Level of visual impairment	Visual acuity in better eye with optical correction (if worn)
Slight, if acuity less than 6/7.5	6/18 or better.
Visual impairment (VI)	Worse than 6/18 up to 6/60
Severe visual impairment (SVI)	Worse than 6/60 up to 3/60 (logMAR 1.1 to 1.3)
Blind (BL)	Worse than 3/60 (worse than logMAR 1.3) to no light perception
	or Visual field < 10 degrees around central fixation

MAR, minimum angle of resolution.

6/7.5 = logMAR 0.10, 20/25, 0.86/18 = logMAR 0.48, 20/60, 0.33

6/60 = logMAR 1.0, 20/200, 0.10

3/60 = logMAR 1.3, 20/400, 1.31.0

Note: Adapted with permission from World Health Organization (WHO). International Statistical Classification of Diseases and Health Related Problems. 10th Revision. Geneva, World Health Organisation, 1992.

Two measures are recognized in clinical practice and research:

1. Functional vision assesses the child’s ability to undertake tasks of daily living dependent on vision, such as navigating independently.

2. Vision-related quality of life elicits the child’s and/or parent’s view of the gap, caused by the visually impairing disorder and its therapy, between the child’s expectations and experiences in terms of his/her physical, emotional/psychological, cognitive, and social functioning.³ Interest in patient-reported outcomes and experience measures (PROMs and PREMs) coincides with the WHO International Classification of Functioning Disability and Health, a classification and measurement of health, and health-related domains which has underpinned the understanding of disability.⁴

Measuring the frequency and burden of childhood visual impairment

The analogy of running a bath (or filling a water trough) illustrates measures of frequency and burden of disease. The speed at which water runs into the bath equates with incidence, i.e. the rate of new occurrence of disease in a given population over a specified time. For example, in the UK the annual incidence of congenital cataract is 2.5 per 10 000 children aged 1 year or less.⁵

The degree to which the bath (or water trough) is full at a particular moment is a balance between how fast water is running in and how much is running out. How full the bath is equates to the prevalence of disease, i.e. the proportion of a given population that has the disease or condition of interest at a particular time. This reflects both the incidence of the disease and its duration, i.e. new cases of disease added to the population while others are “lost” from it through death, cure, or migration. For example, the UK prevalence in childhood of amblyopia with an acuity of worse than logMAR 0.3 (6/12, 20/40, 0.5) is about 1%.

The comparison of how a bath is “valued” more broadly, versus a shower or staying unwashed, might equate with measures of utility such as disability-adjusted life years (DALYs) or quality-adjusted life years (QALYs).⁶ These incorporate morbidity and mortality into a single measure used to compare states of health within and between countries to identify economic and other priorities in health-care provision. Throughout the world, blindness is categorized in the penultimate class of increasingly severe disability.⁶

Prevalence and incidence data provide complementary information. Incidence identifies and monitors trends which reflect changing exposure to risk factors, or emergence of new exposures and in provision of services and planning research, e.g. estimating likely recruitment time in clinical trials. Prevalence measures the magnitude of the problem in a community at a given time. It helps allocate resources and can be used to evaluate services, if changes in prevalence can be attributed solely to changes in outcome or duration of disease as a result of treatment rather than changes in underlying incidence.

Sources of information on frequency and causes of visual impairment

There are a number of sources of epidemiological information about childhood VI or blindness but, in reality, only a few are available in most countries. This explains the currently incomplete picture of VI (Box 1.3).

1. Population-based prevalence studies. These represent a source of precise, representative estimates of burden (frequency) and causes. However, the few studies of whole populations of children with VI, such as the British national birth cohort studies.^2,⁷ need to be very large (a study of 100 000 children is required in an industrialized country to identify 100 to 200 children with VI or blindness): costly and difficult!

2. Population-based incidence studies. Studies of incident (newly occurring) VI are even more difficult, explaining their rarity.¹¹

3. Special needs/disability registers, surveys, and surveillance. Specific studies and/or surveillance systems ⁸ or registers of childhood disability can provide information about VI, but it is important to recognize the potential for bias as certain visually impaired children may be over-represented in these sources, e.g. those with multiple impairment.

4. Studies of schools for the visually impaired. In developing countries, studies of children in special education provide information on causes, but these are biased because many blind children (particularly with additional non-ophthalmic impairments) may not have access to special education. With other facility-based studies, e.g. from clinic attendees, the intrepretation of findings and their extrapolation to other populations needs to take these biases into account.

5. Visual impairment registers. These exist in many industrialized countries but, if registration is voluntary and not a prerequisite for accessing special educational or social services, then registers may be incomplete as well as biased, reflecting differences in parental preferences and professionals’ practices regarding registration of eligible children.⁹

6. Visual impairment teams. Increasingly children in industrialized settings are evaluated by multidisciplinary teams and if these serve geographically defined populations then useful information can be derived.

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