe obesity develop CO2 retention while others do not is not fully understood,” he said, noting that patients with OHS can normalize their CO2 quickly when instructed to take deep breaths. “What we know is that the centers in the brain responsible for augmenting ventilation when CO2 goes up are somehow blunted.”
In a study of obese mice led by Vsevolod Y. Polotsky, MD, PhD, of Johns Hopkins University, Baltimore – and highlighted by Dr. Mokhlesi as an example of important, recent research – leptin delivered intranasally alleviated hypoventilation (and upper-airway obstruction), while intraperitoneally administered leptin did not, seemingly overcoming “central leptin deficiency.” (Am J Respir Crit Care Med. 2019;199[6]:773-83).
“This proved that there is some level of resistance in this animal model ... and has potential for therapeutics in the future,” Dr. Mokhlesi said.
Understanding the role of insulin resistance in OSA is another research focus. Some data suggest that insulin resistance, which is more common in obesity, is more prevalent in populations with OSA, Dr. Patil said. Researchers have discussed a bidirectional relationship for years, but it’s likely that insulin resistance is a precursor, he said.
In a mechanistic study published in 2016, Dr. Patil and his coinvestigators found that obese individuals with insulin resistance but without frank diabetes or sleep apnea demonstrated preclinical elevations in pharyngeal collapsibility during sleep. The findings suggest that insulin resistance could play a causal role in OSA pathogenesis by “generating requisite elevations in pharyngeal collapsibility,” they wrote (Eur. Respir J. 2016;47[6]:1718-26).
More recently, Dr. Patil noted in the interview, there is increasing appreciation in academia that the type of fat may be important to predicting OSA. “Visceral fat has a completely different cytokine-secretion profile than subcutaneous fat ... it is the more metabolically active fat that may secondarily impact upper airway function though a neuroinflammatory mechanism,” he said. “That is one of the working hypotheses today.”