Asthma: what causes it?

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Both parents and children need to have a good understanding of the causes and management of asthma. With appropriate treatment, the great majority of children with asthma can lead perfectly normal lives. Indeed, the goal of treatment is to allow the child to lead a normal, uninterrupted lifestyle.

It is probable that a predisposition to asthma is genetically inherited, and that early in a child’s life something (an ‘inducer’) switches on the disease. In children with such a predisposition, asthma can be brought on by infections, allergens or irritants. It is likely that passive smoking may be one of the irritants that leads to asthma in vulnerable children, as may exposure to certain allergens early in life. Current research is trying to identify some of these factors, but our knowledge is not yet at the stage where we can prevent asthma from developing.

The tendency to develop asthma has a strong genetic basis. If one or both parents or other family members have asthma, hay fever or allergies, or eczema, then there is an increased chance that the child will also have asthma.
The wheeze and cough seen in asthma are due to narrowing of the small air passages (bronchial tubes) in the lungs. The narrowing is caused by a tightening of the muscle in the wall of the bronchial tubes. The underlying inflammation causes swelling in the lining of the bronchial tubes and this, together with an increased production of mucus, results in further narrowing of the air passages. This narrowing reduces the flow of air in and out of the lungs, and results in wheezing, coughing and difficulty in breathing.

Once a child has asthma, acute attacks can be brought on by a variety of triggers. Children with asthma have airways that are ‘hyper-reactive’, and narrow down in response to triggers which do not affect other children. These trigger factors vary from child to child. They include viral infections, pollens, exercise, changes in the weather, cold air and exposure to cigarette smoke, dust or pets. By far the most common trigger is a viral infection, usually the common cold. While exposure to these factors may have little effect on most children, those with asthma will react by wheezing, coughing and becoming short of breath.

Is there a test?

Most children will not need any special investigation. The diagnosis of asthma is usually made on the basis of history and physical examination. Children who have more severe asthma, or who have frequent attacks, may require a chest X-ray. Some children may have special breathing tests often arranged by a paediatrician or respiratory specialist. This usually applies to those who have ongoing, persistently severe asthma.

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