University of Bradford eThesis This thesis is hosted in Bradford Scholars – The University of Bradford Open Access repository. Visit the repository for full metadata or to contact the repository team © University of Bradford. This work is licenced for reuse under a Creative Commons Licence. STRUCTURAL AND FUNCTIONAL ASPECTS OF MYOPIA IN YOUNG ADULTS An investigation of nearwork-induced transient myopia and accommodation in relation to refractive stability Alison ALDERSON Submitted for the degree of Doctor of Philosophy Bradford School of Optometry and Vision Science University of Bradford 2011 Alison ALDERSON Structural and Functional Aspects of Myopia in Young Adults Keywords: Myopia, Accommodation, Ocular Biometry, Nearwork, Orthokeratology SUMMARY This thesis has investigated nearwork-induced transient myopia and accommodation responses in relation to refractive stability, multichromatic stimuli and orthokeratology. Five individual studies have been carried out. Initially an investigation into the temporal and dioptric aspects of nearwork-induced transient myopia was undertaken, suggesting that increased task duration does not increase the level, or slow the regression of post- task NITM, however an increase in the dioptric demand of the task does. In the second study, a longitudinal myopia progression study, these findings were related to short term myopia progression. The third investigation demonstrates the feasibility of measuring the biometric correlates of nearwork-induced transient myopia using a low coherence reflectometry device (LenStar, Haag Streit Koeniz, Switzerland). Fourthly, a comparison of the differences between static and dynamic accommodative responses, microfluctuations and nearwork-induced transient myopia produced when viewing a black/white target as oppose to a red/blue target has suggested the possibility of four accommodative responses to this multichromatic stimulus. Further investigation will be necessary to investigate if any of these response types are related to myopia progression. 1 The final study investigates the effect of two different designs of orthokeratology contact lenses (C5 and polynomial) on visual function. It appears to be the case that although the polynomial lens design has a larger refractive effect than the C5 lens it reduces both high and low contrast corrected visual acuity to a greater extent. The higher the baseline mean spherical equivalent refractive error the larger the detrimental effect. 2 CONTENTS PAGE SUMMARY 1 CONTENTS 3 DEDICATION 15 ACKNOWLEDGEMENTS 16 LIST OF TABLES 17 LIST OF FIGURES 19 CHAPTER 1 MYOPIA 25 1.1 INTRODUCTION 25 1.2 PREVALENCE 25 1.3 IMPACT 26 1.4 EMMETROPIZATION 28 1.5 OCULAR COMPONENTS 30 1.6 RISK FACTORS 31 1.6.1 Genetics verses environment 31 1.6.2 Nearwork 33 1.6.1.1 Nearwork and early onset myopia 34 1.6.1.2 Nearwork and late onset myopia 34 1.6.3 Accommodation 35 1.6.4 Accommodative stimulus response function 36 1.6.5 Nearwork-induced transient myopia 37 1.6.6 High AC/A ratio 40 1.6.7 Sports and outdoor activities 40 1.6.8 Aberrations 41 1.6.9 Relative peripheral hypermetropia 43 1.7 MYOPIA CONTROL 44 1.7.1 Under-correction 44 3 1.7.2 Correction with plus power for near work 45 1.7.3 Pharmaceutical agents 46 1.7.4 Contact lenses 47 1.7.5 Orthokeratology 48 1.7.6 Correction of relative peripheral hypermetropia 49 1.8 CONCLUSION 50 CHAPTER 2 INSTRUMENTATION 52 2.1 MEASUREMENT OF REFRACTIVE ERROR 52 2.1.1 Shin-Nippon SRW-5000 autorefractor 52 2.1.1.1 Validity and repeatability of the Shin-Nippon SRW-5000 53 2.1.2 Shin-Nippon NVision-K autorefractor 55 Validity and repeatability of the Shin-Nippon NVision-K 56 2.2 MEASUREMENT OF ACCOMMODATION 57 2.2.1 Shin-Nippon SRW-5000 for continuous recording of accommodation 57 2.2.1.1 Calibration of the Shin-Nippon SRW-5000 autorefractor for continuous recording of accommodation 59 2.2.1.2 Accommodation measurement 60 2.2.1.3 Precision of continuous recording 63 2.3 BADAL OPTOMETER 65 2.3.1 Presentation of accommodative stimulus during Shin-Nippon autorefractor measurements 66 2.3.2 Shin-Nippon SRW-5000 autorefractor in the measurement of the accommodative stimulus-response function (ASRF) 66 2.4 MEASUREMENT OF OCULAR PARAMETERS 68 2.4.1 IOLMaster 68 2.4.1.1 Measurement of corneal radius 68 4 2.4.1.2 Measurement of anterior chamber depth 67 2.4.1.3 Measurement of axial length 67 2.4.1.4 Validity and repeatability of the IOLMaster 69 2.4.2 LenStar 70 2.4.2.1 Validity and repeatability of the LenStar 71 2.5 MEASUREMENT OF MONOCHROMATIC ABERRATIONS 72 2.5.1 Shack-Hartmann aberrometer 72 2.5.1.1 Control panel of the Shack-Hartmann aberrometer 75 2.5.1.2 Presentation of accommodative stimuli within the Shack-Hartmann aberrometer 78 2.5.1.3 Calibration and verification of the Shack- Hartmann aberrometer 79 2.5.1.3.i Artificial eye calibration 79 2.5.1.3.ii Real eye calibration 80 2.6 MEASUREMENT OF VISUAL ACUITY 81 2.6.1 Bailey Lovie Chart 81 CHAPTER 3 TEMPORAL FACTORS AND DIOPTRIC DEMAND IN NEARWORK-INDUCED TRANSIENT MYOPIA 84 3.1 INTRODUCTION 84 3.1.1 Measurement of NITM 89 3.1.2 Refractive correction 90 3.1.3 Age of participants 91 3.1.4 Task paradigm 91 3.1.5 Comparison of results between NITM studies 92 3.1.6 Temporal factors and NITM 93 3.1.7 Dioptric value and NITM 94 5 3.1.8 Symptoms of NITM 95 3.1.9 Aim of the study 96 3.2 METHOD 97 3.2.1 Instrumentation 97 3.2.1.1 Refraction 97 3.2.1.2 Accommodation 97 3.2.1.3 The task 97 3.2.2 Procedure 100 3.2.2.1 Pre-task 100 3.2.2.2 Task 100 3.2.2.3 Post-task 101 3.2.3 Participants 101 3.2.4 Analysis 102 3.3 RESULTS 103 3.3.1 Experiment 1 103 3.3.1.1 Within-task accommodation 103 3.3.1.2 Post-task NITM values 104 3.3.1.3 Post-task group mean NITM values 105 3.3.1.4 Comparison between post-task NITM values of asymptomatic and symptomatic individuals 108 3.3.1.5 Regression 110 3.3.1.6 Post-task regression quotient for the whole cohort 111 3.3.1.7 Comparison between post-task NITM regression quotient for asymptomatic and symptomatic individuals 112 3.3.2 Experiment 2 114 3.3.2.1 Post-task group mean NITM values for the whole cohort 115 6 3.3.2.2 Comparison between post-task NITM values of asymptomatic and symptomatic individuals 116 3.3.2.3 Post-task regression quotient for the whole cohort 117 3.3.2.4 Comparison between post-task NITM regression quotient for asymptomatic and symptomatic individuals 118 3.4 DISCUSSION 120 3.4.1 Within-task accommodation values 120 3.4.2 Task duration and NITM 121 3.4.3 Dioptric demand and NITM 123 3.4.4 Sympathetic system and NITM 124 3.4.5 Experimental limitations 126 CHAPTER 4 MYOPIA PROGRESSION IN OPTOMETRY STUDENTS, OCULAR CORRELATES AND ASSOCIATION WITH NEARWORK-INDUCED TRANSIENT MYOPIA 127 4.1 INTRODUCTION 127 4.1.1 Prevalence of myopia in the general population 127 4.1.2 Prevalence of refractive errors in student populations 128 4.1.3 Difficulty in comparison of refractive error studies 129 4.1.3.1 Classification of refractive error 129 4.1.3.2 Measurement of refractive error 129 4.1.3.3 Age range 130 4.1.3.4 Analysis of data 130 4.1.4 Ocular correlates of myopia progression 130 4.1.5 Myopia progression 131 4.1.6 Myopia progression and nearwork-induced transient myopia 133 4.1.7 Aim of the study 134 7 4.2 METHOD 137 4.3 RESULTS 138 4.3.1 Participants 138 4.3.2 Analysis of initial data collection 139 4.3.2.1 Refractive error 139 4.3.2.2 Comparison of refractive error between males and females 140 4.3.2.3 Ocular components 141 4.3.2.4 Correlation of ocular components 142 4.3.2.5 Familial myopia 144 4.3.2.6 Nearwork and sport 145 4.3.2.7 Ethnic origin 146 4.3.3 Two year follow up data analysis 146 4.3.3.1 Refractive error 146 4.3.3.2 Myopia progression 147 4.3.3.2.i Progression in all participants 147 4.3.3.2.ii Myopia progression in refractive groups 148 4.3.3.2.iii Myopia progression and association with AL/CR 149 4.3.3.2.iv Myopia progression with nearwork and sporting activities 150 4.3.3.2.v Myopia progression and ethnic origin 151 4.3.3.2.vi Myopia progression and spectacle wear for nearwork 151 4.3.3.2.vii Subjectively reported nearwork-induced transient myopia and myopia progression 151 4.3.3.2.viii Myopia progression and objective measurement of NITM 154 4.4 DISCUSSION 156 4.4.1 Myopia prevalence in a student population 157 4.4.2 Ocular biometric correlates of myopia 158 8 4.4.3 Familial myopia 159 4.4.4 Myopia, nearwork and sporting activities 159 4.4.5 Ethnic origin and myopia 159 4.4.6 Myopia progression in a student population 160 4.4.7 Myopia progression and subjectively reported nearwork-induced transient myopia 161 4.4.8 Myopia progression and objective measurements of nearwork- induced transient myopia 162 4.4.9 Experimental limitations 162 CHAPTER 5 A BIOMETRIC INVESTIGATION OF NEARWORK-INDUCED TRANSIENT MYOPIA 164 5.1 INTRODUCTION 164 5.1.1 Accommodation 164 5.1.2 Biometric assessment of the accommodating eye 164 5.1.2.1 A-scan ultrasonography 165 5.1.2.2 Partial coherence interferometry 165 5.1.2.3 Scheimpflug technique 166 5.1.2.4 Magnetic resonance imaging 166 5.1.2.5 Optical coherence tomography 167 5.1.3 Change in axial length with accommodation 167 5.1.4 Change in anterior chamber depth with accommodation 168 5.1.5 Change in lens thickness with accommodation 168 5.1.6 Change in the position of the lens with accommodation 169 5.1.7 Structural changes of the eye during accommodation and the relation to myopia 169 9 5.1.8 Changes in ocular structure associated with nearwork-induced transient myopia 171 5.1.9 Aim of the study 171 5.2 INSTRUMENTATION 172 5.2.1 Measurement of refractive error 172 5.2.2