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Epidemiology and visual impairment

Epidemiology and its specific application to visual impairment.
This presentation on the epidemiology of  visual impairment has four learning outcomes: Relate to the epidemiology of  blindness and visual impairment Describe the main causes of blindness globally Explore the trends of blindness  with socio-economic variations   globally and within a population Apply the global perspective to understand  the local burden of visual impairment
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? Control - What can we do about it? We will use these factors to determine  appropriate public health control measures. In 2020, 1.1 billion people are affected  by some form of visual impairment. An estimated 43 Million are blind and 596 million  people had distance vision impairment worldwide, of whom 295 million have a moderate to  severe level of distance visual impairment. Another 510 million people had  uncorrected near vision impairment, simply because of not having reading spectacles.
The Lancet Commission used  data from world bank regions. When we compare the distribution  of visual impairment across the   world we find that it is unevenly distributed. Globally, 91.75% of people who are  blind  and 87.68% of those with Moderate to Severe Visual Impairment live  in Low and Middle income countries. There are large interregional differences  in crude and age standardised prevalence.
Western sub-Saharan Africa has the  highest age-standardised prevalence of blindness 1.11% , which means  over 1 person in hundred is blind  ….Whereas  in high-income North  America has the lowest prevalence of blindness at 0.12%, approximately  1 in 1000 persons is blind.
The largest number of blind people  live in South Asia 11.9 million, followed by East Asia (9.1 million),  and southeast Asia (5.9 million), because of the large regional populations.  A similar pattern is seen for moderate to  severe visual impairment for distance vision.  In 2020, of the 43 million estimated  blind, over 33 million were aged 50 years and over, highlighting the  importance of age related blindness.  Data on visual impairment in children  and adults under the age of 40 is scarce. Measuring visual acuity in  young children is a challenge. Therefore the data is derived from a proxy  method using under-5 mortality since 1999.
Using the same method in 2020 it was estimated  that 1.02 million children (0-15 years) are blind, representing 4.8 children per 10,000 population.  The importance of childhood  blindness is not in the numbers affected 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 These trends are continuing—largely  driven by population ageing and changes in disease patterns.​
Globally, for every 100 men with blindness or MSVI   there are 108 women with blindness  and 112 women with MSVI.​ These imbalances are attributed to demographic  factors as women live longer then men.​ However, in some the settings the differences  are also due to socially determined factors as​ women often do not have equal  access to eye care services and are at a higher risk of blindness  from certain conditions, for example trachoma in endemic countries.​ ​  Gender inequities are an important consideration  when pursing universal health coverage. ​  
Vision loss has many causes that  require promotional, preventive,   treatment, and rehabilitative interventions. Globally, the main causes of Visual Impairment are Cataract, uncorrected refractive error, glaucoma, age related macular degeneration, and diabetic retinopathy . ​  ​The leading cause of blindness is ​ ​  cataract affecting 17 Million people  and  Uncorrected refractive error related blindness   affecting 3.70 million.  ​ ​  Notably, 37% of all blindness affecting over  16 million people are from other causes.
There are regional variation on  the main causes of blindness, affected by population demography as  well as access to eye health care.  Understanding these regional  variation in disease patterns allows health systems to prioritise service  provision and resource allocation.  In South Asia the main cause of blindness is  from Cataract and Uncorrected Refractive Error, where as in western Europe the  main causes of blindness are due   to Glaucoma and Age Related Macular Degeneration.  Globally, Cataract is the main treatable cause  of blindness as we have a relatively simple,   and effective, surgical treatment for it.
In 2020, the leading causes of Moderate  to severe visual impairment globally  were from uncorrected refractive error   affecting the distance vision  of over 157·49 million people.  Poor eye health and impaired vision have  a negative effect on quality of life   and restrict equitable access  to and achievement in education… …and the workplace. Vision loss has substantial financial  implications for affected individuals,   families, and communities. Visual Impairment from  refractive error is avoidable, by providing a pair of spectacles we have  simple and cost effective management.  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 3 billion people, in 2020 it was 7.8 billion and by 2060  it is predicted to be nearly 10 billion. A second risk factor is ageing populations. Numbers of people aged over 60 are increasing,  especially in low- and middle-income countries. 80% of older people will be living  in low and middle income countries. These two issues have huge  implications for age-related blindness and appropriate eye health service planning. 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 ensuring 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. 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. Health systems also need to be prepared to   appropriately manage cases  identified by screening. To address these conditions,   planning appropriate infrastructure  for service delivery is a key priority. 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 at the global and local levels. We know that 80% of visual  impairment is due to avoidable causes and that it is mostly found 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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