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Epidemiology and visual impairment

Epidemiology and its specific application to visual impairment.
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SPEAKER: This presentation on the epidemiology of vision impairment has four learning outcomes– Relate to the epidemiology of blindness and vision impairment. Describe the main causes of blindness globally. Explore the trends of blindness with socio-economic variations both globally and within a population. And apply the global perspective to understand the local burden of visual impairment.
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We will look at four aspects of the epidemiology of visual impairment. Magnitude, how many are affected, distribution, where is it, determinants, what are the main causes, and control, what can we do about it? We will use these factors to determine appropriate public health control measures. The magnitude of visual impairment worldwide is currently estimated to be about 285 million people. 39 million people are blind, and 246 million have low vision. When we compare the distribution of visual impairment across the world, we find that it is unevenly distributed with 90% of it in low- and middle-income countries. The highest prevalence of visual impairment between 3.34 and 5.61% is found in India, China, the Middle East, and parts of Southeast Asia.
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Most of sub-Saharan Africa has a prevalence of between 3.18 and 3.33%. The lowest prevalence of visual impairment is found in higher-income countries and in Latin America. Prevalence data is important. It allows health workers to identify the number of blind and visually impaired people in their country or region. The burden of blindness varies by region. This graph shows the numbers of blind people in each World Health Organization region.
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Africa, which has about 11% of the world’s population, has a disproportionately high number of blind people, about six million, compared to China which has 20% of the world’s population and about 8 million blind people. The distribution of visual impairment is affected by age. Globally, 82% of all blind people are aged 50 years and over. The importance of childhood blindness here is not in the numbers effected, but in the number of years a child has to live with that blindness. This is sometimes referred to as blind years. Distribution of blindness is affected by gender. Most surveys around the world have found more blind women than men.
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A study in 2001 estimated the gender distribution of global blindness is 64% women and 36% men. The main reasons for this are there are more women than men over the age of 50 in most populations. Women often do not have equal access to eye care services, and women are at a higher risk of blindness from certain conditions, for example trachoma. Causes of visual impairment. Global data has identified that refractive errors and cataract cause over 75% of all visual impairment in the world. These two diseases occur across all populations, and are closely linked with ageing. Other causes of visual impairment, such as trachoma, corneal opacities, and childhood blindness, occur among specific at-risk populations.
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Diabetic retinopathy, glaucoma, and age-related macular degeneration are diseases that are on the increase globally. Finally, it is difficult to identify a single main cause for about 18% of visual impairment.
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Causes of blindness. Globally, cataract is the main treatable cause of blindness as we have a relatively simple and effective surgical treatment for it. Risk of blindness is closely linked to poverty. Very poor individuals and communities are at the highest risk of blindness. Poor communities are also more at risk from treatable and preventable causes of blindness. To address this issue, it is important to ensure that eye care services are accessible by all. Population related risk factors are likely to increase global visual impairment in the future. One risk factor is increasing global population growth. In 1960, the global population was approximately three billion people. In 2010, it was 6.9 billion, and by 2060, it is predicted to be nearly 10 billion.
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A second risk factor is ageing populations. Numbers of people aged over 60 are increasing and especially in low and middle-income countries. These factors have huge implications for age-related blindness and appropriate service delivery growth needs to be in place.
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Epidemiological data is used to select appropriate control measures for eye disease. Cataract and refractive errors are global diseases, and cause 75% of all avoidable visual impairment. There are clear treatment strategies for these two diseases, which can be implemented at the local level alongside practical measures to strengthen eye care services and ensure access for all. Corneal blindness, trachoma, and childhood blindness are considered focal diseases, because they affect vulnerable high-risk groups of people. There are methods in place to treat, and more importantly, to prevent many of these conditions. The challenge here is to find the cases and implement services at a local level. Understanding local needs at the community level is important for developing strong prevention strategies.
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Diabetic retinopathy and glaucoma fall into the chronic diseases category. Patients need to be detected early and treated to prevent visual impairment. Screening strategies are needed to find the patients as early as possible, and health systems also need to be prepared to appropriately manage cases identified by screening.
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Implementing these control measures can avoid about 80% of visual impairment. In summary, epidemiology allows eye health workers to understand the burden of need, and the main causes and determinants of visual impairment of the global and local levels. We know that 80% of visual impairment is due to avoidable causes. And that it is mostly find in low- and middle-income countries. Strengthening services, improving access, and addressing inequity at the local level are an urgent priority for global eye care. Prevention programmes are essential to achieve these goals, and reduce the burden of visual impairment in the world.
World Health Organization data from 2010 estimated that approximately 285 million people across the world had some form of visual impairment (Pascolini & Mariotti, 2011). In 2017, a new review of the data by the Vision Loss Expert Group and published in The Lancet Global Health estimated that there were 253 million people with visual impairment in 2015 (Bourne et al. 2017).
90% of visually impaired people are in low and middle-income countries. Strengthening services, improving access, and addressing inequity at the local level are an urgent priority for global eye care. Prevention programmes are essential to achieve these goals and reduce the burden of visual impairment across the world.
In this step we use the 2010 data to introduce the epidemiology of blindness and visual impairment, looking at four key concepts:
  • Magnitude – how many people are affected?
  • Distribution – where is it?
  • Determinants – what are the main causes?
  • Control – what can we do about it?
Try to consider the main causes of visual impairment in your own community and the control strategies that local eye care programmes carry out.
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Global Blindness: Planning and Managing Eye Care Services

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