Asthma brought on by exercise

Written by Dr Nicola Wilson, honorary consultant paediatrician



Asthma brought on by exercise (asthma on exercise) happens to almost all people with asthma, or at least it would if their treatment was reduced or stopped.

If you have never experienced asthma when exercising it may be wise to reconsider the diagnosis. In some people it is the only symptom of their asthma.

Since we all exercise, it is a potential problem for all those with asthma. However, in the vast majority it can be readily controlled with medication and other measures.

What are the symptoms of asthma on exercise?

The symptoms of asthma on exercise, either in combination or alone, include:

cough, but asthma is not the only cause.

chest tightness and difficulty in breathing (wheezing).

shortness of breath (this is different from being unfit because it gets worse, not better, after exercise has finished).

symptoms usually begin after exercise has stopped, and are at their worst about five minutes afterwards.

more severe attacks may start during exercise and as a result the activity has to stop.

wheezing or chest tightness gradually lessens over the next 10-30 minutes if the asthma attack is untreated. Recovery can take longer if the attack is very severe.

What causes asthma on exercise?

Nobody really knows the exact reason why asthma is brought on by exercise. The mechanism may be quite complicated and involve sensory nerves and the redistribution of blood flow throughout the lung. The process is thought to include the events outlined below.

When we exercise, we breathe faster and more heavily and this leads to the following situation:

the lining of the airways dries out, concentrating the contents of the cells in that area. This is called increased osmotic load.

the airways cool rapidly because of evaporative heat loss.

In untreated, or under-treated, asthma the lining of the airway is inflamed. The drying and perhaps the cooling of the airways act as physical triggers to cause cells in the inflamed airways to release 'constrictor' chemicals, which in turn cause muscles in the airways to contract and narrow, and give rise to the typical symptoms of asthma.

Evidence for the mechanism of asthma on exercise

Asthma on exercise can be mimicked by voluntarily increasing the breathing rate.

Breathing warm, humid air reduces or prevents asthma on exercise. The reason seems to be that that there is less dryness and cooling of the airway lining. Therefore, there is less release of constrictor chemical and less narrowing of the airways, which leads to reduced symptoms of asthma on exercise.

Breathing cold, dry air increases asthma on exercise. The dryness and increased cooling factors lead to greater release of constrictor chemical and more narrowing of the airways resulting in the typical symptoms of asthma.

Who gets asthma on exercise?

Nearly everyone with under-treated asthma who exercises.

A few people with severe asthma, despite receiving the best treatment.

Children are particularly affected as they tend to run around more than adults.

When tested, 2.5-30 per cent of school children have evidence of asthma on exercise 1,2. The actual number depends on the geographical locality.

When tested 12-15 per cent of unselected athletes reported asthma on exercise3.

At the 1996 Olympic games 20 per cent of athletes reported asthma on exercise4.

Reduced lung function may occur after exercise in some people with hay fever (without actual diagnosed asthma). This can represent unrecognised asthma on exercise. It occurs because the airways of people with hay fever are often inflamed and so will respond to the effects of airway drying5.

Severity of symptoms

The severity of the symptoms depends on:

an individual's response (some people are more sensitive than others). This may vary according to the situation (eg worse during colds or when suffering from hay fever).

how cold and dry the air is. The colder and dryer the air, the greater the changes in the lining of the airways and the larger the stimulus for release of constrictor chemical. This results in more severe symptoms.

if the exercise is continuous (eg cross-country running), in which case the symptoms may be more severe.

if the exercise is intermittent (eg tennis), when the problem may be less troublesome and symptoms less severe.

how strenuous the exercise is. The more energetic the exercise, the greater the need to breathe heavily and the greater the degree of airway drying and cooling, which can result in more severe symptoms.

So asthma on exercise can occur in:

people who are very sensitive to airway drying, even after minimal exercise.

those who are not very sensitive to airway drying, but who exercise very vigorously, such as amateur or professional athletes.

people somewhere in between the two.

Can I exercise if I get asthma on exercise?

Yes! There are many athletes who have asthma on exercise and there are many steps you can take to reduce or avoid it.

There are effective medications such as your reliever inhaler, eg salbutamol, terbutaline, or your sodium cromoglicate inhaler, which can be used immediately before exercise.

Recommended medications, such as salbutamol, terbutaline and sodium cromoglicate, are allowed by sports councils (check with your doctor if you take an unusual preparation, as some contain substances, such as ephedrine, that are not allowed).

Those with troublesome asthma may have to choose a type of exercise that suits them, such as swimming or bowls, or avoid too much exercise at the times of the year when their asthma is a particular problem.

It is important to remember that in some people under-treated asthma on exercise can lead to a dangerous attack in certain situations, such as when there are high levels of pollution in the atmosphere or very cold days.

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