Asthma prevalence in Great Britain. 93/6

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It is perhaps not widely appreciated that comprehensive information on the prevalence of asthma in the UK is not available. Reports of increases in asthma mortality or hospital admissions related to asthma often lead to speculation about possible increases in asthma prevalence or severity. However, indirect indicators such as these may reflect many other factors, including medical care, which are unrelated to prevalence. The only reliable information on prevalence comes from epidemiological surveys. Some of these -such as the national birth cohort studies- have had national coverage, but most have been local surveys of a specific age group, most commonly school-age children from a small geographical area.

Definitions

The prevalence of a disease is the proportion of the population who suffer from the condition at a given point in time. Measurement of the prevalence of asthma is not straightforward for a number of reasons (see footnote).

Figure 1: Summary of morbidity statistic relating to asthma and symptoms of wheeze in children

Most estimates of asthma prevalence are based on self-reported symptoms, most frequently the occurrence of wheezing and whistling in the chest over the last 12 months. Some surveys measure airflow variability or bronchial hyper-responsiveness.

Children

Estimates vary, but in 1993 between 12% and 15% of children suffered episodes of wheezing characteristic of asthma. These varied considerably in frequency and severity and probably less than 5% suffer persistent or repeated attacks of wheezing. Prevalence is higher in boys than in girls at least until adolescence.

Results from a series of separate epidemiological surveys carried out since the mid 1960s show that the prevalence of wheezing illness in children increased substantially by about half between the early 1970s and mid 1980s. However, methodological and other differences between the studies make results difficult to interpret. More recent trends are less clear cut. These studies also show that the proportion of children diagnosed as having asthma increased over the last decade. This recent increase in diagnosed asthma does not appear to be directly related to an increase in wheezing.

Figure 2: Trends in the prevalence of wheeze in the past 12 months and of a diagnosis of asthma in school age children

Results from the National Study of Health and Growth, in which 30,000 primary school children were studied between 1973 and 1986, similarly showed a small annual increase in the proportion of children with wheezing illness over that period. A 50% increase was found in the proportion of children with persistent wheeze ("wheeze on most days and nights") suggesting a possible increase in the prevalence of more severe forms of asthma.

The prevalence of asthma in children shows some geographical variation. In the 1970 Birth Cohort Study, 10% of 5-year olds were found to have suffered an attack of wheezing on the chest in the last 12 months, with prevalence varying from 6% in Scotland to 11% in Wales and 13% in the South West.

Figure 3: Prevalence of wheeze at age 5 by Regional Health Authority in 1975

Adults

Asthma has a good prognosis in childhood. In the 1958 Birth Cohort Study, 30% of 23 year-olds had been wheezy at some stage, but only 4% reported that they had suffered from asthma and/or wheezy bronchitis in the past year. At age 33, however, 18% of subjects reported wheezing or whistling in the chest during the past 12 months and 8% had used drugs prescribed for asthma during the last year.

Figure 4: Prevalence of asthma and wheeze in adults

Measurement of the prevalence of asthma in older adults in difficult because other conditions causing wheeze, such as chronic bronchitis, become increasingly common with age. These may either co-exist with asthma or produce similar symptoms. From early middle-age onwards, more reliable indicators of asthma are: night time breathlessness or, improvements in lung function following the use of bronchodilator drugs.

One study suggests that approximately 5% of older men and 2% of older women suffer from asthma. Because there have been so few comparable studies of asthma in adults, it is impossible to estimate time trends in this age group.

Summary

• Comprehensive information on asthma prevalence in the UK is not available.

• Asthma prevalence is usually estimated from survey data; in children and young adults, wheezing is most commonly used as an indicator of asthma although estimates based on wheezing will include trivial illness; estimates based on wheezing alone are unreliable in older adults.

• 12-15% of children suffer episodes of wheezing characteristic of asthma; less than 5% suffer persistent or repeated attacks.

• The prevalence of asthma in children increased by about half between the early 1970s and mid 1980s but recent trends are less clear cut; the proportion of children diagnosed as having asthma increased over the last decade.

• 15-20% of adults experience wheezing, but probably less than 5% suffer night time breathlessness or reversible air flow limitation characteristic of asthma.

Footnote

The prevalence of asthma reflects both the incidence of the disease (the rate at which new cases of asthma arise) and the duration of the condition. An increase in the prevalence of asthma indicates that either more people are developing asthma, or that the people are tending to suffer the condition for longer, or both. Increases in the reported prevalence of wheezing may also reflect changes in respondents' perception of symptoms perhaps influenced by increased public awareness of asthma.

Measurement of the prevalence of asthma is not straightforward:

• Variation in the way in which asthma presents, making development of a standardised case definition difficult.

• Lack of objective diagnostic test for the physiological or pathological characteristics of asthma which can be cheaply and easily administered in a large survey.

• Under-reporting of the condition by people with asthma (or their parents). They may be unaware that they have the condition or may attribute asthma symptoms to other conditions, such as respiratory infections and other wheezy conditions in young children, or chronic bronchitis in adults.

• Lack of reliable and valid indicators of severity.

• Difficulty in knowing whether someone has "grown out" of asthma.