Etiologi

All chronic cough begins as sub-acute, and differential diagnosis includes all causes of sub-acute cough. Post-infectious cough is the most common aetiology of sub-acute cough. [2] Most cases will be self-limiting. Once cough duration has exceeded 8 weeks, a systematic approach to elucidating cause and best treatment is needed. For non-smoking adults with a normal CXR who do not take ACE inhibitors, chronic cough is usually caused by upper airway cough syndrome (UACS, formerly postnasal drip syndrome), asthma, GORD, or non-asthmatic eosinophilic bronchitis (NAEB).

Common aetiologies

In most non-smoking adults with a normal CXR who do not take ACE inhibitors, chronic cough is caused by one or more of 4 conditions: [3] [4] [5]

• Upper airway cough syndrome (UACS, formerly postnasal drip syndrome): 34%

• Asthma: 25%

• Gastro-oesophageal reflux disease (GORD): 20%

• Non-asthmatic eosinophilic bronchitis (NAEB): 13%.

More than one cause of chronic cough is often present. Truly idiopathic cough is rare and is a diagnosis of exclusion. [4] [6] [7]

Cough as a sole symptom of asthma, known as cough-variant asthma, is present in a sub-group of patients. [8]

These commonest causes account for most patients presenting to specialty clinics with chronic cough and should generally be considered first if there are no signs or symptoms pointing to alternative diagnoses.

Other common causes include:

• ACE inhibitors: dry cough, typically associated with a tickling or scratching sensation in the throat, appears in 5% to 35% of ACE inhibitor users. ACE inhibitor-induced cough is more frequent in women, non-smokers, and people of Chinese origin. [9] [10]

• Post-infectious cough: the most common aetiology of sub-acute cough. [2] A history typical for post-infectious cough should prompt watchful waiting and symptomatic therapy as necessary.

• Bronchitis: chronic bronchitis may be considered when an adult has history of chronic cough lasting for more than 3 months and for at least 2 consecutive years when other diagnoses have been ruled out. Predisposing factors may include smoking and environmental exposure to toxins. [3]

• Bordetella pertussis: when local epidemiology indicates a high rate of pertussis infection, testing for Bordetella pertussis is recommended. If tests are supportive of pertussis, specific antimicrobial therapy is indicated.

Less common aetiologies

Diagnoses to consider are those that impart cough through stimulation of airway mechanical and chemical receptors that feed into the vagus nerve, including afferent nerves located in the chest wall, diaphragm, oesophagus, abdominal wall, and external auditory meatus. [11] Other potential causes therefore are:

• Disorders that distort or irritate the airway (e.g., bronchiectasis, chronic suppurative lung disease, endobronchial tumours, granulomatous disease, foreign bodies)

• Disorders of lung parenchyma (e.g., interstitial lung disease resulting from hypersensitivity pneumonitis, occupational/environmental exposure)

• Other diseases that involve systemic processes (rheumatoid arthritis, sarcoidosis), autoimmune diseases such as SLE, or diseases that stimulate afferent nerves mentioned above

• Irritation of the external ear canal by an infection, wax, or hearing aids may produce cough, through a reflex mediated by Arnold's nerve.

Oral-pharyngeal dysphagia that results in recurrent aspiration of foods and liquids may also cause cough. Patients with cough who report difficulty swallowing should be further evaluated for such aetiology. [3]

Bronchiolitis should also be considered, and may result from infection or may be drug/toxin related. Diffuse panbronchiolitis should be considered in patients who have recently lived in Japan, Korea, or China. [3]

In areas of endemic infection with fungi or parasites, diagnostic evaluation for these should be undertaken when more common causes of cough have been ruled out. [3]

Psychogenic cough may be diagnosed after thorough evaluation has ruled out all other causes. [3]