There is a growing body of evidence suggesting common impairments of various laryngeal functions in laryngeal hypersensitivity disorders, such as chronic cough, muscle tension dysphonia, and vocal fold dysfunction. This program of research investigated novel methods for objective measurement of voice and respiratory-swallowing coordination, with the aim of assessing laryngeal function in individuals with laryngeal hypersensitivity.

It is well known that respiration and swallowing must be well coordinated for safe and efficient ingestion of food and fluids. Respiratory-swallowing coordination (RSC) has most commonly been measured using a nasal cannula to estimate respiration from the nose during swallowing tasks. However, there are reports to suggest that mouth breathing is common in populations characterised by laryngeal hypersensitivity, suggesting that nasal flow measures may misrepresent respiration in these individuals. The first experimental study presented in Chapter 5 was completed to examine the utility of a novel, partitioned oronasal facemask, simultaneously measuring nasal and oral respiratory airflow across a range of swallowing tasks in healthy individuals. Results revealed that the frequency of respiratory phase patterns was minimally altered when nasal respiratory data was supplemented with oral data, with some bolus conditions being more influenced than others. This study incidentally revealed high variability of respiratory phase patterns within individuals across two repeated trials of the same swallowing task. Additionally, a novel presentation of swallowing non-respiratory flow (SNRF) was identified, characterised by pre- and post-swallow inward and outward airflow, suggesting that increased sensitivity in capturing airflow increases the ambiguity of respiratory signals surrounding swallowing.

Following on from the study presented in Chapter 5, a secondary study was developed to further investigate the novel findings of the impact of oral airflow on RSC, respiratory phase pattern variability, and SNRF. In a study of healthy participants, the same oronasal facemask was used to obtain respiratory phase patterns, with comparisons made to measures obtained from a separate nasal mask. This was done to determine whether the differences observed in respiratory phase pattern frequencies and SNRF were due to genuine physiologic alterations based on addition of oral flow, or if changes were due to respiratory or flow changes created by the use of a novel oronasal facemask for both nasal and oronasal estimates. Additionally, this study was designed to thoroughly explore variability of respiratory phase patterns across an increased number (four) of trials of the same swallowing condition, in order to give robust data on variability of respiratory phase patterns. The timing and position of SNRF was assessed across a larger dataset from the oronasal and nasal measures, to determine if addition of oral flow altered the presentation of non-respiratory artefact. Results supported that the findings from the first RSC study were related to the addition of oral respiratory airflow, rather than a confound of the novel mask used, in that oral flow has a larger impact on ingestive swallowing conditions, particularly of liquid boluses. The notion that there is variability in respiratory phase patterns across repeated trials of the same swallowing condition was confirmed. When assessed in isolation, post-swallow respiratory direction was considerably more stable. The variability in overall respiratory phase patterns, alongside the relative consistency of post-swallow respiratory patterns, suggests that this variability is caused by inconsistent pre-swallow respiratory direction, indicating pre-swallow respiration may be altered to facilitate optimal lung volumes for efficient swallowing. Further evidence of pre- and post-swallow, inward- and outward-directed SNRF is presented. Use of an enclosed mask increases sensitivity to SNRF signals, detecting non-respiratory airflow that has not been revealed using traditional, non-sealed flow systems.

Supraglottic constriction commonly occurs across a range of laryngeal hypersensitivity disorders. However, it is challenging to quantify; despite numerous attempts to develop objective measurements for quantification of supraglottic constriction, studies still rely on subjective measures to determine the degree of constriction. In particular, measures are limited due to the difficulty associated with calibrating laryngeal anatomy, given unknown proportions and distance from the endoscopic camera. This severely limits the reliability and validity of findings across the literature. The study reported in Chapter 7 was completed to assess novel measures of the vocal folds and supraglottic structures in participants with muscle tension dysphonia (MTD) and healthy controls, using anatomic structures for calibration. The reliability of these measures was compared to that of a subjective rating scale. To provide some indication of which method offers more validity in estimating dysfunction, subjective and objective measures of supraglottic constriction were compared to acoustic estimates of vocal quality. The unexpected small sample size of the study resulted in statistical limitations, significantly restricting the conclusions that were possible. Despite these issues, several of the proposed measures looked to hold promise based on reliability estimates. However, there was no significant difference in the degree of supraglottic constriction between the healthy and control groups, possibly supporting the notion that degree of supraglottic constriction does not predict level of dysphonia. This was supported by a lack of difference in acoustic vocal quality across groups.

Exploration of laryngeal function in hypersensitivity conditions offers insight into the interplay of respiration, airway protective mechanisms, swallowing and voice in the presence of altered sensation. Chronic cough is thought to result from laryngeal hypersensitivity, with previous research highlighting deficits in swallowing, voice, and cough sensitivity in individuals with chronic cough. Therefore, the study presented in Chapter 8 utilised this patient group as a model of hypersensitivity in which to explore a range of laryngeal functions. A study was completed to characterise broad laryngeal function as it pertains to respiration, airway protection, swallowing and voice, using a range of outcome measures in chronic cough patients and healthy controls. Statistical analyses and conclusions were limited by an unexpectedly low sample size. Descriptive analyses indicated that chronic cough patients present with increased cough sensitivity when assessed using a citric acid aerosol, respiratory and swallowing alterations characterised by mouth breathing surrounding swallowing, and lengthening of airway closure during swallowing. This presentation likely reflects attempts to prevent overstimulation of the larynx, and supports previous observations that swallowing impairments in this population are subclinical. Measures of vocal quality and supraglottic constriction did not differ across the groups. This may be due to a lack of specificity of the measures used, but may also highlight a lack of diagnostic utility in the degree of laryngeal tension between individuals with and without voice disorders, as identified in Chapter 7.

This research program has provided further support for the notion of broad dysregulation of laryngeal functions in laryngeal hypersensitivity disorders. Use of novel measures of respiratory phase patterns have highlighted the need for more sensitive measures of respiratory flow to advance understanding of swallowing physiology and RSC behaviour, particularly in those prone to mouth breathing and with laryngeal hypersensitivity. The investigations of RSC have also demonstrated the need to consider variability of respiratory phase patterns within healthy individuals. Additionally, this research has proposed a novel approach for quantifying supraglottic constriction in disordered and healthy individuals, but also further suggests that distinction between the two groups may be of little diagnostic meaning in individuals with hypersensitivity conditions.