Long-term secular trends in dermatomyositis and polymyositis mortality in the USA from 1981 to 2020 according to underlying and multiple cause of death mortality data

Main findings

In this study, we identified two times as many DM/PM deaths using the MCD data as identified using the UCD data. Persistent downward DM mortality trends for both sexes over the past 4 decades were noted and the patterns were similar according to UCD and MCD. With regard to PM mortality trends for both sexes, the year of significant decline were in late 1990s according to UCD; nevertheless, were in early 1990s according to MCD. The magnitude of decline in PM mortality was larger than that in DM mortality.

Interpreting the findings in the context of previous studies

The main results of two previous population-based cohort studies of DM and PM mortality are summarized in Table 5 [6, 9]. We calculated the mortality rate ratios between early and late observation periods in the two studies and found that the extent of decrease in DM mortality was greater than that in PM mortality in the UK study; no obvious differences were noted in the BC study. However, in the present study, the extent of decrease in PM mortality was more prominent than that in DM mortality.

Table 5 Numbers of deaths (No), mortality rates (deaths per 1000 person-years), and rate ratios (RRs) in two periods according to two population-based cohort studies

Several caveats should be noted in interpreting the differences between the results of the present study and those of the two cohort studies. First, the mortality rates calculated in the cohort studies were actually case fatality rates; that is, the denominators were the numbers of patients with DM/PM diagnoses. However, the denominator of mortality rate in the present study was that of the general population, the mortality rate was affected by two components: the incidence (prevalence) rate and the case fatality rate. According to a population-based study (Rochester Epidemiology Project) in Olmsted County, the incidence of DM was 1.2 (per 100,000 person-years) in 1995–2007 and 1.1 in 2008–2019, no evidence of a change over time [22]. Therefore, the decline in DM/PM mortality rates observed in this study was mainly due to the reduction in case fatality rate. As indicated by Li et al., the early use of disease-modifying antirheumatic drugs (rituximab, methotrexate, azathioprine, and mycophenolate mofetil) and the increasing use of intravenous immunoglobulin might be key factors affecting the decline in DM/PM mortality in recent decades [6].

The findings of this study further indicated that the extent of decline in mortality rates was larger in PM (APC for males was − 4.6% and − 4.3% according to UCD and MCD, respectively) than those in DM (APC for males was − 1.2% and − 1.9%, respectively). However, no such difference was noted in UK and BC study (Table 5) [6, 9]. One plausible explanation was the differences in the robustness of diagnosis of DM/PM between those based on the death certificate versus those based on hospital records (will be discussed later in the limitation section).

The second caveat was that the years of observation differed between the two cohort studies with this study. The time span was 1996 through 2014 in two cohort studies and was 1981 through 2020 in the present study, which hindered valid comparisons. The trends in DM and PM mortality rates might change across study periods. An early study conducted in the USA reported that the age-adjusted DM/PM mortality rates increased from 1968 to 1978, [10] which is in contrast with the decreasing mortality trends since 1981 revealed in the present study.

With regard to the interpretation of the mortality rates estimated according to UCD and MCD data, it is better to examine the instruction depicted in the US standard death certificate: “Enter the chain of events – diseases, injuries, or complications – that directly cause death in Part I” and “Enter other significant conditions contributing to death but not resulting in the underlying cause given in Part I” (Fig. 1). If DM/PM were recorded by certifying physicians in Part II, DM/PM were less likely been designated as the UCD. The proportion of DM/PM as the UCD among those with mention of DM/PM could be a proxy measure of case fatality rate of DM/PM.

In this study, the proportion of DM/PM as UCD among MCD was 48% for DM and 46% for PM and about the same through the study period. The proportion was lower than that in Brazil, which was 57% for DM and 54% for PM [15]. The first probable explanation for the difference was that people with DM/PM in the USA were better treated than their counterparts in Brazil and therefore had lower DM/PM case fatality rates. The second possible explanation was that the case fatality rates of DM/PM were similar in two countries; nevertheless, the US physicians were more likely than their counterpart Brazil physicians to record DM/PM in part II of the death certificate. Study has indicated that physicians in different countries had different habits in recording diabetes in the part II of the death certificate [23].

Strengths and weaknesses

One strength of the present study is its use of nationwide population-based mortality data collected across 40 years to examine long-term trends in DM and PM mortality. This is also the first study to compare the DM and PM mortality trends between those according to UCD versus those according to MCD.

However, this study has several limitations that should be noted. First, some physicians might underreport DM/PM on death certificates. There was no study specifically examined the magnitude of underreporting of DM/PM on the death certificate. According to two studies assessed the underreporting of systemic lupus erythematosus (SLE) on the death certificate, only 40% of people with SLE died, the physicians recorded SLE on the death certificate. The underreporting was higher as the age increased and among people with cancer [24, 25]. However, because the main aim of this study was to examine mortality trends, the underreporting rate is unlikely to have systematically biased the results over time if there were no specific interventions on the certification behaviors.

Second, the validity of using ICD codes to identify DM/PM should be concerned. As it is common that other systemic inflammatory diseases and inherited muscle diseases including muscular dystrophies and metabolic myopathies could be misdiagnosed as idiopathic inflammatory myopathies (IIM). Therefore, it is possible that a decedent assigned with an ICD code of DM/PM as the underlying cause of death or indirect cause of death might actually have no DM/PM. According to a validity study of using ICD code to identify DM, the sensitivity and positive predictive value (PPV) for multiple ICD-9 codes 710.3 in the outpatient setting were 0.89 and 0.35, respectively. The PPV for primary and secondary inpatient codes of 710.3 was 0.95 and as high as 0.80 [26]. An UK study assessed the validity of using ICD-10 codes to identify IIM in hospital episode statistics data indicated sensitivity of 0.73 and PPV of 0.73 [27]. That is to say that the validity of ICD codes in inpatient data to identify people with IMM is acceptable and most of the people died with DM/PM diagnosis recorded on the death certificates were issued by the physicians in hospital. The over-diagnosis of DM/PM on the death certificate might not be high.

Third, as the population in the USA is aging, the proportion elderly decedents with DM/PM increased across the four decades. Study has indicated that the use of age distribution in 2000 in the USA (a relatively younger age structure than in 2020) as standard for calculation of AAMRs would result in a lower estimation of the real mortality rates [28].

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