Neural correlates of cough hypersensitivity in humans: evidence for central sensitisation and dysfunctional inhibitory control

Altering cough reflex sensitivity with aerosolized capsaicin paired with behavioral cough suppression: a proof-of-concept study

Background

The purpose of this prospective, quasi-experimental, single cohort proof-of-concept study was to determine feasibility and proof-of-concept of programmatically decreasing cough sensitivity through use of cough suppression strategies following inhalation of aerosolized capsaicin, in gradually increasing doses, across repeated treatment sessions.

Methods

Five healthy adults, ages 20-32 years of age, enrolled and completed the study. The study commenced in three phases. Phase I consisted of baseline cough sensitivity testing using pharmaceutical-grade aerosolized capsaicin, delivered via a Koko DigiDoser with nebulizer. The single-inhale, dose-response method was used. Doses that elicited two coughs (C2) and five coughs (C5) were recorded. Testing ceased when participants met the C5 threshold or when they had been given the maximum dose of 1,000 µmol/L. Phase II consisted of 5-6 treatment sessions, during which participants were exposed to increasing doses of aerosolized capsaicin while implementing behavioral cough suppression strategies. In phase III, cough sensitivity was re-tested at 1 and 3 weeks post-treatment. Participants were given explicit instructions to not try to suppress their cough. Participants who did not reach the C2 or C5 threshold at 1,000 µmol/L were assigned a score of 1,250 µmol/L.

Results

Each participant demonstrated a gradual increase in maximum capsaicin dose suppressed during each treatment session, with each successfully suppressing at 1,000 µmol/L by the final treatment session. C2 was greater than baseline in 4 of the 5 participants at 1 week post-treatment, and in 3 of the 5, at 3 weeks post-treatment. C5 was greater in all 5 participants at both post-treatment time points. In fact, 4 of the 5 participants did not reach the C5 threshold during either post-treatment testing sessions. Wilcoxon's Signed Rank Test, using the logC2 and logC5 values, revealed a significant difference relative to baseline in logC5 at 1 week (z=-2.02, P=0.04) and 3 weeks (z=-2.03, P=0.04) post-treatment. The difference in logC2 neared significance at 1 week post-treatment (z=-1.77, P=0.077), but was insignificant at 3 weeks post-treatment (z=-1.46, P=0.144).

Conclusions

This study demonstrates the potential of treating patients with refractory chronic cough (RCC), due to cough hypersensitivity, with a progressive desensitization approach paired with behavioral cough suppression. Additional research is needed using a randomized, placebo-controlled trial with patients with RCC.

Central adenosine A1 receptors inhibit cough via suppression of excitatory glutamatergic and tachykininergic neurotransmission

BACKGROUND AND PURPOSE

The adenosine A1 receptor is reported to mediate several excitatory effects in the airways and has inhibitory effects in the CNS. In this study, we investigated the role of peripheral and central A1 receptors in regulating cough and airway obstruction.

EXPERIMENTAL APPROACH

Drugs were administered to guinea pigs via inhalation or i.c.v. infusion. Following the administration of different drugs, cough was induced by exposing guinea pigs to aerosolized 0.4 M citric acid. An automated analyser recorded both cough and airway obstruction simultaneously using whole-body plethysmography.

KEY RESULTS

The A1 receptor agonist, cyclopentyladenosine (CPA, administered by inhalation), dose-dependently inhibited cough and also inhibited airway obstruction. Similarly, CPA, administered i.c.v., inhibited both the citric acid-induced cough and airway obstruction; this was prevented by pretreatment with the A1 receptor antagonist DPCPX (i.c.v.). Treatment with DPCPX alone dose-dependently enhanced the citric acid-induced cough and airway obstruction. This effect was reversed following treatment with either the glutamate GluN1 receptor antagonist D-AP5 or the neurokinin NK1 receptor antagonist FK-888.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that activation of either peripheral or central adenosine A1 receptors inhibits citric acid-induced cough and airway obstruction. The data also suggest that tonic activation of central adenosine A1 receptors serves as a negative regulator of cough and airway obstruction, secondary to inhibition of excitatory glutamatergic and tachykininergic neurotransmission.

The double-sidedness of cough in the elderly

Cough is a physiological reflex to protect airways against aspiration, but also it is one of the most frequent problems that lead patients to seek medical care. Chronic cough is more prevalent in the elderly than younger subjects, and more challenging to manage due to frequent comorbidities and possible side effects from drug treatment. Meanwhile, cough reflex does not decrease with natural aging but is often impaired by pathologic conditions like stroke. The impairment in cough reflex may lead to fatal complication like aspiration pneumonia. In this paper, we reviewed epidemiology and clinical considerations for chronic cough in the elderly, and summarized aging-related changes in cough reflex and also possible ways to normalize cough reflex and prevent aspiration pneumonia.

KAAACI Evidence-Based Clinical Practice Guidelines for Chronic Cough in Adults and Children in Korea

Chronic cough is common in the community and causes significant morbidity. Several factors may underlie this problem, but comorbid conditions located at sensory nerve endings that regulate the cough reflex, including rhinitis, rhinosinusitis, asthma, eosinophilic bronchitis, and gastroesophageal reflux disease, are considered important. However, chronic cough is frequently non-specific and accompanied by not easily identifiable causes during the initial evaluation. Therefore, there are unmet needs for developing empirical treatment and practical diagnostic approaches that can be applied in primary clinics. Meanwhile, in referral clinics, a considerable proportion of adult patients with chronic cough are unexplained or refractory to conventional treatment. The present clinical practice guidelines aim to address major clinical questions regarding empirical treatment, practical diagnostic tools for non-specific chronic cough, and available therapeutic options for chronic wet cough in children and unexplained chronic cough in adults in Korea.

Airway expression of Transient Receptor Potential (TRP) Vanniloid-1 and Ankyrin-1 channels is not increased in patients with Idiopathic Pulmonary Fibrosis

Dry cough is a common symptom described in patients with Idiopathic Pulmonary Fibrosis (IPF) and impairs quality of life. The exact mechanisms causing cough in IPF remain unclear, however evidence suggests altered cough neurophysiology and sensitisation plays a role; IPF patients have an enhanced cough reflex sensitivity to inhaled capsaicin. The Transient Receptor Potential Vanniloid-1 channel (TRPV-1) has a role in the cough reflex and airway expression is increased in patients with chronic cough. The Ankyrin-1 receptor (TRPA-1) is often co-expressed. It was hypothesised that, like chronic cough patients, IPF patients have increased airway TRP receptor expression. Bronchial biopsies were obtained from 16 patients with IPF, 11 patients with idiopathic chronic cough and 8 controls without cough. All other causes of cough were rigorously excluded. Real-time quantitative Polymerase Chain Reaction was used to detect TRPV-1 and TRPA-1 mRNA expression with Immunohistochemistry demonstrating protein expression. Mean TRPV-1 and TRPA-1 gene expression was higher in IPF patients compared with controls, but the difference did not reach statistical significance. Immunostaining supported these findings. This study suggests that structural up-regulation of central airway TRP receptors is not the key mechanism for cough in IPF patients. It is probable that IPF cough results from altered neuronal sensitivity at multiple levels of the cough pathway.

A neuroanatomical framework for the central modulation of respiratory sensory processing and cough by the periaqueductal grey

Sensory information arising from the airways is processed in a distributed brain network that encodes for the discriminative and affective components of the resultant sensations. These higher brain networks in turn regulate descending motor control circuits that can both promote or suppress behavioural responses. Here we explore the existence of possible descending neural control pathways that regulate airway afferent processing in the brainstem, analogous to the endogenous descending analgesia system described for noxious somatosensation processing and placebo analgesia. A key component of this circuitry is the midbrain periaqueductal grey, a region of the brainstem recently highlighted for its altered activity in patients with chronic cough. Understanding the nature and plasticity of descending neural control may help identify novel central therapeutic targets to alleviate the neuronal hypersensitivity underpinning many symptoms of respiratory disease.

Cough sensors from dental pulp

Mechanisms which alter sensory neural activity, in particular those rendering nerves hyper-responsive have been implicated in the pathophysiology of common clinical syndromes including chronic cough, itch and pain. However, experimental study of human sensory neurons is challenging because the cell bodies of peripheral neurons are housed in neuronal ganglia which are not accessible using peripheral biopsy techniques. While important advances have been made from studies conducted in animal models, there are interspecies differences. There is a need for development of a new generation of in vitro neuronal models based on human biology. In this article the propensity for human dental pulp stem cells to differentiate towards a neuronal phenotype and the potential of such a model to study altered sensory neural function will be discussed.

Laryngeal Dysfunction: Assessment and Management for the Clinician

The larynx is one of the most highly innervated organs in humans and serves a number of vitally important, complex, and highly evolved biological functions. On a day-to-day basis, the larynx functions autonomously, addressing several roles including airway protection, swallowing, and phonation. In some situations the larynx appears to adopt a functional state that could be considered maladaptive or "dysfunctional." This laryngeal dysfunction can underpin and account for a number of respiratory symptoms that otherwise appear incongruous with a clinical disease state and/or contribute to the development of symptoms that appear "refractory" to treatment. These include conditions associated with a heightened tendency for inappropriate laryngeal closure (e.g., inducible laryngeal obstruction), voice disturbance, and chronic cough. Recognition of laryngeal dysfunction is important to deliver targeted treatment and failure to recognize the condition can lead to repeated use of inappropriate treatment. Diagnosis is not straightforward, however, and many patients appear to present with symptoms attributable to laryngeal dysfunction, but in whom the diagnosis has been overlooked in clinical work-up for some time. This review provides an overview of the current state of knowledge in the field of laryngeal dysfunction, with a focus on pragmatic clinical assessment and management.

Physiotherapy and Speech and Language therapy intervention for chronic cough

There are few effective pharmacological therapies available to treat refractory chronic cough. Functional MRI studies of the brain have recently shown that patients with chronic cough have dysfunctional inhibitory control of cough. Self-management therapies delivered by physiotherapists or speech therapists are effective at suppressing cough. They enable patients to consciously suppress the urge to cough. The intervention consists of education, laryngeal hygiene, cough suppression and distraction measures and behaviour modification. The efficacy of Physiotherapy and Speech And Language Intervention (PSALTI) has been confirmed in two randomised control trials. In one trial, there was a 41% reduction in cough frequency with PSALTI, assessed objectively with the Leicester Cough Monitor, and a clinically significant improvement in quality of life. Importantly, the improvement in cough was sustained when therapy was discontinued. The addition of the Speech Pathology Treatment to neuromodulator drug therapy, Pregabalin has also been evaluated in a clinical trial. There was a clinically significant improvement in quality of life, and this was sustained when therapy was discontinued. The mechanism of action of PSALTI is not known and this should be investigated in future. Further studies are needed to identify the components of PSALTI that deliver the most benefit, and determine whether PSALTI is effective in cough associated with other chronic lung disorders.

Psychological profile of individuals presenting with chronic cough

Chronic refractory cough (CRC) is a common problem in respiratory clinics. Adverse effects on quality of life are documented in the literature, but relatively little is known about the underlying psychological factors in this patient population. We aimed to investigate the association of psychological factors with chronic cough, comparing CRC to explained cough and non-cough groups. 67 patients attending a specialist cough clinic (CRC, n=25; explained cough, n=42) and 22 non-cough individuals participated. All participants completed the Hospital Anxiety & Depression Scale, Big Five Inventory (Personality), Chalder Fatigue Scale and Patient Health Questionnaire-15. Cough patients also completed the Illness Perception Questionnaire-Revised. Appropriate statistical analyses were used to compare participant groups. Chronic refractory coughers displayed significantly higher levels of anxiety, depression, fatigue and somatic physical symptoms than non-cough participants. Compared to explained coughers, there were higher depression and fatigue scores and significantly more negative illness representations (specifically, strong beliefs regarding negative consequences, lower illness coherence and higher emotional representations). "Explained" coughers reported significantly increased fatigue and somatic symptoms in comparison to non-coughers. The prevalence of fatigue, low mood, negative illness beliefs and increased physical symptom reporting should be considered in consultations and in developing novel interventions for CRC patients.

Advances in mechanisms and management of chronic cough: The Ninth London International Cough Symposium 2016

At the Ninth London International Cough Symposium held in June 2016, advances in chronic cough were presented. Chronic cough has been labelled as a cough hypersensitivity syndrome (CHS) with neuroinflammatory mechanisms likely to be the underlying mechanisms. The concept is that there is a stage of peripheral sensitisation induced by inflammatory factors setting up the scene for a central component that can be visualised by functional magnetic resonance imaging. There has also been progress in assessing CHS patients in the clinic in terms of measuring cough, with an increasing interest in assessing different types of cough associated with respiratory diseases such as asthma, COPD, bronchiectasis and pulmonary fibrosis. There is an emerging area of new antitussives in the form of neuromodulators. These advances have been paralleled by improvements in the management of patients with chronic cough. However, more work is needed but the future looks promising.

'Chronic cough, cause unknown': A qualitative study of patient perspectives of chronic refractory cough

Chronic refractory cough patients have persistent, unexplained, treatment-resistant symptoms. Very little is known about non-physiological processes underlying chronic refractory cough or patients' experience of the condition. In all, 14 patients participated in semi-structured interviews. Interviews were based on the comprehensive cognitive behavioural model and analysed thematically. Eight key themes emerged illustrating that the experience of the onset and persistence of chronic refractory cough is complex, often involving multiple interlinking factors. Themes highlighted the involvement of biological and psychological factors, and the prominent role of the social dimension in how the cough is experienced, perceived and managed. Implications for intervention development are discussed.

Cough Hypersensitivity Syndrome: A Few More Steps Forward

Cough reflex is a vital protective mechanism against aspiration, but when dysregulated, it can become hypersensitive. In fact, chronic cough is a significant medical problem with a high degree of morbidity. Recently, a unifying paradigm of cough hypersensitivity syndrome has been proposed. It represents a clinical entity in which chronic cough is a major presenting problem, regardless of the underlying condition. Although it remains a theoretical construct, emerging evidence suggests that aberrant neurophysiology is the common etiology of this syndrome. Recent success in randomized clinical trials using a P2X3 receptor antagonist is the first major advance in the therapeutics of cough in the past 30 years; it at last provides a strategy for treating intractable cough as well as an invaluable tool for dissecting the mechanism underpinning cough hypersensitivity. Additionally, several cough measurement tools have been validated for use and will help assess the clinical relevance of cough in various underlying conditions. Along with this paradigm shift, our understanding of cough mechanisms has improved during the past decades, allowing us to continue to take more steps forward in the future.