Asthma - extent of asthma in the UK

- chronic bronchitis
- emphysema
- cystic fibrosis
- virus-associated wheeze in pre-school children.

Some patients may be difficult to classify, and doctors may resort to such non-diagnoses as 'asthmatic bronchitis'.

Professor Joad would certainly have said, 'It all depends what you mean by wheeze.'

- Wheeze is a whistling noise.
- It comes from the chest.
- It is most marked on expiration.

It has been shown by workers in both London [1] and Sheffield that the general public has great difficulty in distinguishing between these noises and wheeze, with the result that the prevalence of wheeze tends to be overestimated.

Vibratory, rattling or purring noises arising in the larynx or larger airways are not wheeze, but are widely but wrongly interpreted as wheeze.

Stridor, though whistling, is a mainly inspiratory noise resulting from obstruction to the flow of air through the larynx.

Doctor-diagnosed asthma

. Many epidemiological surveys include a question on whether or not the subject has ever been diagnosed by a doctor as suffering from asthma. On the face of it, this might seem to be a perfectly reasonable means of ascertaining the prevalence of a disease, but asthma is both underdiagnosed and overdiagonsed.

The response to this question is more likely to reflect the diagnostic fashions of the time than the true prevalence of asthma.

Thus, in Aberdeen, the prevalence of doctor-diagnosed asthma [2] has increased twice as fast as the prevalence of wheeze, reflecting the increasing tendency to offer a diagnosis of asthma to explain wheezing and coughing in childhood.

Asthma has now become such a popular diagnosis that one of the main tasks at hospital asthma clinics is to 'undiagnose' it. For instance, the diagnosis is often made in patients with:

- recurrent cough
- wheeze caused by other pathology
- shortness of breath due to cardiovascular unfitness, that is wrongly attributed to exercise-induced asthma.

Doctor-diagnosed asthma is therefore a heavily tarnished gold standard for the diagnosis of asthma. Nevertheless, individuals whose symptoms are severe enough for medical help to be sought are more likely to be suffering from asthma than those with milder wheeze.

Reversible airways obstruction

This definition of asthma depends on the demonstration of a positive response to a bronchodilator drug.

- The peak flow or other lung function test is measured before and after inhaling salbutamol.
- A convincing improvement indicates reversible airways obstruction.

This definition demands that the patient should be wheezy when seen; otherwise there will be nothing to reverse.

- It is useful in clinical practice, when the patient can be asked to report back next time he or she is wheezy.
- It is much less useful in epidemiological studies, which seldom have the resources for repeated examinations.

Bronchial hyper-reactivity

- histamine
- methacholine
- water mist
- cold air.

Such tests are:

- usually positive in more severe asthmatics, so they tend to identify individuals whose asthma is clinically important.
- sometimes positive in individuals who are apparently completely well.

Disadvantages of tests of bronchial reactivity include:

- the same individual may react differently on different occasions.
- the presence of an upper respiratory tract infection commonly increases bronchial hyper-reactivity.
- they are time-consuming (and therefore expensive), and are thus less suitable for application in large population surveys.

An exception is the 'free-running exercise test', which exposes the bronchi to cold air that has not been humidified by passage through the nose. Such exercise tests have been widely used to support the diagnosis of asthma in children, though they lack diagnostic precision [3].

In clinical practice, bronchial challenge tests have their advocates [4] and their critics [5].

Allergic airways disease

. Many asthmatics show signs of allergy such as:

- eczema
- hay fever.

Their symptoms may be triggered by exposure to allergens such as:

- house dust mite
- cat saliva
- pollens
- moulds
- feathers
- foods such as dairy products.

If evidence of allergy is insisted upon for a diagnosis of asthma, then this will exclude many non-allergic individuals who have exactly the same symptoms and signs, and who respond to anti-asthma therapy.

It is however perfectly reasonable to classify asthma as:

- non-allergic ('intrinsic')
- allergic ('extrinsic').

A fuller discussion on the relationship between asthma and allergy appears in a recent review in the British Medical Journal [6].