The increasing prevalence of asthma has made it one of the
major noncommunicable diseases worldwide. It is most common
in children.1 Asthma is a chronic inflammatory condition of the
airways which is usually allergic in origin. It is characterised by
hyperresponsive airways that constrict easily in response to a
wide range of stimuli.2 Although the pathophysiology of asthma is
complex, it is understood that airway inflammation, intermittent
airflow obstruction and bronchial hyperresponsiveness are
involved. The mechanism of inflammation in asthma may be
acute, subacute or chronic. The presence of airway oedema and
mucous secretion also contributes to airflow obstruction and
bronchial reactivity.3
Airway hyperresponsiveness in asthma is an exaggerated
response to numerous exogenous and endogenous stimuli. The
mechanisms involved include direct stimulation of airway smooth
muscle and indirect stimulation by pharmacologically active
substances. The degree of airway hyperresponsiveness generally
correlates with the clinical severity of asthma.3
Although the fundamental causes of asthma are not completely
understood, genetic predisposition combined with environmental
exposure to allergens have been identified as the strongest risk
factors.