A structural and functional profile of high myopia as a function of eye size
AuthorJaworski, Alexandra Anne
AffiliationOptometry and Vision Sciences
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2020-12-24. This item is currently available to University of Melbourne staff and students only, login required.
© 2018 Dr. Alexandra Anne Jaworski
The prevalence of myopia and its associated sight threatening pathology is anticipated to increase, making high myopia a global health concern, especially in our ageing population. Although pathological sequelae and visual dysfunction have been attributed to excessive elongation in the highly myopic eye, a link between axial elongation and visual dysfunction in the absence of significant pathology is less well described. Furthermore, emmetropia and high myopia are variably defined, and in most instances, defined according to refractive error and not eye size. This thesis was designed to define and distinguish emmetropic and highly myopic eyes with regard to axial length and vitreous chamber depth and to apply these definitions to studies exploring the impact of ocular expansion on visual function and ocular structure. The first experiment (Chapter 2) used meta-analysis to predict the axial length (23.47 ± 0.07 mm) and vitreous chamber depth (16.12 ± 0.14 mm) of an emmetropic eye. Based on gender matched studies, male eyes not selected for refractive error were found to be larger with respect to both axial length (+0.52 ± 0.04 mm) and vitreous chamber depth (+0.40 ± 0.04 mm) relative to female eyes. Male emmetropic eyes displayed longer vitreous chamber depths (+0.47 ± 0.15 mm) relative to female emmetropic eyes, but axial lengths of emmetropic eyes did not vary with gender. Linear and non-linear meta-regressions predicted minimum dimensions of -5, -6 and -8 Dioptre (D) eyes, with a – 5 D high myope characterised by an axial length and vitreous chamber depth of at least 24.66 and 16.94 mm, respectively. Refractive errors of -5, -6 and -8 D were selected as they have previously been used to define high myopia in other studies, and were the refractive error groupings investigated in Chapters 3 and 4 of this thesis. Systematic review showed that vitreous chamber depth is presented less frequently than axial length in published literature. This may reflect instrumentation used to perform biometry and suggests that vitreous chamber elongation is mostly assumed but not demonstrated by researchers. In such cases, the role of posterior segment elongation in the development of structural and/or functional sequelae is hypothetical. The second experiment (Chapter 3) contrasted luminance and S-cone pathway spatial processing of axial high myopes ( -8 D) and emmetropes using psychophysical tools. Luminance and S-cone pathways were both probed to discriminate between pathway selective and non-selective visual dysfunction, given the known redundancy of neuronal elements comprising the S-cone pathway and reports of altered colour vision in high myopia. High spatial frequency loss and increased critical area indicated increased separation of neural elements. Critical area enlargement was consistent with a non-uniform posterior pole model of ocular expansion. Models previously reported in the literature have included non-uniform, posterior pole and global expansion. However, unlike the wrok presented here, these studies did not assess visual function with regards to vitreous chamber depth. Reduced contrast sensitivity for spatial summation tasks in the presence of retained sensitivity at lower spatial frequencies suggested non-selective post-receptoral dysfunction due to ocular enlargement and either normal or enhanced photoreceptor sensitivity. The third experiment (Chapter 4) utilised readily available clinical tools, customised automated perimetry and optical coherence tomography, to investigate structure-function relationships in enlarged highly myopic eyes ( -5 D). Generalised choroidal thinning for the central 4 mm and localised retinal thinning confined to the central ±1 mm were evident for high myopes. Although overall visual sensitivity decreased with increasing eye size, regional relationships between sensitivity and structural thickness or eye size were not evident. However, the nasal region appeared predominantly thinner with ocular enlargement suggesting that it may be more susceptible to visual dysfunction in longer eyes. The finding of marked nasal thinning of the ocular structural layers has been reported previously and is consistent with choroidal watershed zones predominantly affecting the nasal posterior pole. This reduced blood supply could underpin structural and functional changes in eyes demonstrating posterior pole expansion. The findings support that vitreous chamber elongation causes high myopia, visual dysfunction, and generalised choroidal and localised retinal thinning. Psychophysical and available clinical tools supported non-uniform ocular expansion in high myopia, with the nasal region predominantly thinned and potentially at risk of visual dysfunction. Although meta-analysis is not a novel tool, its application to defining emmetropia and high myopes with regards to axial length and vitreous chamber depth is novel, and has application in subsequent research and clinical settings.
KeywordsHigh myopia; emmetropia; eye size; refractive error; axial length; vitreous chamber depth; visual function; choroid; retina; OCT
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