Pharyngeal airway wall mechanics using tagged magnetic resonance imaging during medial hypoglossal nerve stimulation in rats

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Abstract
To better understand pharyngeal airway mechanics as it relates to the pathogenesis and treatment of obstructive sleep apnoea, we have developed a novel application of magnetic resonance imaging (MRI) with non-invasive tissue tagging to measure pharyngeal wall tissue motion during active dilatation of the airway. Eleven anaesthetized Sprague-Dawley rats were surgically prepared with platinum electrodes for bilateral stimulation of the medial branch of the hypoglossus nerve that supplies motor output to the protrudor and intrinsic tongue muscles. Images of the pharyngeal airway were acquired before and during stimulation using a gated multislice, spoiled gradient recalled (SPGR) imaging protocol in a 4.7 T magnet. The tag pulses, applied before stimulation, created a grid pattern of magnetically imbedded dark lines that revealed tissue motion in images acquired during stimulation. Stimulation significantly increased cross-sectional area, and anteroposterior and lateral dimensions in the oropharyngeal and velopharyngeal airways when results were averaged across the rostral, mid- and caudal pharynx (P < 0.001). Customized software for tissue motion-tracking and finite element-analysis showed that changes in airway size were associated with ventral displacement of tissues in the ventral pharyngeal wall in the rostral, mid- and caudal pharyngeal regions (P < 0.0032) and ventral displacement of the lateral walls in the mid- and caudal regions (P < 0.0001). In addition, principal maximum stretch was significantly increased in the lateral walls (P < 0.023) in a ventral–lateral direction in the mid- and caudal pharyngeal regions and principal maximum compression (perpendicular to stretch) was significantly increased in the ventral walls in all regions (P < 0.0001). Stimulation did not cause lateral displacement of the lateral pharyngeal walls at any level. The results reveal that the increase in pharyngeal airway size resulting from stimulation of the medial branch of the hypoglossal nerve is predominantly due to ventral displacement of the ventral and lateral pharyngeal walls.