Cough is the commonest respiratory symptom, indeed it is probably the
commonest of all symptoms which results in a consultation. The ten yearly
morbidity Statistics in General Practice Survey reveal that consultation for
cough and upper respiratory tract infection outweighs any other presenting
condition by an order of magnitude.
In the UK
the market for cough remedies, most of which are at best poorly effective, is
ten million pounds, whereas in the USA the cough/cold market is a
staggering thousand million dollars. This massive health care burden is
poorly understood in terms of etiological mechanisms and is very poorly
treated, even with those drugs currently available. Because of a lack of
knowledge of the pharmacology of these agents many patients are under
treated.
Acute cough
The vast majority of acute cough is caused by upper respiratory tract
viral infection. It makes a perfect sense for respiratory viruses and
bacteria to have evolved the ability to cause cough. Once an organism has
successfully invaded the respiratory tract its main problem for continued
survival is how to spread to the next host. Some viruses do this by
producing intense coryza and transmitting themselves from person to person by
manual transmission of infected secretion.
However, the majority are
required to transmit themselves in aerosolised droplets. The ability to
produce a cough is therefore a vital part of the pathogenic armamentarium of
the respiratory tract virus. From the host’s point of view there is
little reason to suggest that the cough produced in an upper respiratory tract
infection is useful. Whilst small amount of secretions may be
expectorated they in no way contribute to the clearing of the airways in normal
subjects.
If one looks at a more extreme example of infective airway
inflammation such as community acquired pneumonia, despite sometimes extensive
consolidation only a few mls of sputum is produced, the vast majority of the
purulent exudate being removed by phagocytosis via the blood stream.
Thus, in normal subjects infective cough is likely to be detrimental and there
is no evidence that effective cough suppression in respiratory tract infection
is harmful, indeed, it may even reduce viral transmission.
The
treatment of acute cough
By definition acute cough is benign and
self-limiting. A case can be made
therefore for withholding treatment and indeed this is what the majority of the
population does. However, some
infections produce cough of such distressing intensity that treatment may be
requested or even demanded. Such
treatment was recently reviewed by the Drug & Therapeutics Bulletin and it
was concluded that there was no effective treatment. This is because there is a
marked lack of clinical trial evidence in this area.
Whilst this observation is
correct I believe the conclusion to be wrong. The problem with determining what
is efficacious in cough arises from the extreme difficulty of performing proper
randomised, double blind studies in a variable symptom such as acute cough. Cough itself is a very variable and episodic
phenomenon and within a given population there is also a marked intersubject
variation in cough intensity and duration.
Finally, in a symptom caused by a wide variety of different organisms of
varying natural history, providing adequate power to compare treatment with the
control group requires vast numbers of patients. Thus, there is no gold standard clinical
study in the literature on which to base treatment strategies. I believe however, absence of evidence is not
evidence of absence (of therapeutic effect).
If one takes second line evidence, such as
those obtained from cough challenge studies and there is a wealth of evidence
suggesting that many treatments available do have a significant effect on the
cough reflex.
Adapted from: http://www.gpplus.com
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