Chapter 50 Loss of Vision and Hearing Joseph Cook, Kevin D. Frick, Rob Baltussen, Serge Resnikoff, Andrew Smith, Jeffrey Mecaskey, and Peter Kilima Although the loss of vision and hearing has multiple causes, account uncorrected refractive errors, but this change has not and the burden of these diseases is complex, three major points yet been approved. emerge from the outset: The major causes of adult-onset blindness are cataract (47.8 percent), glaucoma (12.3 percent), macular degeneration • Impairments of the essential senses of vision and hearing (8.7 percent), diabetic retinopathy (4.8 percent), trachoma contribute to early demise and are important causes of mor- (3.6 percent), and onchocerciasis (0.8 percent). Uncorrected bidity for individuals who are blind or deaf. refractive errors are also a major cause of morbidity related to • Cost-effective interventions are available to address several vision, but this type of disability is not included in the global causes of these burdens now. burden of disease by definition. It has been estimated to be on • The number of cost-effectiveness analyses of interventions the order of 15 percent of the total blind population and could to preserve hearing or vision in developing countries is quite add 50 percent to the low-vision population. However, there limited. are no published data to do more than speculate. The major causes of childhood vision loss have marked Table 50.1 summarizes the conditions causing the sensory regional variations. They include vitamin A deficiency (xeroph- deficits, the proposed interventions and sites of delivery, and thalmia) and ophthalmia neonatorum in low-income coun- the cost and effectiveness of these interventions to the extent of tries, retinopathy of prematurity and hereditary conditions in current knowledge. Earlier work by Evans and others (1996) in middle-income countries, and congenital cataract and glau- Myanmar does not appear because the cost data are quite old coma everywhere. Table 50.3 shows the estimated number of and because the cost-effectiveness data were in dollars per case blind persons worldwide in 2002. of blindness averted rather than dollars per disability-adjusted Vision loss is chronic and, almost invariably, without remis- life year (DALY) averted, which the latest information provides. sion. The extent of morbidity is related to the level of alteration of vision function. However, 80 percent of cases are avoidable, either through treatment (cataract and refractive errors) or NATURE, CAUSES, AND EPIDEMIOLOGY through primary prevention (onchocerciasis, trachoma, glau- OF VISION LOSS coma, and diabetic retinopathy). Strictly speaking, blindness attributable to glaucoma and diabetic retinopathy can be pre- Table 50.2 provides definitions of visual impairment, vented. However, prevention depends on the availability of a blindness, and low vision according to the International simple, cheap, and efficacious diagnostic test and rigorous Classification of Diseases, Injuries, and Causes of Death (WHO treatment. These are not readily amenable to public health 1993). At this time, the World Health Organization (WHO) is programs even in the most technologically advanced countries, considering changing the classification in order to take into especially in the case of glaucoma. 953 Table 50.1 Cost and Effectiveness Data for Vision and Hearing Care Interventions Incremental cost- Cost data effectiveness data Condition Intervention (2004 US$) Effectiveness data (2004 US$/DALY averted) Trachoma Trichiasis surgery 7.14 per village-based surgerya 77 percent cure rate over two yearsb 4–82c Tetracycline — 51 percent cure rate in children at Ͼ9,600d six months following treatmentd Azithromycin — 88 percent cure rate in children at Ͼ4,100d six months following treatmente Cataract Extracapsular surgery — — Ͻ200 (low- and middle- income countries); Ͻ2,400 (high-income countries)b Onchocerciasis Ivermectin — — 40f Source: Authors. — ϭ not available. a. Frick, Keuffel, and Bowman 2001. b. Baltussen, Sylla, and Mariotti 2004. c. Baltussen and others (2005). Cost-effectiveness calculations are based on data from Frick and others (2001) for mass treatment of children only, not greater efficacy reported by Bowman and others (2000) and Solomon and others (2004) for mass treatment of entire communities. The greater efficacy reported in mass treatment of entire communities may lead to better cost-effectiveness. d. Bowman and others 2000. e. Reacher and others 1992. f. Waters, Rehwinkel, and Burnham 2004. Table 50.2 Definitions of Visual Impairment Levels Table 50.3 Number of Blind Worldwide in 2002 from Various Conditions Degree of Visual impairment impairment Definition categories Condition Number blind (millions) Low vision Visual acuity of less than 6/18 1 and 2 Cataract 17.6 (Snellen 20/70) but equal to or Glaucoma 4.5 better than 3/60 (20/400) Age-related macular degeneration 3.2 in the better eye with best possible correction Corneal opacity 1.9 Diabetic retinopathy 1.8 Blindness Visual acuity of less than 3/60 3, 4, and 5 (20/400) or corresponding visual Childhood blindness 1.4 field loss of less than Trachoma 1.3 10 degrees in the better eye with Onchocerciasis 0.3 best possible correction Other causes 4.8 Visual Blindness as well as low vision 1, 2, 3, 4, and 5 Total 36.8 impairment Sources: Pascolini and others 2004; Resnikoff and others 2004. Source: Authors, based on current international definitions by WHO 1993. Burden of Loss for Vision and Risk Factors (ROP) is an important cause in middle-income countries The risk factors for loss of vision are age, gender, poverty, and (Gilbert and Foster 2001). Unfortunately, screening for ROP in poor access to health care. The overall prevalence of vision loss, preterm infants, as well as the organization and provision of which mainly affects the population above age 40, is a function low-vision services, is a tertiary-level function (requiring a well- of age. It is estimated that more than 82.2 percent of all blind equipped clinic or hospital with the most modern facilities),and individuals are 50 or older. Increasing life expectancy results in no data on cost-effectiveness of interventions are available. a growing number of cases of age-related blindness (for exam- More disease-specific factors are poor hygiene, overcrowding, ple, cataract, glaucoma, macular degeneration). Among the 50 ultraviolet radiation, diabetes mellitus, drugs, micronutrient and older age group, cigarette smoking is a clear risk factor for deficiency, heredity and ethnic background, and consanguinity. both cataract and macular degeneration. Childhood vision loss Estimates of the global burden of visual impairment in 2002 represents approximately 4 percent of the total number of visu- were updated using the most recent available data on blindness ally impaired. However, it is the second largest cause of “blind- and low vision (Pascolini and others 2004). The global number person years,” following cataract. Retinopathy of prematurity of people who are visually impaired is in excess of 161 million, 954 | Disease Control Priorities in Developing Countries | Joseph Cook, Kevin D. Frick, Rob Baltussen, and others of whom 36.8 million are blind (Resnikoff and others 2004). Several population-based surveys reported higher risk of Because the international definition refers to the best-corrected mortality among people with visual impairment. Relative risk visual acuity (table 50.2), these figures actually underestimate of mortality among blind and low-vision people varied from the magnitude of the global burden of the visual impairment, 1.5 to 4.1 and from 1.1 to 1.6, respectively. In industrial coun- especially in developing countries, where most of the refractive tries, the relative risk of mortality varied from 1.6 to 2.0. The errors are not corrected (Dandona and Dandona 2003; Fotouhi effect may vary by gender (Lee and others 2002; Taylor and oth- and others 2004; Naidoo and others 2004). A WHO working ers 1991). The link between visual impairment and mortality group has recommended the use of the more accurate “pre- remains poorly understood and cannot be attributed to known senting vision,”recognizing that many people do not have their associations with underlying disease. best-corrected vision. This recommendation is under review The burden from visual impairment accounts for approxi- and, if approved, would substantially increase the estimates of mately 3 percent of the total global burden of disease and 9 per- the burden of disease attributable to impaired vision. cent of total years lived with disability in 2001. Table 50.4 shows The number of women with visual impairment, as estimated the global burden by vision-related cause in DALYs. Globally, from the available studies, is higher than that of men, even after half of the burden from visual impairment is due to cataract. adjustment for age. Female-to-male prevalence ratios indicate The burden of visual impairment is not distributed uni- that women are more likely to have a visual impairment than formly throughout the world; the least developed regions carry men in every region of the world: the ratios from past studies the largest share, as shown by World Bank region in table 50.5. range between 1.5 to 1 and 2.2 to 1. (Resnikoff and others Local and in-country variations, as well as regional variations, 2004). The major reported reason is women’s reduced access to are related to the following factors: eye care services. Higher exposure to risk factors also con- tributes in the case of trachoma. (Abou-Gareeb and others • Epidemiology of cause (for example, onchocerciasis, 2001; Nirmalan, Padmavathi, and Thulasiraj 2003). trachoma). Table 50.4 Global Burden of Visual Impairment, by Major Cause, 2002 Visual impairment Blindness Low vision Percentage Percentage (thousands (thousands Thousands of total of total Condition of DALYs) of DALYs) of DALYs YLDs DALYs Cataract 8,798 15,053 25,251 4.5 1.7 Glaucoma 1,202 2,442 3,866 0.7 0.3 Trachoma 1,403 772 2,329 0.4 0.2 Onchocerciasis 203 146 484 0.1 0.0 Other 4,657 8,814 14,191 2.5 1.0 Total 16,263 27,227 46,121 8.2 3.2 Source: Pascolini and others 2004; Resnikoff and others 2004.