Tongue stiffness is lower in patients with obstructive sleep apnea during wakefulness compared with matched control subjects

Abstract

Study objectives: This study aimed to determine whether tongue stiffness (shear modulus) in patients with obstructive sleep apnea (OSA) is different for controls matched for age, sex, and body mass index (BMI), and to investigate the effect of continuous positive airway pressure (CPAP) on stiffness.

Design: Controlled experimental study.

Setting: Medical research institute.

Participants: Patients with OSA and age-, sex-, and BMI-matched healthy controls.

Measurements: Magnetic resonance elastography was performed in nine patients with OSA (apnea-hypopnea index (AHI) > 15 events/h) and seven controls (AHI < 10 events/h) matched for age, sex, and BMI. Six of these OSA subjects were also scanned while 10 cmH2O CPAP was applied. Mean isotropic shear modulus and anisotropic shear moduli parallel and perpendicular to the muscle fascicles in the tongue were calculated.

Results: Tongue shear modulus in patients with OSA was lower than that in matched controls (2.68 ± 0.35 (mean ± standard deviation) kPa versus 2.98 ± 0.44 kPa, P < 0.001). Shear modulus decreased with increasing AHI (R = -0.496, P = 0.043), but not age, BMI, or percentage tongue fat. Anisotropic analysis revealed that reduction in stiffness was greatest parallel to the muscle fibers. CPAP had no significant effect on tongue shear modulus.

Conclusions: In awake subjects with obstructive sleep apnea, the tongue is less stiff than in similar healthy subjects and this difference occurs in the muscle fiber direction. CPAP did not significantly reduce tongue stiffness. Thus, any change in neural drive to genioglossus during wakefulness is insufficient to restore normal tongue stiffness.

Keywords: collapsibility; elastography; obstructive sleep apnea.

Figure 1

Sample diffusion tensor imaging (DTI) results. The DTI angle image (right panel) shows variation in fiber angle, where each pixel is color-coded with the mean fiber orientation at that location. The angle scale is in radians (2π = 360°, zero is horizontal). Indicative angles for the genioglossus and geniohyoid are shown as blue arrows. Note that fibers in a given direction may be represented by either an angle between 0 and π, or an angle between π and 2π. The left panel shows the anatomy from a high-resolution anatomical scan in the same subject. There is signal dropout from the mouth guard (white region) on the DTI angle image (red arrow).

Figure 2

Sample isotropic elastography data in a patient with OSA. Anatomy (left panel), shear modulus representing stiffness (G' – center panel), and loss modulus representing viscosity (G'' – right panel) showing the outline for analysis of the tongue and soft palate in red. The most anterior portion of the tongue was not included in the area of interest because of artifact from the mouth guard in the raw data. The scale (far right) is in kPa.

Figure 3

Isotropic shear storage moduli of patients with obstructive sleep apnea (OSA) compared to matched controls. Mean isotropic shear moduli storage modulus (G') and loss modulus (G''). Error bars represent the standard deviation.

Figure 4

Anisotropic shear modulus and mechanical anisotropic ratio of the tongue and soft palate in patients with obstructive sleep apnea (OSA) and matched controls.

Figure 5

Shear storage modulus with and without continuous positive airway pressure (CPAP). Left panel is the raw anatomical image with the tongue and uvula marked. Shear storage modulus of the tongue (black outline) and soft palate (red) without (center panel) and with (right panel) CPAP in one subject. Mean stiffness of the tongue did not change with CPAP but the stiffness of the soft palate increased.