Does blindness count? Disability weights for vision loss

How important is blindness? Is being blind 17% or 60% as bad as being dead? More importantly, why is there such disagreement? These numbers are from disability weights. They were introduced by the Global Burden of Disease (GBD) Study (1990) to give a new population health measure, the disability adjusted life year (DALY). DALYs aimed to capture a societal assessment of the burden of disease resulting from premature mortality and the non-fatal consequences of disease and injury. Their concern was for social justice and the association between the health states resulting from disease, and lost welfare, subjective well-being and quality of life. DALYs differed from quality-adjusted life years, which measure individual preferences for time spent in different health states. DALYs aimed to facilitate a more explicit and consistent comparison of health outcomes for health sector evaluation and resource allocation. DALYs are the sum of years of life lost due to premature mortality and years lived with disability. Calculation of the latter includes the disability weight – a number on a scale from 0 to 1.0. A weight close to 0 indicates a state of minimal impact, and a weight close to 1.0 indicates a state so severe its impact is almost as bad as death. Disability weights are obtained from ordinal measurement of preferences (paired health state comparisons). Advanced modelling transforms these data into weights. To date, eight studies have estimated disability weights for blindness (Table 1), using different approaches. These weights vary from 0.60 in the original GBD study to 0.19 in the 2010 GBD study. This threefold reduction in the recent GBD disability weight reduces the apparent importance of cataract blindness, questioning the validity of the disability weights. Applying the weights from the original and 2010 GBD studies gives very different estimates of the effectiveness of cataract surgery: in one study, from 2599 DALYs averted (disability weight 0.60 for blindness) to just 156 DALYs averted (disability weight 0.033 for moderate distance vision impairment). There are a number of possible explanations for the discrepancy. In our opinion, the most significant is the change from rating ‘disability’ to rating ‘health’. Health, as conceptualized by the World Health Organization, is a multidimensional construct, defined as ‘a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.’ In the original GBD Study, six weights captured ‘loss of well-being’, blindness was assigned a weight of 0.60. These were defined in reference to limitations in ability to perform activities of procreation, occupation, education and recreation or needing assistance with activities of daily living. Fundamentally, this is the measurement of disability. In contrast, the recent GBD Studies framed questions about ‘loss of health’. Although this resulted in only small changes to the disability weights for most disease states, for disabling conditions, including vision and hearing loss, the reduction was dramatic, attributed by some to the change in construct. This is not surprising; blind people often say, ‘I am not sick, I just can’t see!’ A second factor is variability in the description of different effects of the ‘disease’. The original GBD Study (1990) defined blindness as, ‘maximal visual acuity of less than 3/60 with the best possible correction’, resulting in ‘limited ability to perform most activities in all of the following areas: recreation, education, procreation or occupation’. The recent GBD studies defined blindness as, ‘completely blind, which causes great difficulty in some daily activities, worry and anxiety, and great difficulty going outside the home without assistance’ (Table 2). After criticism of some of the GBD 2010 disability weights, including those for vision loss, the GBD 2013 study tested a revised lay definition for some conditions. For example, the revised definition for deafness included a more explicit description of social isolation. When retested, the weight changed dramatically from 0.09 to 0.32, leading to the conclusion that, ‘in some cases, responses are evidently highly sensitive to particular details in these descriptions’. The definition for blindness was not modified in the GBD