Effect of Weight Loss on Upper Airway Anatomy and the Apnea-Hypopnea Index. The Importance of Tongue Fat

Abstract

Rationale: Obesity is the primary risk factor for obstructive sleep apnea (OSA). Tongue fat is increased in obese persons with OSA, and may explain the relationship between obesity and OSA. Weight loss improves OSA, but the mechanism is unknown.Objectives: To determine the effect of weight loss on upper airway anatomy in subjects with obesity and OSA. We hypothesized that weight loss would decrease soft tissue volumes and tongue fat, and that these changes would correlate with reductions in apnea-hypopnea index (AHI).Methods: A total of 67 individuals with obesity and OSA (AHI ≥ 10 events/h) underwent a sleep study and upper airway and abdominal magnetic resonance imaging before and after a weight loss intervention (intensive lifestyle modification or bariatric surgery). Airway sizes and soft tissue, tongue fat, and abdominal fat volumes were quantified. Associations between weight loss and changes in these structures, and relationships to AHI changes, were examined.Measurements and Main Results: Weight loss was significantly associated with reductions in tongue fat and pterygoid and total lateral wall volumes. Reductions in tongue fat were strongly correlated with reductions in AHI (Pearson's rho = 0.62, P < 0.0001); results remained after controlling for weight loss (Pearson's rho = 0.36, P = 0.014). Reduction in tongue fat volume was the primary upper airway mediator of the relationship between weight loss and AHI improvement.Conclusions: Weight loss reduced volumes of several upper airway soft tissues in subjects with obesity and OSA. Improved AHI with weight loss was mediated by reductions in tongue fat. New treatments that reduce tongue fat should be considered for patients with OSA.

Keywords: apnea–hypopnea index; obstructive sleep apnea; upper airway; weight loss.

Figure 1.

The relationship of percentage change in tongue fat volume with percentage change in weight and apnea–hypopnea index (AHI). The associations between the percentage change in tongue fat and weight loss (left panel) and AHI change (right panel) are illustrated among obese subjects with apnea undergoing surgical or medical weight loss. Strong positive correlations were observed between tongue fat change and both measures (Pearson’s partial rho = 0.62, P < 0.0001) in covariate adjusted analyses. Mediation analyses suggest that percentage change in tongue fat was the primary upper airway mediator between percentage weight loss and percentage reductions in AHI.

Figure 2.

Changes in upper airway soft tissue structures with weight loss. Three-dimensional reconstructions derived from axial magnetic resonance imaging (T1-weighted, spin echo, 3-mm slice thickness), demonstrating changes in selected upper airway soft tissue structures between baseline and 6-month follow-up in a male patient with sleep apnea. Structures include: tongue, defined as the genioglossus muscle (red); soft palate (magenta); parapharyngeal fat pads (yellow); and lateral pharyngeal walls (green). The region of interest extends from the superior appearance of the tongue to the appearance of the hyoid bone. AHI = apnea–hypopnea index; BMI = body mass index.

Figure 3.

Change in tongue fat volume with weight loss. Three-dimensional reconstruction of tongue (red) and tongue fat (yellow) derived from axial magnetic resonance imaging (MRI; T1-weighted, spin echo, 3-mm slice thickness) and Dixon fat-only MRI (3-mm slice thickness), demonstrating loss of tongue fat between baseline and a 6-month follow-up visit in the same male subject with apnea as shown in Figure 2. The tongue is defined as the genioglossus muscle, and tongue fat is defined as all fat within the genioglossus. AHI = apnea–hypopnea index; BMI = body mass index.

Figure 4.

Change in abdominal fat volumes with weight loss. Three-dimensional reconstructions of abdomen derived from axial magnetic resonance imaging (T1-weighted, spin echo, 10-mm slice thickness) showing fat loss between baseline and a 6-month follow-up visit in the same male subject with apnea as shown in Figures 2 and 3. Subcutaneous fat (cyan), visceral fat (yellow), and the liver (brown) have been highlighted. Subcutaneous fat is defined as all fat superficial to the abdominal fascia. Visceral fat is defined as all fat within the abdominal fascia that is not part of the spinal column. The region of interest extends from the superior appearance of the liver to the L5–S1 intervertebral disc. AHI = apnea–hypopnea index; BMI = body mass index; Subcut. = subcutaneous; Visc. = visceral.

Figure 5.

Mediation of percentage change in weight and apnea–hypopnea index (AHI) by percentage change in tongue fat. Results of the single mediator model of percent change in tongue fat volume mediating the relationship between percent change in weight and in AHI are shown. Unstandardized path coefficients, interpreted as the expected percentage change in outcome for a 1% change in predictor, of the relationships between percent weight change and percent change in tongue fat (path a = 1.168), between percent change in tongue fat and percent change in AHI (path b = 1.074), and the remaining direct effect between percent change in weight and percent change in AHI (path c′ = 2.337) are also shown. Significance of path coefficients is denoted as: *P < 0.05, **P < 0.01, and ***P < 0.001.