Epidemiology of Pediatric Cataracts and Associated Blindness
Rupal H. Trivedi
M. Edward Wilson
Cataract is numerically the largest cause of preventable and treatable visual handicap in childhood.1 Children who are born blind or who become blind after birth have a lifetime of blindness ahead of them that includes all the associated socioeconomic impacts to the child, the family, and the society. Visual defect in childhood is numerically small when compared with blindness at other ages, but since the survivors are likely to live for a long time, when expressed in blindness-years, child blindness becomes very significant. The control of blindness in children is one of the main priorities of Vision 2020: The Right to Sight, the global initiative of the World Health Organization (WHO) and the International Agency for the Prevention of Blindness.2
The major preventable causes of blindness in children (e.g., those causing corneal opacification) are declining in poor countries as a result of large-scale public health interventions, and cataract is becoming a relatively more important avoidable cause.3 Using a standardized classification and coding system, Gilbert et al. evaluated 9,293 children in 40 countries and reported that the lens is responsible for 12% (range, 7%-20%) of anatomical abnormalities in children. Over 200,000 children are blind from disorders of the lens, principally due to unoperated cataract, but also as a result of dense amblyopia following delayed surgery, complications of surgery, or from associated ocular abnormalities.3 Because of the high incidence and treatable nature of the condition, it is reasonable to think that an improved approach to the management of childhood cataracts would have a large impact on childhood blindness as a whole. In this chapter, we describe the epidemiology of childhood cataracts and the blindness associated with them.
PREVALENCE AND INCIDENCE
Prevalence can be defined as the total number of cases of a disease in a given population at a specific time, while the incidence is the number of new cases that develop in a population during a specified time interval. Several articles have been published that attempt to quantify blindness from childhood cataract. Such studies either describe the frequency of lens opacity irrespective of its impact on vision or report the prevalence of blindness due to lens opacity. The reliability of these studies and their usefulness for comparison are uncertain for various reasons, which include the following:
Noncataractous reasons for blindness. Most of the studies define blindness as being caused by pediatric cataracts. However, besides cataracts, there are many conditions that can prevent the achievement of normal visual acuity (e.g., amblyopia, secondary opacification of the visual axis, residual refractive error). It should also be noted that lesser degrees of visual loss, visual field defects, or unilateral blindness could also result in significant visual disability in children, which has not been accounted for in most studies.
Varying standards between studies. Standards for “visual impairment” and “age at childhood cataract” vary between studies. This makes comparison of data difficult. WHO defines the visually impaired child as having a corrected visual acuity of <20/60 in the better eye, severe visual impairment as having a corrected visual acuity of <20/200, and blindness as having a corrected visual acuity of <20/400. Gilbert et al.4 tried to improve the overall collection of data by publishing standardized severe visual impairment forms for childhood data collection. Widespread use of these standardized forms will hopefully lead to more meaningful data collection that will allow comparison between studies.
Different methods for collecting data. Registration data, population surveys, and studies of children in schools for the blind have been found to vary widely in the methods used for data collection. Differences may underestimate or overestimate the severity of disease. Comparisons between these studies are therefore unreliable.
Difficulty in assessing childhood visual function. Visual function can be difficult to assess accurately in children even under ideal conditions. This can lead to variations between examinations of the same children and unreliable visual function reporting.
Childhood Cataracts
The prevalence of childhood cataracts has been reported as 1 to15 per 10,000 children.5 The wide range is because of the reasons described above: variety of methods, different age groups, and varying case definitions used in the studies, as well as true differences between populations.5 Birth prevalence of congenital bilateral cataracts in industrialized countries is 1 to 3 per 10,000 children. Foster, Gilbert and Rahi have calculated that approximately 4 children per million total population per year will be born with bilateral cataracts in industrialized countries, and the figure from developing countries is likely to be 10 children per million total population per year.5 The birth defect monitoring program (BDMP) in the United States reported the prevalence of congenital cataracts as 0.8/10,000 births for 1970 to 1987.6 A cluster of areas with a significantly high prevalence was found in Michigan in a geographic band from the southwestern to the east-central section of the state. Edmonds and James,7 examining the BDMP data, have shown that from 1979/1980 to 1988/1989 there was a 6.8% rate increase (range, 0.7-1.3 per 10,000 births). James6 reported the 1988/1989 BDMP/Commission on Professional and Hospital Activities prevalence of 1.2/10,000 births, with the highest rates found in the Northeast (1.7/10,000). The Metropolitan Atlanta Congenital Defects Program surveillance reported a congenital cataract prevalence of 2.1/10,000 live births from 1988 to 1991.8 Screening of 2,447 four-year-olds yielded a rate of 7.7 cataracts/10,000 live births,9 while two other cohort birth studies suggested a prevalence of 5.3 and 4.4 cataracts/10,000 live births.10,11 A prospective collaborative perinatal project conducted by 12 U.S. universities reported the prevalence of infantile cataract as 13.6/10,000 infants.12 The report further noted that isolated infantile cataracts occurred 3.8 times as often among infants born at weight ≤2,500 g than among those born at >2,500 g. In 2003 Holmes et al.13 reported, in a retrospective population-based medical record retrieval in the U.S. population, that the birth prevalence of visually significant cataracts was 3.0/10,000 (infantile cataract) to 4.5/10,000 (possible infantile cataract, defined as a cataract diagnosed after the 1st year for which there is no evidence of an acquired etiology). The authors estimated a total of 1,774 cases a year, with a prevalence rate of 4.5/10,000 live births. The Nordic registers of the blind suggest that the prevalence of visual impairment as a result of cataract is 0.6/10,000 children aged 0 to 17 years.14
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