The Effect of Letter Size on the Accommodative Response a Thesis
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The Effect of Letter Size on the Accommodative Response A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By: Brian T. Landrum, BS Vision Science Graduate Program The Ohio State University Master’s Examination Committee: Donald O. Mutti, OD, PhD, Advisor Gilbert E. Pierce, OD, PhD Thomas W. Raasch, OD, PhD ABSTRACT Accommodative response was measured on fifteen subjects as they read successive lines on a standard Bailey-Lovie acuity chart at varying levels of defocus. Letter size had a measurable effect on the accuracy of the accommodative response. At large letter sizes the average accommodative response at a 4 D demand was 2.97 ± 0.36D. At smaller letter sizes the average response was significantly larger at 3.44 ± 0.24D (F3.2,45.3 = 22.4, p<0.0001, repeated measures ANOVA). The first significant increase in accommodative response was noted at a letter size of 9.7 minutes of arc (3.27 ± 0.22D; t14= 5.0, p=0.014). The relationship between logMAR acuity and myopic defocus was linear between 0 and –3D. The results were similar under cycloplegia; myopic and hyperopic defocus had roughly the same effect on visual acuity. Taking into consideration all of these factors; excessive lag during reading might be in the range of 1.75 to 2.25D. ii ACKNOWLEDGMENTS First, I would like to thank my advisor Dr. Don Mutti for his continued support, wisdom, guidance and willingness to help me achieve greater things throughout the time of completing this research and thesis. The T-35 grant (T35-EY07151) for providing the funds to complete the research project. The Vistakon Student Travel Fellowship which allowed me the funding to travel to the Academy meeting in 2007 and present this project. iii VITA Born…………………………….….April 21, 1983 BS…………………….…………........March 2001 Optometry Student @ OSU…Sept. 2005- Present T35 Grant Recipient………...……….…June 2001 Field of Study Major Field: Vision Science iv DEDICATION This thesis is dedicated to my mother Tammy who always encouraged me to strive to be the best and nurtured me along the path to success. v TABLE OF CONTENTS Abstract………………………….….…….………………………………………..ii Acknowledgements………………..……………….…………………………...…iii Vita……... ………………………..………………………………………………..iv Dedication…………………….….……………….…………………….………….v List of Figures……………….…………………….………………………………vii List of Tables……………….……………………………………….……………viii Chapters: 1. Introduction……………………………………………………………………1 2. Methods………………………………………………………………………16 2.1 Subjects………………………………………………………………..16 2.2 Procedure for Non-cyclopleged Data Collect…………………………17 2.3 Badal System………………………………………………………….19 2.4 Equipment Setup……………………………….……………………...20 2.5 Calibration of Instrument Vergence Level…………………………….24 2.6 Magnification of the Badal System…………………………………...25 2.7 Magnification at each Blur Level……………………………………...26 2.8 Protocol for Data Analysis…………………………………………….27 2.9 Procedure for Data Collection under Cycloplegia…………………….28 2.10 Cycloplegia…………………………………………………………...29 2.11 Protocol for Cycloplegic Data Collection…………………………….29 2.12 Data Analysis…………………………………………………………29 3. Results………………………………………………………………………...33 3.1Non-cycloplegic Visual Acuity Results…………………….…………..33 3.2Non-cycloplegic Lag Results………………….………………………..35 3.3Non-cycloplegic Pairwise Comparisons Letter to Letter…………….…36 3.4Acuity, Defocus: Myopic vs. Hyperopic Blur during Cycloplegia…….42 3.5Cycloplegic Visual Acuity Results……………………………………..44 3.6Myopic Comparisons Cyclopleged and Non-cyclopleged……………..46 4. Discussion…………………………………………………………………….49 5. Bibliography………………………………………………………………….57 vi LIST OF FIGURES Figure 1.1 Breakdown of Hyperopia………………..…….. 3 Figure 2.1 The Badal Track………..…..…………………. 22 Figure 2.2 View of the System…….…..……….…………. 23 Figure 2.3 Schematic with and without pinhole….………. 24 Figure 2.4 Exam Sheet….……………..………………….. 30 Figure 2.5 Scoring Sheet..……………..………………….. 31 Figure 2.6 Scoring Sheet.……….……..………………….. 32 Figure 3.1 Threshold Acuity vs. Focus Error….…….……. 34 Figure 3.2 Myopic Result Comparison...………….……… 35 Figure 3.3 Lag vs. Letter Size……….....…………………. 37 Figure 3.4 Cycloplegic Defocus………..……………........ 43 Figure 3.5 Complete Data Set of Defocus...…………...…. 44 Figure 3.6 Myopic Regression Cycloplegia………………. 45 Figure 3.7 Hyperopic Regression Cycloplegia………….... 46 Figure 3.8 Myopic Regression Non-cyclopegia.…………. 47 Figure 3.9 Comparing Myopia Regression……………….. 48 vii LIST OF TABLES Table 2.1 Magnification/Minification……………………..27 Table 3.1 Pairwise Comparisons..……..……………....38-41 viii CHAPTER ONE INTRODUCTION Refractive error occurs when light from a distant source does not come to a sharp focal point on the retina. The American Optometric Association (AOA) defines hyperopia as the refractive error where the axial length is shorter than the refracting components of the eye require for light to focus precisely on the photoreceptor layer of the retina 1. Hyperopia may result in combination with or in isolation from a relatively flat corneal curvature, insufficient crystalline lens power, increased lens thickness, short axial length, or variance in the normal separation of the optical components of the eye relative to each other 1. In the case of myopia, the light rays incident on the cornea are focused in front of the retina because either the cornea or the lens supplies too much power for the length of the eye. The only way to compensate for myopia is with optical correction. The options for correcting myopia include spectacles, contact lenses, or refractive surgery such as LASIK. In the case of hyperopia, the eye has a virtual far point. Parallel light incident on the cornea is focused behind the retina. Accommodative effort is needed to bring the image back 1 into the plane of the retina to maintain a sharply focused image. Low amounts of hyperopia or asymptomatic hyperopes are often left uncorrected. If the amount of hyperopia exceeds a clinician’s criterion or the patient is symptomatic, the patient has headaches, asthenopia or near blur, hyperopia can be corrected with convex lenses 1. Hyperopia, also termed farsightedness or hypermetropia, can be classified by the degree of refractive error or by the accommodative state of the eye. If it is classified corresponding to refractive error; it is said to be low if it is below +2.00D, moderate between +2.25D and +5.00D and high above +5.25D. If classified by accommodative status it can be facultative hyperopia, latent hyperopia, absolute hyperopia or manifest hyperopia 1. Facultative hyperopia is the amount of hyperopia that can be overcome by accommodation. Latent hyperopia is the amount of total hyperopia that is obscured by a failure to fully voluntarily relax accommodation. It is clinically revealed by a cycloplegic drug such as tropicamide or cyclopentolate. Absolute hyperopia cannot be overcome by accommodation and manifest hyperopia can be measured with plus lenses without the effects of a cycloplegic drug. Total hyperopia is the sum of latent hyperopia and manifest hyperopia or the sum of absolute and facultative hyperopia (Figure 1.1). 2 Total Hyperopia Manifest Latent Absolute Facultative = AMP Figure 1.1 This figure represents how total hyperopia may be broken down into its subparts. The sum of manifest and latent hyperopia or absolute and facultative hyperopia comprise total hyperopia. Hyperopia is less common in the United States population than myopia or astigmatism. Vitale et. al. showed that hyperopia in the United States, classified in their study as a spherical equivalent of greater than 3.0 diopters had a prevalence of 3.2-4.0% in those people twelve years and older while myopia comprised 33.1% of the population and astigmatism accounted for 36.2% 2. Williams showed that in the United Kingdom hyperopia is more common in those who are socioeconomically disadvantaged. Those who fell into this disadvantaged category were 1.82 times more likely to become hypermetropic 3. Hyperopia affects both children and adults. Bolinovska showed that in young people age 3-18 years the most frequent refractive error in the examined children was hyperopia with astigmatism, while anisometropia, a difference in the refractive error between the eyes, was found in 22% of children. It was also shown that the prevalence of hyperopia decreased in older children. A positive family history was correlated with refractive error, myopia or hyperopia, in 60.50% of children 4. The prevalence of hyperopia is age-related. When born, most full term babies are mildly hyperopic on the order of +2.4D ± 1.2D 5. Those infants who are born 3 prematurely or those of low birth weight tend to be less hyperopic (approximately +0.24D) 5. One study showed that in infants born prematurely, nearsightedness increased as gestational age decreased 6. At 12 weeks, 64% of all infants, all of whom were born at 31-33 and 34-36 weeks gestation, had normal vision. Nearsightedness prevalence at 1 year was less than it was at 6 months (16% vs. 32%), while the prevalence of farsightedness decreased (34 vs. 10 infants). Nearsightedness was inversely related to gestational age and birth weight. Nearsightedness and anisometropia existed only in infants weighing no more than 2000 g at birth. Anisometropia was also inversely related to gestational age. Astigmatism at 6 and 12 months was also associated with low birth weight 6. Refractive error generally decreases toward plano through a process called emmetropization. Emmetropization is the process that coordinates the growth of the eye’s