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- MATERIALS AND METHODS
Summary: Purpose: To demonstrate the application of Disability Adjusted Life Year (DALY) as an aid in health outcome measures to evaluate the epilepsy disease burden in rural China and to provide Chinese data to achieve a better understanding of disease burden due to epilepsy.
Methods: The DALY is the sum of the number of years of survival with disability (Years Lived with Disability, YLD) and the number of years lost because of premature mortality (Years of Life Lost, YLL). We calculated the YLD based on the prevalence survey of epilepsy among 66,393 people sampled in Heilongjiang, Henan, Jiangsu, Ningxia, Shanghai, and Shanxi provinces in 2000. The epilepsy mortality data from Chinese literature provided the YLL due to epilepsy. We applied sensitivity analysis to evaluate the influence of uncertainty on the epilepsy mortality value and disability weight in the study.
Results: In 2000, epilepsy caused 1.83 and 2.48 DALY lost per 1,000 population in Henan and Ningxia province, which had the lowest and the highest DALY lost among the six study areas. Overall, epilepsy caused 1.41 YLLs and 0.67 YLDs per 1,000 population; thus the DALYs lost because of epilepsy was 2.08 per 1,000 population, representing the epilepsy disease burden in rural China.
Conclusions: The DALY measure, which includes the extent of disability from epilepsy, provides a useful tool for the epilepsy disease burden assessment. The disease burden of epilepsy in China is considered higher than previous estimations.
Both disease-specific measures, such as prevalence and incidence, and the non–disease-specific denominator, the mortality, have been used to measure the disease burden of epilepsy in China. The lifetime prevalence of epilepsy is between three and five per 1,000 population, and the incidence of epilepsy is between 30 and 40 per 100,000 population (Sichuan Medical School, 1981; Li et al., 1985; Zhang, 1986; Xue et al., 1987; Kong et al., 1989; Li, 1989; Yang et al., 1989; Li et al., 1989; Liu et al., 1990; Shan et al., 1992; Kleinman et al., 1995; Zhang et al., 1997; Li et al., 1997; Da, 1997; Hong et al., 2000; Huang et al., 2002; Meng and Hong, 2002). A few studies carried out in China indicate that epilepsy-related mortality is between 3 and 7.9 per 100,000 population (Sichuan Medical School, 1981; Li et al., 1985; Li et al., 1989).
The Global Burden of Disease (GBD) study evaluated the global burden of >200 diseases or injuries by a nonmonetary composite index, Disability Adjusted Life Year (DALY), which could be used to evaluate the impact of mortality and morbidity together. One can also use DALY to compare the impacts of various interventions and health care approaches (Murray and Lopez, 1996). The GBD study estimated the DALYs lost because of epilepsy in the world and in the WHO subregions. The Chinese data used by the GBD study, however, were limited and relied largely on experts' opinions. These limitations are expected to affect the reliability of the results. Additionally, China is a large country with 33 provinces, which have highly variable climate, geography, and socioeconomic characteristics. The epidemiologic statistics for epilepsy differed greatly in previous studies (Sichuan Medical School, 1981; Li et al., 1985; Zhang 1986, Xue et al., 1987; Kong et al., 1989; Li, 1989; Yang et al., 1989; Li et al., 1989; Liu et al., 1990; Shan et al., 1992; Kleinman et al., 1995; Zhang et al., 1997; Li et al., 1997; Da, 1997; Hong et al., 2000; Huang et al., 2002; Meng and Hong, 2002). It may be more appropriate to calculate and analyze the disease burden in different provinces rather than to use one number to represent all of China.
From 2000 to 2004, a WHO supported demonstration project, “Epilepsy Management at Primary Health Level” (EMPHL), was carried out in six study areas in rural China. The prevalence of epilepsy was investigated in each study area with populations about 10,000. Using the latest and most reliable prevalence data obtained from the epidemiologic survey, our study assessed the burden of epilepsy by DALY measurement in these six study areas. The study applied the DALY as an aid to evaluate disease burden and to provide data for a better understanding of the disease burden due to epilepsy in China.
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- MATERIALS AND METHODS
We used the prevalence-based calculation methods and estimated that the DALYs lost due to epilepsy per 1,000 population in the six study areas in China ranged from 1.83 in Henan province to 2.48 in Ningxia province in 2000. The overall DALYs lost due to the disease was 2.08 per 1,000 population, representing the epilepsy disease burden in rural China.
Previous reports relied on incidence, prevalence and mortality data, to reflect the disease burden of epilepsy. The GBD study is the only assessment of the epilepsy disease burden using the DALY measurements. Leonardi et al. reported the only published disease burden for epilepsy which presented detailed YLL, YLD and DALY lost worldwide (Leonardi and Ustun, 2002). Their results were referenced from GBD 2000 study. In 1990, the GBD study estimated the DALYs lost due to epilepsy as 0.81 per 1,000 population in China (Murray and Lopez, 1996). No other DALY-related data in China, however, have been published.
In China, few epidemiologic investigations of epilepsy were conducted between 1980 and 2000 due to difficulties in administration and training for the survey for large sample populations. The studies that were conducted, with different study objectives, screening tools, and diagnostic criteria, showed widely divergent prevalence and incidence of epilepsy (Sichuan Medical School, 1981; Li et al., 1985; Zhang, 1986; Xue et al., 1987; Kong et al., 1989; Li, 1989; Yang et al., 1989; Li et al., 1989; Liu et al., 1990; Shan et al., 1992; Kleinman et al., 1995; Zhang et al., 1997; Li et al., 1997; Da, 1997; Hong et al., 2000; Huang et al., 2002; Meng and Hong, 2002). Even using the same method and criteria in one survey, prevalence still differed among localities (Li et al., 1985; Wang et al., 2002; Ding et al., 2004). This kind of prevalence difference might be related to different economic levels and medical conditions treating causes of epilepsy, such as trauma, infection of the CNS, and cerebrovascular diseases, etc.
The EMPHL study provides reliable epidemiologic data due to its study design and diagnostic criteria with respect to epilepsy. The screening questionnaires of the study were based on the ICBERG screening instrument and the WHO screening questionnaires previously used in China. These questionnaires were validated at the Beijing Neurological Institute with a specificity of 78.5% and a sensitivity of 100%. The epilepsy cases were screened by trained physicians, diagnosed by neurologists and experts at a diagnostic workshop. Both convulsive types and nonconvulsive types of epilepsy were diagnosed in the study (Wang et al., 2002). The study areas were representative not only of the different geographical areas (middle [Henan and Shanxi], southeast [Jiangsu and Shanghai], northeast [Heilongjiang], and northwest [Ningxia]), but also of the different economic levels of China (high [Jiangsu and Shanghai], middle [Henan, Shanxi, and Heilongjiang], and low [Ningxia]). Furthermore, the EMPHL epidemiologic survey provided the age-specific epilepsy prevalence that could be directly used for the prevalence-based calculation of DALY lost due to epilepsy. Comparing to the relatively low epilepsy prevalence and mortality used as the parameters for the DALYs assessment in GBD study, our data seems more reliable and representative of the disease pattern in China. Therefore, the disease burden of epilepsy in China is considered higher than the estimation of GBD study.
There are potential limitations to our study. YLL, which is calculated based on mortality data, was a very important part of DALY assessment. In Chinese literature reports, the mortality of epilepsy is between 3 and 7.9 per 100,000 population, which is higher than that used in the GBD study (1.07/100,000). We therefore performed a sensitivity analysis to consider the impact of this plausible range of mortality to the total disease burden (Table 4). Since the EMPHL study could not survey epilepsy mortality and other recent studies did not provide age and sex specific mortality, we could increase the accuracy of the YLL estimate by obtaining mortality data from investigations of larger sample sizes and longer follow-up in multiple areas of China.
The EMPHL epidemiologic survey was a cross-sectional study in rural areas in China. Although the screening tool had high specificity and sensitivity, and the diagnosis procedure was strict, the prevalence of epilepsy might be underestimated if the subjects conceal or deny their seizure history because of barriers of traditional prejudice, or, for children under 4 years old, partial seizure can not be noticed by the parents. Our results, therefore, might underestimate the disease burden by DALY measurement.
The classification of the disability weight of epilepsy offered by the GBD study seems inappropriate as it did not consider the different types of epilepsy (convulsive and non-convulsive) and the seizure frequency, which affect the quality of life. In the sensitivity analysis, we found 33% enhancement of the baseline of the disease burden by DALY measurement if we used higher DW parameters with double value of that in GBD study, indicating that the result of disease burden was impacted by the DW of epilepsy. Although we used the DW provided by GBD in our study for data comparison with other studies, the DW of epilepsy should be reconsidered.
For the complete assessment of the epilepsy disease burden, DALY measurements should be acquired not only for neurological diseases, but also for all the diseases in different areas of China. Unfortunately, this has not been done. Compared to the disease burden due to neurologic diseases in WPRO B subregion provided by GBD study, however, our result of 2.08 DALYs lost per 1,000 population is lower than that of Cerebrovascular disease (9.86 DALYs lost per 1,000 population), and higher than that of Alzheimer's disease and other dementia (1.76 DALYs lost per 1,000 population) or Parkinson's disease (0.21 DALYs lost per 1,000 population) (GBD 2000).
Our study demonstrates a method which can be used to assess disease burden and which can be the basis for economic analysis of epilepsy treatment. According to our measurement, there was an overall size of burden of 1,681,410 DALYs lost in rural China based on the rural population of 808,370,000 in 2000 (National Bureau of Statistics of China, 2000). This entire burden can be massively reduced at low cost via effective treatment such as Phenobarbital (Chisholm, 2005; Wang, 2006). The information provided in this study can help policy planners to allocate resources and identify strategies and interventions for the reduction of the burden of epilepsy in China.