8 Quality of life and markers of asthma control
Traditional measures of disease impact, such as prevalence, mortality and hospitalisation rates, are important but are of limited use in understanding the extent of the effect of a disease on an individual. Health-related quality of life (HRQoL) is a term often used to describe an individual’s perception of how a disease or condition affects their physical, psychological and social wellbeing. It is often used to measure the impact of a disease, such as asthma, on a person’s health and everyday functioning (ACAM 2004). HRQoL measures can be used to describe and predict health outcomes, guide and assess clinical management, and direct clinical policy and the allocation of health resources. The effect that a health condition has on physical, psychological and social wellbeing depends upon the features of the condition and also on individual factors such as perception of health and the relative importance of each domain, which is based on their beliefs, experiences and expectations.
Measures of HRQoL may focus on impacts that are relevant to a specific disease (disease-specific) or, alternatively, on impacts that are relevant to a broad range of health conditions (generic). Both generic and disease-specific measures have a role in the assessment of HRQoL. Measures that are generic are most frequently used in health surveys to assess the overall impact of a person’s health status on their quality life. Measures of HRQoL can be both brief and broadly focused, such as asking someone to rate their overall health status. Alternatively, they can be more complex and precise, such as a HRQoL profile, which measures impacts on physical, psychological and social wellbeing using a series of specifically targeted questions. The broadest measures endeavour to summarise the domains of HRQoL globally in a single question (global measures). A widely used example is the question ‘In general, would you say your health is excellent, very good, good, fair or poor?’ This question, which is the first question of the 36-item Medical Outcomes Study Short-Form (SF-36), is often referred to as the SF-1. It measures global HRQoL with less precision than the entire SF-36. However, the single question is more feasible than the 36-item question for use in large, multi-purpose surveys.
The SF-36 is an example of a HRQoL profile that has been widely used (McHorney et al. 1993, 1994). It measures eight dimensions of physical and psychological health referred to as: physical functioning, role physical, bodily pain, vitality, general health, social functioning, role emotional, and mental health. The questions can be summarised into a physical component summary score (PCS) and mental component summary score (MCS). Information from generic measures can be used to assess the quality of life of subgroups, such as those with asthma, relative to members of the general population or relative to reference values. The limitation of these generic questionnaires is that they may not adequately focus on those aspects of HRQOL that are particularly relevant to the people with specific diseases, such as asthma. Disease-specific measures, on the other hand, focus on the impacts that are relevant to a specific disease. These measures are designed for specific diagnostic or population groups, such as people diagnosed with asthma. The rationale for these questionnaires is that they will be more relevant and more sensitive to the differences between population subgroups with the disease and responsive to changes over time (Patrick & Deyo 1989).
Among people with asthma, disease severity, the level of disease control and the impact of the disease on HRQoL are interrelated. People with inherently severe asthma can be expected, on average, to have worse outcomes and, hence, worse HRQoL than people with less severe disease. The extent to which asthma severity is modified by environmental factors and treatment reflects asthma control. During periods of poor asthma control, people with asthma report poorer HRQoL (Vollmer et al. 1999). Markers of asthma control such as increasing frequency and severity of asthma symptoms, increased used of ‘relievers’ and being woken up frequently at night due to asthma can, therefore, be used as predictors of asthma outcomes.
A number of aspects of the physical impact of disease and its effect on social functioning or role performance can be considered markers of disease control. These include reduced activity days, restricted physical activity, reduced functioning ability, and days lost from work or school. This chapter presents information on HRQoL and markers of control for asthma using data from the ABS National Health Survey and state health surveys.
8.1 Impact of asthma on self-assessed health [back to top]
The presence of asthma is associated with a worse self-assessed health status. In the 2001 National Health Survey, respondents with asthma rated their health significantly worse than respondents without asthma (p trend <0.001). Although the definitions of asthma varied, in all surveys listed in Table 8.1, the distribution of responses on self-assessed health status was shifted towards a more adverse health status among people with asthma.
This disparity was evident in all age groups but it was greatest in the oldest age group, in both males and females, and least among young males (Figure 8.1).
8.2 Impact of asthma on the domains of HRQoL [back to top]
Health-related quality of life measures are commonly described in terms of dimensions that fall into the physical, psychological and social domains. Available evidence suggests that in most dimensions, the HRQoL of people with asthma is worse than that observed in people without the disease. In a survey conducted in South Australia in 1998 (Wilson et al. 2002) , people with asthma had lower (worse) scores than people without asthma for all eight dimensions of the SF-36 (Figure 8.2).
The tables in the following sections summarise the available data on the three core domains of HRQoL (physical, psychological and social) measured in people with asthma. Where available, comparative data from the same survey in people without asthma are also provided.
Physical domain of HRQoL
In the South Australian survey (Wilson et al. 2002), adults with asthma had lower (worse) scores in the three physical components of the SF-36 health questionnaire (physical functioning, role: physical, and bodily pain) than people without asthma (Figure 8.2). Two other population surveys found that adults with asthma had lower physical component summary scores from the SF-12 (an abbreviated version of the SF-36) than those without asthma (Table 8.2).
Psychological domain of HRQoLPeople with asthma report worse psychological health than people without asthma. When general measures of psychological health are used, such as those in the SF-12 (Table 8.3) and SF-36 (Figure 8.2), these differences are small but statistically significant. Specific measures of anxiety and depression levels have identified greater differences between people with and without asthma. A recent study from South Australia reported a higher prevalence of depression among people with asthma compared to people without asthma (Goldney et al. 2003) . Furthermore, this study found that people with more severe symptoms of asthma (shortness of breath, waking at night with asthma symptoms or morning symptoms) were more likely to suffer from major depression than those without severe symptoms.
Social domain of HRQoL
The social domain of HRQoL refers to the ability to perform roles and activities. This has most commonly been measured as time away from work or other usual activities.
Asthma accounts for a large proportion of days lost from work or study (Table 8.4). In the 2001 ABS National Health Survey, the proportion of people with current asthma who had taken time off work or study in the previous 2 weeks because of any illness (11.4%) was higher than the proportion of people without asthma who had taken time off for any illness (7.9%, p<0.001). The proportion of people with asthma who actually attributed days off work or study to asthma was 2.6%. Among children aged 2 to 12 years with asthma, 58% were limited in their normal activity in the last year, resulting in an average of 9.3 days of reduced activity in 2001.
Among participants in the 2001 National Health Survey who had current asthma, more males than females had taken days off work or study because of asthma in the previous 2 weeks (p=0.02) (Figure 8.3). There was no difference in the proportion of males and females who had other days of reduced activity due to their asthma (p=0.39).
More people with asthma (17.5%) than people without asthma (10%) reported having reduced activity days, other than those related to work or study, in the previous 2 weeks (Figure 8.4) (p<0.001). Only 3.2% of people with asthma attributed these reduced activity days to asthma. These observations imply either that people with asthma underestimate the impact of asthma on their ability to undertake activities, or that people with asthma are more likely to have other illnesses that interfere with these activities.
8.3 Markers of asthma control [back to top]
There is a clear relationship between asthma severity and asthma control. The underlying severity of asthma in an individual may be modified by changes in the environment (e.g. reduction in exposure to known triggers of asthma such as dust mites). It may also be influenced by treatment for asthma. Ultimately, the changes in these environmental and treatment factors will impact on the individual’s symptoms and their ability to function. This outcome is referred to as ‘asthma control’. It reflects the combined effect of underlying disease severity, environmental exposures, and the effectiveness of treatment.
Several markers of asthma control have been used in population and clinical studies. These include increasing frequency and severity of asthma symptoms, increased use of bronchodilators (‘relievers’), being woken up frequently at night due to asthma, reduced days of activity, restricted physical activity, reduced functioning ability, and days lost from work or school. These last four markers of asthma control overlap with the impact of asthma on the social domain of HRQoL reported in the previous section.
Assessment of the severity of asthma
The National Asthma Council (NAC 2002) recommends using a number of indicators to classify asthma severity at diagnosis.
Children with asthma are grouped into three broad patterns of asthma, which can be considered to reflect disease severity, using the NAC guidelines:
The supplementary (SAND) asthma module of the BEACH general practice survey (see Appendix 1, Section A1.3) was used on four occasions between 1999 and 2002, to estimate the distribution of severity of asthma among patients attending GPs (AIHW GPSCU 2000, 2001). Severity was categorised based on the groupings in Table 8.5, with the addition of a ‘very mild’ category for those with episodic asthma only.The distribution of patients among the levels of severity did not change significantly between 1999 and 2002 (Henderson et al. 2004). Thirty-two per cent of adult patients with asthma were classified by GPs as having moderate and severe asthma. In that same period, 23% of children with asthma were classified as have frequent episodic or persistent asthma. Most children and adults attending GPs were assessed as having infrequent episodic asthma and mild or very mild asthma, respectively (Table 8.6).
In the New South Wales Health Survey in 1997, 54% of adults with asthma met one or more of the following criteria, which were adapted for survey use from the NAC criteria for moderate to severe asthma (Marks et al. 2000):
There is a large difference in the proportion of people classified as having moderate or severe asthma when the GP data (Table 8.6) and general population data from the New South Wales Health Survey are compared. This can be partly explained by the differences in the methods used for assessing asthma severity, the populations surveyed, and years when the surveys were conducted. The difference reflects uncertainty about the true proportion of people with moderate to severe asthma in the population.
Sleep disturbance due to asthma
People with severe and/or poorly controlled asthma may be awoken from sleep with asthma symptoms. This sleep disturbance due to asthma is an important adverse outcome of the illness and is also regarded as a valuable marker of disease control. Population surveys confirm that this is a common problem in both adults and children with asthma (Table 8.7). In New South Wales in 2001, 48.2% of children with current asthma had disturbed sleep in the last month that was attributed to asthma (Centre for Epidemiology and Research 2002).
Asthma has a measurable impact on how people assess their overall health status. Most of the impact of asthma is on physical functioning and on the ability to perform social roles, such as work or study. Recent evidence suggests there is an important association between depression and asthma.
There are limited data on the prevalence of various levels of asthma severity and control in the general community. It is likely that between one-third and a half of adults with asthma have moderate or severe disease.
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