A recent study analyzed population-based data to determine the global causes of blindness and visual impairment and made projections by cause to 2020.
Visual impairment and blindness are important global public health issues. According to World Health Organization (WHO) estimates, 36 million people worldwide are blind and another 217 million are visually impaired. Although the prevalence of visual impairment has declined since the 1990s, the number of people with vision impairment is increasing worldwide because of population growth and ageing. Interestingly, it is estimated that more than 80% of vision impairment and blindness are caused by preventable or curable conditions.
A recent study published in the Lancet Global Health reviewed and analyzed population-based data to determine the leading causes of blindness and vision impairment and used improved statistical analysis to make projections by cause to 2020. The researchers of the Vision Loss Expert Group used the updated Global Vision Database from 1990 to 2015. The systematic review included 3,983,541 participants in 288 studies from 98 countries.
The Global Causes of Blindness and Visual Impairment
Visual impairment is a decreased ability to see that cannot be corrected by usual means such as glasses. It is usually defined as visual acuity of worse than 6/18. The term blindness is used for complete or nearly complete vision loss. This study defined blindness as a vision of less than 3/60 in the better eye.
The leading global causes of blindness are cataracts, uncorrected refractive error, and glaucoma (damage to the optic nerve due to increased pressure in the eye), while the top three causes of visual impairment are an uncorrected refractive error, cataracts, and age-related macular degeneration.
Based on the findings of this review, 55% of blindness and 77% of vision impairment in adults over 50 years is caused by cataract and uncorrected refractive error. Glaucoma was found to be the cause of 8.5% of blindness and age-related macular degeneration for 4.4% of vision impairment.
Distribution of Blindness and Visual Impairment
Large disparities were observed between regions for the prevalence of cataract and uncorrected refractive error as the leading cause. In high-income regions, blindness due to cataract was less than 22%, whereas it contributed to more than 43% of blindness in East Asia, Southeast Asia, and most of Sub-Saharan Africa. It is not surprising, therefore, that more than 90% of world’s visually-impaired live in developing countries.
Females were found to be at a higher risk of visual impairment or blindness due to cataract, uncorrected refractive error, and diabetic retinopathy, than males. On the other hand, glaucoma and corneal opacity (scarring of the cornea) were the more common causes of blindness and visual impairment in men.
Blindness is a Preventable Global Problem
The WHO reports that 80% of all vision impairment and blindness is avoidable (vision impairment caused by a preventable or curable condition). The researchers estimated that 2.5% of the global population in 2010 had avoidable blindness due to cataract and uncorrected refractive error. This number is projected to increase to 2.7% by 2020.
There has been a marginal decrease in vision loss due to avoidable causes such as cataract, uncorrected refractive error, trachoma, glaucoma, diabetic nephropathy, and corneal opacity from 83% in 1990 to 81% in 2015 and it is projected to decrease to 80.8% by 2020.
A total of 217 million people of all ages were visually impaired in 2015. Of these, 78% had an impaired vision because of cataract or uncorrected refractive error which is an entirely treatable cause. Despite a decrease in the prevalence of blindness from 4.6% in 1990 to 3.4% in 2015, the population growth and ageing has led to an increase in the number of people with avoidable visual impairment and blindness. The review results suggest that this trend is going to continue up to 2020.
The study has some limitations. Definitions of blindness and vision impairment were different between studies and country level data was absent for many studies. In addition, most studies did not include age and ethnicity information. The main strengths of the study include the use of the updated Global Vision Database with 61 new studies, analysis of trends in the causes of vision impairment, and projections of the causes to 2020.
This important study has demonstrated that the most common global causes of blindness are avoidable. The findings of the study indicate the need for collaborative efforts at the global level for increased and improved eye care services. Raising community awareness about preventable blindness and monitoring the utilization of eye health care services will help to slow down the prevalence of avoidable vision impairment and blindness. Urgent action is required to address this largely preventative global health concern.
Written by Preeti Paul, MS Biochemistry
(1) Seth R Flaxman et al., Global causes of blindness and distance vision impairment 1990-2020: a systematic review and meta-analysis. Lancet Glob Health, October 2017.
(2) WHO. Vision impairment and blindness. Media centre, Fact sheet, October 2017, www.who.int/mediacentre/factsheets