Genioglossus activation during maximal sniff manoeuvres: Is upper airway function relevant in the clinical assessment of inspiratory and expiratory muscle strength?

Measurements of maximal inspiratory and expiratory pressures serve as a useful surrogate for inspiratory and expiratory muscle strength, with widespread use in experimental and clinical settings. Peak pressures during maximal voluntary manoeuvres are reflective of global respiratory muscle strength; importantly, such measures have strong prognostic value in a variety of diseases. Mouth pressures are commonly recorded but may be limited in clinical application when individuals have difficulty performing manoeuvres; for example, in persons with significant orofacial muscle weakness. In such circumstances, nasal respiratory-related pressures can serve as an important proxy measure of respiratory muscle force-generating capacity. Indeed, sniff tests have long-standing clinical use in the assessment of diaphragmaticmovement and transdiaphragmatic pressure generation. Maximal sniff nasal inspiratory pressure (SNIPmax) is a predictor of mortality in amyotrophic lateral sclerosis (Capozzo et al., 2015), and SNIP measures are reliable and easy to perform in children with neuromuscular disease, where they prove to be an indicator of the loss of respiratory muscle strength, earlier than that revealed by the measurement of vital capacity by spirometry (Nève et al., 2013). Sniff nasal inspiratory pressure also has application in the longitudinal assessment of chronic lung disease. In this issue of Experimental Physiology, Ichikawa et al. (2018) examined respiratory muscle activation in young healthy male participants during short, sharp, maximal inspiratory and expiratorymanoeuvres, generating flow through the nosewith themouth closed. A balloon catheter inserted into one nostril connected to a pressure transducer afforded the measurement of nasal respiratory pressures with the contralateral nostril remaining patent to allow inspiratory and expiratory flow at rest and during voluntary efforts. Participants were instrumented with fine wire electrodes inserted in a systematic fashion into the parasternal intercostal and scalene muscles (inspiratory) and the transversus abdominus (expiratory) muscle to record respiratory-related EMG activities. In addition, electrodes were inserted into the genioglossus (the principal pharyngeal airway dilator), using a submental approach. The SNIPmax andmaximal ‘reverse sniff’ expiratory pressure (RSNEPmax) were determined in all participants in a seated position, with measurements during graded efforts of increasing intensities also recorded in a subset of participants. The RSNEPmax provides an index of expiratory muscle strength, a significant factor in the generation of cough and airway clearance manoeuvres, whichprotect against aspirationpneumonia, amajor causeofmorbidity andmortality. Allmanoeuvreswereperformedbeginning at end-expiratory volume (functional residual capacity). The principal focus of the study was the characterization of genioglossus muscle activation during SNIP and RSNEPmanoeuvres, primarilymotivated by the observation of reduced SNIPmax inmotor neuron patientswith bulbar involvement comparedwith non-bulbar disease (Chaudri, Liu,Watson, Jefferson, & Kinnear, 2000), whichmight relate to impaired control of pharyngeal airway calibre. In each participant, genioglossus EMG increased linearly with increasing SNIP and RSNEP. Genioglossus activation was coincident with activation of parasternal intercostal and scalene EMGs during SNIP manoeuvres. Peak inspiratory EMG activities occurred during SNIPmax. Genioglossus EMG activity was also increased during RSNEP manoeuvres coincident with activation of transversus abdominus EMG activity. Peak expiratory EMG activities occurred during RSNEPmax, but genioglossus activation was significantly less during RSNEPmax compared with SNIPmax.GenioglossusEMGactivationduring graded inspiratoryefforts is consistentwith studies in a varietyof species, includinghumans, showing inspiratory-related upper airwaymuscle activity and reflex activation that enhances cranial motor outflow to the striatedmuscles of the upper airway, effectively stiffening and/or dilating the compliant conducting airway, promoting patency and pulmonary airflow. Expiratory-related activity of the genioglossus has been described previously, and characterization of human genioglossus motor units has revealed a population of units that showpeak discharge activities during expiration (Nicholas et al., 2010). Positive airway pressure activates laryngealmechanoreceptors and evokes reflex alterations in genioglossus activity. Although this literature is unsettled, there is evidence in humans that expiratory positive airway pressure increases genioglossus EMG activity, but interestingly, this effect is not abolished by anaesthesia of the upper airway (Deegan, Nolan, Carey, & McNicholas, 1996). It thus appears from the study of Ichikawa et al. (2018) that activation of the genioglossus and the presumed mechanical consequences arising thereof is an inherent feature of maximal manoeuvres used to assess respiratory muscle strength. This raises the question as to whether impaired upper airway motor and muscle function has implications for the clinical assessment of inspiratory and expiratory muscle strength in the relevant patient populations. Clearly, aberrant control of the upper airway is problematic in a variety of neuromuscular diseases, predisposing to the development of sleep-disorderedbreathing anddysphagia.However, does ineffective control of the upper airway influence the estimation of global respiratory muscle strength? Flow limitation or partial obstruction of the upper airway would compromise the reliability of the measurement of