Description of the condition
Cough is defined as an airway defensive reflex comprising an inspiratory phase, followed by a forced expiratory effort initially against a closed glottis (Fontana 2008). The duration of cough can be acute (less than three weeks), subacute (between three and eight weeks), or chronic (longer than eight weeks) (Irwin 2006). Details regarding the theory and treatment of cough can be found in several different clinical fields, including otolaryngology, respiratory medicine, speech and language therapy, respiratory physiology, and physiotherapy (Vertigan 2016a).
Unexplained (idiopathic/refractory) chronic cough (UCC) is a cough that persists after common causes have been evaluated and ruled out and medical management options have not offered any convincing relief from symptoms. This condition is a diagnosis of exclusion following a careful diagnostic workup to exclude other causes of cough such as underlying lung/airways disease, gastro-oesophageal reflux, drugs (such as angiotensin-converting enzyme inhibitors (ACEis)), and rhinosinusitis (Gibson 2016a; Morice 2006; Morice 2007; Morice 2007a; Vertigan 2016a).
Evidence suggests that UCC may be caused by neural hyper-responsiveness. Patients with this condition describe an 'irritation' or 'tickle' within the throat and a heightened urge or need to cough (Hilton 2015). In addition, lower concentrations of tussive agents are required to induce cough in patients with UCC compared with healthy controls (Prudon 2005). It is known that vagal afferent bronchopulmonary C-fibres are important in cough regulation (Canning 2014). Novel drugs targeting C-fibre receptors have been recently trialed in humans. An oral transient receptor potential vanilloid subtype 1 (TRPV1) antagonist had no effect on cough frequency in patients with UCC (Belvisi 2016). However, another trial demonstrated a 75% reduction in cough reflex sensitivity in patients with UCC using a novel P2X3 antagonist (Abdulqawi 2015), supporting the hypothesis of neural hyper-responsiveness and hypersensitive central cough reflex in UCC.
Furthermore, an overlap has been postulated between chronic cough and other upper airway dysfunction presentations, such as inducible laryngeal obstruction (ILO), globus pharyngeus, hyperfunctional muscle tension, voice disorders, and dysphagia symptoms (Ryan 2009; Vertigan 2006; Vertigan 2016a). It has been suggested that these entities may represent different manifestations of an underlying hypersensitive and hyper-responsive upper airway (Vertigan 2007; Vertigan 2010; Vertigan 2016a).
Description of the intervention
Speech and language therapy (SLT) offers a non-pharmacological intervention for people with UCC who may have exhausted medical treatment for their condition, or who wish to pursue a non-pharmacological treatment option (Ryan 2010; Ryan 2014; Vertigan 2006; Vertigan 2016a). SLT has been incorporated into the Treatment of Unexplained Chronic Cough: CHEST guideline and expert panel report (Gibson 2016a), a report detailing treatment of UCC, and the Australian cough guidelines summary statement (Gibson 2010). The aim of SLT intervention is to improve an individual's control over cough, as well as to address symptoms associated with dysphonia and ILO (Vertigan 2016a).
Stemple 2009 states that speech and language therapists possess the correct skill set to treat chronic cough via a combination of detailed knowledge of anatomy and physiology of the upper airway, experience in education and training on respiratory physiology and clinical voice disorders, and proven ability to modify laryngeal behaviour (Altman 2000; Stemple 2009). As there is a hypothesised and clinically recognised overlap between cough and other upper airway and laryngeal disorders, such as ILO, globus pharyngeus, muscle tension dysphonia, and dysphagic symptoms, SLT professionals have the skills required to identify, assess, and manage these overlapping conditions effectively by applying techniques traditionally used to treat muscle tension voice disorders (Blager 1998; Blager 2000; Carney 1997; Chhetri 2014; Dunn 2015; Estill 2009; Estill 2009a; Ryan 2010; TitzeI 2006; Vertigan 2007). SLT assessment can also investigate patient-reported symptoms of dysphagia to identify clinical signs of aspiration that may warrant further investigation.
Furthermore, the trained SLT professional can provide biovisual feedback with videolaryngoscopy, which has been found to be useful for assisting patients in applying techniques and in helping to monitor response to SLT (Balkissoon 2012; Belafsky 2001; Christopher 2010; Hull 2016; Olin 2017).
During assessment, the SLT professional evaluates cough characteristics, the urge to cough, and indicators suggestive of ILO (Ryan 2014; Traister 2014). Phonation is often cited as a trigger for coughing episodes, and, given the overlap with dysphonia, a clinical voice assessment can use recognised descriptive rating scales of perceptual voice characteristics, such as rough, breathy, and strained vocal qualities. Hyperfunction in the larynx during phonation can signify laryngeal dysfunction and can serve as a contributory factor to cough reflex sensitivity (Hirano 1981; Omori 2011; Ryan 2014; Vertigan 2007).
When available, the SLT professional can assess instrumental voice (Ryan 2010; Zelcer 2002), for example, by using the multi-dimensional voice analysis programme (Computerised Speech Lab; Pentax Medical, Montvale, NJ, USA) or visualising the larynx via videolaryngoscopy during voice use according to a recognised protocol (RCSLT 2008). Manual assessment of tension in the extrinsic laryngeal musculature is a useful tool for assessing hyperfunction, which can be detected in muscle tension voice disorders (Rubin 2000; Vertigan 2016a).
Education is identified as a crucial element of the SLT approach with focus on encouraging adherence, with the aim of acceptance that patients can control their cough and take ownership of implementing strategies to ignore the urge to cough and stop themselves from coughing. Educators provide information on the importance of sustained application of cough suppression techniques (Vertigan 2012).
Education on the perpetuating nature of the cough is provided, potentially leading to increased sensitivity, laryngeal trauma, and tension. Specific goals for therapy are planned with the patient. The ultimate goal of SLT is for the patient to control the cough, even in the presence of the sensation/urge to cough.
Symptom control techniques
The aim of these strategies is to ask the patient to prevent, stop, or interrupt the cough despite having the triggering sensation. The patient is then asked to substitute the cough with a competing response (e.g. distraction, cough suppression swallow, relaxed throat breathing, a sip of fluids, laryngeal deconstriction) (Estill 2009; Vertigan 2007). Laryngeal control techniques for any coexisting inducible laryngeal obstruction are also included in the SLT therapy intervention (Blager 2000; Boone 1993; Chhetri 2014; Kotby 1993).
A hierarchy for applying control techniques is devised with the patient; this can include graded exposure to desensitise the patient to particular triggers (Gibson 2015)
Reducing laryngeal irritation via vocal hygiene
Vocal hygiene advice is commonly used in the SLT approach to clinical voice disorders (Blager 1988), and it may be beneficial in the treatment of UCC (Vertigan 2012). Vocal hygiene consists of advice on effects of smoking, mouth breathing (up to 50% of people with UCC habitually mouth-breathe), caffeine, and alcohol on laryngeal mucosa. Easy voicing and adequate hydration are emphasised and demonstrated to reduce the risk of phonotrauma (Boone 1993; Kotby 1993; Murry 2004; Solomon 2014; Vertigan 2007). Information on diet and behavioural management of gastroesophageal and laryngopharyngeal reflux is provided (Koufman 2011).
The therapist assesses readiness to engage in SLT, as this may impact the efficacy of therapy (Prochaska 1982). Acceptance that the patient can control the cough (internal locus of control), self-efficacy (Bandura 1986), and the effort required of the patient are made explicit, with emphasis on treatment being "hard work". Realistic, targeted goal setting helps the therapist to direct interventions and monitor progress (Murry 2004; Ryan 2010; Vertigan 2012).
Overall goals of speech and language therapy interventions can be summarised as follows (Gibson 2015).
Reduce the sensitivity of the cough reflex.
Encourage improvement in voluntary control of cough.
Reduce irritation of the larynx.
Whilst previous studies assessing the effectiveness of interventions for UCC have involved input from other allied health professionals (e.g. Chamberlain Mitchell 2017), it has been noted that the added benefit of contributions of other professionals, in addition to SLTs, is unclear (e.g. Smith 2017).
How the intervention might work
SLT interventions aim to improve individuals' control over cough and symptoms associated with any overlapping dysphonia and ILO (Vertigan 2016a), but the mechanism by which the above-detailed, multi-modal SLT intervention may reduce cough severity and frequency, leading to improvement in health-related quality of life, is poorly understood.
It has been postulated that SLT interventions may reduce cough sensitivity in patients with UCC (Smith 2005; Vertigan 2006). However, a multi-centre randomised controlled study demonstrated no differences between intervention and control groups with capsaicin cough challenge (Chamberlain Mitchell 2017). The psychoeducational and control strategies component may support subjective improvement in chronic cough management techniques and may reduce upper airway muscle tension (Canning 2006; Gibson 2009). Improved laryngeal hygiene attained via hydration and education on reduction of laryngeal injury may support lower phonation threshold pressure, thereby reducing stimulation of cough receptors (Casper 2003; Solomon 2014).
Why it is important to do this review
Chronic cough is a significant unmet need. Cough is the most common reason why patients seek medical advice (Morice 2006), and for chronic cough, most patients seek advice three or more times (Chamberlain Mitchell 2017). The prevalence of UCC in the population varies in the most recent studies from 4% (Colak 2017) to 9.6% (Song 2015). Cough presents a considerable financial burden, with acute cough costing approximately £979 million in the UK, including £875 million in loss of productivity and £104 million in healthcare costs (Morice 2006). The cost of chronic cough to the economy remains unclear.
The negative impact of chronic cough on quality of life is far reaching and has been well described (Decalmer 2007; French 1998). Urinary incontinence associated with chronic cough is a particularly distressing symptom (Hrisanfow 2013).
Few effective medical treatments for individuals with UCC are known. For this group, current guidelines advocate the use of gabapentin (Gibson 2016a; Vertigan 2016a). One study demonstrated a reduction in subjective cough scores in response to slow-release morphine sulphate (Morice 2007; Morice 2007a). Another study examined amitriptyline in patients with postviral vagal neuropathy and cough, but guidelines do not currently recommend this treatment (Jeyakumar 2006). SLT has been advocated as an attractive non-pharmacological option for managing UCC without the risks and side effects associated with pharmacological agents; SLT is the focus of this review (Krakowiak 2017).