For readers who may not be aware of the Global Burden of Diseases, Injuries, and Risk Factors study (GBD), this is a regional and global research program of disease burden that assesses mortality and disability from major diseases, injuries, and risk factors. With its first report in 1990, it introduced the disability-adjusted life year (DALY) as a new metric to quantify disease burden to aid comparisons.1 The authors of this paper have accessed the publicly available DALY data for glaucoma from the GBD in 20162 and used this to look at sex differences in age-standardized DALY rates. Additionally, the authors attempted to explore associations of glaucoma DALY burden with three covariates, socioeconomic status (Human Development Index), education (using mean Years of Education), and inequality (Gini coefficient), all measured at country level.
GBD2019 will show a reduction in age-standardized prevalence of glaucoma blindness globally but an increase in moderate and severe vision impairment, over the past decade
Although not explained in the paper, DALYs are a composite of prevalence of vison-impairing glaucoma (derived from population-based studies of eye disease) and the disability weight associated with that severity of vision impairment. In this case, the authors have used glaucoma DALYs which are a combination of DALYs for glaucoma blindness (< 3/60) and glaucoma vision impairment (< 6/18 to 3/60). The authors report that the glaucoma burden (measured in total DALYs) had increased from 1990 to 2016. This had already been reported by the GBD [in DALYs]3 and the Vision Loss Expert Group [VLEG; in cases and crude prevalence rates]4 and would be expected given the ageing of populations. It is interesting that the increase in age-standardized DALYs (and correspondingly age-standardized prevalence) observed between 1990 and 2010 appears to have reversed in the last decade (2010- 2019). GBD 2019 (working with VLEG's more comprehensive Global Vision Database with more glaucoma data sources) will shortly report an 11% reduction in age-standardized DALY rate for glaucoma burden over the past decade.5 By concentrating on all DALYs due to glaucoma in this paper by Wu et al., one cannot disentangle the temporal change in differing severities of glaucomatous vision impairment. Interestingly, GBD2019 will show a reduction in age-standardized prevalence of glaucoma blindness globally but an increase in moderate and severe vision impairment, over the past decade.5
Despite more sources of population-based glaucoma data since 2016, the sparsity of data still precludes precise country-specific glaucoma vision impairment prevalence rates, which is why this is not displayed in Vision Atlas, the vision loss mapping tool hosted by the International Agency for Prevention of Blindness.6 In spite of this country-level uncertainty, the authors then tested for relationships between glaucoma DALY rate and the aforementioned three covariates. Given that these covariates are closely related to a factor called Socio-Economic Index (a composite of education, fertility rates, and economic status) that is actually used by the GBD's statistical model to calculate DALYs, this collinearity presents the considerable risk that the analyses and conclusions drawn concerning socioeconomic disparities in glaucoma burden will be distorted.