| Author: | Zhao, Mei |
| Title: | Chorioretinal structure, blood supply and visual function in high myopia |
| Advisors: | Cheong, Allen (SO) Lam, Andrew (SO) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Myopia Eye -- Blood-vessels Eye -- Diseases Vision Hong Kong Polytechnic University -- Dissertations |
| Department: | School of Optometry |
| Pages: | xviii, 207 pages : color illustrations |
| Language: | English |
| Abstract: | The chorioretinal structure, vasculature, and visual function may be altered adaptively in highly myopic eyes, as a result of excessive eyeball elongation. Structural changes related to high myopia have been widely reported in previous studies, because of their better visibility and detectability compared with vascular and functional changes. A sufficient blood supply is essential for normal visual function. This PhD study aimed to investigate the effect of high myopia on ocular vasculature and visual function. Chapter 1 summarizes previous findings on myopia-related vascular changes from both perfusion and hemodynamic perspectives. Perfusion changes (in macular, peripapillary, and peripheral regions, in particular retinal and choroidal layers) of different refractive groups, especially high myopes, were discussed. Hemodynamic findings were categorized into retrobulbar, pulsative, peripapillary, and parafoveal segments. This chapter also reviewed the myopia-associated functional changes, with focus on the three most important and commonly investigated visual functions: visual acuity, visual field, and contrast perception. Chapter 2 evaluates the agreement between manual and semi-automated choroidal segmentation using a customized program. Manual segmentation is challenging and time-consuming, especially for naïve examiners. An automated segmentation program followed by manual verification, that is a semi-automated approach, was employed to enhance the efficiency and objectivity of choroidal segmentation. Images of 37 healthy eyes obtained from optical coherence tomography were retrospectively reviewed by two naïve and two experienced examiners to measure choroidal thickness. An excellent within-examiner (i.e. semi-automated vs. manual segmentation) and between-examiner agreements (i.e. naïve vs. experienced examiners) demonstrated that inexperienced practitioners could obtain choroidal thickness measurements with accuracy similar to experienced practitioners with the help of automated segmentation software. Chapter 3 reports the investigation of the ocular vasculature of 51 young high myopes and 67 age-matched non-high myopes from the perspectives of perfusion and hemodynamic using a multimodal imaging approach. Reduced retrobulbar flow velocity of the nasal posterior ciliary artery was observed in high myopes compared to non-high myopes. Superficial capillary perfusion decreased, but deep capillary perfusion increased with increasing axial length in the macular area. Macular structure and perfusion density showed topographic correlations, and both were significantly associated with retrobulbar hemodynamics. Peripapillary retinal arteries branching from the central retinal artery showed increased wall thickness and wall to lumen ratio with increasing axial length. Retrobulbar hemodynamics, macular perfusion, and peripapillary artery morphologies all altered with axial elongation and were associated with each other in this young population. Chapter 4 reports the investigation of contrast perception in 17 high myopes and 19 emmetropes, from two different domains: detection and discrimination. Correlations between retinal structure (cone morphometrics, retinal thickness) and function (contrast parameters) were evaluated. Detection performance of high myopes was comparable to that of emmetropes in the central and 20° nasal field, but showed a significant loss of sensitivity (i.e. threshold elevation) of medium spatial frequencies at the 10° nasal field, which might be explained by the optical deterioration at mid-periphery and neural dysfunction in the visual cortex. With regards to the discrimination performance, the high myopia group had comparable thresholds across various pedestal contrast levels with the emmetropia group at central vision. However, their discrimination thresholds elevated for the 10° and 20° nasal fields, especially at low contrast levels, which may be resulted from reduced local detectability and greater intrinsic noise in myopes. Weak structure-function correlations were found between parafoveal cone metrics and central discrimination parameters, and between retinal thickness and contrast detection parameters in the 10° nasal field. This PhD study demonstrated various alterations in ocular vasculature and contrast performance in young high myopes, compared with age-matched non-high myopes or emmetropes. Myopia-related vascular and functional alterations were correlated to changes in chorioretinal structures, which may contribute to the knowledge of the mechanisms of myopia-related fundus pathologies and their potential effects to daily visual activities. Further studies are required to determine the interaction between aging and high myopia, as well as the causal relationships among myopia-related changes in chorioretinal structure, ocular vasculature, and visual function, for the purpose of understanding the natural progression of myopic complications in depth. |
| Rights: | All rights reserved |
| Access: | open access |
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