Skip main navigation

£199.99 £139.99 for one year of Unlimited learning. Offer ends on 28 February 2023 at 23:59 (UTC). T&Cs apply

Find out more

Defining visual impairment

Providing an agreed definition of visual impairment.
SPEAKER: By the end of this presentation, you should be able to describe the World Health Organization’s classification of visual impairment. This classification is based on Section H54 of the ICD-10, the International Classification of Diseases. ICD-10 is the standard diagnostic tool for epidemiology, health management, and clinical purposes. It was endorsed by the 43rd World Health Assembly in May 1990, and it came into use in WHO member states in 1994. Visual acuity is an objective, quantitative measure of visual function, and it 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.
Various charts are used across the world. The most commonly used are the Snellen chart, Landolt C, E chart, and the LogMAR chart. The characters on the charts, known as optotypes, are arranged in a standardised way to test distance vision. The most recent arrangements is a LogMAR chart. This is based on proportional spacing, geometric progression, Sans-serif letters, and five letters to each line. Visual acuity measurements made using one chart can be converted to other chart measurements. For example, a visually 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 will allow 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, and blindness. Each of these four levels has a clear, quantitative measure. Moderate visual impairment and severe visual impairment are grouped together under the term low vision. And when low vision is taken together with blindness, it 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 six metres 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. WHO categories are based on the visual acuity of the better eye in presenting vision. Pinhole is not used when using this method. Presenting visual acuity provides an understanding of refractive need, and is important, particularly for epidemiological surveys and for planning. The WHO’s four levels 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 vision 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’s important to remember that visual status categories are based on visual acuity as measured in the better eye. Blindness is also used if the visual field is less than 10 degrees from the point of fixation.
In summary, visual acuity management is an objective measure of visual function, and it is used to categorise and define vision impairment in epidemiological research, public health planning, and screening programmes. Finally, by using the ICD-10 standard definition and categorisation of visual impairment, we are able to compare data and understand the epidemiology of visual impairment.

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 step 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?

Visual acuity conversion chart

Throughout the course we use the Snellen (metre) measurement of visual acuity. In other contexts Decimal or LogMAR measurement systems are preferred. The chart shown below allows you to convert from one system to another.

Visual acuity conversion chart

This article is from the free online

Global Blindness: Planning and Managing Eye Care Services

Created by
FutureLearn - Learning For Life

Our purpose is to transform access to education.

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

Learn more about how FutureLearn is transforming access to education