This month (October 2014) I wrote an editorial in the medical journal SLEEP about a research study that appeared in the same issue. The study is fascinating because it provides a deeper explanation not only of why weight gain is associated with worse sleep apnea and poorer outcomes for many sleep apnea treatments but why not all people develop sleep apnea when they gain weight. Here is a summary of my editorial:
In today’s society that overemphasizes physical beauty and thinness, few have escaped the dreaded experience of a spouse or friend inquiring about their appearance, typically when wearing a new article of clothing. Many have learned the hard way that there are no correct answers in the interrogation that can ensue, and it is usually best to avoid the question at all costs. In contrast, in the current issue of SLEEP, a study published by Richard Schwab and his team at the University of Pennsylvania demonstrate that answering the slightly-different question posed in the title may provide valuable information to patients at risk for obstructive sleep apnea (OSA).
Exploring the link between weight gain and sleep apnea
Weight gain and obesity are major risk factors for sleep apnea, but the reasons why this is true are not entirely clear. In 2007, a study of human cadaver tongues showed that increases in body mass index (a measure of weight compared to height that increases with weight gain) were associated with a greater proportion of fat within the tongue, especially in the back of the tongue. This suggested that weight gain resulted in fat deposition in the back of the tongue, which would be expected to produce physical enlargement of the tongue and narrowing of the space for breathing behind the tongue.
Clinical experience suggests the association between body weight and OSA is more complex than simply related to body weight, as many people gain weight without developing sleep apnea. The study reported in this issue compared overweight and obese individuals with and without OSA, adding a comparison of those with and without sleep apnea that were otherwise very similar (based on body mass index, age, gender, and race). In both evaluations, the overweight and obese individuals with OSA had greater tongue volumes and a greater volume of tongue fat than those without OSA, correcting for differences in body mass index, age, gender, and race. The difference in the proportion of tongue fat was greatest in the tongue base, although there was substantial variation among individuals within the apneic and control groups. Importantly, they evaluated a muscle that should not be important in sleep apnea (the masseter muscle) and saw no such differences.
The implications of this study for sleep apnea and sleep apnea surgery
Understanding the reasons why weight gain contributes to sleep apnea is important, as it explains what we see in taking care of patients and may offer new directions for treatment. This study does both. It explains why a sleep study has to be done to diagnose OSA, as many people who gain weight do not develop sleep apnea. Treatment outcomes for many surgeries and oral appliances are generally worse among individuals with greater body mass index, although there are many individuals who respond well to these treatments even with high body mass index. Variation in tongue size and tongue fat deposition, whether objectively or relative to the space afforded by craniofacial structure, may explain some of this variation and may direct clinicians in their choice among treatment options for individuals who do not tolerate positive airway pressure therapy. It is interesting that a recent multi-center study of tongue base resection for OSA had a mean resection volume of 10.3 mL, with substantial improvement in sleep study results and subjective measures like sleepiness and quality of life; this volume is consistent with the difference in tongue volume between the OSA and control groups in the study reported this month in SLEEP. Also, in a randomized trial of oropharyngeal exercises to treat OSA, reduction in neck circumference was associated with reduction in the apnea-hypopnea index; whether this reduction in neck circumference was due to loss of fat tissue alone is unclear, but it is consistent with the results of the study published this month. Schwab and his team have published a number of studies over the past 20 years, and there are many other potential studies that can be done. We all look forward to the results of future investigations.
Fat deposition within the tongue—and potentially other structures surrounding the upper airway—may transform our understanding of the link between weight gain and OSA, with wide-ranging implications for diagnosis and treatment. Who knows whether we might someday want to ask the question of whether a tongue looks fat?