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

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.

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.

Efficacy and Tolerability of Gefapixant for Treatment of Refractory or Unexplained Chronic Cough: A Systematic Review and Dose-Response Meta-Analysis

Importance

Gefapixant represents an emerging therapy for patients with refractory or unexplained chronic cough.

Objective

To evaluate the efficacy and tolerability of gefapixant for the treatment of adults with refractory or unexplained chronic cough.

Data Sources

MEDLINE, Embase, Cochrane Central Register of Controlled Trials, and Web of Science from November 2014 to July 2023.

Study Selection

Two reviewers independently screened for parallel and crossover randomized clinical trials (RCTs) that compared, in patients with refractory or unexplained chronic cough, either gefapixant with placebo, or 2 or more doses of gefapixant with or without placebo.

Data Extraction and Synthesis

Two reviewers independently extracted data. A frequentist random-effects dose-response meta-analysis or pairwise meta-analysis was used for each outcome. The GRADE (Grading of Recommendations, Assessment, Development, and Evaluation) approach was used to rate the certainty in whether patients would perceive the effects as important (greater than the minimal important difference [MID]) or small (less than the MID).

Main Outcomes and Measures

Cough frequency (measured using the VitaloJAK cough monitor; MID, 20%), cough severity (measured using the 100-mm visual analog scale [VAS]; higher score is worse; MID, 30 mm), cough-specific quality of life (measured using the Leicester Cough Questionnaire [LCQ]; score range, 3 [maximal impairment] to 21 [no impairment]; MID, 1.3 points), treatment-related adverse events, adverse events leading to discontinuation, and taste-related adverse events.

Results

Nine RCTs including 2980 patients were included in the primary analysis. Compared with placebo, gefapixant (45 mg twice daily) had small effects on awake cough frequency (17.6% reduction [95% CI, 10.6%-24.0%], moderate certainty), cough severity on the 100-mm VAS (mean difference, -6.2 mm [95% CI, -4.1 to -8.4]; high certainty), and cough-specific quality of life on the LCQ (mean difference, 1.0 points [95% CI, 0.7-1.4]; moderate certainty). Compared with placebo, gefapixant (45 mg twice daily) probably caused an important increase in treatment-related adverse events (32 more per 100 patients [95% CI, 13-64 more], moderate certainty) and taste-related adverse events (32 more per 100 patients [95% CI, 22-46 more], high certainty). High-certainty evidence suggests that gefapixant (15 mg twice daily) had small effects on taste-related adverse events (6 more per 100 patients [95% CI, 5-8 more]).

Conclusions and Relevance

Compared with placebo, gefapixant (45 mg orally twice daily) led to modest improvements in cough frequency, cough severity, and cough-specific quality of life but increased taste-related adverse events.

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.

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.

Cough hypersensitivity and chronic cough

Chronic cough is globally prevalent across all age groups. This disorder is challenging to treat because many pulmonary and extrapulmonary conditions can present with chronic cough, and cough can also be present without any identifiable underlying cause or be refractory to therapies that improve associated conditions. Most patients with chronic cough have cough hypersensitivity, which is characterized by increased neural responsivity to a range of stimuli that affect the airways and lungs, and other tissues innervated by common nerve supplies. Cough hypersensitivity presents as excessive coughing often in response to relatively innocuous stimuli, causing significant psychophysical morbidity and affecting patients' quality of life. Understanding of the mechanisms that contribute to cough hypersensitivity and excessive coughing in different patient populations and across the lifespan is advancing and has contributed to the development of new therapies for chronic cough in adults. Owing to differences in the pathology, the organs involved and individual patient factors, treatment of chronic cough is progressing towards a personalized approach, and, in the future, novel ways to endotype patients with cough may prove valuable in management.

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.

A descending pathway emanating from the periaqueductal gray mediates the development of cough-like hypersensitivity

Chronic cough is a common refractory symptom of various respiratory diseases. However, the neural mechanisms that modulate the cough sensitivity and mediate chronic cough remain elusive. Here, we report that GABAergic neurons in the lateral/ventrolateral periaqueductal gray (l/vlPAG) suppress cough processing via a descending pathway. We found that l/vlPAG neurons are activated by coughing-like behaviors and that tussive agent-evoked coughing-like behaviors are impaired after activation of l/vlPAG neurons. In addition, we showed that l/vlPAG neurons form inhibitory synapses with the nucleus of the solitary tract (NTS) neurons. The synaptic strength of these inhibitory projections is weaker in cough hypersensitivity model mice than in naïve mice. Important, activation of l/vlPAG GABAergic neurons projecting to the NTS decreases coughing-like behaviors. In contrast, suppressing these neurons enhances cough sensitivity. These results support the notion that l/vlPAG GABAergic neurons play important roles in cough hypersensitivity and chronic cough through disinhibition of cough processing at the medullary level.

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GABAergic neurons in the l/vlPAG inhibit coughing-like behaviors

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The l/vlPAG sends predominately inhibitory projections to the NTS

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l/vlPAG GABAergic neurons modulate coughing-like behaviors via descending projections

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l/vlPAG-NTS projections mediate cough hypersensitivity via disinhibitory mechanisms

Effect of centrally and peripherally acting GABAB agonism on the healthy human cough reflex

BACKGROUND

Currently there are no effective licensed anti-tussive therapies. Understanding how the neuronal mechanisms mediating the cough reflex in animal models translate to humans is important for the development of effective therapies. Pre-clinical studies suggest that the activation of GABA_B receptors in both the peripheral and central nervous systems inhibit cough.

OBJECTIVE

To compare the effect of central and peripherally acting GABA_B agonists (lesogaberan and baclofen) on the cough reflex in healthy volunteers.

METHODS

We performed a single center, double-blind, double-dummy, three-way crossover trial in healthy controls comparing single doses of lesogaberan (120 mg MR), with baclofen (40 mg) and placebos. Cough responses to inhaled capsaicin were assessed at screening and 2h post-dose on each study day. The primary endpoint was the maximum number of coughs evoked at any concentration of capsaicin (Emax) and the secondary endpoint was the concentration evoking 50% of the maximal response (ED50).

RESULTS

Fifteen participants enrolled onto the study (median age 29 (IQR 25-44) years; 7 females, mean BMI 24.6(±3.0). Lesogaberan treatment produced a small, statistically significant increase in Emax compared with placebo [mean 13.4coughs (95%CI 10.1-17.9) vs. 11.8coughs (8.8-15.9), p = 0.04], but had no effect on ED50 [geometric mean 47.4 μM (95%CI 24.4-91.7) vs 37.6 μM (95%CI 19.2-73.5), p = 0.37]. In contrast, baclofen had no significant effect on Emax (11.1, 95%CI 8.1-15.4) (p = 0.23), but significantly increased ED50 compared with placebo (geometric mean 75.2 μM (95%CI 37.2-151.8), p = 0.002).

CONCLUSION

This data suggests the anti-tussive actions of GABA_B agonists, in healthy volunteers, occur in the central rather than the peripheral nervous system.

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.

Chronic cough is associated with increased reporting of autonomic symptoms

Background

Patients with some neuronal hypersensitivity syndromes experience increased autonomic symptoms. Chronic cough is thought to be a neuronal hypersensitivity disorder and, therefore, may be associated with increased autonomic symptoms.

Methods

96 chronic cough subjects were recruited from the tertiary cough clinic based at Wythenshawe Hospital, Manchester, UK; 76 healthy controls were also recruited. Subjects were aged >18 years. Those with significant respiratory disease, significant smoking history or taking medication known to affect cough or autonomic function were excluded. Subjects completed the Composite Autonomic Symptom Score (COMPASS) 31 autonomic symptom questionnaire, the Cough Quality of Life Questionnaire (CQLQ) and a cough severity visual analogue scale (VAS).

Results

96 chronic cough subjects and 76 healthy volunteers were included in the final analysis. Mann–Whitney U-tests comparing COMPASS 31 scores in both groups showed that the total COMPASS 31 score was significantly higher in the patient group (median 18.4, interquartile range (IQR) 7.5–32.0) than the control group (median 3.6, IQR 1.1–9.5; p<0.001). The chronic cough subjects had significantly higher symptom scores than the healthy volunteer groups in all domains (p≤0.001) except vasomotor symptoms (p=0.770). There was a positive association between COMPASS 31 and CQLQ in the patient group (p<0.001, r=0.432) but not COMPASS 31 and VAS (p=0.227).

Interpretation

Chronic cough patients do indeed report more frequent and severe autonomic symptoms than healthy volunteers, indicating that this population may suffer from dysautonomia. At present, it remains unclear whether this occurs as a result of the cough or whether both the cough and dysfunction are part of some wider vagal pathology.

Chronic refractory cough patients report a greater range and severity of autonomic symptoms when compared to healthy volunteers. This may suggest that the cough is part of a wider vagal pathology.https://bit.ly/33hzJEt

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.

Prevalence, incidence and characteristics of chronic cough among adults from the Canadian Longitudinal Study on Aging

The global prevalence of chronic cough is highly variable, ranging from 2% to 18%. There is a lack of data on the prevalence and incidence of chronic cough in the general population. The objective of this study was to investigate the prevalence and incidence of chronic cough in a sample of Canadian adults, and how these are influenced by age, sex, smoking, respiratory symptoms, medical comorbidities and lung function.

Participants with chronic cough were identified from the Canadian Longitudinal Study on Aging (CLSA) based on self-reported daily cough in the past 12 months. This is a prospective, nationally generalisable, stratified random sample of adults aged 45–85 years at baseline recruited between 2011 and 2015, and followed-up 3 years later. The prevalence and incidence per 100 person-years are described, with adjustments for age, sex and smoking.

Of the 30 097 participants, 29 972 completed the chronic cough question at baseline and 26 701 did so at follow-up. The prevalence of chronic cough was 15.8% at baseline and 17.6% at follow-up with 10.4–17.1% variation across seven provinces included in the CLSA comprehensive sample. Prevalence increased with age and current smoking, and was higher in males (15.2%), Caucasians (14%) and those born in North America, Europe or Oceania (14%). The incidence of chronic cough adjusted for age, sex and smoking was higher in males and in underweight and obese subjects. Subjects with respiratory symptoms, airway diseases, lower forced expiratory volume in 1 s (% predicted), cardiovascular diseases, psychological disorders, diabetes and chronic pain had a higher incidence of chronic cough.

The prevalence and incidence of chronic cough is high in the CLSA sample with geographic, ethnic and gender differences, influenced by a number of medical comorbidities.

Canada has one of the highest prevalences of chronic cough in world and it is more common in Caucasians of European descent. Ageing, smoking, sex, clinical comorbidities and lung physiology all influence the prevalence and incidence of chronic cough.https://bit.ly/3qSBkdp