Assessment and modulation of the sensorimotor cough response.
Thesis DisciplineSpeech and Language Sciences
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Impairments of coughing (i.e. dystussia) and swallowing (i.e. dysphagia) frequently co-occur. This co-morbidity increases the risk of developing aspiration pneumonia. The aim of this research program was to enhance methods of assessment and explore an option for rehabilitation of the sensorimotor cough response. This gave rise to a series of studies that evaluated (1) methods of citric acid cough reflex testing (CRT) (2) modulation of the sensorimotor cough response through sensory stimulation, and (3) the use of acoustic intensity as a measure of cough strength.
There is lack of standardization and inadequate data on test-retest variability of citric acid CRT. These issues necessitated two methodological studies on citric acid CRT. A systematic review that summarized and appraised methods of citric acid CRT across disciplines was completed. Data across studies were translated to standardized units of measurement to streamline comparison across studies. A total of 136 citric acid CRT protocols were retrieved and evaluated by two independent investigators. The study revealed lack of standardization and substandard reporting of instrumentation and citric acid CRT protocols across studies, preventing the full replication of many. It is anticipated that these findings will contribute to the development of standards of methods of citric acid CRT, and highlight implications of methods of citric acid CRT on the outcome of the test.
Test-rest variability of citric acid CRT was evaluated in healthy individuals (n = 16) across three alternate days (i.e. Monday, Wednesday, Friday). Methods of citric acid CRT were chosen to optimize test-retest reproducibility. Estimated increases of 0.43 and 0.32 doubling concentrations of citric acid, per day (p < 0.05), were identified for natural (NCT) and suppressed (SCT) citric acid cough thresholds, respectively. These data suggest a habituationeffect occurs with repeated exposure to citric acid CRT. Quantification of this habituation effect enabled the effects of a sensory stimulation protocol in the subsequent study to be evaluated against the artefact of repeating the citric acid CRT, which was used as an outcome measure.
A prospective, pseudo-randomized control trial was conducted to evaluate the safety and efficacy of a 4-day sensory stimulation protocol, involving inhalations of distilled water to modulate cough sensitivity in healthy adults (n = 24). Evaluation of safety was necessary due to the known risks of bronchoconstriction following distilled water inhalation. Participants were randomly assigned to one of three groups: (1) high intensity stimulation (inhalations of distilled water at high flow rate), (2) low intensity stimulation (inhalations of distilled water at low flow rate), and (3) a control group (inhalations of 0.9% saline). A citric acid CRT was completed at baseline (Day 1), and after the sensory stimulation protocol on alternate days (i.e. Day 3 and Day 5) to determine participants NCT and SCT. The sensory stimulation protocol, and spirometry (to monitor the safety of the distilled water inhalations on the respiratory system) were completed on days two to five. The study revealed that the sensory stimulation protocol did not induce bronchoconstriction in any participant. SCTs changed differently across days in the high and low intensity sensory stimulation groups, compared to the control group (p < 0.05). In the control group, citric acid cough thresholds increased across days, resembling the habituation effect observed upon repeated exposure to CRT. In contrast, an absence of habituation to citric acid CRT was observed following both of the sensory stimulation protocols, suggesting a possible sensitization effect of distilled water.
The final two studies represent a clinical adjunct to this research program. There are no clinically applicable, objective measures of cough strength, as it relates to clearance of penetration and/or aspiration. This prompted two studies that used acoustic intensity to (1) determine a cut-off value of effective/ineffective clearance of penetration and aspiration on videofluoroscopic swallowing studies (VFSS) and (2) compare citric acid induced cough strength between healthy individuals and patients with dysphagia. Acoustic intensity was chosen as a measure of cough strength as it represents a non-invasive, clinically applicable means of measuring coughing. In the first study, patients referred for VFSS were recruited (n = 88). Data were included from patients who coughed to penetration and/or aspiration during their VFSS (n = 13). An important, yet unexpected finding, was that no patient effectively expelled aspirate material from the laryngeal vestibule (n = 10). Coughing expelled penetration (n = 7). However, definitive cut-off values of cough effectiveness could not be made due to the limited number of observations. There are modifications of the study design that must be thoroughly investigated prior to making conclusions regarding the role of coughing in airway clearance of aspirate, and the validity of acoustic intensity as a measure of cough strength.
In the second acoustic intensity study, patients with dysphagia (n = 12) and aged-matched healthy controls (n = 16) were recruited. Audio recordings of citric acid induced coughing were completed for all participants. The study revealed a difference in the acoustic intensity of citric acid induced coughing between the two groups (p < 0.05). Healthy individuals were found to have a louder cough to CRT than patients with dysphagia. Whether this translates to a functional difference between the two groups, in terms of strength of cough, remains to be directly tested. The issues detailed above, encountered when attempting to validate acoustic intensity for airway clearance in patients with dysphagia on VFSS, prevent inference of the current results to functional cough strength.
In summary, this research program enhances understanding of assessment and modulation of the sensorimotor cough response and provides important groundwork for future studies. Future research should evaluate test-retest variability of citric acid CRT and the safety and efficacy of the distilled water sensory stimulation protocol in patients with dysphagia. Additionally, the role of coughing in expelling aspiration from the airway, and the validity of acoustic intensity in predicting effective/ineffective clearance, should be further investigated.