Prevalence and causes of visual impairment in a Brazilian population: The Botucatu Eye Study

In 2002, the World Health Organization (WHO) estimated that there were in excess of 161 million people worldwide with visual impairment (corrected VA < 6/18 in the better eye), including 37 million with blindness (corrected VA < 3/60 in the better eye) [1, 2]. If this definition is expanded to include uncorrected refractive error then it is estimated that 259 million people are visually impaired [3]. The burden of blindness is particularly severe in South-East Asia and India; however in countries such as Brazil, the prevalence of blindness is estimated to be 1.3% of those over the age of 50 years [2]. This represents not only a significant burden to those who are affected, but it also represents a large social and economic cost to the State [4].

There are a paucity of population-based data relating to the level of visual impairment and its causes within Brazil. It is therefore imperative that accurate ophthalmic epidemiological data from this region is collected in order to optimize the allocation of limited resources. There is some evidence from retrospective data that the leading causes of visual impairment in Brazil are uncorrected refractive error and cataract, closely followed by age-related macular degeneration and glaucoma [5].

This paper presents population-based information on the prevalence and causes of visual impairment in Botucatu, Brazil. Unlike many other studies into the prevalence of visual impairment, this study also included children. These data will be of assistance in planning for the delivery of eye health care and the implementation of programs to further reduce the burden of blinding disease by the year 2020.

Participants of European descent comprised 80.6% (95% CI 79.0–82.1) of the sample, followed by people of other races (14.1%; 95% CI 11.9–15.5), African-Brazilian (4.9%; 95% CI 4.1–5.8) and East Asian (0.4%; 95% CI 0.2–0.7) origin. Manual workers were the most common occupation group (25.6%; 95% CI 23.9–27.4), followed by students (22.5%; 95% CI 20.9–24.2), service workers (18.5%; 95% CI 17.0–20.1), professionals (13%; 95% CI 11.7–14.4), retirees (11.1%; 95% CI 9.9–12.4), and trades people (9.3%; 95% CI 8.2–10.5). The majority of the participants had no medical co-morbidities, (81.95%; 95% CI 80.4–83.4) the remainder of the sample suffered from 1 or more disease(s).

Blindness according to the WHO criteria was identified in 10 subjects and low vision in 32 subjects. The sampling design adjusted prevalence of WHO defined blindness was therefore, 0.4% (95% CI: 0.2–0.7) and low vision was 1.3% (95% CI: 0.9–1.7). The prevalence of presenting low vision was 5.2% (4.3–6.1) and the prevalence of presenting blindness was 2.2% (1.6–2.8). The age-specific prevalence of presenting and WHO-defined low vision, blindness, and visual impairment is presented in Additional File 1.

The causes for low vision, blindness and visual impairment are presented in Additional File 2. Refractive error (72.3%; 95% CI 63.8–79.8) and cataract (15.5% 95% CI 9.7–2.8) were the leading causes of presenting low vision. This was also true for presenting blindness where refractive error accounted for 66.7% (95% CI 52.5–78.9) and cataract led to 18.5% (9.3–31.4) of presenting blindness. The majority of WHO-defined low vision was ascribed to cataract (50.0%; 95% CI 31.9–68.1), and this was also true for WHO-defined blindness (50.0%; 95% CI 18.7–81.3).

There were no children with presenting low vision; however there was one child with presenting blindness due to high myopia that was correctable with spectacles. Again there were no children with WHO-defined blindness, but there was one child with WHO-defined low vision due to albinism.

Although there has been a considerable reduction in the infective causes of blindness, the global burden of blindness has not significanlty altered for over a decade [7, 8]. The prevalence of blindness has been perpetuated by a number of factors including: increasing life expectancy in developing countries and the consequent increase in cataract and glaucoma; and the maldistribution of ophthalmic health care. The prevalence of blindness in Brazil is not immune to these factors and retrospective data has previously demonstrated the leading causes of visual impairment to be refractive error and cataract, followed by age-related macular degeneration and glaucoma [5]. These results were determined from the "Cataract Free Zone" project that was simultaneously commenced in Brazil and Peru in 1986 [9]. This paper highlights more recent population-based data on visual impairment in Botucatu, Brazil.

The level of presenting visual impairment of 7.4% in this paper is reduced compared to that found in regions across Brazil between 1986 and 1995 [5]. It may be that the continued efforts of the programs from which these data were drawn have been pivotal in the further reduction of visual impairment. The cause profile of this visual impairment has; however, not altered to a great extent.

In this study, refractive errors and cataract contributed to over 85% of presenting visual impairment and over 60% of WHO-defined visual impairment. These findings echo those previously demonstrated by Arieta et al [5] and are also reflected in data on visual impairment from around the world [2], not only in developing nations, but also in developed countries [10]. The correction of refractive error alone would lead to a significant decrease in the rates of both low vision and blindness. The uncorrected VA was better than 20/30 in just over 50% of the sample. The proportion of individuals with VA better than 20/30 increased to over 70% with correction. It is important to note the high level of presenting visual impairment among younger people aged 20–29 years (7.9%) and 30–39 years (10.4%). This seems disproportionately high but when the best corrected data are examined this falls to 1.4% and 0.6% respectively. This highlights the significant burden of visual impairment due to refractive error that is present is this younger, productive age group.

Aside from refractive errors and cataract, other causes of presenting visual impairment were: age-related macular degeneration (4.9%); glaucoma (2.2%); macular toxoplasmosis (2.7%); other retinopathy, which included diabetic retinopathy, albinism, retinal detachment, macula hole, retinitis pigmentosa, central retinal vein occlusion and central serous retinopathy (2.7%), and; optic neuropathy (0.5%). WHO-defined visual impairment demonstrated a similar profile; however, the relative frequencies of age-related macular degeneration (7.1%); glaucoma (9.5%); macular toxoplasmosis (7.1%), and; retinopathy (9.5%) were higher. It is likely that as cataract and uncorrected refractive error become less frequent the relative frequencies of conditions such as age-related macular degeneration and glaucoma will become more prevalent.

It is encouraging to note that only one patient (0.04%) under the age of 18 years had WHO-defined visual impairment, which was due to albinism. There were; however, eleven children (0.44%) who had presenting visual impairment. Each of these cases was due to a refractive error. Table 1; however, demonstrates that the proportion of children under 10 years sampled was lower than the proportion of the children in this age group in the wider population. This may have led to an under-estimation of the level of visual impairment in this age group.

There is a significant effect of age upon visual acuity, whereby the odds of being blind increase by 3% for each year of increased age. (P = 0.002) This is an effect that has been demonstrated previously [2] and alludes to the need for efforts to be focussed on the ageing population, a population for whom it is of paramount importance to achieve their best vision in order to prevent the social and economic costs of visual impairment.

It was demonstrated that females had higher odds of presenting visual impairment than males (OR 1.5, p = 0.02); however there was no effect from general health or ethnic origin. Ethnic origin was not likely to be a factor in the present study due to the widespread mixture of races found in Brazil. However, American studies have shown differences between races where age-related macular degeneration has been more prominent in Caucasians; and cataract and glaucoma have been more significant in African-Americans [11, 12].

The effect of work-type on visual impairment found one significant result. It was demonstrated that people who were retired were 2.2 times more likely than professionals to develop visual impairment (P = 0.02). This is likely to reflect the higher prevalence of cataract and retinal disease such as age-related macular degeneration among this population, and again highlights the significant burden that visual impairment will have upon an ageing population.

There are a number of limitations to this study and several methodological issues that could be improved upon in future surveys. Firstly it is possible that individuals with visual problems could have been those more likely to participate in the research. This selection bias could have increased the number of cases and, consequently, overestimated the prevalence of disease in our sample compared to the population from which it was drawn. This is highlighted by the age distribution of the sample. Nearly 50% of the sample was aged over 40 years, whereas in the wider Botucatu population and in the municipality of Sao Paulo approximately 30% of the population was over 40 years (Table 1). More specifically it was also noted that whilst 35.2% of people from the Botucatu census data were aged between 1–19 years, the study sample had only 23.5% of participants in this age category. Similarly 17% of the sampled participants were aged 60 years or over, whereas only 11.2% of the people were in this age category according to census data. The older age of the sample may have lead to an over-estimation of visual impairment and could have artificially inflated the prevalence estimates biasing the observed significant increase in visual impairment with age. These errors in sampling could have been avoided by more accurately collecting the number and age of people within each of the households at the time of visiting the households. This would ensure that only those within the household attended and also afford the opportunity to follow up younger children or adults who may have been at school or work at the time of the examination. A mechanism should have been in place to guarantee that only members of the enumerated households attended for the examination and not neighbours or non-household older relatives who may have been enticed by the offer of ophthalmic assessment.

The methodology of this survey could have been further improved by using the LogMAR visual acuity chart. This has been recommended by the WHO and provides a more accurate assessment of visual acuity and better comparability among studies [13]. Comparability would also have been improved by the employment of a more standardized method to ascribe a primary cause of visual impairment. The causes of visual impairment identified by this study may have been slightly different if WHO definitions and algorithms had been used and glaucoma may have been under-diagnosed without the use of visual field testing. There was also an element of non-random selection introduced by selecting neighboring houses where the actual house occupants refused to participate. It would have been more appropriate to move to the next selected house. This, however; occurred on less than 5% of occasions.

This study demonstrates that non-corrected refractive errors were responsible for the majority of cases of blindness and low vision in this Brazilian population closely followed by cataract. A focus on the optical correction of refractive error and operative intervention in the case of cataract will significantly alleviate the burden of visual impairment in Botucatu, and perhaps wider Brazil. This will be facilitated by greater access to refraction services and a rejuvenation of government funding for the provision of cataract surgery.