Rising to the challenge: estimates of the magnitude and causes of vision impairment and blindness

The year 2020 marked the end of the global initiative Vision 2020: The Right to Sight launched by WHO in 1999 to intensify activities for the prevention of blindness. The estimates of the global magnitude and causes of vision impairment and blindness presented by the GBD 2019 Blindness and Vision Impairment Collaborators in this issue of The Lancet Global Health^{1, 2} offer important insights into how actions and investments in the field of eye care have delivered on the goals of this initiative.

It is clear from the data that concerted efforts over the past decades have yielded considerable dividends, with a 27% reduction in the age-adjusted prevalence of blindness reported between 1990 and 2020. Despite this, the total number of individuals with both blindness and moderate and severe vision impairment has increased substantially (by 51% and 92%, respectively) during the same period. A key reason for this increase is that eye care services have been unable to keep pace with population ageing and growth, along with behavioural and lifestyle changes that have led to an increase in the number of eye conditions that cause vision impairment and blindness such as cataract, glaucoma, age-related macular degeneration, myopia, and diabetic retinopathy. It is also evident from the GBD Collaborators' estimates that significant inequalities in access remain between regions: the age-standardised prevalence of blindness in regions of western and eastern sub-Saharan Africa (11·1 and 10·7 per 1000, respectively) and southeast Asia (10 per 1000) are over five times higher than in all high-income regions (<2·0 per 1000).

The authors should be commended for their work, which makes a major contribution towards the understanding of the global epidemiology of vision impairment and blindness. These data will prove pivotal in increasing advocacy and offer important insights into the effectiveness of public health strategies and new treatments in the field. The take-home messages are clear: (i) uncorrected refractive error and unoperated cataract remain major items on the unfinished agenda of public health, and (ii) new strategies are needed to cater for the complex health system requirements associated with the rapid emergence of non-communicable chronic eye conditions, such as diabetic retinopathy, glaucoma, age-related macular degeneration, and complications of myopia.

Although a considerable amount of evidence on vision impairment exits, we should continue to strive to strengthen the type of data that is collected and reported in epidemiological surveys. First, and most importantly, a change is required in the way data on vision impairment are collected and reported. As recommended in WHO's recently published World Report on Vision (2019),³ the field must move away from reporting solely on the impairment of “presenting” visual acuity (ie, vision as measured with spectacles or contact lenses if worn to the assessment) to also measuring and reporting on the impairment of “uncorrected” visual acuity (ie, without spectacles or contact lenses). While the measure of presenting visual acuity is useful for estimating the unmet needs of refractive error correction, individuals with refractive errors have an ongoing need for eye care services and therefore information on both the unmet and the met needs are important to plan services effectively. To this end, the definitions of vision impairment within the 11th revision of the International Classification of Diseases will be updated in 2021.

Second, data on the magnitude and causes of vision impairment in younger populations need to be strengthened: most surveys have been undertaken in adults aged 50 years and over. Behavioural and lifestyle trends have led, and will continue to lead, to a significant increase in the number of children and working-aged adults with eye conditions, such as myopia (a subtype of refractive error) and diabetic retinopathy, that can cause vision impairment. As an example, the number of people globally with myopia is estimated to exceed 2 billion alone,⁴ with a large proportion of these cases occurring in childhood and adolescence.⁵ As a result, the estimated 86·1 million people aged 50 years and over with vision impairment due to uncorrected refractive error presented by the GBD Collaborators undoubtedly represents only a fraction of the total unmet need for refractive error services globally. Addressing the gap in data among younger populations will be important to effectively target the eye care needs of people at critical periods throughout the life course. To this end, opportunities should be taken to incorporate eye care modules within childhood and general health surveys.

As the eye care sector transitions from Vision 2020: The Right to Sight to new strategies in the coming years, there is a need to rise to the challenge of strengthening the type of data and information that is collected in epidemiological surveys. For these data to be more effectively used to drive policy decisions about eye care service planning, a progressive shift will be required towards the systematic collection of data on the total population eye care needs, both met and unmet.