Increases in age at death were observed among decedents with MDs, as expected due to evolving health care treatments for MDs such as corticosteroids and NIV, with an older age at death among whites than blacks with MDs, among both males and females, and a greater increase in age at death among white males than black males with MDs. The distribution of age at death is consistent with the mix of MDs classified together in the ICD system. The small peak at zero years of age and the peak in the 60-year age range are present in both sexes. In the latter group, there were more male than female decedents, consistent with a mixture of autosomal MDs such as FSHD and LGMD, and X-linked MDs such as BMD and EDMD. Among male decedents, there is an additional peak around 20 years of age, which is likely to be composed predominantly of deaths from DMD. The overall MD-associated mortality rate was lower among blacks than among whites, which might be due to a lower prevalence of MDs, underdiagnosis of MDs, or underascertainment through death records for MDs among blacks.
Cardiomyopathy, a major contributor to morbidity and mortality for MDs, was more often reported among blacks than among whites, even early in the study period before the widespread use of corticosteroids and NIV were likely to have had an effect on patients with DMD. Cardiomyopathy is not associated with all types of MD, but is common in DMD,20
and some of the LGMDs.23
Therefore, this finding may be indicative of racial differences in prevalences or natural histories of the different types of MDs. It might also be related to the higher frequency of cardiomyopathy among blacks in general.24
Although it might have been difficult to distinguish clinical heart failure with or without cardiomyopathy, the differences in age at death between decedents with and without reported cardiomyopathy suggested clinically relevant differences between the 2 groups. From 1986 through 2005, the frequency of cardiomyopathy increased steadily among white males, likely the result of increased age at death associated with NIV in patients with DMD and health care improvements for people with MDs in general. At the same time, the age at death of white males with cardiomyopathy gradually increased, consistent with the increased age at death among white males with MDs in general. Neither the frequency of cardiomyopathy nor age at death among those with cardiomyopathy increased among black males.
The racial differences in changes in age at death and frequency of cardiomyopathy might have been related to different prevalences of the various types of MD, different natural histories, or differences in environmental, genetic, or behavioral risk factors. However, it is possible that the use of corticosteroids and NIV was less common among black than white patients, or that these treatments were less effective in blacks because of differences in types of MD or other factors that affect natural histories. While differences in use of interventions might be due to racial or geographic preferences in balancing quality of life against potentially life-extending treatments, differences in access to health care may also be a factor, as suggested by racial differences in locations of MD-associated deaths (appendix e-1).
We looked at the frequencies of cardiac and pulmonary conditions among males who died at ages 10–34 years, which we presume to be mostly comprised of cases of DMD. The increasing frequency of cardiomyopathy and decreasing frequency of pulmonary complications among those without cardiac problems are consistent with the improvements in DMD treatment, which reduce pure pulmonary failure and therefore increase the age at death and the likelihood of cardiomyopathy.
Cardiomyopathy was less common among male decedents over the age of 35 years than among those aged 10–34 years. This finding was expected because males in the older age group are more likely to have one of the autosomal MDs that have lower frequencies of cardiomyopathy, such as FSHD.
As individuals with MDs lived longer, age-related conditions became apparent. The same trends were seen among both races and sexes, although the trends were not significant among blacks and females, likely due to smaller sample sizes. The frequencies of cardiomyopathy and ischemic heart disease were higher among white males than females with MDs, as they are among decedents in the general population.24
Along with the racial difference in cardiomyopathy among all decedents, this suggests that risk factors for heart disease in the general population might also affect the natural histories of MDs.
There were several limitations to this study. This dataset did not include information about quality of life, so increases in age at death do not necessarily equate with improvements in quality of life.29
Interpretation was complicated by the coding of MDs as a group in the ICD system, and by the lack of age-, race-, and sex-specific prevalence and age at death estimates for MDs. The sample sizes for female and black decedents limited the power of some race- or sex-specific analyses, especially when the need to dichotomize year of death could have masked small gradual changes. The dataset did not include age at onset or age at diagnosis; thus, we could not assess disease-related longevity relative to these events. While the database contains some possible social effect modifiers, such as education and marital status, it does not include information to assess sufficiently other sociocultural variables, such as health insurance and family structure.
There was the potential for bias resulting from inaccurate recording of information on death records. While the accuracy of physician-provided race data on death records of black and white decedents was about 98% when compared to information gathered from next of kin,30,31
the reported cause of death and underlying conditions might have been incomplete or inaccurate.32,33
It is not known if accuracy of death record data differed by race, so it is possible that race-specific biases existed because of uniform differences in the reporting of cause of death and underlying conditions for black and white decedents.
There was a greater increase in median age at death among whites than blacks with MDs in the United States, particularly among males without cardiomyopathy. The trends of increasing age at death and frequencies of secondary and age-related conditions were consistent with a slowing of the progression of MDs from corticosteroid use, respiratory support provided by NIV, and improvements in MD-related health care. More studies are needed, such as those ongoing in the Muscular Dystrophy Surveillance Tracking and Research Network,34
to understand the racial differences in age at MD-associated death in the United States.