Family doctor

Background

Cough is a defensive, protective reflex. It may signal the presence of a problem in the respiratory system and may serve to eliminate the condition, which is stimulating the cough as occurs with an infection in the lower airways. However, when a cough persists and no longer serves this function it becomes a concern to both patient and physician.

The prevalence of chronic cough (persisting for more than 3 weeks) in non-smoking adults varies from between 2-23%, depending on the study performed, i.e. it is very common.

A comprehensive history, good examination and appropriate investigations will identify the cause in 95% of people with a chronic cough. It is very important to look for the underlying cause and to direct therapy to eliminate the aetiology rather than to use empirical non-specific cough medicine, which may simply lead to elimination of an important protective reflex and with resultant harmful effects.

Physiology

A cough results from stimulation of the cough reflex. Receptors which activate the cough reflex are situated mainly in the proximal breathing tubes (airways) within the soft tissues and smooth muscle. Cough receptors are also located in the vocal cords, pharynx (at the back of the throat), ear canals, tympanic membranes (ear drums) and diaphragm. A cough can therefore be suppressed in some circumstances by the application of a local anaesthetic to the site of stimulation of the cough receptors.

Animal studies suggest there is a cough centre. Interference with the cough centre is thought to be a mechanism by which narcotic medicines (e.g. morphine) or codeine, act as cough suppressants.

Stimulants of cough receptors can be mechanical (e.g. narrowing of the airways or foreign body, or cancer in the airways), chemical or osmotic. Stimulation of a locally occurring reflex between the ear and the lungs, and the oesophagus and the lungs may be the mechanism by which abnormalities in the ear drum and oesophagus (gullet) lead to cough.

A cough consists of a deep breath in, followed by closure of the vocal cords and rapid increase in pressure within the chest cavity by contracting the muscles between the ribs and the diaphragm muscles. Following opening of the vocal cords air is expelled at very high flow rates. The accompanying vibration helps dislodge mucus or foreign bodies from the airways.

Whilst cough is predominantly a protective mechanism, recurrent or persistent cough can be harmful. The pressures within the chest cavity generated during cough may be high enough to cause a number of problems (Table I)

Table I - Adverse Effects of Chronic Cough

Respiratory

• Wheeze in patients with asthma
• Trauma to upper breathing tubes and vocal cords causing swelling, redness and irritation
• Barotrauma - pneumothorax i.e. air escaping to outside of the lung

Brain

• Cough syncope (loss of consciousness due to concussion)

Chest wall

• Rib fractures, intercostal muscle strain or stress of costochondral junction (cartilage between ribs and sternum "breast bone") causing chest pain

Miscellaneous

• Urinary incontinence (stress),
• Hernias
• Back pain

A very carefully taken history contributes to an accurate diagnosis in about 80% of cases. The main causes are listed in Table II.

Table II

Pulmonary

Nonpulmonary

• Drugs - "ACE inhibitors" in the treatment of high blood pressure
• Gastro-oesophageal reflux (with or without aspiration) - symptomatic or silent
• Congestive heart failure
• Functional (psychogenic cough)

Some important points

Coughing up blood always requires further investigation. A normal chest x-ray does not exclude the presence of a cancer and further investigation with a CT scan of the chest or bronchoscopy (direct examination of the airways) is frequently required.

A history of choking, with possible aspiration at the onset of cough, is an important clue of possible aspiration of a foreign body and which requires urgent bronchoscopy. A normal chest x-ray does not exclude the presence of a foreign body.

Rhinosinusitis with post-nasal drip is the most common cause of a chronic cough. Although frequently obvious sometimes nasal symptoms can be relatively mild and it is only with a CT scan of the sinuses or endoscopic examination by an ear, nose and throat specialist that the diagnosis can be made.

Inflammation (redness and swelling) within the airways is the second most likely cause for cough. Asthma is the most likely diagnosis and cough may be the only symptom of asthma. The absence of wheeze does not exclude the diagnosis. People developing asthma in later life are at greater risk if there is a family history of asthma, or allergy (mother, father, brothers and sisters) or a personal history of allergy (eczema, hay fever). To make a diagnosis requires a careful history, examination, and either a trial of therapy or detailed breathing function tests (we can induce wheeze or bronchospasm by use of certain medications when administered through a nebuliser).

People who have bronchiectasis may have a productive cough (of discoloured mucus) which may only occur adjacent to viral infections. The condition is most likely to arise as a result of damage to the small airways (often in early childhood as a result of whooping cough or adenovirus infection) and the first symptoms may not appear for many years after the initial damage. Occasionally it can occur in adulthood after a nasty pneumonia or bronchitis.

Chronic bronchitis as a consequence of smoking is another common cause of a chronic cough usually associated with clear but thickened mucus.

The third most common cause of chronic cough is indigestion/heartburn (oesophageal reflux). Occasionally this can occur without any overt symptoms of reflux. The mechanism is due to either recurrent aspiration (acid refluxing up to the level of the back of the throat and into the airways) or as a consequence of a local nerve mediated reflex (much more common).

A history of an upper respiratory tract infection preceding the cough may be important. The cough can sometimes last for months before improving spontaneously. It may be as a result of increased sensitivity of cough receptors following infection and sometimes as a result of increased twitchiness within the airways (post infective bronchial hyperresponsiveness). Occasionally it is a consequence of sinusitis with postnasal drip or acute or chronic bronchitis, or infection in someone with bronchiectasis.

Certain anti-hypertensive medications (ACE inhibitors) can be incriminated as a cause for persistent cough, although the exact mechanism is not known.

Interstitial lung disorders (inflammation and swelling within the lung tissue itself) can cause a low-grade dry cough. Diagnosis is made by way of chest x-ray or CT scan.

Investigations

Anyone presenting with a cough for which there is no obvious cause should have standard investigations including: chest x-ray and spirometry (pre and post bronchodilator). The decision as to which investigations to subsequently perform will depend on the likely cause of cough. (See Table III).

Table III - Diagnostic Tests

Treatment of the underlying condition will, in most instances, lead to resolution of the cough.

Sometimes cough suppressants are required and cough syrups are usually of little help. The most commonly employed cough medicine is codeine phosphate, 15-30mg 3 x day. Side effects of therapy include drowsiness and constipation but may only be required for a few days to break the cycle.

Atrovent inhalers can sometimes be useful for coughs, particularly those associated with conditions which inflame the lower respiratory tract.

Excellent practical advice on cough suppression techniques can be acquired from speech therapists associated with Ear Nose and Throat Specialists/Departments.

Getting help:

See your doctor for an assessment if you have a cough that is persisting.