Hospital admissions for asthma may be used as an indirect indicator of the burden of more severe asthma and the efficacy of care. However the factors underlying variations in hospital admission rates are poorly understood and need more research.
HOSPITAL ADMISSIONS FOR ASTHMA
Hospital admissions for asthma have been proposed as a target indicator of improvements in asthma care, but the factors underlying variations in hospital admission rates are poorly understood. Admission to hospital during an asthma attack may indicate the first episode in the disease or a failure of preventive care for established asthma. Hospital care may be important to prevent a fatal outcome in severe or troublesome asthma. Historically, the relationship between asthma prevalence, severity, admissions, and mortality rates in high-income countries has been complex. Changes in the admission rate over time correlate (albeit imperfectly) with changes in the prevalence and severity of childhood asthma. However, the relative ranking of national admission rates for asthma is not consistent between children and adults.
Many attacks of asthma are mild and self-limiting and never present for hospital treatment. The proportion of acute episodes which result in hospital admission varies greatly between countries, depending upon the accessibility and affordability of the health care system, the local thresholds for referral from community to hospital, and from outpatient or emergency visits to inpatient care.
National hospital admission statistics are mainly limited to high-income countries in Europe, North America and Australasia. Data are lacking for most low- and middle-income countries. In European countries, among all age groups, asthma contributes 0.6% of hospital admissions and 0.4% of all inpatient bed-days. Figure 1 shows an almost tenfold variation in age-standardised admission rates for asthma between European countries in recent years.
These all-ages rates conceal considerable variation in hospital admission rates between children (where rates are generally higher) and adults. Caution should be exercised when interpreting geographical differences and trends over time in asthma admission rates for pre-school children (where diagnostic overlap with acute bronchitis and bronchiolitis may occur) and for older adults (where chronic obstructive pulmonary disease may be confused with asthma).
Among 23 European countries, in recent years, there were close correlations between the national admission rates for younger and older children, and between younger and older adults (see Appendices Figures 1-3), but the correlations between rates for adults and children are less impressive (Figure 2).
Trends over time
In most, but not all, European countries, age-standardised asthma admission rates declined through the last decade (Figure 1). In some countries, the reduction was two-fold or greater, a larger change than has been proposed as a target indicator of improvements in asthma care, for example by the Global Initiative for Asthma (GINA) and the Global Asthma Network. This recent decline is largely due to a reduction in admission rates among children, which is part of a longer-term rise and fall, peaking in the early 1990s. This is shown schematically in Figure 3 (based on data from several European countries, the United States of America, Canada, Australia, New Zealand, Hong Kong and Singapore).
Taking a 50-year perspective, the “epidemic” of asthma admissions bears no temporal relationship to two epidemics of asthma mortality (in the 1960s and the 1980s, related to the use of older asthma relievers with potentially toxic side effects), nor to time trends for self-reported asthma prevalence (Figure 3). However, data from the United Kingdom show a peak of primary care contacts for acute asthma, particularly among children, in the early 1990s, similar to that of asthma hospital admissions. This suggests a rise and fall in the incidence of asthma attacks in the community, rather than simply a change in patterns of referral to secondary care, or a reduction in the severity threshold for admission to the hospital ward.
An international comparison of time trends in asthma admissions and asthma drug sales in 11 countries during the 1990s found that increased sales of inhaled corticosteroids (“preventer” medication) were associated with a decline in rates of hospital admissions for asthma. However, inhaled corticosteroids became more widely used for asthma during the 1980s, a period of increasing hospital admission rates among children. Thus, it is not possible to draw firm conclusions about the extent to which uptake of effective “preventer” medication has reduced hospital admission rates for asthma in high-income countries.
Relationship of hospital admissions to other measures of the burden of asthma
When national asthma admission rates for children were compared with the asthma symptoms prevalence and severity data for centres (but not whole countries) participating in the International Study of Asthma and Allergies in Childhood (ISAAC) Phase One study around 1995, a highly significant positive correlation was found between national admission rates and the prevalence of more severe asthma symptoms in 13-14 year olds (14 countries), but not in 6-7 year olds (11 countries). However, a similar analysis (prepared for this chapter) of ISAAC Phase Three data (collected around 2002) for 15 European countries with data in the older age-group, and 11 European countries in the younger age-group, found no statistically significant correlations between how the countries ranked against each other for national admission rates in children and how they ranked for any measure of wheeze or asthma prevalence, including more severe symptoms.
Such comparisons need to be interpreted with caution, because ISAAC centres are self-selected and are not necessarily representative of the countries in which they are located. Additionally, between-country comparisons at a single point in time are potentially biased in many ways. However, some of these biases become less relevant if within-country changes are examined over time.
For countries with ISAAC study centres participating in both Phase One and Phase Three, Figure 4 plots the annual change in childhood hospital admission rates (~1995-2002) against the change in the prevalence of wheeze causing 13-14 year old children to wake at night at least once a week. Over this period, admission rates declined in all these countries except Hong Kong and Poland. There was a significant positive correlation between the decline in prevalence of severe asthma symptoms between Phase One and Phase Three and the decline in the corresponding national admission rates for childhood asthma over a similar period.
Asthma admission rates have been proposed as a target indicator for monitoring progress towards improved asthma care. Large reductions in admissions have occurred already over the last decade in several countries.
However, currently routinely collected information is almost entirely restricted to high-income countries, limiting the value of admission rates for surveillance of the global burden of asthma. Large unexplained changes in admission rates have occurred over the past 25 years, particularly for childhood asthma, but international correlations of within-country change in prevalence versus within-country change in admission rates provide some support for the concept that changes in hospital admission rates can be used as an indirect indicator of the burden of more severe asthma in the community.
In countries which routinely collect admissions data, changes in hospital admissions over time may be used as an indirect indicator of the burden of more severe asthma. Before admission rates can be used as an indirect indicator of the global burden of severe asthma, more countries need to collect admissions data.
Health authorities in all countries should collect counts of hospital admissions in children and adults from defined catchment populations, to monitor trends in asthma over time.