Please use this identifier to cite or link to this item: http://hdl.handle.net/10397/91863
Title: Effect of optical defocus characteristics in the living environment and interaction with peripheral refractive error on myopia progression
Authors: Choi, Kai Yip
Degree: Ph.D.
Issue Date: 2021
Abstract: Myopia prevalence has been soaring in recent decades, reaching extremely high levels worldwide. This increased the cases of pathological myopia, which causes irreversible visual impairments and socioeconomic burdens. Myopia was once thought to be a genetic disease, because of its apparent hereditary characteristics. However, genetics have been found to only weakly contribute to myopia development. In contrast, numerous factors have been identified to be associated with myopia, such as education, activity pattern, and living environment. The consensus is that myopia development is multi-factorial. In Southeast Asia, including Hong Kong, the prevalence of myopia is amongst the highest globally. The city is characterised by an intense education modality, a dense population, and small living space. Education attainment has long been associated with refractive error. In Hong Kong, near work tasks are the mainstream of schoolwork, for which children have to spend hours studying. However, the contribution of the amount of near work is controversial, possibly due to the lack of comprehensive quantification of the near work environment other than one-dimensional working distance. Furthermore, the urban environment was found to be associated with myopia in places such as Australia and China. People in Southeast Asian regions generally live in small flats, with Hong Kong ranked amongst the most crowded in terms of living space per capita. Understanding the mechanisms of myopia development is the key to providing adequate control of its progression. Animal experiments have shown that the eye itself is able to respond to visually driven signals to compensate for the defocus blur induced by the optical lens. Negative (or hyperopic) defocus falls behind the retina and drives the eye to become myopic, while positive (or myopic) defocus falls in front of the retina, inducing hyperopia. This response occurs not only when the defocus is in the central retina, but also in the peripheral retina. However, the role of peripheral refractive error in control of myopia progression has been controversial in clinical studies. Epidemiology studies have revealed independence of myopia progression from peripheral refraction, whilst clinical trials of optical devices inducing peripheral myopic defocus successfully retarded myopia progression in children. Objectives: In this thesis, Chinese schoolchildren in Hong Kong, who are prone to developing myopia, were targeted. Study I aimed to evaluate the association between living environment, mostly in terms of housing, and refractive error. Study II aimed to investigate the relationship between on-axis refractive status and peripheral refraction other than peripheral spherical equivalent refraction. Finally, Study III aimed to evaluate the effect of environmental scene defocus at home, mainly the child's reading desk, on myopia progression, and its interaction with peripheral refraction.
Study I Methods: A total of 1,075 (age: 10.0 ± 1.0 years, 54.5% boys) subjects were recruited by random-cluster sampling according to the population density of the Hong Kong political districts. A self-reporting questionnaire was used to collect information on demographics, living environment, and near work related parameters. The data were analysed to assess the association with axial length and non-cycloplegic refractive error using univariate and multivariate analyses. Results: Population density of the residential district and home size were found to be associated with axial length and non-cycloplegic refractive error, but not the type of housing. Children living in high population density districts had 0.22 mm longer axial length and 0.49 D more myopic refractive error than those living in low population density districts; while children living in small homes had 0.23 mm longer axial length and 0.47 D more myopic refractive error than those living in large homes. The effect of near work posture reached statistical significance, but was not conclusive. However, other factors including near working distance, resting frequency, and participation in extra-curricular activities were independent of the axial length and refractive error in this study. Study II Methods: The same subjects in Study I also participated in Study II, excluding those with small pupil sizes, who were unable to perform peripheral refraction, reducing the sample size to 1,052. Peripheral refraction was measured at ±10° vertically and horizontally. A further ±20° horizontally was measured in a 603-subject subset. The relative peripheral refractions, including spherical equivalent refraction (M), J0, J45, and radiality, which was defined as the absolute difference between P(90) and P(180), were compared between groups of different axial-length-to-corneal-radius-of-curvature (AL/CR) ratios. Multiple correlation analysis was used to assess the relationship between AL/CR and each peripheral refraction vector. Orientation bias was defined as the clearer meridian of the peripheral astigmatism in sphero­cylindrical form, which was compared among AL/CR groups. Results: The results showed that M was more hyperopic with increased AL/CR both horizontally and vertically. In contrast, the magnitude of J0 and J45 became smaller with increased AL/CR along horizontal and vertical visual fields, respectively. Radiality, which represented the quality of focus of the radial component of the retinal image, decreased with increasing AL/CR along the horizontal field. In multiple correlation analyses, M (r = 0.50) and radiality (r = 0.35) demonstrated a moderate, while J0 (r = 0.20) and J45 (r = 0.12) demonstrated a weak correlation with on-axis AL/CR. Regarding orientation bias, radially oriented bias was over-represented in the low AL/CR group, but under-represented in the high AL/CR group. In contrast, tangentially oriented bias was under-represented in the low AL/CR group, but over-represented in the high AL/CR group. Study III Methods: Fifty subjects (age: 9.3 ± 1.2 years, 44% boys) were recruited from the Optometry Clinic of The Hong Kong Polytechnic University, and their homes visited. Demographics, parental myopia, activity pattern, and home size were obtained in a parental interview. Kinect was used to measure the three-dimensional distances of the child's reading desk from their eyes, in order to use these measurements to construct a scene defocus profile. The home scene parameters were calculated as dioptric volume (DV) and standard deviation of the scene defocus (SDD), which represented the total amount of net defocus in the scene and the dispersion of the scene defocus, respectively. On-axis and peripheral refractions were measured at baseline, and myopia progression was measured one year later (ΔM). Peripheral refraction was measured ±30° horizontally, and the M, J0, P(90), and P(180) values were fitted in quadratic regressions and analysed. The correlation between ΔM and home scene parameters was calculated. Stepwise multiple linear regression was also used to assess the relationship between ΔM and home scene parameters along with other co-variates. The correlation between ΔM and peripheral refraction was calculated. Following this, the partial correlation between ΔM and peripheral refraction, controlled for the home scene parameters, was also calculated. Multiple linear regression was also used to assess the relationship between ΔM and peripheral refraction, adding home scene parameters as co-variates. Results: The findings revealed that faster myopia progression was associated with a more dispersed scene defocus profile (ρ = -0.42) and a more hyperopic scene defocus at the para-central field (B = -0.18). The results did not show that any quadrants of the scene, or a non-linear spatial summation between myopic and hyperopic scene defocus, had a better correlation with ΔM. In contrast, it was determined that peripheral refraction was independent of the ΔM after controlling for baseline M. However, after adding home scene parameters as co-variates, peripheral refraction was again associated with ΔM (M: ρ = -0.30; J0: ρ = -0.49; P(180): ρ = ­0.35). Finally, children living in a small home had 0.41 D and 0.49 D faster myopia progression than those living in medium and large homes, respectively. Conclusions: Myopia development is complex and multi-factorial. Among the environmental factors, the living environment, specifically the home size, and the near working scene at home were associated with myopia progression. In contrast, despite the controversy in the myopia literature, the results showed a significant effect of peripheral refractive error in myopia development, but only when it was controlled for the home scene parameters. Therefore, it may be speculated that there is an interaction between external and internal factors, in terms of the scene defocus stimulation and peripheral refractive error, respectively. When the external factors were strong, the eye would depend on the external stimulation and emmetropise accordingly. However, when the external factors were weak, the eye would rely on the central and peripheral refractive profile internally. As home size is a difficult factor to modify, indoor scene modification can be further studied to assess the myopia control effect.
Subjects: Myopia
Eye -- Refractive errors
Hong Kong Polytechnic University -- Dissertations
Pages: 215 pages : illustrations
Appears in Collections:Thesis

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