- Email: [email protected]
The effect of pH on citric acid cough challenge: A randomised control trial in chronic cough and healthy volunteers
Accepted Manuscript Title: The Effect of pH on Citric Acid Cough Challenge: A Randomised Control Trial in Chronic Cough and Healthy Volunteers Authors: Z.L Rai, H.E. Fowles, C. Wright, H. Joseph, A.H. Morice PII: DOI: Reference:
S1569-9048(17)30366-X https://doi.org/10.1016/j.resp.2018.02.013 RESPNB 2933
To appear in:
Respiratory Physiology & Neurobiology
Received date: Revised date: Accepted date:
22-10-2017 25-1-2018 21-2-2018
Please cite this article as: Rai, Z.L, Fowles, H.E., Wright, C., Joseph, H., Morice, A.H., The Effect of pH on Citric Acid Cough Challenge: A Randomised Control Trial in Chronic Cough and Healthy Volunteers.Respiratory Physiology and Neurobiology https://doi.org/10.1016/j.resp.2018.02.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The Effect of pH on Citric Acid Cough Challenge: A Randomised Control Trial in Chronic Cough and Healthy Volunteers. 1) Z.L Rai [email protected] Castle Clinical Castle Cottingham Cottingham, UK HU16 5JQ
Hill Trials Hill
Hospital Unit Hospital
IP T
07507573313 2) H.E Fowles
SC R
[email protected]
Hull York Medical School Centre for Cardiovascular and Metabolic Research, Respiratory Castle Hill Hospital Cottingham, UK HU16 5JQ
U
3) C Wright [email protected]
A
N
Hull York Medical School Centre for Cardiovascular and Metabolic Research, Respiratory Cottingham, UK
[email protected]
M
4) H Joseph
ED
Hull York Medical School Centre for Cardiovascular and Metabolic Research, Respiratory Cottingham, UK 5) A.H Morice
PT
[email protected]
CC E
Castle Head of Academic UK HU16 5J
Hill Academic Department
Department
of of
Key words; Chronic Cough, Cough Challenge, Cough Hypersensitivity
A
Word Count 1863
Hospital Medicine Medicine
Highlights
IP T
1) What is the key question? Do patients with chronic cough have an altered response to inhalational cough challenge? 2) What is the bottom line? In normal subjects the predicted concentration response to pH was seen but in chronic cough patients there was a heightened response which was unpredictable and individually variable.
N
Abstract
U
SC R
3)Why read on? These findings demonstrate experimentally the clinically observed phenomenon of hypersensitivity that chronic cough patients report. These insights are useful in our understanding of the mechanism of cough hypersensitivity, thus aiding patient communication.
A
Introduction
ED
M
Citric acid has been used for over six decades to induce cough; however the mechanism of its pro-tussive effect is still not fully understood. We assessed the response to inhalation of citric acid at varying levels of acidity to determine if the pH of the solution plays a role in the induction of cough. Data was collected from both healthy volunteers and patients with chronic cough. Methods
CC E
PT
20 chronic cough patients and 20 healthy volunteers were recruited and underwent three cough challenges on separate days. Each visit involved 5 repeated one second inhalations of 300mM citric acid solution. The concentration of the citrate cation remained constant, but the pH of the solution altered by the addition of sodium bicarbonate to 3, 5 and 6, representing the pKa values of the individual acid moieties. The total number of coughs elicited was recorded for each inhalation.
A
Results
Two subjects withdrew and were not included in the analysis. Participants were gender matched, each group consisting of 12 females. 74% of chronic coughers coughed at pH 3 (mean coughs 16), 89% coughed at pH 5 (18) and 63% coughed at pH 6 (7). In healthy volunteers, 60% of subjects coughed at pH 3 (9), 30% of subjects coughed at pH 5 (3), and 10% of subjects coughed at pH 6 (0). Thus chronic cough patients coughed more than healthy volunteers and did not exhibit a clear pH concentration response. There was also a greater variability in their response to individual challenges.
Introduction
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Citric acid has been used since 1954 to induce a tussive response (Bickerman 1954). The citric acid cough challenge model has become a standard methodology for investigating cough reflex sensitivity (Wright CE 2010). The challenge is highly reproducible and has become one of the favoured techniques for studying the pharmacokinetics and pharmacodynamics of anti-tussive medications. Despite its extensive use, the mechanism whereby citric acid produces cough has never been fully elucidated.
M
A
N
U
Previous studies have shown that citric acid along with other complex acids demonstrates a variable response within the population (Wong CH 1999). Indeed, a proportion of both normal volunteers and chronic cough patients fail to cough even at high inhaled concentrations. Within individuals tussive response between the different complex acids is consistent. In contrast other tussive challenges, such as those using capsaicin are not correlated with acid challenge. It has therefore been suggested that unlike capsaicin, citric acid does not act solely via the TRPV1 receptor. Other candidate receptors for producing a tussive response by citric acid include the acid sensing ion channels (ASIC) (Kolarik M 2002) and other members of the TRP family such as the TRPA1 receptor (Garrity PA 2011).
CC E
PT
ED
We hypothesized that the pH of the nebulized solution may be the important factor in precipitating the tussive response in man. We therefore constructed a challenge methodology based around the respective pKa (symbol for acid dissociation constant at logarithmic scale) of the three acid moieties in the citric acid molecule. We have previously observed that patients with a chronic cough have an increased sensitivity associated with a greater variability in their pattern of coughing (Kastelik JA 2002). We sought to characterize this further to help our understanding of the phenomenon of cough hypersensitivity. Methods
A
Study Protocol
This was a randomized, double blind single centre study. Ethical approval was obtained to complete this study from the U.K. R.E.C (REC reference: 14/SS/1071, Protocol number: ACADMED240913 clinicaltrials.gov reference NCT02039999. Two study populations were recruited. Normal volunteers were recruited from the hospital staff and chronic cough patients were recruited from the Hull Cough Clinic. The two groups were matched for gender by stratification during recruitment. More females were recruited deliberately in the Healthy Volunteers group to better match the demographic of chronic cough patients, which has a significant for female
preponderance (Morice AH 2014). Healthy Volunteers were required to be free from significant respiratory illness and both the groups were using no regular medication known to affect cough. Chronic cough patients were also required to be stable on other medication for a month before recruitment and exhibit a Hull Airway Reflex Questionnaire (HARQ) (Morice AH 2011) score of 20 or above. A HARQ below the upper limit of normal (13 or less) on entry was required by the Healthy Volunteers.
IP T
Citric acid solutions of pH 3.13, 5.05 and 5.99 were prepared by diluting 1 molar solution of citric acid with sodium hydroxide and distilled water (Goldberg RN 2002).
SC R
Following informed consent subjects were challenged on 3 separate study days, one for each of the three pH solutions and each challenge was separated by at least 48 hours. The order of the pH challenge was randomized. Randomisation codes were generated via sealedenvelope.com. Vials of pre-formed citric acid were then numbered, according to the code by staff not involved in the study.
A
N
U
Subjects were initially challenged with normal saline, followed by 5 repeated inhalations of 300mM citric acid solution at 1 minute intervals. The challenges were delivered via dosimeter (KoKo) as per the European Respiratory Society recommended methodology (Wright CE 2010) . Subjects were challenged at the same time of day to eliminate any diurnal variation. Study personnel counted the number of coughs for 15 seconds after each inhalation.
M
Statistical Analysis.
Results
PT
ED
Data was analysed with SPSS (IBM Version 22). Within groups the comparison of total number of coughs at each pH was performed using the Friedman’s test with Bonferroni correction. Between groups comparison was performed using the Wilcoxon Sign Rank test
CC E
Twenty chronic cough patients (median age 74 range 23 to 77) and 20 Healthy Volunteers (median age 42 range 50 to 83) were prospectively recruited into this study. Participants were gender matched and included 12 females in each group. All subjects were non-smokers. One participant in each group withdrew their consent and their data was not included in the final analysis.
A
Initial inhalation of normal saline provoked cough in a single subject in the Healthy Volunteers where as in the chronic coughers 10 subjects coughed to normal saline on at least one challenge day and 3 coughed on all 3 challenge days. As has been previously observed, a number of subjects did not cough to citric acid inhalation even at pH 3. One patient in the chronic cough group and 4 in the Healthy Volunteers did not cough to any challenge. The individual cough responses to challenge at the 3 different pH values is illustrated in figures 1a and 1b. Mean responses are demonstrated in figure 2. The mean total cough response for the 5 challenges at pH 3 was similar (16 vs 9),
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whereas at pH 5 the cough response in the Healthy Volunteers had dropped to mean of 3 but in chronic coughers actually rose to 18 (p<0.01). At pH 6 mean cough response fell in both groups approaching 0 in Healthy Volunteers (0.6) whereas chronic coughers still had substantial cough response (8) (p<0.01). Thus there appeared to be a much greater variability in response in the chronic cough patients. We further analysed this by looking the variation in the number of coughs produced at each of the 5 inhalations at a given pH value in the chronic cough group and compared this with the Healthy Volunteers. The variability of response to challenge in each subject at a given pH is illustrated in Figure 3 showing the standard deviation of an individual subject’s response. Discussion
A
CC E
PT
ED
M
A
N
U
SC R
Our objective in this investigation was to characterise the degree of hypersensitivity to citric acid seen in patients with chronic cough. Overall our patients coughed 240% more illustrating what has now come to be called the Cough Hypersensitivity Syndrome. Patients with chronic cough present with hypersensitivity to a wide range of mechanical, thermal and chemical stimuli such as perfumes, bleaches and change in temperature and this syndrome has become an overarching diagnosis covering the pathophysiology of a number of respiratory illnesses such as asthma, chronic bronchitis and chronic idiopathic cough (Morice AH 2014). Three basic hypotheses have been generated to explain this phenomenon. Firstly excessive external stimulation such as atmospheric pollution has been invoked, but this does not explain the generality of cough hypersensitivity within the population even though particular environments such as bottle manufacturer and dust exposure have been shown to alter cough reflex sensitivity (Gordon SB 1998). Secondly upregulation of afferent sensory receptors has been hypothesised to contribute, with some studies suggesting an increase for example of TRPV1 receptors in chronic cough (Groneberg DA 2004). Given that the other known putative cough receptors such as TRPA1 and ASIC would also have to be upregulated in parallel to account for generalised cough hypersensitivity perhaps this is an unlikely explanation. The best evidence against this mechanism has an important aetiological factor is the failure of specific TRPV1 and A1 antagonists in the clinic (Khalid S 2014). Finally central sensitisation or lack of inhibition either in the ganglia, brainstem or cortex clearly could up-regulate the cough reflex (Ando A 2016). The striking preponderance of women presenting with chronic cough and the known hypersensitivity of women to inhaled tussive challenges which has recently been shown to be associated with a greater activation of the somatosensory cortex validates the likely importance of such central mechanisms sensitivity (Morice AH 2014). In studying the effects of citric acid at different pH we have illustrated in the Healthy Volunteers the previously described reproducibility of citric acid challenge in measuring tussive response. However for the first time we have demonstrated that
U
SC R
IP T
this relationship does not hold true for patients suffering from chronic cough. Thus in Healthy Volunteers the stimulus activating the cough reflex appears to have a more or less linear relationship to the concentration of protons in the nebulised solution. Unfortunately we have not elucidated which of the several potential receptors have been activated to generate the stimulus for coughing and this would need specific antagonists of the putative acid sensor within the airways. In contrast, the striking lack of relationship to pH in the chronic cough patients was mimicked by their tussive response to normal saline. This was again an erratic phenomenon, some patients coughing vigorously to normal saline with subsequent citric acid challenge evoking no cough. The stimulus for this normal saline ‘hypersensitivity’ is unclear but jet nebulisation such as occurs in the dosimeter used causes a fall in temperature and so a thermal stimulus may be the cough provoking factor. Cough with normal saline does however illustrate both the increased sensitivity to minimal stimulation and the apparent random nature of the excessive response to such stimulation. Patients in the clinic describe paroxysms of severe cough (excessive response ) induced by exposure to ordinarily innocuous agents (TernestenHasseus E 2011). ‘There is no pattern to it Doctor’
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PT
ED
M
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The considerable difference in response of patients suffering from chronic cough is illustrated by a two measures of variability in the citric acid challenge. Firstly there is a lack of concentration response over the pH spectrum studied. Secondly at a given pH a patient with chronic cough responds in a highly idiosyncratic manner, sometimes not coughing to the first few inhalations and then coughing excessively to what is an exactly similar stimulus. These altered responses bear conspicuous parallels to that seen in chronic pain where the phenomenon of ‘wind up’- the perceived increase in intensity over time when a given non painful stimulus is delivered repeatedly and the concept of allodynia –central sensitization following painful, often repetitive, stimulation (Troels SJ 2014). Allodynia can lead to the triggering of a response from stimuli which do not normally provoke pain. These concepts have previously been discussed in the context of cough where term allotussia has been coined (Vertigan AE 2011). However the naming of the phenomenon does not necessarily help in our understanding. Cough is essentially a vagal phenomenon with the afferent pathway relaying in a complex fashion through nodose and jugular ganglia where the expression of neurotransmitters and receptors may be altered by disease (Undem BJ 2015) . In addition the concept of simple afferent neurone to a brainstem ‘cough centre’ is unlikely to reflect the complex neurophysiology of plastic neurone to neurone interaction. Our study has several limitations. Whilst we matched the gender between our two study groups we did not match age. Our normal volunteers were considerable younger than the chronic cough population. Of these demographics there is a large body of evidence supporting heightened cough sensitivity in women, but there is little evidence of an important age related change in cough sensitivity. Indeed if age
related neurological conditions such as stroke dementia or Parkinson’s disease are excluded from population surveys of cough reflex sensitivity there appears to be little difference in adults (Ebihara S 2003). Additionally in studying citric acid we have potentially used an agent which is subject to tachyphlaxis (Morice AH 1992). However there was no evidence of an order effect in our chronic cough subjects and in both arms of the study the order of challenge solution was randomised.
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In conclusion, we have demonstrated the explosive nature of cough response to the tussive stimulus citric acid in chronic cough patients. Hypersensitivity is not simply a shift in the dose response curve to citric acid but a fundamental alteration in the pattern of response to a given stimulus.
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References Ando A, Smallwood D, McMahon M, et al. Neural correlates of cough hypersensitivity in humans: evidence for central sensitisation and dysfunctional inhibitory control. Thorax 2016;71(4):323-9. Bickerman HA, Barach AL. The experimental production of cough in human subjects induced by citric acid aerosols; preliminary studies on the evaluation of antitussive agents. Am J Med Sci 1954;228(2):156-63.
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Ebihara S, Saito H, Kanda A, et al. Impaired efficacy of cough in patients with Parkinson disease. Chest 2003;124(3):1009-15.
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Garrity PA. Weakly acidic, but strongly irritating: TRPA1 and the activation of nociceptors by cytoplasmic acidification. J Gen Physiol 2011;137(6):489-91.
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Goldberg RN KNaLR. Thermodynamic Quantities for the Ionization Reactions of Buffers. J Phys Chem 2002;1(31):231-370. Groneberg DA, Niimi A, Dinh QT, et al. Increased expression of transient receptor potential vanilloid-1 in airway nerves of chronic cough. Am J Respir Crit Care Med 2004;170(12):1276-80. Kastelik, J. A., et al. (2002). "Gender related differences in cough reflex sensitivity in patients with chronic cough." Am J Respir Crit Care Med 166: 961-964. Khalid S, Murdoch R, Newlands A, et al. Transient receptor potential vanilloid 1 (TRPV1) antagonism in patients with refractory chronic cough: a double-blind randomized controlled trial. J Allergy Clin Immunol 2014;134(1):56-62. Kollarik M, Undem BJ. Mechanisms of acid-induced activation of airway afferent nerve fibres in guinea-pig. J Physiol 2002;543(Pt 2):591-600. Morice AH, Faruqi S, Wright CE, et al. Cough hypersensitivity syndrome: a distinct clinical entity. Lung 2011;189(1):73-9. Morice AH, Higgins KS, Yeo WW. Adaptation of cough reflex with different types of stimulation. Eur Respir J 1992;5(7):841-7. Morice AH, Jakes AD, Faruqi S, et al. A worldwide survey of chronic cough: a manifestation of enhanced somatosensory response. Eur Respir J 2014;44(5):1149-55 Morice AH, Millqvist E, Belvisi MG, et al. Expert opinion on the cough hypersensitivity syndrome in respiratory medicine. Eur Respir J 2014;44(5):1132-48. SJ Troels, Finnerup N. Allodynia and hyperalgesia in neuropathic pain: clinical manifestation and mechanisms. The Lancet 2014;13(9):924-35. Ternesten-Hasseus E, Larsson S, Millqvist E. Symptoms induced by environmental irritants and health-related quality of life in patients with chronic cough - A cross-sectional study. Cough 2011;7:6. Undem BJ, Zaccone E, McGarvey L, et al. Neural dysfunction following respiratory viral infection as a cause of chronic cough hypersensitivity. Pulm Pharmacol Ther 2015;33:52-6. Vertigan AE, Gibson PG. Chronic refractory cough as a sensory neuropathy: evidence from a reinterpretation of cough triggers. J Voice 2011;25(5):596601.
Wong CH, Matai R, Morice AH. Cough induced by low pH. Respir Med 1999;93(1):58-61. Wright CE, Jackson J, Thompson RL, et al. Validation of the ERS standard citric acid cough challenge in healthy adult volunteers. Cough 2010;6:8.
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Bibliography Bibliography Bibliography Bibliography
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Fig 1 a 50
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Fig 1.b
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_ Figure 3. Standard deviation of the 5 inhalations for each subject
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Figure 3. Standard deviation of the 5 inhalations for each subject
S1569-9048(17)30366-X https://doi.org/10.1016/j.resp.2018.02.013 RESPNB 2933
To appear in:
Respiratory Physiology & Neurobiology
Received date: Revised date: Accepted date:
22-10-2017 25-1-2018 21-2-2018
Please cite this article as: Rai, Z.L, Fowles, H.E., Wright, C., Joseph, H., Morice, A.H., The Effect of pH on Citric Acid Cough Challenge: A Randomised Control Trial in Chronic Cough and Healthy Volunteers.Respiratory Physiology and Neurobiology https://doi.org/10.1016/j.resp.2018.02.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The Effect of pH on Citric Acid Cough Challenge: A Randomised Control Trial in Chronic Cough and Healthy Volunteers. 1) Z.L Rai [email protected] Castle Clinical Castle Cottingham Cottingham, UK HU16 5JQ
Hill Trials Hill
Hospital Unit Hospital
IP T
07507573313 2) H.E Fowles
SC R
[email protected]
Hull York Medical School Centre for Cardiovascular and Metabolic Research, Respiratory Castle Hill Hospital Cottingham, UK HU16 5JQ
U
3) C Wright [email protected]
A
N
Hull York Medical School Centre for Cardiovascular and Metabolic Research, Respiratory Cottingham, UK
[email protected]
M
4) H Joseph
ED
Hull York Medical School Centre for Cardiovascular and Metabolic Research, Respiratory Cottingham, UK 5) A.H Morice
PT
[email protected]
CC E
Castle Head of Academic UK HU16 5J
Hill Academic Department
Department
of of
Key words; Chronic Cough, Cough Challenge, Cough Hypersensitivity
A
Word Count 1863
Hospital Medicine Medicine
Highlights
IP T
1) What is the key question? Do patients with chronic cough have an altered response to inhalational cough challenge? 2) What is the bottom line? In normal subjects the predicted concentration response to pH was seen but in chronic cough patients there was a heightened response which was unpredictable and individually variable.
N
Abstract
U
SC R
3)Why read on? These findings demonstrate experimentally the clinically observed phenomenon of hypersensitivity that chronic cough patients report. These insights are useful in our understanding of the mechanism of cough hypersensitivity, thus aiding patient communication.
A
Introduction
ED
M
Citric acid has been used for over six decades to induce cough; however the mechanism of its pro-tussive effect is still not fully understood. We assessed the response to inhalation of citric acid at varying levels of acidity to determine if the pH of the solution plays a role in the induction of cough. Data was collected from both healthy volunteers and patients with chronic cough. Methods
CC E
PT
20 chronic cough patients and 20 healthy volunteers were recruited and underwent three cough challenges on separate days. Each visit involved 5 repeated one second inhalations of 300mM citric acid solution. The concentration of the citrate cation remained constant, but the pH of the solution altered by the addition of sodium bicarbonate to 3, 5 and 6, representing the pKa values of the individual acid moieties. The total number of coughs elicited was recorded for each inhalation.
A
Results
Two subjects withdrew and were not included in the analysis. Participants were gender matched, each group consisting of 12 females. 74% of chronic coughers coughed at pH 3 (mean coughs 16), 89% coughed at pH 5 (18) and 63% coughed at pH 6 (7). In healthy volunteers, 60% of subjects coughed at pH 3 (9), 30% of subjects coughed at pH 5 (3), and 10% of subjects coughed at pH 6 (0). Thus chronic cough patients coughed more than healthy volunteers and did not exhibit a clear pH concentration response. There was also a greater variability in their response to individual challenges.
Introduction
SC R
IP T
Citric acid has been used since 1954 to induce a tussive response (Bickerman 1954). The citric acid cough challenge model has become a standard methodology for investigating cough reflex sensitivity (Wright CE 2010). The challenge is highly reproducible and has become one of the favoured techniques for studying the pharmacokinetics and pharmacodynamics of anti-tussive medications. Despite its extensive use, the mechanism whereby citric acid produces cough has never been fully elucidated.
M
A
N
U
Previous studies have shown that citric acid along with other complex acids demonstrates a variable response within the population (Wong CH 1999). Indeed, a proportion of both normal volunteers and chronic cough patients fail to cough even at high inhaled concentrations. Within individuals tussive response between the different complex acids is consistent. In contrast other tussive challenges, such as those using capsaicin are not correlated with acid challenge. It has therefore been suggested that unlike capsaicin, citric acid does not act solely via the TRPV1 receptor. Other candidate receptors for producing a tussive response by citric acid include the acid sensing ion channels (ASIC) (Kolarik M 2002) and other members of the TRP family such as the TRPA1 receptor (Garrity PA 2011).
CC E
PT
ED
We hypothesized that the pH of the nebulized solution may be the important factor in precipitating the tussive response in man. We therefore constructed a challenge methodology based around the respective pKa (symbol for acid dissociation constant at logarithmic scale) of the three acid moieties in the citric acid molecule. We have previously observed that patients with a chronic cough have an increased sensitivity associated with a greater variability in their pattern of coughing (Kastelik JA 2002). We sought to characterize this further to help our understanding of the phenomenon of cough hypersensitivity. Methods
A
Study Protocol
This was a randomized, double blind single centre study. Ethical approval was obtained to complete this study from the U.K. R.E.C (REC reference: 14/SS/1071, Protocol number: ACADMED240913 clinicaltrials.gov reference NCT02039999. Two study populations were recruited. Normal volunteers were recruited from the hospital staff and chronic cough patients were recruited from the Hull Cough Clinic. The two groups were matched for gender by stratification during recruitment. More females were recruited deliberately in the Healthy Volunteers group to better match the demographic of chronic cough patients, which has a significant for female
preponderance (Morice AH 2014). Healthy Volunteers were required to be free from significant respiratory illness and both the groups were using no regular medication known to affect cough. Chronic cough patients were also required to be stable on other medication for a month before recruitment and exhibit a Hull Airway Reflex Questionnaire (HARQ) (Morice AH 2011) score of 20 or above. A HARQ below the upper limit of normal (13 or less) on entry was required by the Healthy Volunteers.
IP T
Citric acid solutions of pH 3.13, 5.05 and 5.99 were prepared by diluting 1 molar solution of citric acid with sodium hydroxide and distilled water (Goldberg RN 2002).
SC R
Following informed consent subjects were challenged on 3 separate study days, one for each of the three pH solutions and each challenge was separated by at least 48 hours. The order of the pH challenge was randomized. Randomisation codes were generated via sealedenvelope.com. Vials of pre-formed citric acid were then numbered, according to the code by staff not involved in the study.
A
N
U
Subjects were initially challenged with normal saline, followed by 5 repeated inhalations of 300mM citric acid solution at 1 minute intervals. The challenges were delivered via dosimeter (KoKo) as per the European Respiratory Society recommended methodology (Wright CE 2010) . Subjects were challenged at the same time of day to eliminate any diurnal variation. Study personnel counted the number of coughs for 15 seconds after each inhalation.
M
Statistical Analysis.
Results
PT
ED
Data was analysed with SPSS (IBM Version 22). Within groups the comparison of total number of coughs at each pH was performed using the Friedman’s test with Bonferroni correction. Between groups comparison was performed using the Wilcoxon Sign Rank test
CC E
Twenty chronic cough patients (median age 74 range 23 to 77) and 20 Healthy Volunteers (median age 42 range 50 to 83) were prospectively recruited into this study. Participants were gender matched and included 12 females in each group. All subjects were non-smokers. One participant in each group withdrew their consent and their data was not included in the final analysis.
A
Initial inhalation of normal saline provoked cough in a single subject in the Healthy Volunteers where as in the chronic coughers 10 subjects coughed to normal saline on at least one challenge day and 3 coughed on all 3 challenge days. As has been previously observed, a number of subjects did not cough to citric acid inhalation even at pH 3. One patient in the chronic cough group and 4 in the Healthy Volunteers did not cough to any challenge. The individual cough responses to challenge at the 3 different pH values is illustrated in figures 1a and 1b. Mean responses are demonstrated in figure 2. The mean total cough response for the 5 challenges at pH 3 was similar (16 vs 9),
IP T
whereas at pH 5 the cough response in the Healthy Volunteers had dropped to mean of 3 but in chronic coughers actually rose to 18 (p<0.01). At pH 6 mean cough response fell in both groups approaching 0 in Healthy Volunteers (0.6) whereas chronic coughers still had substantial cough response (8) (p<0.01). Thus there appeared to be a much greater variability in response in the chronic cough patients. We further analysed this by looking the variation in the number of coughs produced at each of the 5 inhalations at a given pH value in the chronic cough group and compared this with the Healthy Volunteers. The variability of response to challenge in each subject at a given pH is illustrated in Figure 3 showing the standard deviation of an individual subject’s response. Discussion
A
CC E
PT
ED
M
A
N
U
SC R
Our objective in this investigation was to characterise the degree of hypersensitivity to citric acid seen in patients with chronic cough. Overall our patients coughed 240% more illustrating what has now come to be called the Cough Hypersensitivity Syndrome. Patients with chronic cough present with hypersensitivity to a wide range of mechanical, thermal and chemical stimuli such as perfumes, bleaches and change in temperature and this syndrome has become an overarching diagnosis covering the pathophysiology of a number of respiratory illnesses such as asthma, chronic bronchitis and chronic idiopathic cough (Morice AH 2014). Three basic hypotheses have been generated to explain this phenomenon. Firstly excessive external stimulation such as atmospheric pollution has been invoked, but this does not explain the generality of cough hypersensitivity within the population even though particular environments such as bottle manufacturer and dust exposure have been shown to alter cough reflex sensitivity (Gordon SB 1998). Secondly upregulation of afferent sensory receptors has been hypothesised to contribute, with some studies suggesting an increase for example of TRPV1 receptors in chronic cough (Groneberg DA 2004). Given that the other known putative cough receptors such as TRPA1 and ASIC would also have to be upregulated in parallel to account for generalised cough hypersensitivity perhaps this is an unlikely explanation. The best evidence against this mechanism has an important aetiological factor is the failure of specific TRPV1 and A1 antagonists in the clinic (Khalid S 2014). Finally central sensitisation or lack of inhibition either in the ganglia, brainstem or cortex clearly could up-regulate the cough reflex (Ando A 2016). The striking preponderance of women presenting with chronic cough and the known hypersensitivity of women to inhaled tussive challenges which has recently been shown to be associated with a greater activation of the somatosensory cortex validates the likely importance of such central mechanisms sensitivity (Morice AH 2014). In studying the effects of citric acid at different pH we have illustrated in the Healthy Volunteers the previously described reproducibility of citric acid challenge in measuring tussive response. However for the first time we have demonstrated that
U
SC R
IP T
this relationship does not hold true for patients suffering from chronic cough. Thus in Healthy Volunteers the stimulus activating the cough reflex appears to have a more or less linear relationship to the concentration of protons in the nebulised solution. Unfortunately we have not elucidated which of the several potential receptors have been activated to generate the stimulus for coughing and this would need specific antagonists of the putative acid sensor within the airways. In contrast, the striking lack of relationship to pH in the chronic cough patients was mimicked by their tussive response to normal saline. This was again an erratic phenomenon, some patients coughing vigorously to normal saline with subsequent citric acid challenge evoking no cough. The stimulus for this normal saline ‘hypersensitivity’ is unclear but jet nebulisation such as occurs in the dosimeter used causes a fall in temperature and so a thermal stimulus may be the cough provoking factor. Cough with normal saline does however illustrate both the increased sensitivity to minimal stimulation and the apparent random nature of the excessive response to such stimulation. Patients in the clinic describe paroxysms of severe cough (excessive response ) induced by exposure to ordinarily innocuous agents (TernestenHasseus E 2011). ‘There is no pattern to it Doctor’
A
CC E
PT
ED
M
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The considerable difference in response of patients suffering from chronic cough is illustrated by a two measures of variability in the citric acid challenge. Firstly there is a lack of concentration response over the pH spectrum studied. Secondly at a given pH a patient with chronic cough responds in a highly idiosyncratic manner, sometimes not coughing to the first few inhalations and then coughing excessively to what is an exactly similar stimulus. These altered responses bear conspicuous parallels to that seen in chronic pain where the phenomenon of ‘wind up’- the perceived increase in intensity over time when a given non painful stimulus is delivered repeatedly and the concept of allodynia –central sensitization following painful, often repetitive, stimulation (Troels SJ 2014). Allodynia can lead to the triggering of a response from stimuli which do not normally provoke pain. These concepts have previously been discussed in the context of cough where term allotussia has been coined (Vertigan AE 2011). However the naming of the phenomenon does not necessarily help in our understanding. Cough is essentially a vagal phenomenon with the afferent pathway relaying in a complex fashion through nodose and jugular ganglia where the expression of neurotransmitters and receptors may be altered by disease (Undem BJ 2015) . In addition the concept of simple afferent neurone to a brainstem ‘cough centre’ is unlikely to reflect the complex neurophysiology of plastic neurone to neurone interaction. Our study has several limitations. Whilst we matched the gender between our two study groups we did not match age. Our normal volunteers were considerable younger than the chronic cough population. Of these demographics there is a large body of evidence supporting heightened cough sensitivity in women, but there is little evidence of an important age related change in cough sensitivity. Indeed if age
related neurological conditions such as stroke dementia or Parkinson’s disease are excluded from population surveys of cough reflex sensitivity there appears to be little difference in adults (Ebihara S 2003). Additionally in studying citric acid we have potentially used an agent which is subject to tachyphlaxis (Morice AH 1992). However there was no evidence of an order effect in our chronic cough subjects and in both arms of the study the order of challenge solution was randomised.
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In conclusion, we have demonstrated the explosive nature of cough response to the tussive stimulus citric acid in chronic cough patients. Hypersensitivity is not simply a shift in the dose response curve to citric acid but a fundamental alteration in the pattern of response to a given stimulus.
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References Ando A, Smallwood D, McMahon M, et al. Neural correlates of cough hypersensitivity in humans: evidence for central sensitisation and dysfunctional inhibitory control. Thorax 2016;71(4):323-9. Bickerman HA, Barach AL. The experimental production of cough in human subjects induced by citric acid aerosols; preliminary studies on the evaluation of antitussive agents. Am J Med Sci 1954;228(2):156-63.
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Ebihara S, Saito H, Kanda A, et al. Impaired efficacy of cough in patients with Parkinson disease. Chest 2003;124(3):1009-15.
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Garrity PA. Weakly acidic, but strongly irritating: TRPA1 and the activation of nociceptors by cytoplasmic acidification. J Gen Physiol 2011;137(6):489-91.
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Goldberg RN KNaLR. Thermodynamic Quantities for the Ionization Reactions of Buffers. J Phys Chem 2002;1(31):231-370. Groneberg DA, Niimi A, Dinh QT, et al. Increased expression of transient receptor potential vanilloid-1 in airway nerves of chronic cough. Am J Respir Crit Care Med 2004;170(12):1276-80. Kastelik, J. A., et al. (2002). "Gender related differences in cough reflex sensitivity in patients with chronic cough." Am J Respir Crit Care Med 166: 961-964. Khalid S, Murdoch R, Newlands A, et al. Transient receptor potential vanilloid 1 (TRPV1) antagonism in patients with refractory chronic cough: a double-blind randomized controlled trial. J Allergy Clin Immunol 2014;134(1):56-62. Kollarik M, Undem BJ. Mechanisms of acid-induced activation of airway afferent nerve fibres in guinea-pig. J Physiol 2002;543(Pt 2):591-600. Morice AH, Faruqi S, Wright CE, et al. Cough hypersensitivity syndrome: a distinct clinical entity. Lung 2011;189(1):73-9. Morice AH, Higgins KS, Yeo WW. Adaptation of cough reflex with different types of stimulation. Eur Respir J 1992;5(7):841-7. Morice AH, Jakes AD, Faruqi S, et al. A worldwide survey of chronic cough: a manifestation of enhanced somatosensory response. Eur Respir J 2014;44(5):1149-55 Morice AH, Millqvist E, Belvisi MG, et al. Expert opinion on the cough hypersensitivity syndrome in respiratory medicine. Eur Respir J 2014;44(5):1132-48. SJ Troels, Finnerup N. Allodynia and hyperalgesia in neuropathic pain: clinical manifestation and mechanisms. The Lancet 2014;13(9):924-35. Ternesten-Hasseus E, Larsson S, Millqvist E. Symptoms induced by environmental irritants and health-related quality of life in patients with chronic cough - A cross-sectional study. Cough 2011;7:6. Undem BJ, Zaccone E, McGarvey L, et al. Neural dysfunction following respiratory viral infection as a cause of chronic cough hypersensitivity. Pulm Pharmacol Ther 2015;33:52-6. Vertigan AE, Gibson PG. Chronic refractory cough as a sensory neuropathy: evidence from a reinterpretation of cough triggers. J Voice 2011;25(5):596601.
Wong CH, Matai R, Morice AH. Cough induced by low pH. Respir Med 1999;93(1):58-61. Wright CE, Jackson J, Thompson RL, et al. Validation of the ERS standard citric acid cough challenge in healthy adult volunteers. Cough 2010;6:8.
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_ Figure 3. Standard deviation of the 5 inhalations for each subject
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Figure 3. Standard deviation of the 5 inhalations for each subject