The distribution of a disease across different regions, periods, and populations is an external representation of how the etiological factors impact on the target population. It reveals epidemiological patterns, providing etiological clues that form the basis for further scientific enquiry and the formulation of research studies. These provide evidence for developing effective prevention and treatment policies, health resource allocation, and disease diagnosis. The main metrics of disease distribution include disease prevalence and incidence.

Prevalence, also known as prevalence rate, is the proportion of a population found to have a certain disease (including old and new cases) at a given time. Period prevalence and point prevalence refer to different time scales. In practice, point prevalence applies to a time period no longer than a month according to the following formula:

Incidence is defined as the frequency of which new cases of a given disease occur in a population, within a specified period of time. It describes the severity of risk factors in a given population and indicates the population’s susceptibility. It is calculated as:

Cumulative incidence is the number of new cases of a certain disease in a particular population over a certain period divided by the size of total population at the beginning of the period. Cumulative incidence reflects the cumulative impact of incidence. It is calculated with the following formula:

Cumulative incidence can be expressed in %, ‰, or ‱ as needed.

There is a paucity of data on the epidemiology of pediatric cataracts with considerable variations due to different study designs, definitions of pediatric cataracts, and age groups used in different reports (Table 5.2). Most existing data on pediatric cataracts is derived from population-based studies of childhood blindness, and relatively few studies have investigated pediatric cataracts primarily as a disease. Failure to adjust for refractive errors in a number of studies also compromises data validity. Currently, data on epidemiology of pediatric cataracts in China is still absent.

The prevalence of pediatric cataracts measures the proportion of children with cataracts in the target population within a particular time frame. Given the difficulty of eye examinations in children, previous epidemiological studies have mainly focused on congenital/newborn cataracts and produced wide-ranging results. Based on the WHO definition of visual impairment, Parikshit and colleagues made projections of different childhood blindness etiologies in countries of varying income levels, using the World Bank income-based country classifications [12]. Their findings show that lens abnormalities (cataracts, aphakia, etc.) are as important as retina and whole-globe lesions in being etiological causes of childhood blindness in China (Table 5.3).

Foster and colleagues reported that the prevalence of pediatric cataracts ranges from 0.01 to 0.15 % [13]. Furthermore, they extrapolated that there were ten new cases of bilateral congenital cataracts/million population/year in developing countries compared to four new cases/million population/year in developed countries, attributing this difference to higher birth rate and increased exposure to rubella and other etiological factors in developing countries. The report also estimated there were 200,000 blind children living globally due to bilateral congenital cataracts.

The Birth Defects Monitoring Program (BDMP) of the United States of America (USA) is a population-based epidemiological study, and from 1970 to 1987, 15,487,449 newborns in 48 states were surveyed. The prevalence of congenital cataracts was reported as 0.008 % [6], with Michigan posting the highest rate and Eastern New York the second highest rate. The Metropolitan Atlanta Congenital Defects Program (MACDP) monitored 696,057 newborns in the five districts of Atlanta from 1968 to 1991 and reported the prevalence of congenital cataracts to be 0.021 % [14]. In this study, the prevalence was 0.018 % for the white population and 0.026 % for other ethnicities. A collaboration study by 12 US universities found that the prevalence of cataracts in newborns was 0.136 %, with 6.5/10,000 for bilateral cataracts, 7.1/10,000 for unilateral cataracts, and 7.6/10,000 with associated congenital anomalies [15].

Cataract is a major cause of childhood blindness in middle-income countries in Europe. A cohort study with 10-year follow-up by Stewart-Brown and colleagues in the UK in 1970 studied 1500 10-year-olds from schools for the blind and other special schools. The prevalence of pediatric cataracts was reported to be 0.047 % [4]. Stayte and colleagues similarly found the prevalence of cataracts in children aged 2–5 years to be 0.044 %[7].

To date, China has yet to produce a systematic and complete dataset for the epidemiology of pediatric cataracts. In a nationwide sampled survey of low vision and blindness in 2001, the prevalence of visual disability in 6024 children aged 0–6 years from six provinces and municipalities was found to be 0.11 %. This is similar to the reported prevalence of many developed countries. The study revealed nine cases of congenital cataracts, which accounted for 14.1 % of the visual disability [16]. The Beijing Childhood Visual Impairment Program conducted in 2004 is a population-based study. Eye examinations were performed in children aged 3–6 years old with visual acuity below 6/18, from eight urban and ten rural communities. Among the 17,699 children examined, there were three cases of congenital cataracts, resulting in a prevalence of 0.0169 % [17].

A population-based epidemiological study in rural southern India by Syril K. Dorairaj and colleagues examined 14,423 children under the age of 16 years and reported the prevalence of lens-related visual impairment including cataracts, aphakia following cataract surgery, and pseudophakia to be 4.5/10,000 [18].

Epidemiological data on pediatric cataracts alone is largely absent in Africa. Lawan and colleagues studied children with congenital eye and adnexal anomalies under the age of 10 years at an ophthalmic clinic of a teaching hospital in Nigeria between 2001 and 2005. It is found that cataracts accounted for 35 % of congenital ocular anomalies [19].

The incidence of pediatric cataracts measures the proportion of new cases of cataracts in the target population over a certain time period. As studies assessing incidence are often time-consuming and resource-intensive, there are currently no specific reports of the incidence of pediatric cataracts. A 25-year (1975–2000) longitudinal cohort study conducted by Wirth MG in Australia found 421 new cases of pediatric cataracts in total and estimated the incidence to be 2.2/10,000 in Australia [20]. A cohort study in Denmark documented the nationwide cumulative incidence of cataracts, providing classifications for 2.6 million children under 17 from 1980 to 2000 [21]. Only inpatients of childhood cataracts were registered from 1980 to 1994. The cumulative incidence of cataracts in the 20-year follow-up was 92.4/100,000; and gender-specific cumulative incidence was 107.9/100,000 in boys and 76.2/100,000 in girls [21]. The overall cumulative incidence of pediatric cataracts from 1995 to 2000 was 108.4/100,000 [21]. The gender-specific incidence was 119.2/100,000 in boys and 97.0/100,000 in girls. The incidence of congenital/newborn cataracts declined with age, whereas the incidence of traumatic cataracts increased with age.

Investigating the etiology of pediatric cataracts is important for targeted prevention and the development of treatment strategies. Due to the low incident rate and the relatively small number of patients, reliable epidemiological studies of pediatric cataracts etiology are rare. In the UK, among the 243 children diagnosed with congenital or infantile cataracts from 1995 to 1996, bilateral cases accounted for 66 % of all cases [22]. In bilateral cases, 61 % were isolated cataracts and 25 % were associated with systemic anomalies. In unilateral cases, 47 % were isolated cataracts and 6 % were associated with systemic anomalies, while the rest had concurrent eye anomalies [22]. Unilateral cataracts were more likely to be associated with other eye anomalies than bilateral cataracts (47 % vs. 14 %). Etiological factors were not identifiable in 92 % of the unilateral cases and 38 % of the bilateral cases. In bilateral cases, 56 % of these children had concurrent hereditary conditions associated with cataracts, while only 6 % of unilateral cases were associated with hereditary conditions.

Haargaard and colleagues classified the etiology of congenital/newborn cataracts in a cohort study registering 1027 cases in children aged under 17 years (Table 5.4) [23]. Persistent fetal vasculature (PFV) was the most common eye anomaly associated with pediatric cataracts, whereas Down syndrome was the most common genetic disorder associated with congenital cataracts. Rubella infection accounted primarily for intrauterine infection-related congenital/infantile cataracts. Low birth weight (LBW) was also a significant risk factor for infantile cataracts. The Collaborative Perinatal Project, a joint study by 12 US university medical centers, found that the incidence of cataracts in newborns of LBW (≤2500 g) was 3.8 times higher than that of newborns with normal weight (>2500 g) [15]. Older maternal age is also a significant risk factor for pediatric cataracts, as it is closely related to adverse pregnancy outcomes, such as premature birth, fetal distress, gestational diabetes mellitus (GDM), increased likelihood of cesarean section, and LBW.