Global Patterns of Blindness



Global Patterns of Blindness


Lalit Dandona1

Professor

Hannah Kuper2

Senior Lecturer

Rakhi Dandona3

Additional Professor


1Public Health Foundation of India, New Delhi, India; Professor, Global Health, Institute for Health Metrics and Evaluation, University of Washington, Seattle, Washington, USA

2International Centre for Eye Health, London School of Hygiene and Tropical Medicine, London, UK

3Public Health Foundation of India, New Delhi, India; Associate Professor, School of Public Health and George Institute for International Health, University of Sydney, Sydney, Australia


Correspondence to Lalit Dandona: lalit.dandona@phfi.org, dandona@u.washington.edu



Definition of Blindness

A key issue in any discussion of blindness is its definition. The elements of this definition that need attention are (i) level of distance visual acuity, (ii) presenting or best-corrected visual acuity, and (iii) visual field constriction. The two levels of distance visual acuity that have been commonly used worldwide to define blindness are <20/400 or <20/200 in the better eye. In the United States, legal blindness is defined as distance visual acuity ≤20/200. The International Statistical Classification of Diseases (ICD) defines blindness as best-corrected distance visual acuity <20/400 in the better eye.1 Recent data, however, suggest that this definition based on best-corrected visual acuity is excluding a substantial amount of blindness globally due to uncorrected refractive error.2,3,4,5 The World Health Organization (WHO) estimate of 37 million blind persons in the world in the year 2002 with the ICD definition of blindness increases by an additional 5 to 8 million blind persons if presenting (functional) distance visual acuity <20/400 in the better eye is used to define blindness instead, which can be improved with refractive correction.3,4,5,6 If the cutoff level of distance visual acuity to define blindness globally were reduced to <20/200 in the better eye, as is commonly used in high-income countries, and presenting visual acuity were used instead of best corrected acuity, the total number of blind persons in the world with this definition would be estimated as 57 million.6 With the increasing realization that the previous common use of best-corrected acuity to define blindness, recommended by the ICD, has resulted in overlooking refractive error as a significant cause of blindness and visual impairment, an increasing number of population-based studies are assessing presenting visual acuity followed by refraction and best-corrected acuity, thereby allowing assessment of blindness and visual impairment due to refractive error as well as other causes. Taking into account the increasing available evidence regarding the limitations of the ICD definition of blindness, which is based on the recommendations of a study group in 1972,1 a revision of this definition was proposed in early 2006.6 However, as of mid-2009, the ICD definition of blindness had not been revised.1

Because visual field constriction causes functional impairment, its inclusion in the definition of blindness is recommended by the ICD.1 However, because of the difficulty in assessing visual fields in large population-based surveys, many surveys have not included visual field constriction in the definition of blindness. On the other hand, some population-based studies have emphasized inclusion of visual field constriction to <10 or 20 degrees of fixation in the definition of blindness.7,8,9

The variations in the definition of blindness have to be taken into account when comparing data from different studies. This chapter discusses the patterns of blindness worldwide by geographic distribution, age, gender, and ethnicity, using published data.


Blindness by Geographic Distribution


Prevalence

The burden of blindness is not distributed uniformly in the different regions of the world. It is highest in the less developed regions (Table 53.1). This is largely due to the lack of availability and utilization of eye care services but is also influenced by variations in environmental factors predisposing to blindness, inherent susceptibility to blinding conditions, and lower socioeconomic status. The reported prevalence of blindness defined with visual acuity <20/400 in the better eye for all age groups in the population considered together vary in Africa between 3.2% in Equatorial Guinea,12 2.2% in the Central African Republic,13 1.7% in Mali,14 1.2% in Nigeria,15 1.0% in South Africa,16 0.9% in Ethiopia,17 0.8% in the Cape Verde Islands,18 and 0.4% in the Gambia.19 These prevalence estimates in the Indian subcontinent have been reported as 1.8% in Pakistan20 and 1.3% in India.21 In other parts of the world, these prevalence estimates have been reported in Asia as 0.4% in China22 and 0.3% in Malaysia23; in the Eastern Mediterranean region as 1.1% in Oman,24 0.6% in Lebanon,25 and 0.4% in Turkey26; and in Eastern Europe as 0.5% in Bulgaria.27








Table 1. Estimated Burden of Blindness in Different Regions of the World in 2008


































































































































GBD Subregiona Countries Populationb (millions) Number Blindc (millions) Prevalence of Blindness (%)
Africa region
AFRO D Algeria, Angola, Benin, Burkina Faso, Cameroon, Cape Verde, Chad, Comoros, Djibouti, Equatorial Guinea, Gabon, Gambia, Ghana, Guinea, Guinea-Bissau, Liberia, Madagascar, Mali, Mauritania, Mauritius, Niger, Nigeria, Sao Tome and Principe, Senegal, Seychelles, Sierra Leone, Somalia, Sudan, Togo 407 4.4 1.1
AFRO E Botswana, Burundi, Central African Republic, Congo, Côte d’Ivoire, Democratic Republic of the Congo, Eritrea, Ethiopia, Kenya, Lesotho, Malawi, Mozambique, Namibia, Rwanda, South Africa, Swaziland, Uganda, United Republic of Tanzania, Zambia, Zimbabwe 416 4.6 1.1
Americas region
AMRO A Canada, United States of America 341 0.5 0.1
AMRO B Antigua and Barbuda, Argentina, Bahamas, Barbados, Belize, Brazil, Chile, Colombia, Costa Rica, Cuba, Dominica, Dominican Republic, El Salvador, Grenada, Guyana, Honduras, Jamaica, Mexico, Panama, Paraguay, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Suriname, Trinidad and Tobago, Uruguay, Venezuela 492 1.7 0.3
AMRO D Bolivia, Ecuador, Guatemala, Haiti, Nicaragua, Peru 80 0.4 0.5
Eastern Mediterranean region
EMRO B Bahrain, Cyprus, Gaza, Iran (Islamic Republic of), Jordan, Kuwait, Lebanon, Libyan Arab Jamahiriya, Oman, Qatar, Saudi Arabia, Syrian Arab Republic, Tunisia, United Arab Emirates, West Bank 158 1.1 0.7
EMRO D Egypt, Iraq, Morocco, Yemen 167 1.5 0.9
Europe region
EURO A Andorra, Austria, Belgium, Croatia, Czech Republic, Denmark, Finland, France, Germany, Greece, Iceland, Ireland, Israel, Italy, Luxembourg, Malta, Monaco, the Netherlands, Norway, Portugal, San Marino, Slovenia, Spain, Sweden, Switzerland, United Kingdom 423 0.6 0.1
EURO B1 Albania, Bosnia and Herzegovina, Bulgaria, Georgia, Montenegro, Poland, Romania, Serbia, Slovakia, the Former Yugoslav Republic of Macedonia, Turkey 172 0.7 0.4
EURO B2 Armenia, Azerbaijan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan 57 0.2 0.4
EURO C Belarus, Estonia, Hungary, Kazakhstan, Latvia, Lithuania, Republic of Moldova, Russian Federation, Ukraine 233 1.2 0.5
Southeast Asia region
SEARO B Brunei Darussalam, Indonesia, Malaysia, Philippines, Singapore, Sri Lanka, Thailand 447 2.5 0.6
SEARO D Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan 1,533 13.8 0.9
Western Pacific region
WPRO A Australia, Japan, New Zealand, Republic of Korea 175 0.2 0.1
WPRO B1 China, Democratic People’s Republic of Korea, Mongolia 1,413 9.1 0.6
WPRO B2 (D) Cambodia, Lao People’s Democratic Republic, Myanmar, Vietnam 154 1.4 0.9
WPRO B3 (D) Cook Islands, Fiji, Kiribati, Marshall Islands, Micronesia (Federated States of), Nauru, Niue, Palau, Papua New Guinea, Samoa, Solomon Islands, Timor-Leste, Tonga, Tuvalu, Vanuatu 9 <0.1 0.6
Total   6,677 44.0 0.7
aGBD stands for the Global Burden of Disease project; the subregions are adapted from this project.10 The letter with each subregion indicates mortality stratum: A is very low child mortality and low adult mortality; B is low child mortality and low adult mortality; C is low child mortality and high adult mortality; D is high child mortality and high adult mortality; and E is high child mortality and very high adult mortality. This classification aims at maximizing the epidemiologic homogeneity of subregions. EURO B and WPRO B are subdivided further to capture epidemiologic differences. The original GBD classification divides WPRO B into B1, B2 and B3, but because several countries in the B2 and B3 groupings have a high child mortality, these could be classified as D, which we show in parentheses in the table.
bPopulation for 2008 from the U.S. Census Bureau International Data Base.11
cBlindness defined as presenting visual acuity <20/400 in the better eye; the number of persons with blindness estimated by the authors based on various sources and using assumptions; it is important to note that these point estimates of blindness are indicative only and would have a plausibility range around them.

The prevalence of blindness varies within a country and is often higher in rural areas.12,20,21,22,25,26,27,28,29,30,31 In the same area, those belonging to lower socioeconomic strata have been reported to have significantly higher prevalence of blindness,9,21,28,29,32,33 with the odds of being blind almost ten times higher in the lowest socioeconomic stratum as compared with the highest stratum in an Indian population.21 Disadvantaged groups such as refugees have also been reported to have a particularly high prevalence of blindness.34

The prevalence estimates of blindness are much lower in high-income countries; therefore, population-based studies are mostly undertaken for the middle- and old-age populations, as blindness in the younger age groups is rare. For example, blindness prevalence with visual acuity <20/400 in the better eye for the population 40 or more years of age has been reported as 0.18% in Australia35 and 0.14% in Japan,36 0.5% in Singapore37 and 0.2% in United States,38 and as 0.5% in the Netherlands for the population 55 or more years of age.39 In contrast, blindness defined with the same acuity level has been reported as 4.5% in the population 40 or more years of age in India.21 This is 25 times higher than the prevalence of blindness in the similar age group in Australia.35 Some underserved communities within the developed countries, however, have relatively high blindness prevalence, examples of which include an American Appalachian community40 and an Australian Aboriginal population.41


Causes

Cataract is the most common cause of blindness globally, accounting for about 40% of total blindness. Uncorrected refractive error is the second most common cause of blindness globally, accounting for about 14% to 18% of total blindness.3,4 These causes of blindness are common in low- and middle-income countries but are relatively rare in high-income countries (Tables 53.2 and 53.3). Because blindness due to cataract usually manifests in old age (Fig. 53.1) and that due to refractive error usually occurs at an early age (Fig. 53.2), it has been estimated that the number of blind-person-years suffered due to all refractive error blindness in an Indian population is about twice as many as due to all cataract blindness.21,42 Of all blind people in low- and middle-income countries, about two thirds of cases are due to cataract or refractive error, which is relatively easily treatable, yet this blindness persists because the logistics of providing treatment to those who need it most have not been put into place.








Table 2. Causes of Blindness from Some Population-based Studies of All Age Groups in Low- and Middle-Income Countries































































































Cause of Blindness Percent Prevalence of Blindnessa in All Ages Considered Together (%)
Country (Publication year) Mali14 (1996) Ethiopia17 (1997) Nigeria15 (2004) India21 (2001) Lebanon25 (1997) Oman24 (2002)
Sample size 5,871 7,423 3,204 10,293 10,148 11,417
Cataract 1.17 0.40 0.56 0.61 0.25 0.34
Refractive errorb — 0.07 0.25 0.20 0.08 0.03
Trachoma 0.21 0.18 — — — 0.26
Other corneal opacityc 0.12 0.04 0.02 0.12 0.05 0.10
Glaucomad 0.14 0.08 0.19 0.12 0.02 0.14
ARMDe — — 0.03 0.02 0.02 0.03
Cataract surgery complicationsf NA NA NA 0.05 NA NA
Others 0.06 0.08 0.15 0.22 0.18 0.20
Total 1.70 0.85 1.20 1.34 0.60 1.10
ARMD, age-related macular degeneration; NA, data not available.
aBlindness defined as distance visual acuity <20/400 in the better eye. The studies from Mali, Ethiopia, Nigeria, and India were each done in a particular part of the country; therefore, these blindness data may not be representative of the entire country. The studies from Lebanon and Oman were national surveys. The selection of studies in this table is illustrative of the causes of blindness in all ages considered together and does not include all surveys reported in recent years.
Presenting or best-corrected visual acuity: All studies used presenting visual acuity to define blindness with exception of the study from Mali, which does not mention the use of presenting visual acuity and does not report blindness due to refractive error. Therefore, it could be assumed that it used best-corrected visual acuity to define blindness.
Visual field constriction: The definition of blindness in the study from India included visual field constriction of <10 degrees assessed with automated perimetry; 7.8% of the blindness in this study was due to visual field constriction. The other studies in this table did not assess visual fields.
bRefractive error blindness includes aphakia. Refractive error blindness reported in the study from India includes 0.05% prevalence of blindness due to refractive error–related amblyopia.
cOther corneal opacities mainly due to vitamin A deficiency, infectious keratitis, and the use of harmful traditional eye medicines.
dThe prevalence of blindness due to glaucoma in many of these studies is likely to be an underestimate because thorough evaluation for glaucoma, including a detailed dilated retinal examination, is mentioned only in the studies from India and Oman and visual field assessment is mentioned only in the study from India. The blindness due to glaucoma in the study from India includes 0.07% prevalence due to primary open-angle glaucoma, 0.04% prevalence due to primary angle-closure glaucoma, and 0.01% prevalence due to secondary glaucomas; the other studies do not distinguish between the types of glaucoma causing blindness.
eARMD could have been underestimated in the studies that did not include a dilated retinal examination.
fThe prevalence of blindness in the population due to complications of cataract surgery is specifically mentioned only in the study from India. It is likely that this cause of blindness is distributed in some other cause categories in the other studies in this table.








Table 3. Causes of Blindness from Some Population-based Studies of Middle and Old Age Populations in High-Income Countries






























































Cause of Blindness Prevalence of Blindnessa (%)
Country (Publication year) The Netherlands39 (1998) Australia35 (2001) United States38 (2006)
Age group 55 years or more 40 years or more 40 years or more
Sample size 6,775 4,744 6,122
ARMD 0.27 0.07 0.05
Glaucomab 0.04 0.04 —
Diabetic retinopathyc — — 0.02
Other retinal 0.05 0.02 0.05
Cataract 0.03 — —
Refractive error NA 0.02 NA
Others 0.08 0.03 0.08
Total 0.47 0.18 0.20
ARMD, age-related macular degeneration; NA, data not available.
aBlindness defined as distance visual acuity <20/400 in the better eye. The blindness data in the study from the Netherlands was on an older population, for which a relatively higher prevalence would be expected. Therefore, this is not directly comparable with the other two studies. All three studies were in populations of large cities. The study from the United States was on a Latino population; thus, these blindness data may not be representative of the entire country. Due to the relatively low prevalence of blindness in these studies, the confidence intervals around the estimates for the causes of blindness would be wide with the sample sizes in these studies. The selection of studies in this table is illustrative of the causes of blindness in middle and old age populations in high-income countries and does not include all surveys reported in recent years.
Presenting or best-corrected visual acuity: The study from Australia used presenting visual acuity to define blindness, but the studies from the Netherlands and the United States used best-corrected visual acuity.
Visual field constriction: The definition of blindness in the study from Australia included visual field constriction of <5 degrees assessed with automated perimetry. The studies from the Netherlands and the United States did not include visual field constriction in the definition of blindness.
bPrimary open-angle glaucoma was responsible for the majority of glaucoma blindness in the Australian study and all of the glaucoma blindness in the Netherlands study.
cDiabetic retinopathy as a cause of blindness was reported only in the study from the United States.






Figure 53.1. A typical bilateral dense cataract, which commonly leads to blindness in many low- and middle-income countries.






Figure 53.2. This child from India is blind without spectacles because of high myopia but has excellent vision with spectacles.

Data have emerged from surveys in many Asian and African countries indicating that poor outcome after surgery is not uncommon, with as many as 20% to 40% of eyes having vision worse than 20/200 following surgery.43,44,45,46,47,48,49,50,51,52 This means that poor-quality cataract surgery may in itself be an important cause of blindness. There is a need for much more attention to the quality of cataract surgery and postoperative care if this iatrogenic blindness is to be reduced.

Glaucoma makes up about 10% of global blindness.4 The absolute prevalence of blindness from glaucoma is substantially higher in low-income countries than in high-income countries, which is partly a consequence of the lack of strategies in place for early detection and treatment of glaucoma,53 but predisposition toward glaucoma also varies markedly geographically. Primary open-angle glaucoma is most prevalent in Africa and among people of African origin53; consequently, the prevalence of blindness due to primary open-angle glaucoma is higher in populations of African origin.54 Primary angle-closure glaucoma is largely confined to Asia, with China containing approximately half of the world’s cases and India one quarter,53 leading to a higher prevalence of blindness due to primary angle-closure glaucoma in people of Chinese or Indian origin.55,56

Trachoma and other corneal opacities are estimated to cause about 7% to 8% of global blindness.4 Blindness due to trachoma is found predominantly in sub-Saharan Africa, but there are also regions affected in Asia and pockets of disease in South America and Australia.57,58 Trachoma is primarily found in the dry and poorer parts of the world, and the availability of water and latrines and health awareness could interrupt the transmission of the causative agent Chlamydia trachomatis.59 Within countries, trachoma is found in the poorer parts, and even within an area it may be most common among people in the lowest socioeconomic strata. Other frequent reasons for preventable corneal blindness in low-income countries include vitamin A deficiency in childhood, other infectious keratitis including that due to measles and ophthalmia neonatorum, and the use of harmful traditional eye medicines.21,60,61,62 Another cause of infectious blindness onchocerciasis (river blindness), which makes up less than a percent of global blindness, is predominantly found in sub-Saharan Africa6 and is largely confined to Central and West Africa where the black fly breeds near fast-flowing water. The majority of these infectious/corneal causes of blindness are preventable.

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Jul 11, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Global Patterns of Blindness

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