Technical Definitions

Peter Ackland - IAPB

GBM roundelThe International Classification of Diseases (ICD) is the standard classification tool for epidemiology, health management and clinical purposes. This includes surveys to asses specific health conditions in population groups. It is used to monitor the incidence and prevalence of diseases and other health problems, providing a picture of the general health situation of countries and populations. Currently version ICD10 is followed, and WHO is presently working to produce an updated version – ICD11.

ICD10 includes considerable detail about eye diseases and conditions. It also classifies the severity of visual impairment.

The full classification is quite detailed and includes categorisation of binocular and monocular visual impairment and also considers the extent of a person’s visual field. The full classification can be viewed here. For those less familiar with the ICD10 categorization, the important points to know and which are relevant to the data presented in the IAPB Vision Atlas are as follows:

Categorisation of visual impairment is based upon visual acuity, which is measured by a person’s ability to read the different sized letters in an eye chart such as the Snellen chart (Figure 1 below); other types of chart, including the Tumbling E chart, are also used. In different countries, acuity is expressed using different scales – as fractions or decimals and with a person standing either 6 metres or 20 feet from the chart. A comparison chart of the different notations is available here.

Figure 1: Representation of how the visual acuity scale relates to the letters on a Snellen chart

Updated on 9th Oct 2016
N.B. This diagram is for illustrative purposes only – an actual Snellen chart is larger than that shown and visual acuity should only be measured using a proper vision chart, viewed at the correct distance and with adequate lighting.

For simplicity, in the IAPB Vision Atlas we are presenting data using the metre scale as this is the scale used in the most recent WHO and VLEG prevalence estimates.

A visual acuity of 6/6 (also known as 20/20) is considered ‘normal’ eyesight, though healthy young people may have a binocular acuity superior to 6/6. As a person’s visual acuity gets worse, so they can only see the larger letters on a vision chart. Visual impairment is categorised as mild, moderate, severe or blind in ICD10. Figure 1 above outlines a simplified version of the full categorisation, and shows how this relates to the letters a person can see on a Snellen chart as well as how someone with the different acuity levels caused by a refractive error might view a typical scene. Two important points to note relating to this categorisation are:

Distance- and near-vision loss

ICD10 categorises visual impairment according to how well someone can see at a distance. It does not include categorisation of impairment relating to near-vision problems. This is a point of some controversy as so much of our work and social life depends on the ability to see clearly close up. It is hoped that ICD11 will include categorisation of visual impairment by near as well as distance vision.

Previous global data has reported distance vision loss only and the exclusion of near-vision loss has led to an under reporting of the true scale of vision loss. The latest VLEG data included in this IAPB Vision Atlas includes estimates for near-vision loss.

Presenting vision

ICD9 measured visual acuity as ‘best corrected’, i.e. a person’s visual acuity was measured after any vision loss due to refractive error (a major cause of declining visual acuity) had been corrected by provision of glasses of optimal prescription. This definition was not very relevant to the real life situation that many people living in the poorest communities face as they are frequently unable to access or afford glasses. Many other people may use glasses that are old and are no longer of optimal prescription. Accordingly ICD10 made the change to measure ‘presenting’ visual acuity i.e. in epidemiological studies the visual acuity of an individual would be measured based upon their life situation – if they did not possess a pair of glasses then their acuity would be measured without any correction, or alternatively measured with the glasses they actually used. This welcome change meant that more recent global data (including that shown in the IAPB Vision Atlas) includes people who have vision loss due to uncorrected or under-corrected refractive error.

Categorisation of visual impairment is based upon visual acuity, which is measured by a person’s ability to read the different-sized letters in an eye chart such as the Snellen chart

Vision Atlas Definitions
Updated on 9th Oct 2016

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