The World-Wide Situation

The prevalence of visual impairment (VI) is the percentage of the general population who have the condition at a given point in time. Usually, the prevalence of VI in a selected but representative sample is determined, and then extrapolated to the entire population. If comparisons are to be made between countries, and over time, then a consistent definition of VI is required, and the World Health Organization (WHO) definitions have been discussed in Chapter 1 . These definitions, part of the 11th revision of the International Classification of Diseases ( ICD-11 ), have changed considerably over time, leading to significant increases in the reported prevalence. This means that comparisons to surveys in earlier years should be made with caution.

The priority for the WHO is to determine the Global Burden of Disease. VI is one of 291 different diseases and injuries ( ) which are investigated to establish the cost on the population in terms of the years of life lost (less relevant for VI than some other diseases) or years lived with a disability. These data can be used to advocate for resources to be allocated to the particular diseases by member states. Monitoring changes over time is necessary to judge the effectiveness of the healthcare infrastructure which has been put in place.

If all age groups in the population are not equally affected, this could be expressed in the form of an ‘age blindness burden’ (ABB). This can be found by dividing the proportion of the blind in the age group, by the proportion of the region’s population which is of the same age.

So,

If not all countries/geographical regions are equally affected, it is possible to express this difference in terms of a regional blindness burden (RBB), found by dividing the proportion of the total world blind in that region, by the proportion of the world population which inhabits that area.

So,

If the region has its ‘fair share’ of the world blind population—for example, it might have 10% of the world’s population and 10% of the world blind amongst that population—it would have an RBB of 1.0. A RBB >1.0 indicates that the region needs to prioritise blindness prevention and treatment when deciding how to divide limited resources. However, it is likely that each region has a different age profile within its population, so the ‘age adjusted’ prevalence could be used to make the regional comparison more realistic. This compares each population as if they had the same age profile.

Prevalence of Visual Impairment

The most recently published data suggest that the age-adjusted prevalence of moderate/severe VI and blindness globally is 4.34%, based on over 338 million cases ( ; ) ( Table 2.1 ) (It should be borne in mind that including mild impairment and uncorrected presbyopia increases this figure to 2.2 billion).

Table 2.1

Age-Adjusted Prevalence of VI and Blindness Globally in 2020

Reprinted with permission from Elsevier. Causes of blindness and vision impairment in 2020 and trends over 30 years, and prevalence of avoidable blindness in relation to VISION 2020: the Right to Sight: an analysis for the Global Burden of Disease Study. The Lancet Global Health, 9 (2), e144–e160.

	Blind	Moderate/Severe VI	Mild VI
Globally	0.53%	3.58%	3.2%
	43.3 million	295 million

 

VI , Visual impairment.

This can be broken down by geographical region ( ) to show the prevalence and regional burden of both blindness and moderate/severe VI ( Table 2.2 ). It can be seen that the regional differences for more severe loss are relatively small but would be much greater if mild impairment and uncorrected presbyopia were included. Rates of unaddressed near vision impairment are estimated to be greater than 80% in western, eastern and central sub-Saharan Africa, while comparative rates in high-income regions of North America, Australasia, Western Europe and Asia-Pacific are reported to be lower than 10% ( ; ).

Table 2.2

Number of People Blind and With Moderate/Severe VI, and Corresponding Share of Global Impairment, by Region, in 2010

From Pascolini, D., & Mariotti, S. P. (2012). Global estimates of visual impairment: 2010. British Journal of Ophthalmology , 96 (5), 614–618. https://doi.org/10.1136/bjophthalmol-2011-300539 .

 

(Left) The percentage of the global population (% Global), the percentage of the blind population (% Blind), and the percentage of the population with moderate and severe visual impairment (% MSVI), by world region, in 2010. (Right) A calculation of the Regional Blindness Burden (RBB) and Regional Moderate and Severe Visual Impairment Burden (RMSVIB) as described in the text. Based on data from Pascolini and Mariotti, 2012.

Fig. 2.1 shows the difference in prevalence by age, which is much more dramatic. As shown in Table 2.3 , throughout the world it is older adults within the population who bear the burden of significant VI, with the global prevalence rising from 10 in every 1000 children to 140 in every 1000 adults over 50 years of age.

The prevalence (%) of distance vision impairment (VI) by age in the global population in 2020.

Reprinted with permission from Elsevier. Trends in prevalence of blindness and distance and near vision impairment over 30 years: An analysis for the Global Burden of Disease Study. The Lancet Global Health , 9 (2), e130–e143; Reprinted with permission from Elsevier. Causes of blindness and vision impairment in 2020 and trends over 30 years, and prevalence of avoidable blindness in relation to VISION 2020: the Right to Sight: an analysis for the Global Burden of Disease Study. The Lancet Global Health , 9 (2), e144–e160.

Table 2.3

Age Distribution of People With Blindness and Moderate/Severe VI Within the Global Population, 2010

From Pascolini, D., & Mariotti, S. P. (2012). Global estimates of visual impairment: 2010. British Journal of Ophthalmology , 96 (5), 614–618. https://doi.org/10.1136/bjophthalmol-2011-300539 .

 

(Left) The percentage of the global population (% Global), the percentage of the blind population (% Blind), and the percentage of the population with moderate and severe visual impairment (% MSVI), by age group, in 2010. (Right) A calculation of the Age Blindness Burden (ABB) and the Age Moderate and Severe Visual Impairment Burden (AMSVIB) as described in the text. Based on data from Pascolini and Mariotti, 2012.

Causes of Visual Impairment

Many of the most common causes of moderate or severe distance vision impairment or blindness globally could be treated or prevented: these are uncorrected refractive error, cataract, glaucoma, corneal opacities, diabetic retinopathy, and trachoma.

A more detailed analysis ( ) shows the relative importance of the major causes of blindness and moderate/severe VI in the different economic/geographical regions in 2015 ( Table 2.4 ).

Table 2.4

Mean Estimated Percentage ^a of the Major Causes of Blindness (Upper Figure) and Moderate/Severe VI (Lower Figure), in Each Economic/Geographical Region in 2015

Reprinted with permission from Elsevier. Global causes of blindness and distance vision impairment 1990–2020: a systematic review and meta-analysis. The Lancet Global Health , 5 (12), e1221–e1234.

Region	Uncorrected Refractive Error	Cataract	Glaucoma	AMD	DR	Corneal Opacity	Trachoma
High-income Asia Pacific	13.13 49.36	20.32 14.66	13.51 3.6	16.66 11.60	3.87 4.07	2.38 0.79	0 0
Central Asia	12.85 48.26	25.94 18.11	14.17 4.05	14.01 10.05	3.60 4.06	3.58 1.41	0 0
East Asia	12.90 47.08	43.58 32.54	7.06 1.56	5.33 3.39	0.51 0.57	4.26 1.54	1.81 1.33
South Asia	36.43 66.39	36.58 23.62	5.81 1.09	1.44 1.31	0.16 0.15	2.43 0.74	0.04 0.03
Southeast Asia	12.57 46.14	45.0 33.95	6.99 1.57	5.24 3.46	0.59 0.71	4.39 1.63	0.13 0.07
Australasia	13.11 49.26	19.65 14.1	13.48 3.59	16.52 11.36	4.48 4.72	2.35 0.78	0 0
Caribbean	12.59 47.85	25.74 18.09	9.61 2.25	5.64 3.76	0.79 0.89	1.65 0.52	0 0
Central Europe	12.98 49.40	25.42 18.16	14.08 3.95	15.92 10.85	3.10 3.12	3.63 1.38	0 0
Eastern Europe	13.00 48.53	20.91 14.84	14.33 4.13	19.53 13.39	4.91 5.06	3.43 1.32	0 0
Western Europe	13.12 49.61	21.42 15.49	13.50 3.58	15.39 10.68	3.30 3.48	2.43 0.81	0 0
Andean Latin America	12.62 46.07	27.48 20.23	9.40 2.22	3.84 2.44	0.39 0.42	1.60 0.53	0 0
Central Latin America	12.71 47.90	23.97 17.03	9.99 2.34	6.19 3.96	0.95 1.02	1.62 0.52	0 0
Southern Latin America	13.00 48.20	21.68 15.95	13.61 3.71	14.34 10.11	3.51 3.93	2.41 0.82	0 0
Tropical Latin America	12.90 49.26	21.88 15.27	10.45 2.41	7.39 4.68	1.33 1.38	1.54 0.49	0 0
North Africa and Middle East	12.34 44.58	28.11 21.37	6.89 1.62	3.16 2.16	1.39 1.61	4.47 1.67	2.62 1.90
High-income North America	13.08 49.53	20.13 14.39	13.45 3.56	15.85 10.86	4.33 4.55	2.39 0.79	0 0
Oceania	12.61 47.76	47.29 34.60	6.72 1.43	3.08 2.07	0.31 0.34	4.24 1.55	0 0
Central sub-Saharan Africa	12.72 46.50	43.62 34.41	14.14 3.63	5.18 3.42	0.47 0.51	4.01 1.55	0.27 0.21
East sub-Saharan Africa	12.16 46.79	44.67 32.35	11.70 2.90	3.16 2.37	0.23 0.31	3.76 1.36	7.02 5.78
Southern sub-Saharan Africa	12.38 48.05	35.18 26.94	15.47 4.12	11.75 8.01	1.56 1.77	4.05 1.53	0.51 0.31
West sub-Saharan Africa	12.36 46.35	43.56 34.14	13.27 3.29	3.98 2.54	0.29 0.38	3.85 1.49	3.43 2.46
World	20.28 52.34	35.15 25.15	8.49 2.05	5.93 4.38	1.06 1.30	3.21 1.14	0.97 0.64

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