What is chronic cough?
A chronic cough is defined as one that lasts for longer than one month in individuals who do not respond to first line treatment. It is an extremely common complaint in the general population.
Having a chronic cough can be embarrassing and frustrating. However, by understanding the underlying mechanisms that drive a chronic cough, 95% of patients can, with specific treatment, have the problem resolved.
Mechanisms of a chronic cough
The respiratory tract has rich innervations of sensory nerve endings, which are normally buried beneath the epithelium, the outer layer of the lining of the airways. Factors that contribute to denudation of the epithelium, such as acute or chronic inflammation, will expose these fibres to inhaled stimuli. This subsequently leads to enhanced sensitivity of the airways to environmental stimuli such as cold air, exercise or emotion, which normally would not trigger the cough reflex. This phenomenon is known as increased airways hyper-reactivity (AHR), and is a major mechanism underlying a chronic cough. It is typically found in people who have uncontrolled asthma, prolonged post-viral bronchitis and chronic obstructive airways disease (COPD) – a condition previously known as chronic bronchitis and emphysema.
A major factor that enhances AHR and subsequently a chronic cough, is an ongoing inflammatory process in the airways. This may have the nature of an allergic bronchitis as found in chronic asthmatics. In a number of smokers and ex-smokers who suffer from COPD, a chemically induced inflammatory reaction of the airways, which is resistant to most forms of treatment, will persist. The release of inflammatory mediators and increased blood supply, with a subsequent “overheating” of the mucosal membrane of the airways, enhances nerve irritability and contributes to the chronic cough. This ongoing chronic inflammatory process predisposes the mucosal membrane and airways to secondary bacterial and viral infections, to the point where both patient and doctor are uncertain as to the exact cause of the ongoing inflammation and chronic cough.
In Third World countries, where endemic tuberculosis and HIV-related lung disease predominate, permanent structural damage of the airways often occurs together with a tendency to recurrent chest infections – as seen in patients with bronchiectasis, a condition of dilatation of the airways with destruction of the normal protective mechanisms. When a chronic cough is associated with an excessive mucus production, brochiectasis should be suspected. The nature of the mucus is also important e.g. a large volume of purulent (containing pus) mucus would indicate bronchial inflammation and be an important guideline as to an underlying cause and possible treatment.
What causes chronic cough?
Numerous factors – many treatable – can contribute to a chronic cough. The following conditions may contribute to this disorder:
- Chronic post-nasal drip is either swallowed or removed by clearing the throat during daytime hours. At night this mucus may invade the upper airways by passing through the vocal cords when individuals are asleep. This will induce irritation and inflammation of the large airways and subsequently contribute to the cough. Frequently the post-nasal drip is induced by allergic rhinitis (inflammation of the membrane lining the nose), which contributes to allergic bronchitis in its own right. Treatment of the allergic inflammatory response in the nasal passages as well as the bronchial tree (the airways that branch into the lungs) could resolve the problem.
- Asthma, particularly in children and young people, frequently presents only with a chronic cough and no symptoms of wheezing. These coughing asthmatics respond very well to the conventional treatment of allergic asthma.
- Chronic obstructive pulmonary disease (COPD) induces a different kind of chronic inflammatory response in the airways, which is accompanied by an above average production of mucus. A characteristic of the COPD patient is an early morning cough, which will carry on in spasms until the sticky mucus has been cleared from the airways. After this, patients may state that they have no further symptoms during the rest of their daily routine.
- Acid reflux. Paradoxically the stomach may be a cause of chronic cough – usually not recognised by the patient or doctor. This cause of cough goes unnoticed until patients are questioned about the presence of heartburn and acid reflux, particularly at night. Chemical irritation of the oesophagus (gullet) by acid reflux through a reflex neural mechanism to the bronchial wall, or direct aspiration of acid stomach content during sleep are common causes of chronic cough. Specific investigation such as a gastroscopy (using an optical tube to view inside the stomach), pH-determination in the oesophagus and occasionally a milk scan may indicate a deficiency of the gastro-oesophageal valve and subsequent leakage of stomach content into the oesophagus.
- Antihypertensive drugs (ACE inhibitors). Medication such as the ACE-inhibitors, a commonly used anti-hypertensive drug, has a particularly bad reputation for inducing chronic cough. Replacement of this group of drugs with other forms of anti-hypertensives can abolish a patient’s coughing.
- Chronic inflammatory conditions such as tuberculosis or HIV-associated chronic pneumonias may also present with a chronic cough. In Third World countries, these diseases are given a high priority and always have to be excluded in patients with respiratory symptoms. In developed countries these conditions may initially not be recognised as a cause of chronic cough.
- Carcinoma (tumour) of the bronchus may induce chronic cough due to the presence of a tumour in the airway. Coughing is induced either due to the mechanical presence of the tumour or due to the inflammatory response as a result of the obstruction.
- Interstitial lung disease. Uncommon but important causes of chronic cough are those conditions that affect the soft tissue of the lung, with a negative influence on oxygen uptake and development of a stiff lung. These are collectively known as interstitial lung disease. A number of causes of interstitial lung disease, particularly one known as extrinsic allergic alveolitis, caused by inhalation of organic antigens (foreign bodies that stimulate an immune response), have as their main symptom a chronic cough, particularly after exposure to the antigen.
Evaluation of patients with a chronic cough
A good medical history and physical examination will frequently suffice for determining the major factors that contribute to a chronic cough. The main purpose of this activity is to determine the trigger mechanisms. Unless these are eradicated, the treatment of the chronic cough will be unsuccessful. Frequently, the associated symptoms and physical findings provide an answer as to the cause. Examples include:
- Visible evidence of post-nasal drip or blockage of the nasal passages indicating that the upper airways are causing the cough.
- Evidence of wheezing of the chest in chronic asthma and COPD.
- Evaluation of sputum for evidence of purulence or chronic infecting organisms such as those that cause tuberculosis or pneumonia may represent chronic infections found in HIV/AIDS. (Sputum is the substance expelled by coughing or clearing the throat.)
More than 80% of chronic coughers will surrender the secret of their disease after good questioning and a physical examination. It is only when doubt still exists that a CT-scan of the sinuses, an x-ray of the chest, evaluation for causative allergens and lung functions will be required. A gastroscopy conducted by an experienced physician or gastroenterologist will be required if reflux is suspected.
How is chronic cough treated?
The success of treating patients with chronic cough lies in the ability of the doctor to evaluate and eliminate the causative factors and does not rely on symptomatic relief measures such as cough suppressants. (A normal cough mechanism is mandatory for lung health. Suppression of the cough reflex for a period in excess of a few hours – usually in unconscious patients – results in immediate and severe consequences. This could include severe lung infections, which may be fatal.)
Examples of treatment for specific causes:
- Treatment should be aimed at eliminating a post-nasal drip. When this cause is associated with allergic asthma, treatment of both the upper and lower airways with inhaled corticosteroids could be a significant step in containing the cough.
- Treatment of acid reflux with powerful antacids known as proton pump inhibitors, or, in severe cases of reflux with surgical correction of the gastro-oesophageal valve, will resolve this source of a chronic cough.
- Treatment of chronic purulent infections, with good attention to underlying destructive lung lesions that can precipitate recurrent chest infections, will prevent this source of a chronic cough from recurring.
Therapeutic failures in chronic cough usually relate to one of the following:
- More than one cause. This can occur in up to 30% of chronic coughers. Not only have the different contributing factors to be evaluated, but they also have to be rated in order of frequency so as to emphasise the specific forms of treatment.
- The duration of the prophylactic treatment for a chronic cough may be too short – a mistake commonly made by patients and practitioners when symptomatic relief starts setting in. In some cases, such as chronic allergic rhinitis and asthma or COPD, prophylactic treatment may have to be continued indefinitely to prevent a chronic cough from recurring. Subsequent inadequate doses or inappropriate therapy may also contribute to ongoing symptoms regardless of whether a correct selection has been made for treatment.
Recurrence of acute exacerbations, particularly in COPD and asthma patients, has to be considered when symptoms recur in individuals who have been asymptomatic for periods of time.
The problem that patients are faced with is that of identifying a general practitioner or a pulmonologist (lung specialist) who is prepared to pursue a cause and administer specific treatment. The success of this approach will be a source of mutual satisfaction for practitioner and patient.
Damage caused by a chronic cough
Complications of chronic cough may include the following:
- Severe bouts of coughing, specifically in the elderly, can cause rib fractures, which should always be suspected if patients experience severe chest pain of sudden onset without a history of trauma (injury).
- Stress incontinence is an embarrassing complication of chronic cough, which occurs commonly in elderly females.
- An infrequent but important complication is that of loss of consciousness due to impaired circulation to the brain during bouts of acute coughing.
- Surgeons and anaesthetists will usually postpone planned surgery in patients who have a chronic cough. The stress placed on fresh wounds by coughing will not only be painful, but may also lead to lack of adhesion or, in the case of eye surgery, to severe post-operative complications which may leave patients worse off than before.
Written by Prof J.R. Joubert, MSc, MBChB (Stell), FCP (SA), MMed (Int. Med), MD (Stell).