Model-based approach for analyzing prevalence of nuclear cataracts in elderly residents

Highlights

• Large-scale computation estimates lens temperature in different populations.

• High temperature in lens in tropical environment appears around the lens nucleus.

• Temperature and solar exposure correlate with prevalence of nuclear cataract.

• Cumulative thermal dose in lens well correlates with prevalence of nuclear cataract.

• Cumulative wet bulb globe temperature is proposed for practical assessment.

Abstract

Recent epidemiological studies have hypothesized that the prevalence of cortical cataracts is closely related to ultraviolet radiation. However, the prevalence of nuclear cataracts is higher in elderly people in tropical areas than in temperate areas. The dominant factors inducing nuclear cataracts have been widely debated. In this study, the temperature increase in the lens due to exposure to ambient conditions was computationally quantified in subjects of 50–60 years of age in tropical and temperate areas, accounting for differences in thermoregulation. A thermoregulatory response model was extended to consider elderly people in tropical areas. The time course of lens temperature for different weather conditions in five cities in Asia was computed. The temperature was higher around the mid and posterior part of the lens, which coincides with the position of the nuclear cataract. The duration of higher temperatures in the lens varied, although the daily maximum temperatures were comparable. A strong correlation (adjusted R² > 0.85) was observed between the prevalence of nuclear cataract and the computed cumulative thermal dose in the lens. We propose the use of a cumulative thermal dose to assess the prevalence of nuclear cataracts. Cumulative wet-bulb globe temperature, a new metric computed from weather data, would be useful for practical assessment in different cities.

Introduction

The eye is highly sensitive to various environmental agents [[1], [2], [3]]. International standardization bodies set the limits for non-ionizing radiation, especially for wavelengths less than 1 mm, based on eye safety [4]. However, the mechanisms are variable, including corneal damage, photochemical reactions, and cataract formation [5,6].

The role of environmental agents in cataractogenesis, which has been observed predominantly in elderly populations, has been a controversial and widely debated topic. Although extensive studies have been conducted, the exact mechanism of cataractogenesis is not yet fully understood. This could be attributed to a number of cofactors influencing cataractogenesis [7] and should be investigated separately and systematically. This study focused specifically on physical factors, among which ultraviolet (UV) radiation and ambient temperature exposure [2,6,8,9] have received much attention.

The three most common types of cataracts are nuclear cataracts (NUCs), which lead to a gradual opacification of the nucleus of the lens; cortical cataracts, which involve the cortex from the periphery toward more central opacification; and posterior subcapsular cataracts, which give rise to distinct opacity on the posterior capsule [10]. Several studies have reported that UV radiation is a risk factor for cortical cataracts. Within the solar radiation spectrum [11], UV is an electromagnetic wave with a wavelength between 10 and 400 nm, corresponding to a penetration depth between 20 and 150 μm in the human skin [12]. A recent epidemiological study suggested that UV radiation is correlated with the prevalence of cortical cataracts [13].

Miranda argued that cataract incidence in tropical and subtropical zones showed a stronger correlation with ambient temperature than with solar radiation [14]. This study also suggested that NUCs are more prevalent in tropical and subtropical zones. Another aspect to be noted is that the prevalence of NUCs increased significantly in the elderly, particularly those living in tropical zones [15,16]. According to a recent epidemiological study, self-reported cataracts were observed 4 times more frequently among people aged ≥70 years than among those aged 50–59 years (95% CI: 2.28 = −0.50) [17]. If cataractogenesis in the tropical zone is closely related to ambient temperature and/or solar radiation, the number of patients may increase in subtropical zones as climate change continues and aging societies persist [18].

Weather data from a measurement-based database can provide useful insights on ambient temperatures and support future research. Ambient temperature and relative humidity (RH) may influence body temperature. Solar radiation, specifically infrared radiation, should also be considered, and it is considered to vary over time. Energy deposition is concentrated around the cortical region of the skin and cornea because of the small penetration depths, and it then diffuses into deeper regions due to heat conduction [19,20].

To the best of the authors’ knowledge, cataractogenic exposure has not been quantified before. Thus, the internal physical quantity needs to be evaluated. It is difficult to evaluate lens temperature because, in addition to a complex heat transfer system around the eye, the thermophysiological response may change with age as well as location (i.e., tropical and temperate areas). In our previous studies, we modeled the thermoregulatory response in the elderly based on the measurement of human subjects [21,22]. In addition, we clarified that the tropical test subject was mainly characterized by parameters corresponding to the countercurrent blood in different body parts and an approximately 20% increase in sweating rate [23].

If we can characterize NUC prevalence in different countries, the results may facilitate the development of future intervention strategies. In this study, we investigated the effect of lens temperature as a potential physical agent for NUC formation. First, we extend a conventional thermoregulatory response model of the test subject to elderly subjects in a tropical region. The temperature in the eye lens, particularly around the center, was computed based on exposure to ambient temperature and infrared radiation in adult males and elderly individuals who grew up in tropical zones. We introduced the cumulative thermal dose in the mid-part of the lens as a potential physical agent of NUC in the elderly population of different locations based on epidemiological studies conducted by our group [13,16]. The corresponding ambient parameter was then correlated with the lens temperature for practical assessment.