Cough is one of the most common symptoms to present to primary and secondary care physicians. Chronic cough can be a difficult condition to treat, and the cough can have a significant impact on a patient’s quality of life. They may also complain of other laryngeal symptoms including dysphonia. Often when these patients have been seen by ear, nose and throat (ENT) specialists in the past, the cough has been attributed to laryngopharyngeal reflux and post-nasal drip. However, more recently chronic cough has been thought to be caused by hypersensitivity of the afferent nerves of the airways. There are a number of potential causes and differentials that need to be investigated. It is now recommended that these patients are treated with a multi-disciplinary approach, with input from ENT, respiratory, speech and language therapists and physiotherapist.
16.1 Cough Reflex
Cough is a reflex action to clear the lower airways by an involuntary expiration against a closed glottis. This protective reflex has a complex pathway. It starts with stimulation of an afferent sensory limb with cough receptors lining the pharynx, larynx and airways. The afferent fibres then travel along the vagus nerve to the cough centre, which is located in the medulla of the brainstem. The reflex arc continues from the brain stem via efferent fibres that pass along the vagus, phrenic and spinal motor nerves to the muscles of the larynx, pharynx, diaphragm and abdominal wall.
There are three main afferent receptors in the airway:
1. Rapidly adapting receptors that respond to mechanical stimuli.
2. Slowly adapting receptors that respond to chemical stimuli.
3. C-fibres that respond to chemical stimuli.
The receptors in the larynx respond to both mechanical and chemical stimuli. In chronic cough, the cough reflex leads to neuroplasticity resulting in peripheral and central sensitisation. This results in an over-sensitive cough reflex, which maintains the chronic cough and known as chronic cough hypersensitivity syndrome and laryngeal hypersensitivity.
An acute cough is defined as being present for less than 3 weeks and most commonly occurs following an upper respiratory tract infection of viral origin. A chronic cough continues for more than 8 weeks and can be due to a number of different causes. The patients referred to ENT have normally already had some investigations in primary care and/or by the respiratory physicians. Patients with normal chest imaging, non-smokers and no underlying respiratory condition are often referred to ENT to exclude any other causes of cough.
16.2.1 Differential Diagnosis
The differential diagnosis of chronic cough caused by an ENT condition:
• Laryngopharyngeal reflux.
• Post-nasal drip.
• Chronic rhinosinusitis.
• Laryngeal hypersensitivity.
• Laryngeal dysfunction.
• Allergic response.
• Airway stenosis.
There are many other causes for cough not listed above such as compression from an enlarged thyroid mass, pharyngeal pouch, oesophageal dysmotility, oesophageal webs and strictures, neuro-laryngological disorders, vasculitidies and cranial nerve palsies—all of which may present to ENT. However, there will be other signs and symptoms attributed to them and they are unlikely to have a chronic cough on its own.
16.3 Clinical Assessment
Clinical assessment entails taking a focussed history, clinical examination of the neck, mouth, oropharynx and flexible nasopharyngolaryngoscopy.