Cough is a predominant feature of respiratory infection and, in tuberculosis, is of prime importance for transmitting infection. Tuberculosis is transmitted by the air, yet the process by which bacilli are aerosolized has received little attention. Features of cough may account for differences in transmission rates from source cases of pulmonary disease. We review the literature on the mechanisms and characteristics of cough in tuberculosis in the context of the dissemination of infection. Coughing is probably more important than other respiratory maneuvers, and characteristics of mucus may have an important role but data are scarce. Direct mechanisms of cough in tuberculosis are unknown, as are temporal and other patterns that correlate with the release of viable airborne bacilli. Other than antituberculous chemotherapy and masks, there are few methods of modulating cough in tuberculosis. This is an increasingly important area for research.

Functional brain imaging in individuals with chronic cough demonstrates reduced activation in cortical regions associated with voluntary cough suppression. Little is known about the ability of patients with chronic cough to suppress cough. This study aimed to compare the ability to voluntarily suppress cough during inhaled capsaicin challenge in participants with chronic refractory cough with that in healthy controls. In addition, this study aimed to assess the repeatability of capsaicin challenge test with voluntary cough suppression.Participants with chronic refractory cough and healthy controls underwent inhaled capsaicin challenge tests while attempting to suppress their cough responses. After 5 days, either a conventional capsaicin challenge test with no cough suppression attempt, or a repeat test with an attempt at cough suppression was performed. Threshold capsaicin concentrations required to elicit one, two and five coughs were calculated by interpolation. Objective 24-h cough frequency was measured in individuals with chronic refractory cough.Healthy controls were able to suppress capsaicin-evoked cough while participants with chronic refractory cough were not. Geometric mean±sd capsaicin dose thresholds for five coughs with (CS5) and without (C5) suppression attempts were 254.40±3.78 versus 45.89±3.95 µmol·L−1, respectively, in healthy controls (p=0.033) and 3.34±5.04 versus 3.86±5.13 µmol·L−1, respectively, in participants with chronic refractory cough (p=0.922). Capsaicin dose thresholds for triggering five coughs with self-attempted cough suppression were significantly lower in participants with chronic refractory cough than in healthy controls; geometric mean±sd 4.94±4.43 versus 261.10±4.34 µmol·L−1, respectively; mean difference (95% CI) 5.72 (4.54–6.91) doubling doses (p<0.001). Repeatability of cough suppression test in both patients and healthy controls was high; intraclass correlation coefficients of log(CS5) values 0.81 and 0.87, respectively. CS5 was associated with objective cough frequency (ρ=−0.514, p=0.029).Participants with chronic refractory cough were less able to voluntarily suppress capsaicin-evoked cough compared to healthy controls. This may have important implications for the pathophysiology and treatment of chronic cough.

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The transmission of tuberculosis is complex. Necessary factors include a source case with respiratory disease that has developed sufficiently for Mycobacterium tuberculosis to be present in the airways. Viable bacilli must then be released as an aerosol via the respiratory tract of the source case. This is presumed to occur predominantly by coughing but may also happen by other means. Airborne bacilli must be capable of surviving in the external environment before inhalation into a new potential host—steps influenced by ambient conditions and crowding and by M. tuberculosis itself. Innate and adaptive host defenses will then influence whether new infection results; a process that is difficult to study owing to a paucity of animal models and an inability to measure infection directly. This review offers an overview of these steps and highlights the many gaps in knowledge that remain.

The widespread use of cough counting tools has, to date, been limited by a reliance on human input to determine cough frequency. However, over the last two decades advances in digital technology and audio capture have reduced this dependence. As a result, cough frequency is increasingly recognised as a measurable parameter of respiratory disease. Cough frequency is now the gold standard primary endpoint for trials of new treatments for chronic cough, has been investigated as a marker of infectiousness in tuberculosis (TB), and used to demonstrate recovery in exacerbations of chronic obstructive pulmonary disease (COPD).This review discusses the principles of automatic cough detection and summarises key currently and recently used cough counting technology in clinical research. It additionally makes some predictions on future directions in the field based on recent developments. It seems likely that newer approaches to signal processing, the adoption of techniques from automatic speech recognition, and the widespread ownership of mobile devices will help drive forward the development of real-time fully automated ambulatory cough frequency monitoring over the coming years. These changes should allow cough counting systems to transition from their current status as a niche research tool in chronic cough to a much more widely applicable method for assessing, investigating and understanding respiratory disease.

Measurement of 24-h cough frequency in pulmonary TB helps predict infectiousness and transmission patterns.

Cough reflex hypersensitivity (CRH) and impaired cough suppression are features of chronic refractory cough (CRC). Little is known about cough suppression and CRH in cough associated with chronic obstructive pulmonary disease (COPD). This study investigated the ability of participants with COPD to suppress cough during a cough challenge test in comparison to participants with CRC and healthy subjects. This study also investigated whether CRH is associated with chronic cough in COPD.Participants with COPD (n=27), CRC (n=11), and healthy subjects (n=13) underwent capsaicin challenge test with and without attempts to self-suppress cough in a randomised order over 2 visits, 5 days apart. For participants with COPD, the presence of self-reported chronic cough was documented, and objective 24-h cough frequency was measured.Amongst participants with COPD, those with chronic cough (n=16) demonstrated heightened cough reflex sensitivity (CRS) compared to those without chronic cough (n=11); geometric mean (sd) capsaicin dose thresholds for 5 coughs (C5) 3.36 (6.88) versus 44.50 (5.90) µmol·L−1 respectively (p=0.003). Participants with CRC also had heightened CRS compared to healthy participants; geometric mean (sd) C5 3.86 (5.13) 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) versus 12.80 (8.33) µmol·L−1 (p<0.001) and 44.50 (5.90) 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) 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) versus 3.34 (5.04) µmol·L−1, p=0.922). C5 and CS5 were associated with objective 24-h cough frequency in participants with COPD; ρ=−0.430, p=0.036 and ρ=−0.420, p=0.041 respectively.Participants with COPD-chronic cough and CRC both have heightened cough reflex sensitivity but in contrast, only participants with CRC were unable to suppress capsaicin evoked cough. This suggests differing mechanisms of cough between participants with COPD and CRC, and the need for disease specific approaches to its management.

Coughs are well recognised by different listeners. The method used to count coughs should be clearly described as visual information has a significant influence. Non-automated cough counting remains the gold standard method of quantifying cough.