07-05-2014, 03:54 PM
Chronic cough in children
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ABSTRACT
Chronic cough, defined as coughing for more than
four weeks, is a common childhood complaint.
With careful history-taking and appropriate
investigations, a single cause can be found in most
cases- these can be successfully treated. Although
we have some understanding about the cough
reflex through animal studies, the full mechanism
and exact location of the responsible neurons in the
human brain have not been completely elucidated.
There are many causes for it but asthma, chronic
rhinosinusitis and gastroesophageal reflux disease
account for most of the cases. Other causes, such
as dysfunctional swallowing, congenital anomalies
and cigarette smoking, are also important. All
children with chronic cough deserve a thorough
and proper evaluation. The current review provides
essential information for medical practitioners to
approach this problem. An algorithm is provided
to aid the diagnostic process.
INTRODUCTION
Cough is a common symptom that brings a child to
medical attention. The prevalence of chronic cough
in Chinese children is about 6.4%(1). In the United
States, it accounts for 3% of medical consultations(2).
Girls seem to have a lower cough threshold, but the
reason for this gender difference is unclear(3). While
much of acute cough is due to viral infection of the
upper respiratory tract, chronic cough, defined as
coughing for more than four weeks duration(4), can be
caused by various conditions other than infection,
some of which may be serious. A single cause can
be found in up to 82% of all cases of chronic cough(4).
The majority of these can be successfully treated.
Thus, it is imperative that one looks for the underlying
cause and treat appropriately rather than use cough
suppressants indiscriminately.
NEURAL PATHWAY OF COUGH REFLEX
Cough is an important reflex defense mechanism
for clearing the airways of inhaled noxious stimuli.
The reflex begins with stimulation of the cough
receptors by viruses, aeroallergens and chemicals.
There are two distinct types of afferent fibres,
namely: the rapidly adapting irritant receptors
(RARs) and the capsaicin-sensitive unmyelinated
bronchial C-fibres(5) . The former type of fibres is
sensitive to mechanical and/or chemical stimuli (5).
They are present throughout the respiratory tract
down to the respiratory bronchioles, especially the
posterior wall of the trachea, carina and branching
points of large airways. They are also present in the
external auditory canals and tympanic membrane,
pleura and pericardium, where they tend to respond
to only mechanical stimuli(6). The bronchial C-fibres
are neuropeptide-containing nerve fibres that are
sensitive to chemical stimulation rather than
mechanical stimulation (7). Stimulation of these
fibres causes bronchial smooth muscle contraction,
vasodilatation, plasma extravasation and mucus
secretion(8). However, the exact role of C-fibres in the
human cough reflex is still unclear
PHYSIOLOGY OF COUGH
Cough is a rapid and ballistic movement. A typical
cough starts with a deep inspiration to at least
50% of the vital capacity. This is followed by the
compression phase, where the glottis closes for
about 0.2 second and there is contraction of the
respiratory musculature. The glottis then opens
suddenly. This unleashes the high intrapleural
pressure that develops during the glottic closure,
creating a high expiratory flow rate and narrowing
the central airways, which can be as high as
12 L/sec and reaches a peak flow of 30 to 50 msec
following the start of the expiratory phase. Finally,
the relaxation phase completes the act when the
respiratory musculature relaxes with a reversal of the
intrathoracic pressure (6,10). Normal function of the
mucociliary apparatus is critical in maintaining an
effective cough, as it brings secretions from the
periphery to the proximal airways where they can
be cleared by coughing.
ASTHMA
The prevalence of childhood asthma is increasing
globally. In Hong Kong, the prevalence of asthma
in children aged between 3 and 10 years is 6%(25);
rising to 11% in the 13- to 14-year-olds(20) . Up to
57% of asthmatics may present with only cough(26).
This is sometimes referred to as cough-variant
asthma or cough-predominant asthma, which is
the preferred term(27). Asthma is a chronic airway
inflammatory disease with bronchial hyper-
responsiveness and reversible airway obstruction.
A typical history of cough that is triggered off by
viral respiratory infection or allergen exposure is
worse at night, and is exacerbated by exercise,
cold air or smoke. Favourable response to anti-
inflammatory therapy may help one to arrive at the
diagnosis(28). Presence of other atopic features, such
as allergic rhinitis, eczema, allergic conjunctivitis
and urticaria, is helpful to confirm atopy and hence,
supports the diagnosis of asthma. Spirometry is
helpful in demonstrating airway hyper-responsiveness
in children over six years of age. Other helpful
investigations include skin prick test and measuring
serum immunoglobulin E level.
POST
-INFECTIOUS/VIRAL COUGH
This is a clinical diagnosis by exclusion. However,
it is frequently misdiagnosed and treated as asthma(58).
It is a recognised entity in most European studies on
chronic cough(59). The specific infection causing the
chronic cough is unknown in most cases but respiratory
viruses, especially respiratory syncytial virus and
parainfluenza virus, Mycoplasma pneumoniae,
Chlamydia pneumoniae and Bordetella pertussis,
have all been implicated. The exact mechanism is
probably due to persistent airway inflammation
with transient airway hyper-responsiveness after
infection(60). The induced sputum shows a lack of
eosinophils, which is not typical of untreated asthma(61).
The disease is self-limited and will resolve without
any treatment. Inhaled corticosteroids may be tried,
but no randomised study has been performed
to date. Ipratropium can also be useful(62) . A brief
course of oral steroid may be necessary in some
protracted cases
CONCLUSION
Careful history-taking and physical examination,
together with appropriate investigations, enable
the correct diagnoses to be made for most cases of
chronic cough within a reasonable time frame.
The diagnostic approach is summarised in Fig. 1.
Most cases can be dealt with using this algorithm.
For the rare cases that elude diagnosis, referral to a
specialist in paediatric respiratory medicine is warranted.