Global, regional, national burden and gender disparity of cataract: findings from the global burden of disease study 2019

Cataract, defined as loss of lens transparency, causes alteration of refractive properties and elevated light scattering, resulting in hazy vision or blindness [1]. Cataract is one of the leading causes of blindness worldwide [2, 3]. The prevalence of cataracts increases with age, ranging from 3.9% among 55–64 years to 92.6% among those 80 years and older [4]. In 2010, there were 10.8 million cataract blind people [5], this number is expected to increase to 40 million in 2025 with the aging of the world’s populations and greater life expectancies [6]. The impact of cataracts on visual loss, especially in an elderly population, exacerbates the risk of dementia [7], increases the likelihood of falls and road traffic crashes [8, 9], can markedly affect the quality of an individual’s life and ultimately leads to higher mortality [10].

As yet, there are no preventative or therapeutic drugs against cataracts have been approved, leaving surgery as the only effective treatment option [11]. Due to refinements in modern cataract Surgical techniques, the procedure is considered to be relatively safe and can improve the visual function of patients well [12]. Research has found that timely and equitable access to cataract surgery can prevent fall-related injuries and support healthy aging [8, 9]. Some studies also suggest that cataract extraction is associated with a lower risk of developing dementia among older adults [7]. Given our globally aging population, the number of people affected with cataracts is predicted to increase worldwide, especially in low-income nations with limited access to surgery [13]. The management of cataracts will become a socioeconomic challenge since the social and economic costs of cataracts are quite staggering and the demand for cataract surgery far exceeds limited public health resources.

Despite the efficacy and advantage of cataract surgery, it is not equally accessible to the world’s population [14, 15]. Especially in developing countries, people might not have the necessary financial means or access to a surgeon who can operate cataract surgeries. Hence, most cataract around the world remains untreated. Disease burden, cataract surgical rate/coverage, and human resources are endorsed as the national indicator for monitoring eye services by the World Health Assembly [16]. The health burden of disease can be quantified using disability-adjusted life years (DALYs). DALYs are the sum of years lived with disability (YLDs) and years of life lost (YLLs) owing to premature death [17]. To better evaluate the global burden of cataracts, we use prevalence and DALYs from the GBD 2019 study as the main measurements to make comparisons across the year, age, region, gender, and socioeconomic status. We aimed to raise the attention of the public to prompt the treatment of cataracts and provide a reference for health policymaking.

This study comprehensively demonstrated the trend of the global burden of cataracts by year, age, gender, and socioeconomic status with measurements of prevalence and DALYs. We found that gender inequality in the global burden of cataracts had persisted since 1990, increased with age, and was greater in countries and territories with lower socioeconomic development. From 1990 to 2019, global DALY numbers and crude rates rose gradually, while age-standardized rates declined. Since age-standardized prevalence rates exclude the effect of population size and age structure, revealing the true burden of disease. It indicates that progress was made in the fight against cataracts, but we still face enormous challenges in avoiding vision impairment caused by cataracts, due to the growing and aging population.

In an era of shifting global agendas, more emphasis is expanded on non-communicable diseases and injuries along with communicable diseases. Ono et al. reported for the first time that cataracts have the most uneven distribution at the global level among non-communicable eye diseases, using disability-adjusted life-year data from the 2004 GBD study [20]. Yan and his colleagues [21] based on the data from the GBD 2017 study, found socioeconomic status was inversely associated with the burden of cataracts, which has been also confirmed in our study [22]. A similar trend has been visible in some previous investigations [22, 23]. HDI (human development index) was mostly used in previous studies, which is a product published by the United Nations Development Programme (UNDP) covering social and economic factors in three areas (health, education, and living standards) [14]. However, changes were made to the definition of HDI before and after 2010. To ensure comparability between different years, the development status of a country/region was evaluated by SDI in our study. We found that age-standardized DALY rates differed among countries with different SDI regions, with cataract burden being more concentrated in countries with lower socioeconomic status. In high-middle SDI countries, a significant negative correlation exists between age-standardized DALY rates and SDI. Similarly, socioeconomic disparity also exists in many eye diseases, Li et al. found less developed countries tend to have a higher burden of uncorrected refractive error [24].

This association may be due to better eye care and greater access to cataract surgery in countries with high SDI. The research found that the prevalence of blindness was negatively correlated with the density of ophthalmologists. Higher national income was associated with a higher density of ophthalmologists, ranging from an average of 76.2 ophthalmologists per million population in high-income countries to an average of 3.7 ophthalmologists in low-income countries, with a difference of 18 times [15]. Wang eal. Documented the strong associations of socioeconomic indices with quantity and quality of cataract surgery, the countries with the highest socioeconomic level had the best cataract surgery outcomes [14]. Low-income countries with lower cataract surgical coverage, in part due to inadequate training opportunities for young surgeons [25].

In 1999, the WHO and the International Agency for the Prevention of Blindness (IAPB) launched the VISION 2020 global initiative to eliminate avoidable blindness by 2020 [26]. Cataracts and uncorrelated refractive errors are considered the main objects of WHO and VISION 2020 initiatives. However, the refractive error can be corrected by spectacles, there is no effective conservative method for cataracts except surgery. Cataract is the leading global cause of blindness in those aged 50 years and older [27], impairs vision, and affects the patient’s quality of life. Improving vision timely facilitates many daily life activities, enables better educational achievements, and increases work productivity, reducing inequality. Studies have also shown that cataract surgery can reduce the risk of dementia and fall. The incidence of falls among older people referred for cataract surgery was 31% lower after first eye surgery, and a further 50% lower after second eye surgery, by restoring binocular vision [9]. Cataract surgery is a disability-preventing and highly cost-effective intervention costing less than $100 per DALY avoided [28]. Therefore, cataract surgery leads to huge socioeconomic benefits and improvement in well-being and quality of life. Nonetheless, cataract surgery is not a panacea for all problems. With the population aging and life expectancy rising, demand for cataract surgery is surging. A total of $5733 million investment was estimated to be required for eliminating blindness due to cataracts between 2010 and 2020, which is a great financial burden and a challenge to public health [29]. Beyond that, the incidence of cataract surgical complications was 1.2%, including posterior capsular rupture, cystoid macular edema, retinal detachment, and endophthalmitis [30]. Most of these complications of surgery may need further intra- or postoperative management. Although DALY is lower in high-income countries, it may require higher environmental costs, against the idea of sustainable development. For example, a phacoemulsification cataract extraction in a UK hospital produced more than 20 times the greenhouse gas emission of the same procedure in an Indian hospital [31]. Progress in improving the global burden of cataracts has been achieved in recent years, but much more remains to be done.

Previous studies mostly concentrated on the overall visual impairment caused by cataracts, but the different levels of visual impairment (moderate, severe vision loss, and blindness) remain unknown. We found moderate vision loss took the majority in terms of age-standardized prevalence rates of cataracts, while age-standardized DALY rates of blindness predominated. DALY reflects the gap between the actual health status and the normative situation. It indicates that cataract blindness damages the healthy life years of patients and quality of life seriously, even at low prevalence rates. What’s more, we found gender inequality in the burden of cataracts had persisted since 1990, and the inequality remained with aging among different stages of vision impairments and regions with different levels of development. Other studies have found similar patterns of gender disparity [23], including but not limited to cataracts, uncorrected refractive error [24], age-related macular degeneration [32], and diabetes retinopathy [33] also share the same pattern.

One possible explanation is that women have a higher prevalence of cataracts and a longer life expectancy. According to the World Health Organization’s official website, the average life expectancy of women in 2020 is generally higher than that of men. Japan ranks first with an average life expectancy of 83.7 years, while the average life expectancy of women is 86.8, while that of men is only 80.5 [34]. Another explanation may have to do with gender inequality, which has been a problem throughout the world for many years. There is evidence suggesting that women are disadvantaged in areas such as education, job opportunities, income distribution, and medical care [35]. Although 60% of cataract patients are women, men are 1.39 times more likely than women to undergo cataract surgery [36]. Compared with men, women have less support from family and less control over finances, which may hinder their access to cataract surgery. For children with bilateral cataracts, girls are also less likely to undergo surgery than boys in low-income countries [37]. Therefore, more attention should be paid to eye care services for women, eliminating gender inequality is an important part of combating the global burden of cataracts.

The limitations of this study should not be ignored. First, the accuracy of health burden information is limited by the source of original data, which the GBD 2019 study noted in their reports [17]. In absence of data, the out-of-sample prediction validity dependent on modeling is bound to bring some deviation. Second, the use of aggregate data at the first level of administrative organization within each country level ignoring data from subnational locations would be a source of bias. There can be great variation inside a country, and extending conclusions to a particular region should be cautious. Moreover, COVID-19 has changed the way people live around the world, which may lead to a decrease in the predictive accuracy of ARIMA models. In the outpatient clinic, due to the lockdown policy, we found that many cataract patients have poor vision when they see a doctor, and the times waiting for cataract surgery are longer, which is bound to affect their quality of life.

In summary, this study showed global health progress in cataracts with age-standardized DALYs rate decreasing in the past few decades. The aging of the population is outpacing the growth of the profession, cataract still reminds a global public health concern. Cataract-related health services should be strengthened for the older population, females, and people in lower SDI regions. Hopefully, our study could raise awareness of the disease burden of cataracts and would be valuable for policy making and program planning.