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Abstract

• In 1997, 27% of Australian children had current wheeze, and this is increasing by 1.4% per year.
  
• The prevalence of wheeze among adults is lower and appears to be stable.
  
• The prevalence of persistent asthma (wheezing episodes with abnormal airway function between episodes) in children has increased from 5% to 9% in the past 20 years. In adults, the prevalence is 5%-6%.
  
• Up to 80% of adults with persistent asthma have abnormal lung function.
  
• Asthma deaths in Australia have fallen 28% since peaking in 1989, but the mortality rate is still twice that of England.

In the same year, the Cooperative Research Centre for Asthma (CRCA) was established. The CRCA is a joint venture between two medical research institutes (the Institute of Respiratory Medicine and the Garvan Institute of Medical Research), three universities (the University of Sydney, Monash University, and the University of Western Australia), six pharmaceutical companies, and the New South Wales Department of Health. Its mission is to reduce the burden of asthma on the Australian community. The three research programs of the CRCA are prevention of asthma, treatment of asthma, and diagnostic, delivery and monitoring devices.

Here we summarise information on the burden of asthma in Australia and make some international comparisons. We do not attempt to develop a single estimate for the "burden" of asthma, but describe data on prevalence, severity, lung function, general practitioner and hospital attendances, management, morbidity, and mortality as related to asthma. Data on the prevalence of atopy are not presented. Although this is the strongest risk factor for asthma,¹ its contribution to the burden of asthma is indirect.

Definitions

There is no agreed definition of asthma. The terms we use here are defined in Box 1. Asthma prevalence can be measured in terms of self-reported wheeze, doctor-diagnosed asthma, or a combination of symptoms and lung function abnormality.² Moreover, asthma can be classified as intermittent or persistent, and persistent asthma can be classified into mild, moderate and severe, based on symptoms and degree of airway hyperresponsiveness.³

There are limitations to the use of questionnaires in the measurement of asthma. These arise because of differences between patients, parents, and doctors in the use of the label "asthma" and, to a lesser extent, "wheeze". Furthermore, substantial problems with recall bias may influence the findings. In evaluating questionnaire-based reports of the prevalence of asthma, it is important to be aware of the questions used to define asthma in the particular study. It is also relevant to be aware of assessments of the reliability and validity of the questionnaire.

Prevalence

Wheeze

Children: Over the past 20 years, there have been at least 26 population-based studies measuring self-reported current wheeze in Australian children (Box 2). The prevalence of current wheeze has been increasing at a rate of 1.4% per year.⁴ The International Study of Asthma and Allergies in Childhood (ISAAC),¹⁹ in 38 countries across all continents, found that among children aged 6-7 years Australia had the second-highest prevalence of self-reported current wheeze (24.6%).

Approximately a quarter of Australian children have wheezed in the past 12 months, and it seems unlikely that they all have intermittent or persistent asthma that requires treatment or is a burden. At present, there is no way to classify asthma on wheeze alone, although, in many studies, children with more than four wheeze episodes per year are regarded as having "asthma". Robertson et al⁴ found that among 6-7-year-olds who report current wheeze 34.7% reported more than four wheeze episodes (about 8% to 9% of the population). In comparison with other countries, Australia has the second-highest percentage of children aged 6-7 years who report more than four wheeze episodes in the previous 12 months.¹⁹ It can be calculated from the data reported by Bauman et al⁵ that children who wheeze are symptomatic about 14% of the time.

Adults: In contrast to the information available for childhood wheeze, there is a paucity of data on the prevalence of current wheeze in Australian adults. Box 3 shows the prevalence of current wheeze to be between 17% and 29% in adults, with no apparent increase over time. In the European Community Respiratory Health Survey (ECRHS), conducted in 22 predominantly European countries, Australia had the fourth-highest prevalence of self-reported current wheeze in populations aged 20 to 44 years.²⁰

Wheeze and lung function

Objective measures of lung function combined with asthma symptoms allow classification of asthma into intermittent and persistent. Persistent asthma is clinically important asthma in that it causes more interference with work and school, requires more treatment, and results in more healthcare utilisation; hence, it represents a greater burden than intermittent asthma.^2,27 In Australia, about 9%-11% of children⁶ and 5%-6% of adults^21,22 have persistent asthma.

Persistent asthma was not measured in phase one of the ISAAC study. The Australian reporting centre for the ECRHS found that 25.5% of its sample of adults in Melbourne had persistent asthma.²⁸ However, the sample used in that study to determine prevalence of airway hyper-responsiveness (and thus persistent asthma) was not random, but was enriched with an additional 27% of symptomatic subjects. Although data on the prevalence of airway hyperresponsiveness have been published for the ECRHS as a whole, no data on the prevalence of symptoms together with airway hyperresponsiveness (ie, persistent asthma) have been published from this survey.

Severity

The measurement of severity of asthma is difficult. The nature of mild, moderate, or severe asthma depends on the perspective of the observer. Questionnaire assessments of severity generally focus on symptoms that are regarded by clinicians as indicative of more severe asthma, such as frequent wheeze episodes, severe episodes, hospital and emergency department admissions, and disturbed sleep. In addition, questions relating to disability and handicap arising from the disease, such as time off work or school and inability to carry out normal activities, are used as indicators of severity. None of these could be considered a "gold" standard. In the Australian data from the ISAAC study, among children aged 6-7 years with current wheeze 55.1% have nocturnal waking and 15.3% report severe wheeze episodes.⁴ In the international comparison, Australia ranked 10th highest for the percentage of children aged 6-7 years who had disturbed sleep in the previous year.¹⁹

Rosier et al²⁹ used a statistical approach (item response theory) to develop a questionnaire-based functional assessment of disease severity in children with wheeze. In a population study, they found that the 14% of children who had current wheeze included 47% low, 18% mild, 30% moderate, and 5% high severity. This scale was validated by demonstrating correlations with school absence, functional impairment, practitioner consultations, and medication requirements.

In the clinical setting, measurements of airway hyper-responsiveness are often used to determine asthma severity. Peat et al⁶ showed that of the Australian children with persistent asthma the percentage of children with severe airway hyperresponsiveness was less than 1%. Thus, it appears from both questionnaires and lung function data that the proportion of children with severe asthma is about 0.4%-0.6% of the population.

The proportion of the burden of asthma that comes from the small group with severe asthma and from the larger group with mild asthma is open to interpretation. Health planners should target those with severe asthma, as they have a greater potential for adverse health outcomes³⁰ and poorer long term lung function than individuals with intermittent asthma, many of whom remit.^30,31

Lung function

A proportion of the burden of asthma comes from symptoms of poor lung function. In their study of Busselton adults, Peat et al³² found that individuals with asthma have a more rapid decline of lung function over time compared with normal subjects. In that study, 86% of females and 82% of males with current asthma had abnormal lung function. A study of children in a suburb of an industrial city and in a rural town in New South Wales found that those who had airway hyperresponsiveness had reduced measurements of forced expiratory volume in one second (FEV₁) over time.³³ The magnitude of the burden of asthma due to poor lung function has not been documented.

 GP Consultation

Data from the Bettering the Evaluation and Care of Health Study, 1998-1999,³⁴ indicate that asthma is the sixth most frequently managed problem by GPs in Australia. Asthma is one of the top 20 reasons for individuals to visit a GP, with a rate of 1.4 per 100 encounters.

Hospitalisation

During the period 1998-1999, there were 53 907 hospital separations and 7464 same-day separations for asthma in Australia, equating to 147 496 patient-days.³⁵ The average length of stay was 2.7 days. Faniran et al found that about 1.8% of children were admitted to hospital for asthma and 6.8% attended the emergency department in the previous year.⁷ In Australia, the ISAAC study found that for 6-7-year-olds who reported current wheeze about 7% were admitted to hospital and 14.4% attended the emergency department.⁴ Between 1998 and 1999, South Australia reported the highest hospitalisation rate (384 per 100 000 people) and Tasmania reported the lowest rate (148 per 100 000 people).^35,36

Compared with countries with a similar high prevalence of wheeze, Australia has a very low rate of hospital admissions.³⁷ The ECRHS study found that Australia had the third-lowest hospitalisation rate in adults with current doctor-diagnosed asthma. Only 1% of adults with asthma were admitted to hospital in the previous year. This might indicate that, comparatively, asthma in Australia is reasonably well managed, although many studies suggest that management is still not ideal.^38,39

Management

Correct prescribing practices and adherence to therapy are core issues in the management of asthma and quality of life, and thus the burden of asthma. One Australian study found that the use of anti-inflammatory medication was unsatisfactory in 26% of children with asthma.³⁸ These children were treated either too aggressively or inadequately.

Box 4 shows the patterns of treatment for children in Australia who reported wheeze episodes in the previous year.⁴ The degree of interventions increased in proportion to the number of wheeze episodes. However, for children who reported more than 12 wheeze episodes, both the use of inhaled steroids and the use of a written asthma action plan were inadequate.

Perceptions of treatment efficacy and management among young adults (20-44 years old) in Victoria were examined in the ECRHS study.³⁹ There was generally poor adherence to therapy among this age group. Adults in New Zealand and the UK also have a high prevalence of wheeze, but are three times more likely than Australian adults to take anti-inflammatory medication daily.³⁷

It is difficult to ascertain how much of the GP and hospital attendances are indicative of poorly controlled asthma. Ruffin et al have shown that there is insufficient ownership of asthma management plans in South Australia.⁴⁰ In 1996, only 33% of patients with asthma had an asthma action plan. Those who had action plans were more aware of asthma severity, used preventer medication more regularly, measured their lung function and understood their asthma. It is acknowledged that self-management plans result in improved health outcomes for patients with asthma.⁸ Furthermore, regular review and having a written asthma action plan contribute to reduction in hospital and emergency department attendances, and to reduced absenteeism from work or school and reduced nocturnal asthma.⁴¹

 Morbidity and quality of life

The Living with Asthma Study, conducted in 1999, found that asthma had a substantial impact upon child and adult lifestyles. Both groups felt tired and frustrated because of their asthma. One in five children did not ride a bike or play at school or with animals, and one in three did not participate in organised sport. One in four adults avoided socialising in restaurants, pubs and clubs because of the smoky environment. Parents of children with asthma were more anxious than parents of children who did not have asthma.

The 1995 National Health Survey, conducted in a sample of the adult Australian population by the Australian Bureau of Statistics, included administration of the SF-36 (Short Form, 36 questions) questionnaire,⁴² which measures physical functioning, role limitations due to physical problems, bodily pain, general health, vitality, social functioning, role limitations due to emotional problems, and mental health. For all these categories, adults with asthma had lower mean scores than adults without asthma. The NSW Health Survey 1997 (Box 5) showed that many adults who have current doctor-diagnosed asthma suffer considerable morbidity.⁴²

Mortality

In 1998, there were 685 deaths from asthma in Australia (Box 6).⁴⁴ This represents a 28% fall in asthma deaths since they peaked in 1989. The declining trend has occurred across all age and sex groups. Improved asthma management, implemented through the National Asthma Campaign, best explains this trend. There has been greater physician and patient testing of lung function and overall use of management plans. Despite improving mortality rates in Australia, the rate of 0.61 per 100 000 people aged 5-34 years is almost double that of England (0.35 per 100 000),⁴⁵ indicating that there is still scope for improvement in asthma management practices.

Discussion

It is widely acknowledged that there are many sources of inaccuracy in studies of asthma prevalence.^46,47 Even allowing for the various asthma definitions used in questionnaires, the data indicate that there is a large burden of asthma in Australia, in both children and adults. Moreover, for many measures of asthma, Australia has a high, if not the highest, prevalence when compared with other countries.

However, large gaps remain in knowledge about the prevalence, severity and morbidity of persistent asthma in Australia and internationally. More information is needed about treatment practices and the groups of people most at risk for poor outcome. To accurately assess the burden, it will be necessary to relate asthma prevalence and severity to the indicators of morbidity and make international comparisons.

There are many things that can be done now to reduce the burden of asthma.

• Patients and families can manage their own asthma by increasing their knowledge and awareness of asthma medication and control and removing barriers to adherence to treatment regimens.

• Clinicians should find efficient ways to diagnose and treat people with asthma, and form partnerships with patients to implement asthma management and action plans.

• Researchers should decide on useful definitions of asthma so that prevalence, severity and health outcomes can be compared with time and with changes in treatment practice.

Although there is no cure for asthma, the disease can be controlled by good management. Improving quality of life and keeping prevalence, mortality and hospital admission rates low is well within the scope of clinicians and patients. This represents the preventable burden of asthma.

Many questions about the burden of asthma, and how best to reduce it, remain to be answered. These include:

• What responsibility for reducing the burden of asthma should be borne by government, the National Asthma Campaign and asthma clinicians?

• How much of the burden is due to poor management by the doctor or individuals with asthma?

• Can asthma be prevented if treated early?

• Are there protective factors that can be introduced to reduce the prevalence of asthma?

The Cooperative Research Centre for Asthma will address these questions and it is hoped that, together with other researchers, it will answer them.

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 Autors' details

Reprints will not be available from the authors.
Correspondence: Ms V A Keena, Institute of Respiratory Medicine, PO Box M77, Missenden Road, NSW 2050.
vakATmail.med.usyd.edu.au

©MJA 2001
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