Global Blindness: Defining visual impairment
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In order to understand the epidemiology of visual impairment we need to use an agreed definition of visual acuity. This will allow us to compare studies from different locations and also measure impact. In this presentation we look at the World Health Organization’s (WHO) classification of visual impairment, which will be used throughout the course. How can this classification be applied to your own work? © 2016 International Centre for Eye Health, London School of Hygiene & Tropical Medicine. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license. This video is part of the free online course: Global Blindness: Planning and Managing Eye Care Services. We encourage the use and adaptation of this resource for teaching and learning. Find more Open Educational Resources for eye care on our website at http://iceh.lshtm.ac.uk/oer
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Global Blindness: Defining visual impairment
- 1. Global Blindness: Planning and managing eye care services Defining visual impairment
- 2. Learning outcomes Describe the World Health Organization’s classification of visual impairment
- 3. Visual acuity is a measure of visual performance
- 4. Types of visual acuity chart • Snellen chart • Landolt C • E chart • LogMAR chart
- 5. LogMAR chart • proportional spacing • geometric progression • sans-serif letters • 5 letters to each line.
- 6. Visual acuity measurements can be converted
- 7. Definition of visual acuity WHO uses 4 levels of visual function: • Mild or normal vision • Moderate visual impairment • Severe visual impairment • Blindness
- 8. Definition of visual acuity WHO uses four levels of visual function: • Mild or normal vision • Moderate visual impairment • Severe visual impairment • Blindness Low vision
- 9. Definition of visual acuity WHO uses four levels of visual function: • Mild or normal vision • Moderate visual impairment • Severe visual impairment • Blindness Visual impairment
- 10. Measurement of visual acuity is based on distance vision or visual field assessment
- 11. WHO categories are based on: Vision of the better eye Presenting vision (not corrected)
- 13. In summary Visual acuity measurement is: • An objective measure of visual function • Used to categorise and define visual impairment
- 14. Summary Use the ICD-10 definition and categorisation of visual impairment to understand the epidemiology of visual impairment.
- 15. Written by Dr Daksha Patel © 2016 International Centre for Eye Health, London School of Hygiene & Tropical Medicine. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license. Taken from the free online course: Global Blindness: Planning and Managing Eye Care Services. We encourage the use and adaptation of this resource for teaching and learning. Find more Open Educational Resources for eye care on our website at http://iceh.lshtm.ac.uk/oer
Editor's Notes
- By the end of this presentation you will be able to describe the World Health Organization's classification of visual impairment. This classification is based on section H54 of the ICD-10, or the International Classification of Diseases (ICD). ICD-10 is the standard diagnostic tool for epidemiology, health management and clinical purposes. It was endorsed by the forty-third World Health Assembly in May 1990 and came into use in WHO Member States in 1994.
- Visual acuity is an objective quantitative measure of visual function, and is one of the first clinical examinations in eye care. It can be carried out in various settings and conditions using a range of standardised charts, distances and illumination. Image: Sarah Polack CC BY-NC 2.0 flic.kr/p/dWcpw5
- Various charts are used across the world. The most commonly used are: - Snellen chart - Landolt C - E chart - LogMAR chart The characters on the charts – known as optotypes - are arranged in a standardised way to test distance vision. Image: U.S. Pacific Fleet (Ted Wendel) CC BY-NC 2.0 https://flic.kr/p/ciuuiN
- The most recent arrangement is the LogMAR chart. This is based on proportional spacing, geometric progression, sans-serif letters and 5 letters to each line. Image: National Eye Institute CC BY-NC-SA flic.kr/p/cuGJMW
- Visual acuity measurements made using one chart can be converted to other chart measurements. For example, a visual acuity of 6/60 on a Snellen chart is 0.1 on a decimal scale and 1.0 on a logMar scale.
- In order to understand the epidemiology of visual impairment we need to use an agreed definition of visual acuity. This allows us to compare studies and also to measure impact. The World Health Organization has agreed on the use of four levels of visual function, as described in the ICD-10. These four levels are: Mild or no visual impairment Moderate visual impairment Severe visual impairment Blindness Each of these four levels has a clear quantitative measure.
- Moderate visual impairment and severe visual impairment are grouped under the term “low vision.”
- Low vision taken together with blindness represents all visual impairment.
- Measurement of visual acuity is based on distance vision or visual field assessment. A Snellen or equivalent chart is used at a distance of 6 m or 20 feet in a well illuminated environment. The patient reads across and down the chart as far as they can. Each eye is tested independently and recorded. Image: IKP Centre for Technologies in Public Health CC BY-NC 2.0 flic.kr/p/8wjiCq
- WHO categories are based on the visual acuity of the better eye and presenting vision. Pin hole is not used when using this method. Presenting visual acuity provides an understanding of refractive need and is important, particularly for epidemiological surveys and planning. Image: Ferdinand Ama CC BY-NC 2.0 https://flic.kr/p/9Gh5Ez
- The WHO’s 4 level of visual acuity are categorised and measured as follows: Category 0 is Mild or no visual impairment. In this category, vision is equal to or better than 6/18 using Snellen chart measurements Category 1 is Moderate visual impairment. In this category, vision is worse than 6/18 but equal to or better than 6/60 Category 2 is Severe visual impairment. In this category, vision is worse than 6/60 but equal to or better than 3/60 The final category, is category 3 - blindness. In this category, vision is worse than 3/60 It is important to recall that visual status categories are based on visual acuity measured in the better eye. Blindness is also used if the visual field is less than 10 degrees from a point of fixation
- In summary Visual acuity measurement is: An objective measure of visual function Used to categorise and define visual impairment in epidemiological research, public health planning and screening programmes
- Finally, by using the ICD-10’s standard definition and categorisation of visual impairment we are able to compare data and understand the epidemiology of visual impairment.