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The Development of a Clinical Trial Protocol and Functional Biomarkers for Age-Related Macular Degeneration

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The Development of a Clinical Trial Protocol and Functional Biomarkers for Age-Related Macular Degeneration Claire McKeague A thesis submitted to Cardiff University for the degree of Master of Philosophy SCHOOL OF OPTOMETRY AND VISION SCIENCES, CARDIFF UNIVERSITY 2014 SUPERVISED BY DR ALISON BINNS & DR TOM MARGRAIN DECLARATION This work has not previously been accepted in substance for any degree and is not concurrently submitted in candidature for any degree. STATEMENT 1 This thesis is being submitted in partial fulfillment of the requirements for the degree of …………………………(insert MCh, MD, MPhil, PhD etc, as appropriate) STATEMENT 2 This thesis is the result of my own independent work/investigation, except where otherwise stated. Other sources are acknowledged by explicit references. STATEMENT 3 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter- library loan, and for the title and summary to be made available to outside organisations. Summary Age-related macular degeneration (AMD) is the leading cause of blindness amongst older adults in the developed world. With the predicted rise in the ageing population over the next decades, the prevalence of this debilitating disease will simply continue to increase. The only treatments currently available are for advanced neovascular AMD. The retina is already severely compromised by this stage in disease development. Therefore, there is a pressing need to evaluate potential novel interventions that aim to prevent the development of advanced disease in people with early AMD, to prevent sight loss from occurring. Furthermore, it is necessary to develop tests that are sensitive to subtle changes in visual function in order to evaluate the efficacy of these emerging treatments. There is a growing body of evidence to suggest that hypoxia contributes to the development of AMD. Hypoxia is most acute at night when the retinal photoreceptors are most metabolically active, due to the demands of the rod dark current. Increasing the light levels at night will cause the oxygen demand, and hence the hypoxia, to be substantially diminished. This leads to the hypothesis that providing low level night time light therapy to people with early AMD may slow disease progression by reducing hypoxia. In order to evaluate the potential effectiveness of such an intervention, it is necessary to select appropriate outcome measures. The inherent variability of the standard test of visual function, visual acuity, renders it inappropriate for use as a primary outcome measure in proof of concept clinical trials. Therefore, the first aim of this thesis was to evaluate the diagnostic validity and repeatability of alternative functional tests that may be used as biomarkers for early macular disease. Dark adaptation was evaluated using three stimuli, a spot of 2o radius and annuli of 7o and 12o radii, in 21 healthy adults (on two occasions) and in 11 participants with early AMD. All stimuli were found to be highly diagnostic for early AMD. The spot of 2o radius provided the best separation between groups with respect to the time constant of cone recovery (area under the ROC curve 0.91). The repeatability of chromatic and flicker thresholds were also assessed in 30 healthy adults. The coefficient of repeatability, expressed as a percentage of the mean threshold, was 17.1% for red-green chromatic thresholds, 31.1% for blue-yellow, 53.4% for 14Hz flicker thresholds, and ranged between 36.4%-53.3% for parameters of dark adaptation. A small learning iii effect was found for both chromatic thresholds and the 14-Hz flicker test, indicating that a control group is needed in studies of new therapeutic interventions. The second aim of this thesis was to develop a protocol for a clinical trial that seeks to determine if low level night time light therapy can prevent the progression of early AMD. The level of retinal illuminance required to suppress the rod dark current, the maximum retinal illuminance which prevents substantial suppression of melatonin secretion, and the most appropriate means of delivering the dose of retinal illumination were evaluated. The final protocol employed an organic LED illuminated light mask, worn during hours of sleep, as the mode of intervention. In conclusion, this thesis has confirmed that cone dark adaptation is a sensitive functional biomarker for AMD, and that all three functional tests have a good inter- session repeatability. These biomarkers will be validated in the prospective clinical trial of low-level light therapy to confirm their prognostic and predictive capabilities. The proposed trial will also evaluate the effectiveness of the low level night time light therapy, delivered by means of an illuminated light mask, at slowing the progression of early AMD. iv Acknowledgements First and foremost, I give thanks to my Saviour Jesus Christ for giving me the knowledge, understanding and strength to undertake and complete this research project. Having spent two years studying just one aspect of the human eye, I have only begun to scratch the surface in understanding its intricate design, and am in awe of the power and wisdom of God and His amazing creation! I would also like to express my deepest gratitude to my wonderful supervisors, Dr Alison Binns and Dr Tom Margrain, for their continual guidance and support at all stages of this research project. I am greatly indebted to them for all the time they have devoted to helping me, and feel honoured to have worked with them over the past two years. I am grateful to all the lovely participants in my studies that have given up their time to provide the data presented in this thesis, with only the promise of a cup of tea and a light at the end of the tunnel! Thank you to all my friends in room 2.10 for helping me perfect the art of procrastination. Thank you to the Eye Clinic staff for all their help throughout this time. Furthermore, a special thank you to Sue and all the lovely staff in the Optometry building for all their help and support. They put a smile on my face every day! Lastly, I would like to thank my family for their constant love, support and encouragement over the past two years. Without them, this work would not have been accomplished. Amazing grace, how sweet the sound That saved a wretch like me. I once was lost but now am found, Was blind but now I see. v Table of Contents Summary ..................................................................................................................... iii Acknowledgements ...................................................................................................... v List of Figures .............................................................................................................. ix List of Tables ............................................................................................................. xii List of Equations ...................................................................................................... xiii 1. Introduction .......................................................................................................... 1 1.1 General Introduction .................................................................................................... 1 1.2 The Healthy Retina ....................................................................................................... 1 1.2.1 Overview of the Retina ............................................................................................ 1 1.2.2 Bruch’s Membrane .................................................................................................. 3 1.2.3 The Retinal Pigment Epithelium .............................................................................. 4 1.2.4 Photoreceptor cells ................................................................................................... 5 1.2.5 The Macula .............................................................................................................. 6 1.2.6 Blood Supply to the Retina ...................................................................................... 7 1.3 Age-Related Macular Degeneration ............................................................................ 9 1.3.1 Background .............................................................................................................. 9 1.3.2 Clinical Classification ............................................................................................ 10 1.3.3 Clinical features of AMD ...................................................................................... 10 1.3.4 Grading scales for severity of AMD ...................................................................... 12 1.3.5 Risk Factors for the Development of AMD ........................................................... 14 1.3.6 Clinical Assessment of AMD ................................................................................ 17 1.3.7 Pathogenesis of AMD ............................................................................................ 22 1.3.8 Management .......................................................................................................... 26 1.4 Hypoxia and AMD ...................................................................................................... 29 1.4.1 Oxygen Demand of the Retina ............................................................................... 29 1.4.2 Oxygen Supply to the Retina ................................................................................
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