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Asthma - extent of asthma in the UK
Written by Dr George Russell, reader in child health, University of Aberdeen
What is epidemiology?
Epidemiology is the study of disease in populations.
Its purpose is to examine variations in disease in relation to differences in (for instance):
Epidemiological studies serve a variety of purposes.
medical facilities match likely demand
provide clues as to the cause
to identify targets for intervention
Epidemiological studies provide valuable clues to the cause of disease. Classical examples include:
It was possible to apply epidemiological techniques to these studies, because:
If everyone drank polluted water, or if everyone had typhoid fever, Salmonella typhi from polluted water would still be the cause of typhoid fever, but the link would not be apparent. Similarly, if everyone smoked, smoking would still be the major cause of lung cancer, but epidemiological methods would fail to demonstrate the association.
It was also possible to study these conditions because the diseases themselves were well defined. You either have typhoid or you don't; carriers may cause some confusion, but during an epidemic (such as we had in Aberdeen in the '60s) there is seldom any doubt. Similarly, the diagnosis of lung cancer is usually unequivocal.
In contrast, the epidemiological study of asthma is difficult because:
Before the extent of asthma can be discussed sensibly, two issues need to be addressed:
Prevalence may be measured:
Clearly, depending on the period studied, the question asked, or the test performed, entirely different answers will be obtained. Thus, the prevalence of wheeze will differ widely depending on whether the period studied is a single day, a week, a year, or the patient's lifetime, and the prevalence of wheeze will not be same as the prevalence of asthma.
Incidence
Thus, because asthma is a chronic disorder, the incidence of new cases during a period of, say, a year, will be vastly less than the prevalence of asthma in the same population.
Incidence is also used to describe the frequency of events such as wheezing attacks and hospital admissions.
At first sight these precise definitions may appear to be pedantic, but they are important. For instance, the great majority of modern asthma surveys (including the International Study of Asthma and Allergy in Childhood - ISAAC) include a question on wheezing during the past year. Some older studies, such as those from Aberdeen with which I have been involved, enquire about wheeze in the past three years, giving a higher prevalence of wheeze than studies conducted using the ISAAC protocol.
Precision is therefore all-important. Before comparing the results of different studies, it is essential to examine them in detail, to ensure that like is being compared with like.
Definitions of asthma
Asthma may not even be a single disease, but the culmination in a common group of symptoms of several distinct pathological processes. There are however certain features that are widely used in defining asthma, each of which will identify a different population, highlighting the impossibility of discussing the extent of asthma with any precision.
Recurrent wheeze is often interpreted as asthma, but asthma is by no means the only cause of wheeze. Other causes include:
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.'
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
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:
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 demands that the patient should be wheezy when seen; otherwise there will be nothing to reverse.
Bronchial hyper-reactivity
Such tests are:
Disadvantages of tests of bronchial reactivity include:
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
Their symptoms may be triggered by exposure to allergens such as:
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:
A fuller discussion on the relationship between asthma and allergy appears in a recent review in the British Medical Journal [6].
The incidence of a condition describes 'The number of new cases arising in a defined population during a specified period'.
Older readers will remember the Brains Trust that ran for many years on the BBC Home Service. One of the Brains, Professor Joad, used to play for time by prefacing every answer with the phrase, 'It all depends what you mean by …' This approach is particularly relevant to asthma, as there is no easily applied or widely accepted definition, and the term means different things to different people.
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.
This definition of asthma depends on the demonstration of a positive response to a bronchodilator drug.
This implies the development of bronchospasm in response to a defined stimulus, such as the inhalation of:
Many asthmatics show signs of allergy such as:
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