Model-based approach for analyzing prevalence of nuclear cataracts in elderly residents
Graphical abstract
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.
Section snippets
Data sources for epidemiological evaluation
The prevalence of NUC has been measured and reported in previous studies from Kanazawa Medical University [13,16]. The subjects comprised 2637 residents aged 50 or over (average 62.4 ± 9.2 years) who participated in epidemiological surveys conducted in five cities (shown in Fig. 1; generated with Wolfram Mathematica 12.1) with varying UV irradiance and ambient temperatures (Table 1). The study was approved by the Institutional Review Board of Kanazawa Medical University and was conducted
Anatomical human model
A Japanese male model based on magnetic resonance images was used as a reference in this study [27]. The height and weight of this model were 1.73 m and 65 kg, respectively, which are close to the mean measurements of Japanese males. This model was segmented into 51 anatomical regions, such as skin, bone, muscle, and fat, including six tissues around the eye (e.g., lens, vitreous, iris, cornea, and vitreous). The resolution of the original model was 2 mm. However, this resolution is not
Fundamental characteristics of lens temperature for heat exposure
The lens temperatures in different test subjects (age and climate) were evaluated at an ambient temperature of 35 °C and RH of 50%. The time required to reach a steady state was 30 min (90% saturation). As shown in Fig. 4, the difference in the lens temperature between the temperate and tropical zones was 0.3 °C, which is mainly attributable to the difference in the core temperatures. In the elderly, the core temperature increased due to the decline in heat sensitivity of the skin and
Discussion
For test subjects in conditions where the ambient temperature is 35 °C and RH is 50%, the core temperature difference of the elderly around 65 years (60s) was 0.3 °C higher than that of healthy adults (50s). As often reported in the field of heat-related illnesses, this can be attributed to a decline in the sweating capacity of the elderly or low evaporative heat loss [66]. This is also different for populations in temperate and tropical areas. Our computational results may explain the
Conclusion
With a large-scale integrated computational method of bioheat modeling and thermoregulation, we evaluated the daily lens temperature change in test subjects with different thermoregulatory responses considering climate and age for the first time. We then discussed its correlation with the prevalence of NUCs. Our computational estimation of the cumulative thermal dose in the lens was found to be highly correlated with NUC prevalence than with the remaining parameters (daily peak lens temperature
Funding
This work is supported by “Joint Usage/Research Center for Interdisciplinary Large-scale Information Infrastructures” in Japan (Project ID: jh190008-NAH, jh200007-NAH).
Declaration of competing interest
The author declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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