Mechanics and neuromuscular activation of the upper airway in obstructive sleep apnoea

Obstructive sleep apnoea (OSA) results from a combination of anatomical and neuromuscular defects. The mechanical properties of the airway and muscular activity in response to neural drive are poorly characterized. Objectives: To quantify the movement and strain of the upper airway tissues using dynamic MRI techniques and to improve understanding of airway mechanics. Methods: Spatial Modulation of Magnetization (SPAMM) and Magnetic Resonance Elastography (MRE) were used to investigate the mechanics of the tissues surrounding the airway in OSA patients. EMG was performed to quantify any respiratory related geniohyoid activity in quiet breathing and in the genioglossus with 90Hz vibration to ensure the vibration stimulus used during MRE was not activating the muscle as a potential confounder. SPAMM allows a grid of saturated magnetization to be superimposed on tissues using MRI and tissue motion to be followed with ultra-fast imaging as the grid deforms. SPAMM was used to image the movement of the tongue and lateral walls of the airway associated with quiet breathing in OSA patients compared to controls matched for age, sex and BMI. It was also used to image the movement of the tissues surrounding the airway associated with mandibular advancement across a spectrum of OSA (AHI range 0 75) and the effect of increasing airway pressure from 5 to 10cmH2O in subjects with moderate or severe OSA. MRE is a method of quantifying the shear storage modulus of tissue by imaging the propagation speed of a shear wave produced by vibrating the tissue. Oscillating motion sensitive field gradients are synchronized with the propagating waves in order to image the wave and a quantitative map of tissue mechanical properties is produced by mathematically reconstructing the wave in three dimensions. The shear modulus of the tissue is related to the wavelength of the wave in the tissue. Viscosity can be measured from the attenuation of the amplitude of the wave. MRE was used to measure the