In the event of airway invasion, coughing is instigated in attempt to clear the misdirected material. Sensory receptors in the airway identify material that has entered (i.e., aspiration), with disruption to the ability to detect invasion or damage to the communicating pathways resulting in silent aspiration. In the absence of a response to aspiration, individuals are at increased risk of morbidity and mortality. Cough testing enables examination of airway sensitivity by irritating sensory receptors to evoke coughing. Methods of cough testing seek to replicate activation of the receptors in response to aspiration. This research programme aimed to increase the scientific rigour of cough testing by examining factors relating to the administration of the test and interpretation of results. The research programme was guided by a scoping review of the cough testing literature within the field of swallowing. From the review, a methodologic study was developed to enhance our understanding of the effect of citric acid presentation order on cough thresholds. A second study was conducted through use of an online survey to further our understanding on the integration of cough testing results in the clinical management of patients with swallowing disorders.

The scoping review explored and documented how cough testing has been used since its translation from the respiratory field into the field of swallowing. The use of cough testing was known to have increased with methodological variability evident across publications. Two independent reviewers completed all aspects of the review. Fifty-seven studies were identified that utilised 59 distinct cough testing protocols. Wide variation in practice was demonstrated across protocols. The review highlighted a need for greater understanding of the implications of different cough testing methods. This is needed to determine the methods that most closely mimic a cough response to aspiration, as well as to prevent inconsistency in the interpretation of results. Further exploration of safety is also required to ensure optimal methods for reducing risk of infection and lung inflammation (bronchoconstriction).

Citric acid was identified to be a commonly used aerosol to evoke coughing in protocols. When using multiple strengths (concentrations) of citric acid, they are typically provided in an ascending order (i.e., from low to high concentrations). A major limitation of cough testing is the potential influence of previous exposure to cough evoking aerosols on airway sensitivity, with an attenuation of cough response possible with repeated exposure. Changes in response have been documented across repeated sessions, suggesting that this presentation order does not control for aspects of attenuation to the stimuli. Therefore, the impact of presentation order required exploration, to refine methods of cough testing and determine the instability previously seen in sensitivity thresholds. The first study in this programme of research examined the effect of ascending and descending presentation orders of citric acid on cough thresholds. Healthy participants (n=22) attended two sessions, receiving nine citric acid concentrations in ascending order in one session, and in descending order for the other. No significant differences were identified for natural (NCT) and suppressed (SCT) citric acid cough thresholds or ratings of urge-to-cough (UTC) across conditions. However, the novel design of this study revealed limitations of current methods of testing that have not been shown previously. The results highlight a need to examine the current definition of a cough sensitivity ‘threshold’, before evaluating the impact of presentation order on NCT and SCT in separate sessions. Thus, further investigation is warranted before the finding of stable thresholds across ascending and descending presentation orders can be interpreted with confidence.

When used in clinical practice, cough testing is conducted as an adjunct to the clinical swallowing evaluation (CSE), providing information on the integrity of airway sensation and patient risk of silent aspiration. Diet recommendations have been shown to be more restrictive with use of cough testing. However, the specific role played by cough testing results in these decisions remains unclear due to multiple uncontrolled factors in previous research. Therefore, this survey study examined the influence of cough testing results and aspiration pneumonia risk factors on diet recommendations, while controlling for patient characteristics. An online survey was sent to speech and language therapy clinicians working in dysphagia management. Two patient cases were presented that differed only in their presence of risk factors known to be predictive of development of aspiration pneumonia (oral hygiene, smoking status and complexity of medical history). For each patient case there were three assessment scenarios: CSE information only; CSE information plus a ‘pass’ result to cough testing; and CSE information plus a ‘fail’ result to cough testing. Clinicians were required to select one of four diet recommendations for each scenario: baseline diet, modified diet, nil-by-mouth, or ‘other’. Clinicians were also asked to provide their management plans and rationale. Ninety-seven data sets were used in the final analysis. Findings demonstrated that a ‘fail’ result led to the most restrictive recommendations, with less restrictive recommendations given with a ‘pass’ result and when only CSE information is given. Aspiration pneumonia risk factors were shown to influence decision making, with the low-risk patient more likely to be recommended a less restrictive diet.

In summary, the scoping review highlighted wide variability in the implementation of cough testing. Also identifying lung function monitoring and methods of solution preparation as key considerations in the administration of cough testing. The results of the methodological study suggest that citric acid presentation order does not affect thresholds, but further studies investigating methodological aspects of cough testing are required before application of this finding can be confirmed with use of specific cough testing protocols. The use of lung function testing alongside cough testing enabled the safety of the protocol to be confirmed but required significant respiratory effort particularly in older participants. Exploration of the interpretation of results revealed that a ‘fail’ cough test result led to more restrictive and cautious patient management. The presence of aspiration pneumonia risk factors also influenced decisions with more restrictive recommendations given for the ‘high-risk’ patient. However, the finding that the patient management decisions made when provided with only CSE information are very similar to patient management decisions made for a ‘pass’ result suggests that patient airway safety is often assumed, despite an absence of information directly relating to risk of silent aspiration.

These novel findings require ongoing exploration, with more questions raised around the impact of other methodological choices, and clinicians’ potential assumption of safety in the absence of information around airway sensitivity. Future research should continue to examine questions around the administration and interpretation of cough testing, but there is a need for specific guidance for exploring airway sensitivity in the field of swallowing. Collation of current knowledge would encourage greater consistency in the field, and enable unanswered questions to be highlighted, whilst also ensuring the most appropriate use of methods are used in cough testing to mimic an aspiration event and evaluate silent aspiration risk.