In the bi-monthly CME & Quiz series, Dr. YK Amdekar writes about cough, why drugs often fail to relieve it, and what to do if symptomatic therapy fails.

To begin, take the quiz below to test your knowledge.

"Cold and cough” – terms that are often misinterpreted

These terms are used by laypersons with a different meaning as per what they perceive. The physician must confirm whether “cold” refers to nasal discharge or blocked nose and whether “a cough” is a sound produced by an attempt at forceful expulsion during expiration. It is not surprising to find there may not be either cold or a cough, even when complained.

Another loose term used by lay people is congestion in the chest. This term may simply refer to noisy breathing that may or may not be accompanied by cold or a cough. Similarly, congestion in the throat is another misrepresented term. After all, congestion is a physical sign and not a symptom and so, the patient will not know congestion. He simply presumes it.

Hence it is necessary for the physician to confirm the intended meaning the patient wishes to convey. Without such clarification, a physician may be misled in the interpretation of such symptoms. What parents or patients say may not mean what they actually want to convey and this is so true about terms like “cold and cough”. Hence doctors must make sure about these terms, lest they get misled.

---

Do “cold and cough” go together?

Often they are together because the most common cause of this combination symptoms is either a viral infection or allergy. Viral infection presents with fever followed by cold and cough while allergy starts with cold and cough and often without fever.

In fact sequence of events guides the doctor to diagnose the disease. Bacterial infection is mostly localised and does not present with cold and cough. However, allergy leading to cold and cough may be secondarily infected with bacteria and so may present as dual disease.

It is clear that physicians must make sure about the presence or absence of cold and/or a cough and also the sequence of appearance of symptoms, by detailed enquiry. It helps in the proper evaluation.

---

What is cough?

Cough is a sound produced by sudden, forceful and often repetitive attempt at expulsion during expiration. It is a protective reflex that is expected to expel any irritant, secretions or foreign particles from larger airways – upper airways (larynx and trachea) and lower airways (proximal or larger bronchi).

Affection of other areas in the respiratory tract such as nose, pharynx, bronchioles, lung parenchyma, pleura and interstitium present with minimal or no cough. Cough as a major symptom suggests larger airway disease, anywhere from the larynx to bronchi. Affection of bronchioles, lung parenchyma, pleura and interstitium may result in a mild cough or no cough. Thus, the severity of a cough helps to localise the lesion.

---

How is cough generated?

Cough starts with initial deep inspiration that is followed by brief powerful expiratory effort with closed glottis resulting in the generation of pressure in the airways and then a sudden opening of glottis with closure of nasopharynx and vigorous expiration through the mouth. It produces sound and it all happens reflexly.

It is clear that a cough due to the affection of bronchioles, lung parenchyma or interstitium cannot be effective because low luminal airflow and velocity at these sites fail to generate enough pressure and hence do not present with a significant cough or often without a cough.

Even if a child cannot initiate deep inspiration or powerful expiration as may happen in case of severely obstructed airways or respiratory muscle paralysis, he/she may not be able to cough even in presence of irritant or secretions in larger airways. Neonates and younger infants are not able to cough effectively and it is obvious that child on mechanical ventilation would not be able to cough.

Besides the presence of irritant in the airways, the ability to produce a cough depends on the site of irritation in airways and extent of muscle power necessary to generate adequate pressure to expel the irritant. Lower the site of affection in airways and younger the child, the cough may not be effective. Thus, the absence of a cough does not rule out airway disease.

---

How is cough mediated?

Cough is mediated exclusively via the vagus nerve. It is worth noting that pharynx is not supplied by vagus and hence isolate affection of pharynx does not cause a cough unless irritant or secretions trickle down to the larynx. This has clinical relevance in that isolated pharyngeal disease as in bacterial pharyngitis would not present with a cough and so cough in pharyngeal disease suggests an extension of disease beyond pharynx as happens in viral infection.

Airways extending from the larynx to larger or proximal bronchi contain rapidly adapting pulmonary stretch receptors that quickly adapt to persistent stimulus thereby interrupting bout of cough transiently, facilitating normal breathing in between bouts of cough.

This is important as cough must be interrupted at least for a few seconds to facilitate the act of breathing. If a single bout of cough continues for more than few seconds, it may lead to cessation of breathing – apnoea and may also be fatal as occasionally happens in an infant suffering from pertussis – whooping cough.

Thus, pertussis is a serious disease in an infant. ever repeated bouts of cough cannot be prevented as nerve endings in mucosal epithelium sense the irritant or inflammatory secretions and produce a cough. Finally, vagal fibres enter brainstem from where cough reflex is generated via second order neurons. It also has clinical relevance as an unconscious patient due to brainstem affection is not able to cough.

Nature has provided cough reflex but also has taken care that bout of a cough is transiently interrupted to facilitate breathing. Functioning neurological circuit from vagus nerve to brain stem is vital to produce a cough.

---

Factors in airways leading to cough

There are several factors in airways that can stimulate cough receptors to produce cough. These receptors are sensitive to mechanical factors such as inhaled foreign body or compression pressure on airways as in case of a mediastinal tumour and these receptors also are sensitive to acid or isomolar solutions such as water. This explains cough due to gastro-oesophagal reflux (GERD) or aspiration of water or food particles into the airways.

Similarly, other events such as smooth muscle contraction as in asthma, vasodilatation and oedema as in cardiac conditions, mucous secretions as in bronchitis and reduced lung compliance as in pneumonia are responsible to produce cough. Inflammation commonly due to infection or non-infective causes is responsible for a cough in routine practice.

Drugs such as angiotensin-converting enzyme inhibitors (ACE inhibitors) and non-steroidal anti-inflammatory drugs (ibuprofen) are known to trigger a cough, though the exact mechanism is not known. Similarly, psychogenic factors, especially in children, present with a cough referred to as “a habit cough”.

It is clear that a cough may result from primary affection of organs other than the respiratory system as happens in cardiac, upper GI disorders and neurological diseases besides also caused by psychogenic factors and few drugs. Different mechanisms operate to irritate airways to produce a cough.

---

Basic concepts of why drugs often fail to relieve cough

There are several components such as neurogenic and mechanical factors that are involved in producing cough. Besides, most of the irritants or inflammatory products in airways are not easy to get rid of as consistency of mucus and efficiency of ciliary function determine ease of expulsion and so cough continues in spite of trial with different drugs.

Even mere symptomatic relief is also difficult to achieve as there is no drug known to science that can control all these variable factors. That is why most cough remedies are cocktails of sedative, expectorant, mucolytics and antihistamines with the hope that one of the constituents may work. But it does not do so.

Temporary relief may be possible with inhaled bronchodilator in case of bronchospasm. Transient relief may occur with the hydration of airways as is done with steam inhalation or sips of warm water or chewable item in mouth secreting more saliva (dryness of airways increases a cough).

Reclining with head high position offers a bit of relief and so also adequate ventilation and comfortable room temperature. Of course, removal of an inhaled foreign body can “cure” a cough.

Cough can be very distressing to a patient but the management of a cough poses a great challenge to every doctor. Unlike fever which results from same mechanism irrespective of the cause of fever and hence transient control is possible with standard drugs, cough is a result of different mechanisms and hence there is no standard symptomatic therapy to suppress a cough. Hence no cough syrups work!

---

If symptomatic therapy does not work, what next?

A significant cough though distressing to a patient has a purpose of expelling the irritant. So, unless irritant is expelled, the cough would never stop. Hence attempt should be made to offer comfort to a child rather than suppressing cough.

During day time, most children remain reasonably comfortable in spite of a cough, simply because they are too busy with play and other activities that they ignore a cough. However, cough disturbs them during sleeping time and hence attempt must be made to ensure proper sleep within limits. One can use with discretion an antihistamine containing cough suppressant in a single dose at night.

A cough suggests bronchospasm, inhaled bronchodilator would offer relief better than oral medicine. There is no reason to use symptomatic drugs through the day and should only be administered when discomfort is intolerable. None of these modalities “cure” a cough and this is the reason why treating a cough is a big challenge to a physician.

The provisional diagnosis of the disease presenting with a cough is vital, without which relief of a cough may not be possible. Though luckily some of the causes of a cough may be self-limiting unfortunately they are often recurrent. The ability of physician to counsel a patient or parent is most important in case of severe airway disease. It is a skill that physicians must develop, especially in conditions where there is no quick fix. It is irrational to go on trying different cough remedies until nature works.

The next article deals with the clinical application of basic facts highlighted in this article with a discussion on live case scenarios, representing day-to-day problems faced by practitioners.

Disclaimer- The views and opinions expressed in this article are those of the author's and do not necessarily reflect the official policy or position of M3 India.