The human upper airway has many important physiological functions including speech, swallowing and breathing. The human tongue forms an important part of the upper airway. It is made up of different muscles with origins both within and external to the tongue. Among these, the genioglossus (GG) is the largest dilator of the upper airway and it has been shown with ‘tagged’ MRI imaging to both depress and protrude the tongue during quiet breathing (Cheng et al 2008). Further understanding of the GG’s role in both healthy people and those with upper airway disorders can be aided by real-time imaging of the tongue’s motion.
Ultrasound does not emit radiation, is non-invasive, gives good resolution of soft tissue structures, and can display cross-sectional anatomy and tissue motion in real time. It also has been used to demonstrate dynamic upper airway motion during swallowing and speech (Shawker et al 1983; Shawker et al 1984). Given these previous studies, we hypothesised that during quiet breathing in an awake subject lying supine, ultrasonography can be used to quantify GG movement with the largest motion in an anterior direction during inspiration. We further hypothesised that the maximal displacement can be seen in the inferoposterior part of the GG and that this technique can be reproduced across different imaging sessions.
WHAT DID WE FIND?
Under ultrasound, the GG was visualized as a large fan-like muscle originating from the mandible and extending upward into the tongue body. As it contracted over the respiratory cycle, it moved the tongue forward and downward. This motion was shown to begin before inspiratory airflow, with the largest displacement observed in the infero-posterior part of the GG.
SIGNIFICANCE AND IMPLICATIONS:
This study demonstrates a novel use of ultrasound to quantify the GG movement in awake humans during quiet breathing. It is a simple, reproducible, safe technique that does not emit radiation. The observed movement is consistent with that observed during quiet breathing with tagged MRI. Given there is evidence from MRI that respiratory motion of GG is markedly impaired in disorders such as obstructive sleep apnea (Bilston & Gandevia 2014), this method, combined with physiological measures, should be broadly applicable to patients with upper airway and sleep disorders to quantify GG motion, providing a new imaging technique for anatomical ‘phenotyping’ in such patients.
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