Surveys variously report "doctor-diagnosed asthma", "self-reported asthma", "current/first episodes of wheeze" and, occasionally, lung function measurements and bronchial reactivity. "Doctor-diagnosed" asthma will vary because of diagnostic uncertainty, fashions over time, and overlap with other conditions (mainly chronic obstructive pulmonary disease (COPD) in adults); for example, social class I and II patients are more likely to be labelled as having asthma rather than chronic bronchitis (Littlejohns et al., 1989). Greater publicity will lead to more patients perceiving themselves as suffering from asthma and using symptoms of wheeze has particular problems in older adults where COPD is increasingly likely. At present it is not possible to determine "cut-off" points in peak flow variability or in bronchial reactivity in response to challenge tests which could be used as diagnostic criteria (Higgins et al., 1992).
Accepting all of these qualifications, the best estimate of asthma 12 month period prevalence in adults sufficiently severe to require regular medical intervention is approximately four percent (Anderson et al., 1994b). Approximately 10% of 15-64 year olds report "current wheezing", going up to 15-20% in the over 65s (Anderson et al., 1994b) (although there will obviously be overlap with COPD in the older age group). Data on incidence in adults is more sparse. In the National Child Development Study 1958 cohort, incidence levelled out at 0.7% per annum by the age of 23 (Anderson et al., 1986; Anderson et al., 1992); a major national study in the USA of over 14,000 subjects found an incidence of 0.2% per annum in people aged 25-74 (McWhorter et al., 1989). About two percent of adults consult their general practitioner annually with asthma, and 10-15% of these are referred for out-patient treatment (Anderson et al., 1994b). About 1% of all adult admissions and 1% of hospital bed-days are related to asthma.
There has been a particularly hot debate about trends in asthma prevalence over time. Relatively little work has been done in adults, but what evidence there is suggest that there is no increase in prevalence in the over 65s, there is conflicting evidence for the middle-aged, and a probable increase in "apparent" prevalence in young adults (Anderson et al., 1994b; Phelan, 1994). Most of the literature relates to children, where there has been a marked increase in "apparent" prevalence and where the debate centres on how much of this is artifactual. An international consensus is now emerging that this is, at least in part, a real increase (Phelan, 1994; Peat et al., 1994; Anderson et al., 1994b) and might therefore be expected to impact on adult prevalence in the future.
As for UK trends in hospitalisation, there has been a modest rise in admissions in young adults whilst there has been no change in older adults; and a marked increase in hospitalisation in children, mainly confined to the under 5s (Anderson et al., 1994a; Hyndman et al., 1994). Again, argument has raged over how much of this is due to changes in medical care rather than in prevalence or severity of asthma; recent studies from East Anglia and Croydon suggest that changes in medical care are the more likely explanation (Anderson et al., 1994a; Hyndman et al., 1994).
There has also been debate about the apparent increase in mortality from asthma in adults over the last 20 years (Burney, 1988). Increased mortality in the over 65s can probably be explained by diagnostic transfer from COPD, but the increase for younger adults cannot be adequately accounted for by changes in diagnostic practice or revisions of ICD coding (Burney, 1988; Weiss et al., 1993). Neither do changes in treatment appear to be responsible (Burney, 1988). However, a more recent analysis of death rates suggests that the increase in young adults' death rates is smaller than previously thought and is confined to females aged 15-24 (Anderson and Strachan, 1991). Overall, mortality is low in children and rises steeply with age, probably related to the overlap with COPD (Burney, 1992). Absolute death rates are, however, low: asthma contributes to 1.3% of deaths in the 15-44 year age group (Anderson et al.,1994b).
Anderson, H.R., Butland, B.K. and Strachan, D.P. (1994a) Trends in prevalence and severity of childhood asthma. British Medical Journal 308:16 1600-1604.
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Burney, P.G. (1992) Asthma: epidemiology. British Medical Bulletin 48:10-22.
Higgins, B.G., Britton, J.R., Chinn, S., Cooper, S., Burney, P.G. and Tattersfield, A.E. (1992) Comparison of bronchial reactivity and peak expiratory flow measurements for epidemiological studies. American Review of Respiratory Disease 145:588-593.
Hyndman, S.J., Williams, D.R., Merrill, S.C., Lipscombe, J.M. and Palmer, C.R. (1994) Rates of admission to hospital for asthma. British Medical Journal 308:1596-1600.
Littlejohns, P., Ebrahim, S. and Anderson, R. (1989) Prevalence and diagnosis of chronic respiratory symptoms in adults. British Medical Journal 298:1556-1560.
McWhorter, W.P., Polis, M.A. and Kaslow, R.A. (1989) Occurrence, predictors and consequences of adult asthma in NHANESI and follow-up survey. American Review of Respiratory Disease 139:721-724.
Peat, J.K., van den Berg,. Green, W.F., Mellis, C.M., Leede, S.R. and Woolcock, A.J. (1994) Changing prevalence of asthma in Australian children. British Medical Journal 308:1591-96.
Phelan, P. (1994) Asthma in children: epidemiology. (Editorial) British Medical Journal 308:1584-1585.
Weiss, K.B., Gergen, P.J. and Wagener, D.K. (1993) Breathing better or wheezing worse? The changing epidemiology of asthma morbidity and mortality. Annual Review of Public Health 14:491-513.