Coughing itself is a reflex stimulated by ‘afferent’ nerves in the airways which carry a message of airway irritation or restriction to the brain stem, which in turn sends a message to muscle tissue to contract to create a cough via the ‘efferent’ nerves. The afferent nerves are sensitive to a range of both chemical (eg cigarette smoke) and mechanical (eg an airway restriction) stimuli.
All cases of chronic cough start as an acute cough, which then become a ‘chronic cough’ when it lasts last longer than a predetermined time, currently generally accepted at 8 weeks, although the term has in the past been applied to coughs lasting longer than 3 weeks.
Research into the causes of chronic cough over the past 15 years generally agrees that there are three primary causes of the vast majority of cases (92%-100%) of chronic cough where both current cigarette smoking and the use of ACE (Angiotensin-Converting Enzyme) inhibitors have been ruled out:
1) Upper Airway Cough Syndrome (UACS)
UACS covers a range of conditions affecting the nose and the sinuses, including postnasal drip syndrome (PNDS) where excess mucus produced in the nose and sinuses drips backwards behind the mouth to irritate the voice box area causing a cough.
The primary cause of chronic cough in children, and the second most common cause of chronic cough in adults, after UACS. Asthma-induced coughs can be triggered by exposure to extreme air temperatures (cold or hot) and fumes or other airborne particles. One type of asthma has a cough as the only symptom (cough variant asthma).
3) Gastroesophageal Reflux Disease (GERD)
This is where enzyme laden acidic content from the stomach travel back (refluxes) up the gullet to spill over into the voice box area causing cough.
Success rates in treatment of chronic cough
There is some disagreement in the medical community as to the success rate in treating chronic cough. Although some specialist centres claim to be able to successfully treat chronic cough in 98% of cases, other medical practitioners report success rates in some cases as low as 58%. Chronic cough that cannot be resolved is often categorised as ‘idiopathic’, ie spontaneously arising or where the cause of the condition cannot be determined.
Researchers speculate that in fact chronic cough may continue due to several factors, including
Non compliance, ie the patient not taking the prescribed medications
Inadequate doses of medication and/or duration of medication
Not taking into account multiple and co-existing causes of chronic cough.
One theory put forward by researchers is that a cough is initially triggered by a specific ‘event’ such as a subclinical viral infection (ie with no other apparent symptoms), or an environmental irritant, and that the cough persists due to another pre-existing condition such as inflammatory disease affecting the lungs or other genetic or hormonal factors. A combination of one or more pre-existing conditions serve to ‘upregulate’ and sustain the initial cough reflex over a longer period.
The difficulty for the practitioner is that one or more of these conditions may be asymptomatic, and if more than one condition is present, all conditions must be treated in order to resolve the chronic cough. This approach, say the researchers, is likely to have a more positive outcome than looking for rarer potential causes of the condition.
The one exception to this approach acknowledged by the researchers is that Non Asthmatic Eosinophilic Bronchitis (NAEB) should also be excluded as a potential cause at an early stage. It is done by examining a sample of sputum to show eosinophilia (>3%) in the absence of bronchial hypersensitivity, and it may be more common than currently recognised.