Impaired cough suppression in chronic refractory cough

Cough and itch: Common mechanisms of irritation in the throat and skin

Cough and itch are protective mechanisms in the body. Cough occurs as a reflex motor response to foreign body inhalation, while itch is a sensation that similarly evokes a scratch response to remove irritants from the skin. Both cough and itch can last for sustained periods, leading to debilitating chronic disorders that negatively impact quality of life. Understanding the parallels and differences between chronic cough and chronic itch may be paramount to developing novel therapeutic approaches. In this article, we identify connections in the mechanisms contributing to the complex cough and scratch reflexes and summarize potential shared therapeutic targets. An online search was performed using various search engines, including PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1983 to 2024. Articles were assessed for quality, and those relevant to the objective were analyzed and summarized. The literature demonstrated similarities in the triggers, peripheral and central nervous system processing, feedback mechanisms, immunologic mediators, and receptors involved in the cough and itch responses, with the neuronal sensitization processes exhibiting the greatest parallels between cough and itch. Given the substantial impact on quality of life, novel therapies targeting similar neuroimmune pathways may apply to both itch and cough and provide new avenues for enhancing their management.

What causes cough in pulmonary fibrosis, and how should we treat it?

PURPOSE OF REVIEW

To review the current understanding of the impact, mechanisms and treatments for cough in patients with interstitial lung disease (ILD). Evidence suggests that cough is a prevalent symptom in patients with ILD and has a significant impact on patients.

RECENT FINDINGS

There is increasing interest in the role of cough hypersensitivity as seen in chronic refractory cough in patients with ILD, and encouraging recent results suggest that ILD-associated cough responds to opiate therapy.

SUMMARY

Understanding the aetiology of cough in patients with ILD is crucial to continue to develop therapies which might be effective in reducing cough and increasing quality of life.

Laryngeal Vibrotactile Stimulation Is Feasible, Acceptable To People With Unexplained Chronic Cough

OBJECTIVES

Unexplained chronic cough (UCC) is common and has significant impacts on quality of life. Ongoing cough can sensitize the larynx, increasing the urge to cough and perpetuating the cycle of chronic cough. Vibrotactile stimulation (VTS) of the larynx is a noninvasive stimulation technique that can modulate laryngeal somatosensory and motor activity. Study objectives were to assess feasibility and acceptability of VTS use by people with UCC. Secondarily, changes in cough-related quality of life measures were assessed.

METHODS

Adults with UCC recorded cough measures at baseline and after completing 2 weeks of daily VTS. Feasibility and acceptability were assessed through participant-reported device use and structured feedback. Cough-related quality of life measures were the Leicester Cough Questionnaire (LCQ) and the Newcastle Laryngeal Hypersensitivity Questionnaire (NLHQ).

RESULTS

Nineteen adults participated, with mean age 67 years and cough duration 130 months. Notably, 93% of planned VTS sessions were logged, 94% of participants found the device comfortable to wear, 89% found it easy to operate and 79% would recommend it to others. Pre-post LCQ change achieved a minimal important difference (MID) (mean 1.3 [SD 2.4, p = 0.015]). NLHQ scores improved, but did not reach an MID.

CONCLUSIONS

Laryngeal VTS use was feasible and acceptable for use by patients with UCC and was associated with a meaningful improvement in cough-related quality of life. Future studies will include VTS dose refinement and the inclusion of a comparison arm to further assess the potential for laryngeal VTS as a novel treatment modality for UCC.

LEVEL OF EVIDENCE

4 Laryngoscope, 2024.

Drugs Targeting Cough Receptors: New Therapeutic Options in Refractory or Unexplained Chronic Cough

Refractory chronic cough is a disabling disease with very limited therapeutic options. A better understanding of cough pathophysiology has led to the development of emerging drugs targeting cough receptors. Recent strides have illuminated novel therapeutic avenues, notably centred on modulating transient receptor potential (TRP) channels, purinergic receptors, and neurokinin receptors. By modulating these receptors, the goal is to intervene in the sensory pathways that trigger cough reflexes, thereby providing relief without compromising vital protective mechanisms. These innovative pharmacotherapies hold promise for improvement of refractory chronic cough by offering improved efficacy and potentially mitigating adverse effects associated with current recommended treatments. A deeper comprehension of their precise mechanisms of action and clinical viability is imperative for optimising therapeutic interventions and elevating patient care standards in respiratory health. This review delineates the evolving landscape of drug development in this domain, emphasising the significance of these advancements in reshaping the paradigm of cough management.

The Therapeutic Landscape in Chronic Cough

In recent years, there has been a substantial increase in the development of antitussive therapies and the first new therapy, gefapixant has been licenced in Europe. This review describes current unlicenced treatments for chronic cough and details treatments currently in development for refractory chronic cough and cough in idiopathic pulmonary fibrosis, as well as compounds previously explored.

Brainstem processing of cough sensory inputs in chronic cough hypersensitivity

BACKGROUND

Chronic cough is a prevalent and difficult to treat condition often accompanied by cough hypersensitivity, characterised by cough triggered from exposure to low level sensory stimuli. The mechanisms underlying cough hypersensitivity may involve alterations in airway sensory nerve responsivity to tussive stimuli which would be accompanied by alterations in stimulus-induced brainstem activation, measurable with functional magnetic resonance imaging (fMRI).

METHODS

We investigated brainstem responses during inhalation of capsaicin and adenosine triphosphate (ATP) in 29 participants with chronic cough and 29 age- and sex-matched controls. Psychophysical testing was performed to evaluate individual sensitivities to inhaled stimuli and fMRI was used to compare neural activation in participants with cough and control participants while inhaling stimulus concentrations that evoked equivalent levels of urge-to-cough sensation.

FINDINGS

Participants with chronic cough were significantly more sensitive to inhaled capsaicin and ATP and showed a change in relationship between urge-to-cough perception and cough induction. When urge-to-cough levels were matched, participants with chronic cough displayed significantly less neural activation in medullary regions known to integrate airway sensory inputs. By contrast, neural activations did not differ significantly between the two groups in cortical brain regions known to encode cough sensations whereas activation in a midbrain region of participants with chronic cough was significantly increased compared to controls.

INTERPRETATION

Cough hypersensitivity in some patients may occur in brain circuits above the level of the medulla, perhaps involving midbrain regions that amplify ascending sensory signals or change the efficacy of central inhibitory control systems that ordinarily serve to filter sensory inputs.

FUNDING

Supported in part by a research grant from Investigator-Initiated Studies Program of Merck Sharp & Dohme Pty Ltd. The opinions expressed in this paper are those of the authors and do not necessarily represent those of Merck Sharp & Dohme (Australia) Pty Ltd.

Chronic cough: is the end nigh?

Chronic cough (lasting more than 8 weeks) is a common condition with substantial psychosocial impact. Despite huge efforts following robust guidelines, chronic cough in many patients remains refractory or unexplained (RU-CC). Recent insights support a significant role for cough hypersensitivity in RU-CC, including neuropathophysiological evidence from inhalational cough challenge testing, functional magnetic resonance imaging, and airway nerve biopsy. Along with improved approaches to measuring cough, this knowledge has developed in tandem with repurposing neuromodulator medications, including gabapentin, and evidence for non-pharmacological treatments. Most significantly, there is now a pipeline for novel classes of drugs specifically for chronic cough. The P2X3 receptor antagonist gefapixant is the first such drug to be approved in Europe. However, challenges persist. The field of chronic cough needs more robust epidemiological data, enhanced diagnostic tools, further well-designed clinical trials accounting for the effects of placebo, and treatments with minimal side-effects. Addressing these challenges are novel chronic cough registries, improved International Classification of Diseases (10th revision) coding, genetic testing options and further mechanistic studies. This Viewpoint article discusses these facets and considers how, whilst the end of chronic cough may not be imminent for all patients, the evolving landscape looks increasingly optimistic.

Chronic Cough as a Genetic Neurological Disorder? Insights from Cerebellar Ataxia with Neuropathy and Vestibular Areflexia Syndrome (CANVAS)

Chronic cough is common, and in many cases unexplained or refractory to otherwise effective treatment of associated medical conditions. Cough hypersensitivity has developed as a paradigm that helps to explain clinical and research observations that frequently point towards chronic cough as a neuropathic disorder. Cerebellar ataxia with neuropathy and vestibular areflexia syndrome (CANVAS) is a recently described neurological condition whose clinical features include gait ataxia, unsteadiness, peripheral neuropathy, and autonomic dysfunction. Chronic cough is also a common feature of the syndrome, with features of hypersensitivity, often preceding core neurological symptoms by up to 30 years or more. The genetic basis in a majority of cases of CANVAS appears to be biallelic variable repeat intron expansion sequences within RFC1, a gene normally involved in the regulation of DNA replication and repair. The same polymorphism has now been identified at an increased frequency in patients with unexplained or refractory chronic cough in the absence of defining clinical features of CANVAS. This review expands on these points, aiming to increase the awareness of CANVAS amongst clinicians and researchers working with chronic cough. We discuss the implications of a link between RFC1 disease and cough. Improved understanding of CANVAS may lead to an enhanced grasp of the pathophysiology of chronic cough, and new approaches to antitussive treatments.

A Non-Pharmacological Cough Therapy for People with Interstitial Lung Diseases: A Case Report

Purpose

To explore the feasibility of a non-pharmacological cough control therapy (CCT) customized for a client with interstitial lung disease (ILD).

Client Description

An 83-year-old female with hypersensitivity pneumonitis, and chronic cough for 18 years treated previously with pharmacological treatment for the underlying lung disease and gastroesophageal reflux disease, as well as lozenges and breathing and relaxation strategies.

Intervention

Four cough education and self-management sessions (45-60 minutes each) facilitated by a physiotherapist and speech-language pathologist via videoconference were conducted. Session topics included mechanisms of cough in ILD, breathing and larynx role in cough control, trigger identification, cough suppression and control strategies, and psychosocial support towards behaviour change using motivational interviewing.

Measures and Outcome

The following assessments were conducted prior to and one week after the intervention: semi-structured interviews, Leicester Cough Questionnaire, King's Brief Interstitial Lung Disease questionnaire, Functional Assessment of Chronic Illness Therapy Fatigue Scale, modified Borg Scale for severity and intensity of cough, and the Global Rating of Change Questionnaire.

Implications

Implementing the CCT was feasible. The client reported increased perceived cough control, a reduction in exhaustion from coughing bouts, and a better understanding of the mechanisms behind cough management and suppression. Improvements were also observed in cough-related quality of life, severity, and intensity.

Altered functional connectivity of the nucleus tractus solitarii in patients with chronic cough after lung surgery: an rs-fMRI study

BACKGROUND

To explore the altered functional connectivity (FC) of the nucleus tractus solitarii (NTS) in patients with chronic cough after lung surgery using resting-state functional magnetic resonance imaging (rs-fMRI), and the association between abnormal FC and clinical scale scores.

METHODS

A total of 22 patients with chronic cough after lung surgery and 22 healthy controls were included. Visual analog scale (VAS), Mandarin Chinese version of the Leicester Cough Questionnaire (LCQ-MC), and Hamilton anxiety rating scale (HAMA) scores were assessed, and rs-fMRI data were collected. The FC analysis was performed using the NTS as the seed point, and FC values with all voxels in the whole brain were calculated. A two-sample t-test was used to compare FC differences between the two groups. The FC values of brain regions with differences were extracted and correlated with clinical scale scores.

RESULTS

In comparison to healthy controls, FC values in the NTS and anterior cingulate cortex(ACC) were reduced in patients with chronic cough after lung surgery (GRF correction, p-voxel < 0.005, p-cluster < 0.05) which were positively correlated with LCQ-MC scores (r = 0.534, p = 0.011), but with VAS (r = -0.500, p = 0.018), HAMA (r = -0.713, p < 0.001) scores were negatively correlated.

CONCLUSIONS

Reduced FC of the NTS with ACC may be associated with cough hypersensitivity and may contribute to anxiety in patients with chronic cough after lung surgery.

Evaluation and management of chronic cough in adults

Background: Chronic cough (CC), a cough that lasts > 8 weeks, has an overall prevalence of 5-11% in adults, peaking between 60 and 80 years of age. Of the 15% of patients who remain undiagnosed or refractory to treatment, two thirds are women. Objective: The objective was to present an updated evidence-based algorithmic approach for evaluating and managing CC, with emphasis on treatment modalities for refractory CC. Methods: A literature search was conducted of medical literature data bases for guidelines, position papers, systematic reviews, and clinical trials from January 2022 to June 2023, on the evaluation and management of CC. Results: The initial assessment should be limited to a detailed history, physical examination, chest radiograph, spirometry, exhaled nitric oxide, blood eosinophil count, and measurement of cough severity and quality of life by using validated instruments. The top diagnoses to consider are asthma, chronic obstructive pulmonary disease, nonasthmatic eosinophilic bronchitis, gastroesophageal reflux disease, and upper airway cough syndrome. Additional studies are only obtained when red flags are present or the patient fails to respond after avoidance of high-risk factors, e.g., smoking and angiotensin-converting enzyme inhibitors, and 4-6 weeks of empiric treatment for the most likely respiratory and gastrointestinal diseases. When diagnostic tests and/or specific directed treatments fail to control CC, low-dose morphine (preferred), gabapentin, pregabalin, and/or cough control therapy are recommended. Non-narcotic purinergic 2×3 (P2×3) receptor antagonists, gafapixant and campilixant, are currently being studied for CC. Conclusion: For the evaluation and management of patients with CC, clinicians should use an algorithmic approach and identify "red flags," reduce high-risk factors, and use empiric treatment for the five top diagnoses before extensive diagnostic testing. Current treatment for refractory cough is limited to symptomatic management.

Management of non-pharmacologic therapy for chronic refractory cough: Mechanism, composition, applicable population, and assessment

Chronic cough is common in the clinic and can seriously affect the quality of life of patients. Following the existing guidelines for treatment, refractory chronic cough is defined as a clinical condition in which the cause of the cough remains unclear after comprehensive examination and treatment, or the cause is clear but symptomatic treatment is ineffective. It has been found that non-pharmacologic therapy can effectively improve the quality of life and reduce the frequency of coughing for some patients with refractory chronic cough. Compared with pharmacological therapy, non-pharmacologic therapy has no obvious adverse effects; therefore, non-pharmacologic therapy has good application prospects in the diagnosis and treatment of refractory chronic cough. This paper summarizes the composition, indication, action and mechanism of non-pharmacologic therapy in the diagnosis and treatment of refractory chronic cough and prospects for research on non-pharmacologic therapy.

From bench to bedside: The role of cough hypersensitivity in chronic cough

BACKGROUND

Chronic cough is a burdensome condition characterized by persistent cough lasting longer than 8 weeks. Chronic cough can significantly affect quality of life, physical function and productivity, with many people troubled with a cough that lasts for months or even years. People with chronic cough commonly report a persistent urge to cough with frequent bouts of coughing triggered by innocuous stimuli, which has led to the concept of cough hypersensitivity.

MAIN BODY

Both central and peripheral neural pathways regulate cough, and although mechanisms driving development of cough hypersensitivity are not fully known, sensitization of these neural pathways contributes to excessive cough triggering in cough hypersensitivity. Effective therapies that control chronic cough are currently lacking. Recent therapeutic development has focused on several ion channels and receptors involved in peripheral activation of cough (e.g., transient receptor potential channels, P2 × 3 receptors and voltage-gated sodium channels) or central cough processing (e.g., neurokinin-1 [NK-1] receptors and nicotinic acetylcholine receptors).

CONCLUSION

These targeted therapies provide novel insights into mechanisms underlying cough hypersensitivity and may offer new treatment options for people with chronic cough. In this review, we explore preclinical and clinical studies that have improved our understanding of the mechanisms responsible for chronic cough and discuss the most promising targeted approaches to date, including trials of P2 × 3-receptor antagonists and NK-1-receptor antagonists.

Multiple Chemical Sensitivity: It's time to catch up to the science

Multiple chemical sensitivity (MCS) is a complex medical condition associated with low dose chemical exposures. MCS is characterized by diverse features and common comorbidities, including fibromyalgia, cough hypersensitivity, asthma, and migraine, and stress/anxiety, with which the syndrome shares numerous neurobiological processes and altered functioning within diverse brain regions. Predictive factors linked to MCS comprise genetic influences, gene-environment interactions, oxidative stress, systemic inflammation, cell dysfunction, and psychosocial influences. The development of MCS may be attributed to the sensitization of transient receptor potential (TRP) receptors, notably TRPV1 and TRPA1. Capsaicin inhalation challenge studies demonstrated that TRPV1 sensitization is manifested in MCS, and functional brain imaging studies revealed that TRPV1 and TRPA1 agonists promote brain-region specific neuronal variations. Unfortunately, MCS has often been inappropriately viewed as stemming exclusively from psychological disturbances, which has fostered patients being stigmatized and ostracized, and often being denied accommodation for their disability. Evidence-based education is essential to provide appropriate support and advocacy. Greater recognition of receptor-mediated biological mechanisms should be incorporated in laws, and regulation of environmental exposures.

Cough desensitization treatment for patients with refractory chronic cough: results of a second pilot randomized control trial

OBJECTIVE

The purpose of this study was to collect pilot efficacy data on a novel treatment for refractory chronic cough (RCC), which we call cough desensitization treatment (CDT).

DESIGN AND METHODS

In this parallel cohort, sham-controlled, randomized controlled trial, 21 adults with RCC were randomly assigned to 12 sessions of either CDT (progressive doses of aerosolized capsaicin while behaviorally suppressing cough; n = 11) or a sham treatment (repeated exposure to aerosolized saline; n = 9). The Leicester Cough Questionnaire (LCQ) was the primary outcome measure. Perceived cough severity with a visual analogue scale and cough challenge testing (for measuring cough-reflex sensitivity) were secondary outcome measures. Data were analyzed with mixed effects linear regression and follow-up contrasts.

RESULTS

Results on all measures favored CDT. Excluding one sham participant, whose baseline LCQ scores were deemed unreliable, mean change in LCQ at 3-weeks post treatment was 6.35 and 2.17 in the CDT and sham groups, respectively. There was moderate to strong evidence of a greater improvement in the CDT group in total LCQ score (p = .058) and LCQ Psychological domain (p = .026) and Physical domain (p = .045) scores. Strong evidence was found for a greater reduction in urge-to-cough during CCT in the CDT group (p = .037) and marginal for a reduction in the capsaicin cough-reflex sensitivity (p = .094). There was weak evidence of a greater reduction in cough severity in the CDT group (p = .103).

DISCUSSION

Although the study is limited due to the small sample size, the data provide additional evidence supporting further research on CDT. CDT resulted in a greater change in the primary efficacy measure (LCQ) than both pharmaceutical and behavioral treatments currently found in the literature.

TRIAL REGISTRATION

This trial (NCT05226299) was registered on Clinicaltrials.gov on 07/02/2022.

Characterization of Ethyl Butyrate-Induced Cough Before and After Breath Control Techniques in Healthy Adults

PURPOSE

Methods for cough elicitation frequently involve aerosolized tussive agents. Here, we sought to determine whether healthy individuals demonstrate a quantifiable cough response after inhaling a volatile ester and if breath control techniques modify this chemically induced cough response.

METHOD

Sixty adult male and female participants inhaled prepared liquid dilutions of ethyl butyrate dissolved in paraffin oil at 20%, 40%, and 60% v/v concentrations in triplicate, with presentation order randomized. We delivered stimuli through a face mask connected to an olfactometer and respiratory pneumotachograph. Participants rated sensations of their urge to cough and pleasantness of the odor while cough airflow was measured. Following baseline testing, participants were randomized to implement pursed-lip breathing or slow-paced breathing after inhaling ethyl butyrate to determine the effects of breath control on cough measures.

RESULTS

Inhaled ethyl butyrate elicited cough in 70% of participants. Higher concentrations of ethyl butyrate resulted in significantly greater sensation of the urge to cough, F(2, 80) = 10.72, p < .001, and significantly more generated coughs, F(2, 63) = 13.14, p < .001. Compared to baseline, participants rated significantly decreased urge to cough during breath control techniques, F(1, 40) = 11.01, p = .0019. No significant changes were observed in the number of generated coughs between baseline and breath control techniques, F(1, 31) = 7.23, p = .01.

CONCLUSIONS

Airborne ethyl butyrate is a tussigenic agent in humans. Our findings provide opportunities for future research directions in normal and disordered cough responses to volatile compounds.

Novel capsaicin cough endpoints effectively discriminate between healthy controls and patients with refractory chronic cough

RATIONALE

Chronic cough is a common problem, substantially affecting quality of life. Effective treatments and diagnostic clinical tools for refractory chronic cough are lacking which remains a diagnosis of exclusion.

OBJECTIVES

To investigate capsaicin evoked cough responses in healthy volunteers and refractory chronic cough patients and assess the discriminatory ability of novel endpoints.

METHODS

Dose-response capsaicin cough challenges were performed, and receiver operating characteristic curves constructed to evaluate the discriminatory value of novel endpoints; Emax (maximum number of coughs evoked by any capsaicin concentration) and ED50 (capsaicin concentration evoking at least half of Emax).

MEASUREMENTS AND MAIN RESULTS

Ninety-three healthy volunteers (median age 39yrs(IQR; 29-52), 47 females) and 51 refractory chronic cough patients (59yrs(53-67), 31 females) were studied. Emax was significantly higher in the patient group compared to healthy volunteers (p < 0.001) and ED50 was significantly lower (p = 0.001). Both parameters were influenced by gender; females had a higher Emax (p = 0.009) and more sensitive ED50 (p < 0.001) but there were no correlations with other patient demographics. There was a significant relationship between Emax and cough frequency in the patient group (p < 0.001). Emax effectively discriminated between the groups (AUC = 0.83, 95% CI; 0.75-0.90, p < 0.001) independently of ED50 which was less favourable (AUC = 0.66, 95% CI; 0.57-0.76, p = 0.002). Emax and ED50 were shown to be repeatable, and the dose-response method well tolerated.

CONCLUSION

Novel capsaicin dose-response endpoints effectively discriminate between healthy controls and refractory chronic cough patients, which may better represent pathophysiological mechanisms and show promise for development as a tool to identify patients with cough hyper-excitability.

CLINICAL TRIAL REGISTRATION

www.isrctn.com; ISRCTN23684347.

Mini-review: Hypertussivity and allotussivity in chronic cough endotypes

In recent years our understanding of the neurophysiological basis of cough has increased substantially. In conjunction, concepts around the drivers of chronic coughing in patients have also significantly evolved. Increasingly it is recognised that dysregulation of the neuronal pathways mediating cough play an important role in certain phenotypes of chronic cough and therefore pathological processes affecting the nervous system are likely to represent key endotypes in patients. Taking inspiration from the study of neuropathic pain, the term hypertussia has been employed to describe the phenomenon of abnormal excessive coughing in response to airway irritation and allotussia to describe coughing in response to stimuli not normally provoking cough. This review aims to summarise current clinical evidence supporting a role for the hyperexcitability of neuronal pathways contributing to chronic coughing and suggest how these might align with the clinical features observed in patients.

Impact of mental health and personality traits on the incidence of chronic cough in the Canadian Longitudinal Study on Aging

Background

Chronic cough is a common troublesome condition, but risk factors for developing chronic cough are poorly understood. The aim of this study was to understand the relationship between mental health disorders, personality traits and chronic cough.

Methods

The Canadian Longitudinal Study on Aging is a prospective, nationally generalisable, random sample of adults aged 45-85 years at baseline recruited between 2011 and 2015, and followed-up 3 years later. Chronic cough was defined as a daily cough over the last 12 months. Incident chronic cough was defined as those participants who reported new-onset chronic cough between baseline and follow-up 1. Current depressive symptoms and psychological distress were assessed using the Center for Epidemiologic Study Short Depression Scale (CESD-10) and Kessler Psychological Distress Scale (K-10), respectively. The "Big Five" personality traits were assessed using the Ten-Item Personality Inventory. Relative risks are reported using a multivariate mutually adjusted model.

Results

At follow-up 1, 2506 participants (11.1%) reported new-onset chronic cough during the ∼3-year interval. Depressive symptoms (CESD-10 ≥10: relative risk 1.22 (95% CI 1.03-1.44)) and psychological distress (K-10 ≥22: relative risk 1.20 (95% CI 1.07-1.36)) at baseline were both independent predictors of a higher risk of incident chronic cough. Prevalent and incident chronic cough were also independently associated with an increased risk of developing depressive symptoms and psychological distress. Personality traits did not influence the development of chronic cough but did increase the risk of depressive symptoms and psychological distress.

Conclusions

This study shows that there is a bidirectional relationship between chronic cough, and depressive symptoms and psychological distress, and personality traits do not independently influence the development of chronic cough.

Evidence for Alpha7 Nicotinic Receptor Activation During the Cough Suppressing Effects Induced by Nicotine and Identification of ATA-101 as a Potential Novel Therapy for the Treatment of Chronic Cough

Studies performed in healthy smokers have documented a diminished responsiveness to tussive challenges, and several lines of experimental evidence implicate nicotine as an antitussive component in both cigarette smoke and the vapors generated by electronic cigarettes (eCigs). We set out to identify the nicotinic receptor subtype involved in the antitussive actions of nicotine and to further evaluate the potential of nicotinic receptor-selective agonists as cough-suppressing therapeutics. We confirmed an antitussive effect of nicotine in guinea pigs. We additionally observed that the alpha-4 beta-2 (α ₄ β ₂)-selective agonist Tc-6683 was without effect on evoked cough responses in guinea pigs, while the α ₇-selective agonist PHA 543613 dose-dependently inhibited evoked coughing. We subsequently describe the preclinical evidence in support of ATA-101, a potent and highly selective (α ₇) selective nicotinic receptor agonist, as a potential candidate for antitussive therapy in humans. ATA-101, formerly known as Tc-5619, was orally bioavailable and moderately central nervous system (CNS) penetrant and dose-dependently inhibited coughing in guinea pigs evoked by citric acid and bradykinin. Comparing the effects of airway targeted administration versus systemic dosing and the effects of repeated dosing at various times prior to tussive challenge, our data suggest that the antitussive actions of ATA-101 require continued engagement of α ₇ nicotinic receptors, likely in the CNS. Collectively, the data provide the preclinical rationale for α ₇ nicotinic receptor engagement as a novel therapeutic strategy for cough suppression. The data also suggest that α ₇ nicotinic acetylcholine receptor (nAChR) activation by nicotine may be permissive to nicotine delivery in a way that may promote addiction. SIGNIFICANCE STATEMENT: This study documents the antitussive actions of nicotine and identifies the α₇ nicotinic receptor subtype as the target for nicotine during cough suppression described in humans. We additionally present evidence suggesting that ATA-101 and other α₇ nicotinic receptor-selective agonists may be promising candidates for the treatment of chronic refractory cough.

Clinical characteristics in adult patients with somatic cough syndrome

Objective:

The data in regard of the clinical characteristics and diagnosis of somatic cough syndrome in adults were limited. The aim of this study was to fill that gap.

Methods:

This was a retrospective analysis of patients with somatic cough syndrome. We described clinical characteristics of adult patients with somatic cough syndrome.

Results:

Twenty-three somatic cough syndrome patients were identified in 543 adult patients with chronic cough. Psychiatric disorder of these patients was identified as anxiety (n = 8), obsessive-compulsive (n = 7), somatoform (n = 6), depression (n = 3), and cognitive bias (n = 1). Twelve patients showed abnormal results of investigations related with common causes of chronic cough, including gastroesophageal reflux, sputum eosinophilia, bronchial hyper-responsiveness, or signs of sinusitis but did not respond to the treatments directed to those conditions. All these patients were ever misdiagnosed as other causes of chronic cough. Compared to 520 non-somatic cough syndrome patients, patients with somatic cough syndrome were younger (32 (29.0–43.0) vs 42.0 (32.0–55.0) years, p = 0.013), longer disease duration (48.0 (19.5–102.0) vs 24.0 (9.0–72.0) months, p = 0.037), more common in dry cough (100% vs 57.6%, p < 0.001), and lower proportion of nocturnal cough (13.0% vs 40.2%, p = 0.009). Common cold (60.9%) was the most common initial trigger of cough and itchy throat (60.9%) was the most common accompanying symptom in patients with somatic cough syndrome. Notably, there were similar distribution in cough triggers and accompanying symptoms between two groups.

Conclusion:

In spite of much higher proportion of dry cough and smaller proportion of nocturnal cough, adult patients with somatic cough syndrome show similar clinical characteristics with other chronic cough patients, in regard of cough triggers, accompanying symptoms as well as abnormal results of investigations, which should be an important reason for misdiagnosis of somatic cough syndrome. Psychiatric disorder should be addressed in clinical management of chronic cough.

WAO-ARIA consensus on chronic cough - Part 1: Role of TRP channels in neurogenic inflammation of cough neuronal pathways

BACKGROUND

Cough features a complex peripheral and central neuronal network. The function of the chemosensitive and stretch (afferent) cough receptors is well described but partly understood. It is speculated that chronic cough reflects a neurogenic inflammation of the cough reflex, which becomes hypersensitive. This is mediated by neuromediators, cytokines, inflammatory cells, and a differential expression of neuronal (chemo/stretch) receptors, such as transient receptor potential (TRP) and purinergic P2X ion channels; yet the overall interaction of these mediators in neurogenic inflammation of cough pathways remains unclear.

OBJECTIVES

The World Allergy Organization/Allergic Rhinitis and its Impact on Asthma (WAO/ARIA) Joint Committee on Chronic Cough reviewed the current literature on neuroanatomy and pathophysiology of chronic cough. The role of TRP ion channels in pathogenic mechanisms of the hypersensitive cough reflex was also examined.

OUTCOMES

Chemoreceptors are better studied in cough neuronal pathways compared to stretch receptors, likely due to their anatomical overabundance in the respiratory tract, but also their distinctive functional properties. Central pathways are important in suppressive mechanisms and behavioral/affective aspects of chronic cough. Current evidence strongly suggests neurogenic inflammation induces a hypersensitive cough reflex marked by increased expression of neuromediators, mast cells, and eosinophils, among others. TRP ion channels, mainly TRP V1/A1, are important in the pathogenesis of chronic cough due to their role in mediating chemosensitivity to various endogenous and exogenous triggers, as well as a crosstalk between neurogenic and inflammatory pathways in cough-associated airways diseases.

Utility of Cough Provocation Tests in Chronic Cough and Respiratory Diseases: A Comprehensive Review and Introduction of New Reference Ranges for the Capsaicin Test

Cough provocation tests (CPTs) are an objective measurement of the sensitivity of the cough reflex arc. However, they are not established in clinical practice because a large variability of response in healthy subjects limits their diagnostic value. There is a paucity of studies that have investigated CPT reference ranges in healthy subjects. This systematic review describes the variability of the responses to CPTs in healthy subjects and factors that influence it. A new analysis of 134 healthy subjects was conducted to create reference ranges for single-breath capsaicin CPT by calculating the interquartile ranges for the provocative concentration of capsaicin to induce 2 and 5 coughs. Female subjects had a more sensitive cough reflex than male counterparts. The ability of CPTs to distinguish various respiratory diseases from healthy subjects was also reviewed. Cough sensitivity was consistently heightened in the following groups: unselected patients with chronic, refractory, or recurrent cough, unexplained chronic cough, gastro-esophageal reflux-associated cough, cough-variant asthma, lower airway symptoms induced by chemical irritants, and fibrotic interstitial lung diseases. In the following groups, hypersensitivity of the cough reflex was present in those individuals whose symptom profile was predominated by cough: asthma, chronic obstructive pulmonary disease (COPD), bronchiectasis, and sarcoidosis. In the following conditions, patients usually cough in order to expectorate mucus from their airways, not because of a hypersensitive cough reflex arc: productive cough, asthma, upper airway cough syndrome, COPD, bronchiectasis, cystic fibrosis, and chronic respiratory infections. CPTs have the potential to identify patients with chronic respiratory symptoms due to cough reflex hypersensitivity, thereby providing a targeted approach for therapy.

Cough symptom severity in patients with refractory or unexplained chronic cough: a systematic survey and conceptual framework

BACKGROUND

Cough severity represents an important subjective endpoint in assessing the effectiveness of therapies for patients with chronic cough. Although cough-specific quality of life questionnaires exist, a widely available cough severity instrument with established measurement properties remains unavailable.

AIMS

To identify and summarise the results of studies reporting on the experience of patients with chronic cough and, in the process, develop a conceptual framework to inform development of a patient-reported outcome measurement (PROM) addressing cough severity.

RESULTS

We identified 61 eligible studies reporting on patient experience with chronic cough. Studies provided 82 potential items, of which 43 proved unique and relevant to cough severity. The urge-to-cough sensation and the cough symptom itself represented broad domains of cough severity. Two subdomains under urge-to-cough included frequency (1 item) and intensity (1 item). Five subdomains under cough symptoms included control (2 items), frequency (6 items), bout duration (1 item), intensity (8 items), and associated features/sequelae (24 items).

CONCLUSIONS

Our systematic survey and conceptual framework identified items and domains of cough severity in patients with refractory or unexplained chronic cough. The results support item generation and content validity for a PROM assessing cough severity.

Clinical characteristics of patients with chronic cough in Guangdong, China: a multicenter descriptive study

Background

The clinical characteristics of patients with chronic cough are reported only in single-center survey in China, being significantly different from that in western countries. Here, we performed a multicenter study to describe the clinical characteristics of chronic cough patients.

Methods

A cross-sectional observational survey was conducted in thirteen tertiary hospitals of Guangdong, South China. Relevant data were recorded using a standardized questionnaire and analyzed, including demographics, educational attainment, cough features, and concomitant symptoms.

Results

Of 933 patients in this study, the median age was 40.0 (IQR 31.0–52.0) years with a peaked age of 30–39 years. The proportion of females (487, 52.2 %) was comparable to that of males (446, 47.8 %). Up to 81.9 % of the patients were non-smokers. More than two-thirds of the subjects with chronic cough had a low educational level. The median cough duration was 6.0 (IQR 3.0–24.0) months, and 73.0 % of chronic cough patients presented with dry cough. Laryngeal paresthesia was the most common concomitant symptom (704, 75.5 %), followed by rhinitis/sinusitis-related (350, 37.5 %) and respiratory symptoms (322, 34.5 %). Rhinitis/sinusitis-related symptoms more frequently occurred in patients with productive cough than in those with dry cough (49.0 % vs. 33.0 %, P < 0.001). Moreover, female patients displayed an older age and a higher prevalence of nocturnal cough compared to male patients (both P < 0.05).

Conclusions

Our results show an equal gender, young profile and laryngeal paresthesia in patients with chronic cough, and different clinical features between females and males.

Limosilactobacillus reuteri DSM-17938 for preventing cough in adults with mild allergic asthma: A double-blind randomized placebo-controlled cross-over study

BACKGROUND

Cough is a common troublesome symptom in asthma which is neuronally mediated. Limosilactobacillus reuteri DSM-17938 (L. reuteri DSM-17938) is a probiotic shown to be effective in pre-clinical models at suppressing neuronal responses to capsaicin, a transient receptor potential vanilloid agonist (TRPV1).

OBJECTIVE

Investigate the effects of DSM-17938 versus matched placebo on capsaicin-evoked coughs in mild allergic asthmatics.

METHODS

We performed a 4-visit, randomized, double-blind, placebo-controlled, two-way cross-over study comparing full dose cough responses with inhaled capsaicin in mild allergic asthmatics after 1 month of treatment with DSM-17938 compared with matched placebo. Randomization and allocation to trial group were carried out by a central computer system. Histamine skin prick testing, airway hyper-responsiveness and inflammatory cells in induced sputum were measured at every visit. Blood was collected to extract PBMCs and stimulated with CD3/CD28 to ascertain the effects of DSM-17938 /placebo on T-cell cytokine responses.

RESULTS

Seventeen subjects were recruited and 15 completed the study (8 females, mean age 27.3 years). There was no difference in the change in maximum capsaicin-evoked coughs (Emax) after treatment with L. reuteri DSM-17938 compared with placebo [mean difference 2.07 coughs (95% CI -2.77 to 6.91, p = .38) or relative changes in geometric mean ratios for the dose evoking at least half the Emax (ED50) [1.05 (95% CI 0.31-3.58, p = .94)], concentration evoking 2 coughs (C2) [0.63 (0.26-1.53), p = .28] and 5 coughs (C5) [0.79 (0.25-2.50), p = .67]. There was no effect on histamine skin prick wheal size, intensity of itch sensation, methacholine PC20, airway inflammation or T-cell responses after stimulation with CD3/CD28. There were no serious adverse events. One subject developed a mild upper respiratory tract infection and another mild transient nausea whilst on DSM-17938.

CONCLUSION

In this small study in adults with mild allergic asthma, we found no evidence that L. reuteri DSM-17938 has any systemic effects on airway nerves, smooth muscle, sputum inflammatory cells, skin responses or T-cell responses after oral consumption.

TRIAL REGISTRATION

Clinicaltrials.gov Identifier: NCT03603522.

Gender Difference in Chronic Cough: Are Women More Likely to Cough?

Chronic cough is a common complaint for patients to seek medical cares all over the world. Worldwide, about two thirds of chronic cough patients are females. However, in some regions of China the prevalence of chronic cough between sexes is roughly the same. Estrogen and progesterone can not only have an effect on transient receptor potential vanilloid 1 channel, eosinophils and mast cells, but also influence laryngeal dysfunction, gastroesophageal reflux disease and obstructive sleep apnea hypopnea syndrome, which may lead to increased cough sensitivity in women. On the other hand, the quality of life was adversely affected more in female patients with chronic cough. Both hormones possibly cause gender difference in chronic cough.

The interrelatedness of chronic cough and chronic pain

Since chronic cough has common neurobiological mechanisms and pathophysiology with chronic pain, both clinical disorders might be interrelated. Hence, we examined the association between chronic cough and chronic pain in adult subjects in the Rotterdam Study, a large prospective population-based cohort study.Using a standardised questionnaire, chronic pain was defined as pain lasting up to 6 months and grouped into a frequency of weekly/monthly or daily pain. Chronic cough was described as daily coughing for at least 3 months duration. The longitudinal and cross-sectional associations were investigated bi-directionally.Of 7141 subjects in the study, 54% (n=3888) reported chronic pain at baseline. The co-prevalence of daily chronic pain and chronic cough was 4.4%. Chronic cough was more prevalent in subjects with daily and weekly/monthly chronic pain compared with those without chronic pain (13.8% and 10.3% versus 8.2%; p<0.001). After adjustment for potential confounders, prevalent chronic pain was significantly associated with incident chronic cough (OR 1.47, 95% CI 1.08-1.99). The association remained significant in subjects with daily chronic pain (OR 1.49, 95% CI 1.06-2.11) with a similar effect estimate, albeit non-significant in those with weekly/monthly chronic pain (OR 1.43, 95% CI 0.98-2.10). After adjustment for covariables, subjects with chronic cough had a significant risk of developing chronic pain (OR 1.63, 95% CI 1.02-2.62) compared with those without chronic cough.Chronic cough and chronic pain confer risk on each other among adult subjects, indicating that both conditions might share common risk factors and/or pathophysiologic mechanisms.

Impact and disease burden of chronic cough

Cough is an important physiological mechanism to protect the lower airways, but it is also one of the most common symptoms that lead patients to seek medical consultations. Chronic cough, usually defined by cough duration longer than 8 weeks in adults, is prevalent in the community and impairs quality of life in individual patients. There are several characteristics that may differentiate pathologic coughs from protective cough responses, including cough duration, frequency, hypersensitivity, impaired cough suppressibility, or treatment refractoriness. However, the impact of cough, or its perceived severity, is a sum of the patient's experience of coughing and is a factor that may finally define cough as a disease. Social isolation is a major impact of chronic cough, which is now worse due to the coronavirus disease 2019 pandemic and a widespread fear of cough as a source of contagion. Emotional impact and economic impact (medical cost, reduced work productivity, and job loss) are also important complications of chronic cough, and they interact in a vicious cycle, aggravated by social isolation and treatment failures. This paper is a narrative review that aims to address the impact and disease burden of chronic cough in adult patients. We review the literature, but also introduce the findings from recent qualitative interviews and the patients' experiences of the impact and burden of chronic cough and discuss unmet needs in these patients.

Cough hypersensitivity and suppression in COPD

Cough reflex hypersensitivity and impaired cough suppression are features of chronic refractory cough (CRC). Little is known about cough suppression and cough reflex hypersensitivity in cough associated with chronic obstructive pulmonary disease (COPD). This study investigated the ability of patients with COPD to suppress cough during a cough challenge test in comparison to patients with CRC and healthy subjects. This study also investigated whether cough reflex hypersensitivity is associated with chronic cough in COPD.Participants with COPD (n=27) and CRC (n=11) and healthy subjects (n=13) underwent capsaicin challenge tests with and without attempts to self-suppress cough in a randomised order over two visits, 5 days apart. For patients with COPD, the presence of self-reported chronic cough was documented, and objective 24-h cough frequency was measured.Amongst patients with COPD, those with chronic cough (n=16) demonstrated heightened cough reflex sensitivity compared to those without chronic cough (n=11): geometric mean±sd capsaicin dose thresholds for five coughs (C5) 3.36±6.88 µmol·L^-1 versus 44.50±5.90 µmol·L^-1, respectively (p=0.003). Participants with CRC also had heightened cough reflex sensitivity compared to healthy participants: geometric mean±sd C5 3.86±5.13 µmol·L^-1 versus 45.89±3.95 µmol·L^-1, respectively (p<0.001). Participants with COPD were able to suppress capsaicin-evoked cough, regardless of the presence or absence of chronic cough: geometric mean±sd capsaicin dose thresholds for 5 coughs without self-suppression attempts (C5) and with (CS5) were 3.36±6.88 µmol·L^-1 versus 12.80±8.33 µmol·L^-1 (p<0.001) and 44.50±5.90 µmol·L^-1 versus 183.2±6.37 µmol·L^-1 (p=0.006), respectively. This was also the case for healthy participants (C5 versus CS5: 45.89±3.95 µmol·L^-1 versus 254.40±3.78 µmol·L^-1, p=0.033), but not those with CRC, who were unable to suppress capsaicin-evoked cough (C5 versus CS5: 3.86±5.13 µmol·L^-1 versus 3.34±5.04 µmol·L^-1, p=0.922). C5 and CS5 were associated with objective 24-h cough frequency in patients with COPD: ρ= -0.430, p=0.036 and ρ= -0.420, p=0.041, respectively.Patients with COPD-chronic cough and CRC both had heightened cough reflex sensitivity but only patients with CRC were unable to suppress capsaicin-evoked cough. This suggests differing mechanisms of cough between patients with COPD and CRC, and the need for disease-specific approaches to its management.

Behavioral and Regional Brain Responses to Inhalation of Capsaicin Modified by Painful Conditioning in Humans

BACKGROUND

Cough is a defense mechanism that protects the airways and lungs in response to airway irritation. The sensory neurons involved in detecting airway irritants and the neural pathways mediating cough share similarities with those that encode pain from the body. Painful conditioning stimuli applied to one body site are known to reduce the perception of pain at another. However, whether the neural regulation of cough is influenced by painful stimuli is not known.

RESEARCH QUESTION

What are the behavioral and neural outcomes of painful conditioning stimuli on urge-to-cough (UTC) and cough evoked by inhaled capsaicin?

STUDY DESIGN AND METHODS

Sixteen healthy participants underwent psychophysical testing and functional MRI while completing a series of capsaicin inhalations to induce UTC and cough. The responses associated with capsaicin inhalation without pain were compared with those after the application of painful conditioning stimuli.

RESULTS

Significant decreases were seen behaviorally of 18.7% ± 17.3% (P < .001) and 47.0% ± 30.8% (P < .001) in participants' UTC ratings and cough frequencies, respectively, during the application of pain. UTC ratings were reduced by 24.2% ± 36.5% (P < .005) and increased by 67% ± 40% (P < .001) for capsaicin and saline inhalation, respectively, during the scanning session. Painful conditioning stimuli were associated with widespread decreases in regional brain responses to capsaicin inhalation (P < .001). Several brain regions showed levels of reduced activation attributable to painful conditioning that correlated with related changes in behavioral responses during scanning (R² = 0.53).

INTERPRETATION

Pain-related decreases of cough and UTC are accompanied by widespread changes in brain activity during capsaicin inhalation, suggesting that pain can modify the central processing of inputs arising from the airways. A mechanistic understanding of how cough and pain processing interact within the brain may help develop more effective therapies to reduce unwanted coughing.

Peripheral and central mechanisms of cough hypersensitivity

Chronic cough is a difficult to treat symptom of many respiratory and some non-respiratory diseases, indicating that varied pathologies can underpin the development of chronic cough. However, clinically and experimentally it has been useful to collate these different pathological processes into the single unifying concept of cough hypersensitivity. Cough hypersensitivity syndrome is reflected by troublesome cough often precipitated by levels of stimuli that ordinarily don't cause cough in healthy people, and this appears to be a hallmark feature in many patients with chronic cough. Accordingly, a strong argument has emerged that changes in the excitability and/or normal regulation of the peripheral and central neural circuits responsible for cough are instrumental in establishing cough hypersensitivity and for causing excessive cough in disease. In this review, we explore the current peripheral and central neural mechanisms that are believed to be involved in altered cough sensitivity and present possible links to the mechanism of action of novel therapies that are currently undergoing clinical trials for chronic cough.

The Effect of Pain Conditioning on Experimentally Evoked Cough: Evidence of Impaired Endogenous Inhibitory Control Mechanisms in Refractory Chronic Cough

The pathophysiology of refractory chronic cough (RCC) is unclear. We hypothesised that endogenous inhibitory control mechanisms, such as those activated by noxiousstimuli inducing pain (conditioned pain modulation (CPM)), may be capable of inhibiting coughing and urge to cough evoked by inhaled capsaicin. Furthermore, these mechanisms may be impaired in patients with RCC.The objective was to investigate the effects of pain on cough and urge to cough (UTC) in healthy volunteers (HV) and RCC. HV and RCC patients underwent a randomised, controlled, 4-way cross-over study comparing the effect of 4 interventions on capsaicin evoked coughing and UTC. The interventions comprised immersing a hand in i) noxious cold-water, ii) warm water, iii) warm water but instructed to voluntarily supress coughing and iv) no intervention. The co-primary outcomes were numbers of evoked coughs and UTC scores.Twenty HV (mean age 50.1(±SD14.2), M:F 10:10) and 20 RCC (age 60.1(±7.9), M:F 9:11) participated. Overall, noxious cold water reduced capsaicin evoked UTC scores and cough numbers compared with warm water (1.6 (95% C.I. 1.3-2.0) versus 2.2 (1.8-2.6) p<0.001; 4.8 coughs (3.7-6.2) versus 7.9 coughs (6.7-9.5) p<0.001, respectively). HV and RCC demonstrated similar reductions in the UTC during noxious cold-water immersion, but noxious cold water and voluntary suppression interventions were less effective in RCC than HV in reducing capsaicin evoked cough (p=0.041).Endogenous inhibitory control mechanisms, specifically those activated by pain, can reduce both coughing and the UTC. Impairment of endogenous inhibitory control mechanisms may contribute to excessive coughing in RCC.

Recent progress in the management of chronic cough

Chronic cough is a common clinical condition with significant impact on quality of life and for which effective therapy remains an unmet clinical need. Over the past decade, there has been a major shift in how we approach this problem, driven by better appreciation of the clinical manifestation of chronic cough and an improved understanding of the associated neurobiology. "Cough hypersensitivity syndrome" has been proposed as a new diagnostic term for chronic cough, encompassing different phenotypes of the condition. Accumulating evidence suggests that this new concept is clinically relevant. However, while it is gaining widespread endorsement within the allergy and respiratory community, raising its profile in routine clinical practice is a priority. Thus, the present paper reviews recent progress in our understanding and management of chronic cough, with focus on mechanistic and clinical studies. It also provides detail on knowledge gaps and future research directions.

Exploring the role of nerves in asthma; insights from the study of cough

Cough in asthma predicts disease severity, prognosis, and is a common and troublesome symptom. Cough is the archetypal airway neuronal reflex, yet little is understood about the underlying neuronal mechanisms. It is generally assumed that symptoms arise because of airway hyper-responsiveness and/or airway inflammation, but despite using inhaled corticosteroids and bronchodilators targeting these pathologies, a large proportion of patients have persistent coughing. This review focuses on the prevalence and impact of cough in asthma and explores data from pre-clinical and clinical studies which have explored neuronal mechanisms of cough and asthma. We present evidence to suggest patients with asthma have evidence of neuronal dysfunction, which is further heightened and exaggerated by both bronchoconstriction and airway eosinophilia. Identifying patients with excessive coughing with asthma may represent a neuro-phenotype and hence developing treatment for this symptom is important for reducing the burden of disease on patients' lives and currently represents a major unmet clinical need.

Exploring the clinical relevance of cough hypersensitivity syndrome

Introduction: Knowledge of the pathophysiology of cough has continued to advance over recent decades. Establishing anatomic-diagnostic protocols, based on the anatomy and distribution of vagus nerve pathways regulating the cough reflex, was the first breakthrough in modern clinical medicine for chronic cough. The unmet clinical need has prompted revised thinking regarding the pathophysiology of and the clinical approach to chronic cough.Areas covered: The paradigm of cough hypersensitivity syndrome (CHS) has been recently proposed, wherein aberrant neuro-pathophysiology is a common etiology. This review covers major findings on chronic cough and cough hypersensitivity, particularly focused on recently-published studies and explores the clinical relevance and applicability of CHS based on current knowledge and discuss knowledge gaps and future research directions.Expert opinion: This paradigm has provided new opportunities in managing chronic cough and evidence is accumulating to support the validity of CHS. It also warrants the re-appraisal of existing clinical evidence and investigation of how to refine our clinical strategy. While CHS highlights the importance of clinical thinking from the viewpoint of cough, the value of anatomic-diagnostic protocols should remain. Moreover, given the considerable heterogeneity in clinical presentation, cough-associated disease conditions, and treatment responses across different patients, precise molecular endotyping remains key to making further to advancing clinical practice .

Current Status and Future Directions of Chronic Cough in China

Chronic cough is one of the most common complaints for which patients in China seek medical attention. However, there are no nationwide data on the prevalence and socioeconomic burden of chronic cough. Although approximately 50% of Chinese men smoke, the vast majority of patients presenting for evaluation of chronic cough are never smokers. An equal sex distribution and a middle-aged predominance have been observed in the Chinese chronic cough population, despite demonstration of a higher cough reflex sensitivity in females and older patients. The role of air pollution in the distinct age and sex distribution requires further study. In terms of the etiologies of chronic cough in China, cough-variant asthma, upper airway cough syndrome, nonasthmatic eosinophilic bronchitis, and atopic cough are the most common causes, comprising 75.2% to 87.6% of cases across different regions. Chinese Guidelines for Diagnosis and Treatment of Cough were initially published in 2005, and updated in 2009 and 2016. In addition, the China Cough Coalition was established in 2016. Great progress has been made in both cough-related clinical practice and research in recent years, however, there are still challenges ahead. To facilitate optimal management of chronic cough in China, efforts promoting the dissemination and application of published guidelines will be essential, especially in community-based healthcare and in rural regions. As chronic refractory cough has been identified as a huge challenge to clinicians worldwide, continued international cooperation will be essential in optimizing evaluation and management of chronic cough.

Altered neural activity in brain cough suppression networks in cigarette smokers

Cough is important for airway defence, and studies in healthy animals and humans have revealed multiple brain networks intimately involved in the perception of airway irritation, cough induction and cough suppression. Changes in cough sensitivity and/or the ability to suppress cough accompany pulmonary pathologies, suggesting a level of plasticity is possible in these central neural circuits. However, little is known about how persistent inputs from the lung might modify the brain processes regulating cough.In the present study, we used human functional brain imaging to investigate the central neural responses that accompany an altered cough sensitivity in cigarette smokers.In nonsmokers, inhalation of the airway irritant capsaicin induced a transient urge-to-cough associated with the activation of a distributed brain network that included sensory, prefrontal and motor cortical regions. Cigarette smokers demonstrated significantly higher thresholds for capsaicin-induced urge-to-cough, consistent with a reduced sensitivity to airway irritation. Intriguingly, this was accompanied by increased activation in brain regions known to be involved in both cough sensory processing (primary sensorimotor cortex) and cough suppression (dorsolateral prefrontal cortex and the midbrain nucleus cuneiformis). Activations in the prefrontal cortex were highest among participants with the least severe smoking behaviour, whereas those in the midbrain correlated with more severe smoking behaviour.These outcomes suggest that smoking-induced sensitisation of central cough neural circuits is offset by concurrently enhanced central suppression. Furthermore, central suppression mechanisms may evolve with the severity of smoke exposure, changing from initial prefrontal inhibition to more primitive midbrain processes as exposure increases.

Are neural pathways processing airway inputs sensitized in patients with cough hypersensitivity?

Patients with cough hypersensitivity exhibit unusually low thresholds for responses to tussive stimuli, exaggerated responses to suprathreshold tussive stimuli, and report spontaneous experiences of urge-to-cough in the absence of exogenous stimulation. These aberrant responses to tussive challenge have the hallmark features of behaviours associated with a sensitized sensory system. Searching for further evidence to implicate neural sensitization in the symptomatology of cough hypersensitivity warrants consideration. If up-regulation of neural circuits involved in processing of airways inputs can be demonstrated in patients with cough hypersensitivity, then strategies to reverse this dysfunctional plasticity can be contemplated and assessed. This review considers the implications of neural sensitization as a factor in the cough hypersensitivity syndrome, reflects on the limited data available in this field, and suggests prospective directions for future research.