The Effect of Letter Size on the Accommodative Response a Thesis

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.

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

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.

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

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

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).

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 optical and axial components resulting in the development of a near emmetropic refractive error. The mechanisms which drive this process are not well understood;

however, several lines of evidence indicate that emmetropization is an active process

that is regulated by feedback associated with optical defocus and the eye’s refractive

state. This process results in a decrease in the amount of hyperopia and a reduction in

the variance of refractive error mostly through axial length changes 7. The process is

understood to take place between infancy and childhood. The main theory behind

emmetropization is that the eye responds to its own refractive error by changing its

rate of growth. Hyperopia is thought to stimulate the eye to grow longer axially and

myopic defocus is thought to be the “stop” signal to axial growth 8. Most of the time emmetropization occurred very quickly; within the first year of life. By age two the average human spherical equivalent is +1.00 diopters 9 10. Over the next one to two decades of life there is a relative decrease in hyperopia and an increase in the prevalence of myopia. Later in life, starting around the age of fifty, there is an increase in hyperopia of +0.48D spherical equivalent for people age 43-59, presumably due to changes in the refractive index of the crystalline lens among adults. This trend continues until the lens changes are so severe that it causes a myopic shift until the cataract is removed, -0.19D spherical equivalent, in those individuals over the age of seventy 11.

Hyperopia in infancy and childhood can result in poor visual development, amblyopia, and/or strabismus. As one of the most frequent amblyogenic factors in children, it represents a public health problem 4. Its signs and symptoms include blurred vision, accommodative dysfunction, asthenopia, amblyopia and strabismus in extreme cases 12 13 3. These symptoms tend to manifest themselves especially while the patient performs tasks up close like reading or computer work. If uncorrected, especially in children, it may affect learning performance in school. Shankar et. al. showed that uncorrected hyperopes lagged behind emmetropes when performing the following tasks: tests of letter and word recognition, receptive vocabulary, and emergent orthography and crowded visual acuity, despite no difference in phonological awareness skills, visual cognitive skills, and other family variables

known to affect the acquisition of literacy skills 14. Grisham et. al. also showed that

hyperopia and anisometropia appear to be related to poor reading progress and their

correction seems to result in improved performance 15. It has also been shown that

long term uncorrected hyperopia may cause motor skill deficits (manual dexterity,

balance, and ball skills) 16. These corrected hyperopes may also perform better in

school activities 17.

The question of whether or not to prescribe glasses for hyperopia is controversial.

It has been previously suggested that prescribing glasses for full time wear in children who do not have amblyopia or accommodative esotropia may interfere with the

process of emmetropization 18 19. However, Atkinson et. al. have shown that after

prescribing spectacles to hyperopic infants, corrected babies still underwent the same

minimal amount of emmetropization as uncorrected babies 20 12. Anker et. al.

compared hyperopes who were not corrected with glasses, those who were corrected

with spectacles as well as a control group without hyperopia. It was shown that

hyperopes who were untreated had a significantly poorer acuity outcome than the

control group at the same age. It was also shown that young hyperopes who were

compliant with spectacle wear had a better acuity outcome than the untreated

hyperopes. Poor acuity was defined as a best-corrected acuity of 6/12 or worse on crowded letters or 6/9 or worse on single letters, at age 4 years. They also showed that children who were significantly hyperopic at eight months old were more likely to be strabismic by 5.5 years of age compared to emmetropes. However, the study

found no difference in the incidence of strabismus between corrected and uncorrected

hyperopes 13. Refractive correction may also put a high bilateral hyperope at a lower

risk for the development of bilateral amblyopia 21.

Correction of childhood hyperopia, however, presents a challenge for the

clinician. The AOA states that the treatment of hyperopia is complex and several

factors need to be analyzed since there is no universal approach to its treatment. Some

patients may need to be corrected and some may not. This determination can be made

based upon specific elements such as the magnitude of hyperopia, presence of

astigmatism or anisometropia, patient’s symptoms, demand on the visual system, as

well as the state of accommodation, visual acuity, and efficiency of the visual system

when performing tasks. The treatment of hyperopia is directed towards achieving four

goals: reducing accommodative demand, providing clear, comfortable vision with

normal binocularity, remediating symptoms, and reducing the future risk of vision

problems 1. Some clinicians make the decision to prescribe based almost entirely on

distance visual acuity and the size of the refractive error 22. Controversies do exist

between optometrists and ophthalmologists over when to prescribe, what amount of hyperopia to prescribe for, and what signs and symptoms must accompany the child

with farsightedness. Lyons et. al. demonstrated different habits between optometrists

and ophthalmologists when it comes to prescribing correction for hyperopia. Her

survey showed that about one-third of optometrists surveyed would prescribe optical

correction for symptom-free 6-month-old infants with +3.00 D to +4.00 D hyperopia,

but fewer than 5% of ophthalmologists prescribe at this level. Furthermore, most ophthalmologists would not prescribe optical correction for symptom-free 2-year-old children unless they had +5.00 D of hyperopia or more. Most ophthalmologists

(71.4%) prescribe the full amount of astigmatism correction and less than the full amount of cycloplegic spherical component, and most optometrists (71.6%) prescribe

less than the full amount of both components. When prescribing less than the full

amount of astigmatism, eye care practitioners do not tend to prescribe a specific

proportion of the cycloplegic refractive error 23.

Despite this valuable information there still appears to be controversy over

correcting hyperopia between optometry and ophthalmology. Recent literature has

examined the prescribing habits of these two professions and compared them side by

side. This literature has found that optometrists prescribe at higher rates compared to

ophthalmologists 24. If an ophthalmologist was asked how he viewed optometrists

prescribing patterns he would deem them unnecessary in many cases 24. This literature is significant because it demonstrates two professions with the same goals in caring for the vision of children not sharing similar treatment plans and standards of care.

After a formal analysis, one can conclude that optometrists are not prescribing disproportionately more often for the lowest levels of refractive error (Mutti, letter to the editor, AAPOS). Eye examination results for referred children in the state of

Tennessee were reviewed to determine the prescribing patterns for children. The children were referred by a photoscreening program conducted by a group of

volunteers from the Tennessee Lions Outreach Program. The photoscreener that was being used to determine amblyogenic factors was the MTI (Riviera Beach, Florida).

A total of 102,508 preschool children were screened. When considering hyperopes between plano and +2.00D, 46.0% of optometrist prescriptions were in this range compared with 47.8% of ophthalmologist prescriptions, that is no association was found between the practitioner type performing the exam and the children’s degree of refractive error (χ² = 0.027, P = 0.87). Both optometrists and ophthalmologists saw the same sorts of children in this sample as there was no association between practitioner type performing the examinations and children’s degree of refractive error (χ2 = 0.75, P = 0.39). Yet optometrists did prescribe more often, 35.1% of the time, than general ophthalmologists (11.7%) or pediatric ophthalmologists (1.8%).

One might ask how we can get these two professions prescribing using the same standard of care. It is frustrating that there is this disagreement and controversy when current guidelines are “consensus and are solely based on professional experience and clinical impressions, because there are no scientifically rigorous data for guidance”

(Preferred Practice Pattern, Pediatric Eye Evaluations, American Academy of

Ophthalmology, p.13). Interestingly, there is far less controversy between the professions when it comes to myopia, also known as nearsightedness. In the case of a myope it is easy to prescribe as even the smallest amounts of nearsightedness will cause a reduction in distance visual acuity. The referral criterion for refractive correction for a child with distance blur due to myopia has long been understood and

very well agreed upon by optometrists, ophthalmologists, and researchers. The AOA states that in most cases of school children 0.50 D of nearsightedness is sufficient enough to warrant correction 25. One half to three quarters of a diopter of myopia decreases visual acuity enough that glasses are indicated and prescribed in a similar fashion amongst members of all the eye care professions. Both professions would agree that -0.75D of myopia would cause a significant visual loss to about 20/30 that would indicate the necessity of spectacle prescribing. This amount of myopia would be enough to decrease visual performance in school aged children and interfere with learning 25. Criteria similar to this may be useful in clinical decision-making when it comes to hyperopia. Such a criterion would also be more likely accepted by both professions if it were made analogous to that of myopia and distance visual acuity.

Refraction and visual acuity are standard tests in the routine eye care of patients.

Prescriptions and referrals are commonly made for nearsightedness based on the amount of distance blur; however, farsightedness mostly affects reading at near and there is no accepted guideline for referral due to the fact that poor reading may be an indicator of a poor reading skill or may be too much blur due to the farsightedness.

This uncertainty makes near visual acuity unreliable for serving as the referral criteria for hyperopia. Objective measurements may become the standard for referral for hyperopia if it is known how much blur creates an unacceptable reduction in a patient’s near visual acuity. Dynamic MEM retinoscopy measurement of accommodative lag may be one solution. It has been argued by Hunter to be a

valuable diagnostic tool in the management of hyperopic children 26. Perhaps clinically significant hyperopia is the amount that results in clinically unacceptable amounts of accommodative lag.

Accommodation is the process by which the eye increases optical power to maintain a clear image on the retina. Accommodative amplitude gradually decreases from about fifteen diopters in early childhood to one diopter before the age of sixty 27.

A hyperope uses accommodation to focus through the refractive error to maintain a

clear image. If hyperopic patients do this constantly the hyperopia can often be

missed by a clinician or not prescribed for unless they are symptomatic, unless a full

battery of near testing is performed, or unless cycloplegia is used. Accommodative

lag occurs when the eye does not accurately focus on the object of regard when there

is a stimulus to accommodate; in other words, the image is behind, not on the retina.

With lags of accommodation, the eye focuses behind the target causing the image to

fall behind the retina. The amount of lag is determined by the difference between the

stimulus value and the amount of the actual accommodative response. Depth-of-focus

is a widely used concept in visual science and it is of high importance when

considering refractive procedures and corrections. Depth-of-focus refers to the image quality and the greatest range of dioptric focusing error which does not result in an objectionable or noticeable subjective degradation in retinal image quality. Studies have shown that depth-of-focus decreases as pupil size increases; as expected based on optical principles 28. Also, as letter size increases depth-of-focus might also

increase. This is thought to happen because small focusing errors do not alter the

critical spatial frequencies of large letters; however, with small letters small amounts of defocus alter these critical spatial frequencies rendering the letter unreadable; for example, greater degradations in the modulation transfer function with defocus

occurred at high compared to moderate or low spatial frequencies. Atchison et al. also

found that the depth of field was much smaller; meaning the effect of blur was more

noticeable when subjects viewed higher spatial frequencies. 28. Majaj et. al. took this

process a little further and asked what spatial frequency channels do people use to

read different size letters. Majaj showed that people use different spatial frequency

channels to read different size letters. For small letters (high stroke frequency, high

spatial frequency) we use a high frequency channel. For large letters, we use a lower

frequency channel. These are not as affected by blur, although it’s a little higher frequency than the stroke frequency meaning we use a slightly higher frequency harmonic compared to the fundamental. People are able to tolerate more blur on larger letters without losing the ability to resolve them. The small amount of blur it takes to render a high spatial frequency letter unreadable does not produce the same

outcome at a lower spatial frequency 29.

Contrary evidence was provided by Lovasik et. al. who found that the

accommodative response was a fairly constant value across a range of different letter sizes. Letters ranged in size from 200 minutes of arc (20/800 equivalent) down to 5 minutes of arc (20/20 equivalent). All accommodative responses were within 0.50D

of one another 30. In other words, the function of accommodative response versus letter size in the aforementioned study was flat.

A recent study 31 measuring contrast sensitivity for up to 20 spatial frequencies ranging from 1 to 20 c/deg was completed under cycloplegia at different levels of positive and negative defocus in myopes and non-myopes. The non-myopes contrast sensitivity was reduced in a systematic fashion as the amount of defocus increased.

This reduction was similar for positive and negative lenses of the same power.

Myopes; however, showed a contrast sensitivity loss that was significantly greater

with positive defocus compared to negative defocus 31. It is possible therefore that

defocus from uncorrected myopia may have a different effect on acuity than

hyperopic defocus from accommodative lag. A referral criterion based on lag may be

different than the standard of 0.50D to 0.75D of blur from myopia.

Previous investigators have studied the relationship between acuity and

accommodative demand. Johnson found that near acuity was constant for

accommodative demands up to three diopters for individuals 22-24 32. Others have

tested acuity at larger accommodative demands, closer distances, and found that there

is a reduction in near visual acuity between three and five diopters of demand 33. The reason that near visual acuity was reduced at these larger demands was due to accommodative lag and near focus error. However, the authors did not quantify the relationship between the degree of near focus error and the drop in acuity at near. A similar relationship was reported by Subbaram and Bullimore in 2002 in a sample of

adults 34. They reported an approximate one line decrease in acuity for every 0.50

diopter of near focus error.

Johnson showed the importance of luminance as it pertains to visual acuity and

the accuracy of the accommodative response. At the high luminance levels

(51.42cd/m2), errors in accommodation were minimized. As the luminance was decreased, accommodation appeared to approach a fixed focus corresponding to an

intermediate distance at low luminance level (0.051cd/m2). Also noted was the

decrease in visual acuity as the luminance level was decreased as one would expect.

Resolution visual acuity also varied with target distance, showing a maximum at an intermediate distance and diminishing for both near and far. This change in acuity and

accuracy of the accommodative response was closely related over a range of

luminance levels and stimulus distances 32.

The purpose of this thesis work is to understand the relationship between blur at

near and visual acuity at near in comparison to the same relationship at distance.

Previous research has suggested that an accommodative lag of 0.75D or greater

measured objectively may need further consideration 35. This information may

provide guidance as to the level of blur that is tolerable for good reading

performance, and perhaps setting a criterion for whether or not to prescribe glasses

for children with hyperopia. The relationship between letter size and accommodative response will also be investigated. This information may help to determine an appropriate target size to use when testing a child’s accommodative state as children

do not read small detailed print in school books. Understanding the relationships between near acuity, accommodative response, and hyperopic blur may improve the management of the hyperopic child by primary care practitioners.

CHAPTER TWO

METHODS

2.1 Subjects

Fifteen adult subjects participated in this study. They were a sample of convenience as they were recruited through a mass email sent to all students, faculty and staff of The Ohio State University College of Optometry. The first fifteen subjects who replied back with interest and met the inclusion criteria were accepted for the study. To be eligible subjects had to be less than 35 years of age, be able to correctly read the 0.0 logMAR line on a standard Bailey-Lovie chart at six meters

with a current best spectacle correction, and have good ocular health (no systemic or

ocular disease that would affect accommodation such as Multiple Sclerosis or

Parkinson’s disease, or accommodative excess among other problems).

The fifteen subjects ranged in age from 20.8 - 31 years old (24.7 ± 2.8 years,

mean ± SD). There were 4 males and 11 females. Subjects ranged in refractive error

from +2.52D to -6.80D spherical equivalent (-2.09 ± 2.85D, mean ± SD). If the

subjects were not emmetropic they wore best spectacle correction for the experiment.

The subjects were asked about their ocular and systemic health as well as current medications and this was recorded on an examination sheet in order to verify that there was no history of accommodative problems as well as health problems that would confound any experimental variables or interfere adversely with cycloplegia

(e.g., cardiac or respiratory problems).

The study consisted of two sessions; one in which the subject was free to accommodate, without cycloplegia, and the second session where the subject was tested following cycloplegia. All fifteen subjects completed the first session. Twelve subjects completed the second session. The Ohio State University’s Biomedical

Institutional Review Board, in accordance with the tenets of the Declaration of

Helsinki, approved the study protocol. Subjects were educated on the purpose and the procedures of the study, and written consent was obtained before enrollment into the study.

2.2 Procedure for Non-Cyclopleged Data Collection

A standard Bailey-Lovie chart (prepared by the Multimedia Center at the School of Optometry, University of California, Berkeley) was placed on the opposite side of the laboratory and inverted six meters away from the WR-5100 Grand Seiko autorefractor (RYUSYO Industrial Co. LTD., Osaka, Japan. Serial No. 41M0040).

The instrumentation was cleaned thoroughly with an alcohol pad. The subject was put

behind the autorefractor without spectacle correction and aligned. The proper alignment consisted of the patient’s chin completely resting in the chinrest with the mouth closed. The forehead was then placed flush against the forehead rest and the patient was asked not to move. The occluder was then placed over the left eye. The chinrest was then moved so that the patient’s lateral canthus was correctly aligned with the marking on the headrest. The patient’s pupil was then brought into a clear focus with the autorefractor target in the center of the pupil and the experiment was ready to be run. A small target was placed on the Badal track and the subject was asked when it came into a sharp focus without their habitual correction on. When it did the subject was then instructed to concentrate on the smallest row of letters and keep it clear while readings with the autorefractor were taken on the right eye only.

These five readings were averaged and served as baseline refractive error data. The subject then put on their habitual refractive correction (if one was necessary to obtain

20/20 vision) and they were positioned in the autorefractor properly with the left eye occluded. Subjects were asked to read successive lines from largest to smallest (37.8 to 1.9 minutes of arc) of a standard Bailey-Lovie acuity chart one letter at a time. As the subject read a letter an autorefractor reading was simultaneously taken. The autorefractor target was in the center of the pupil which was kept in focus each time a reading was taken. If there was an error or an incorrect reading (for example the autorefractor would give the reading “cyl over”) the subject was asked to reread the letter and the response was measured again before continuing. Subjects were

encouraged to proceed onto the next line until three of five letters were missed within

a row. Guesses were mandatory until the subject failed to name three of five letters

correct on a row. Measurements were only taken on the right eye of each subject.

This procedure was completed a total of seven times per subject during this first session. The chart was presented at a total of five different stimuli levels in the following order: 0D, -1D, -2D, -3D of myopic blur (+1D, +2D, +3D lenses) followed by +4D of hyperopic blur (-4D lens) presented three times in a row. Two different

Bailey-Lovie charts were alternated between stimulus levels to help prevent memorization by the subject. This procedure was both audio and video recorded so that it could be analyzed at a later date to determine accommodative response and acuity threshold.

2.3 Badal Setup

A Badal lens system was utilized because it was realized that neither a computer monitor nor a printer had the ability to produce/print quality letters as small as 5 minutes of arc. From a viewing distance of 0.25m a letter subtending 5 minutes of arc would be 0.36mm tall (see calculation).

ATAN(x/0.25m) = 5 minutes of arc = 0.0015radians

X=0.36mm

The lab has a HP4050 printer. This printer produces and image containing 1200 dots per inch (per HP). This means each letter would have 17.0 dots (see calculation).

This would make for a pixilated letter and for poor viewing quality.

Dots/Letter = (1200/inch) x (1 inch/25.4mm) x (0.36mm/letter) = 17.0

A full HD (1080p) 15 inch monitor could also not be used as it would produce a poorer letter quality than the printer would. A monitor would produce a letter quality of 1.02 dots per letter (see calculation).

(1080lines/15 inch monitor) x (1 inch/25.4mm) x (0.36mm/letter) =

1.02 lines/letter

2.4 Equipment Setup

A Badal Track was mounted on the WR-5100 Grand Seiko autorefractor because, as noted above, neither a computer screen nor a laser printer could produce font size small enough to test threshold visual acuity at near. The Badal Track was bolted to the tray on the autorefractor to ensure that it remained steady and aligned for each subject. It was also equipped with an extra +6.25D lens which served to produce the image of the distant Bailey-Lovie chart for the Badal lens so that it was possible to test near threshold acuity. The Bailey-Lovie charts were six meters from the autorefractor at the end of the room where they were mounted to the wall to make sure that they remained at a constant height between trials and between subjects.

Sufficient lighting was supplied by overhead lights as well as a desk lamp and a stand lamp. The luminance of the chart without the Badal system in place 1 meter from the chart was 91.5 Footlamberts (313.5cd/m2). The luminance of the chart through the

Badal lenses and the autorefractor was 49.5 Footlamberts (169.6cd/m2). Both of these values were measured with a photometer (Pritchard Photometer, Model 1980A

(Kollmorgen Corporation). This value is well within the photopic range.

The different stimuli levels were marked on the Badal track and the +6.25D lens was aligned with the appropriate mark to create the corresponding blur level. During all testing the occluder on the autorefractor was in front of the left eye. A VCR was connected with the autorefractor and a TV monitor. Each session was taped and audio recorded. The microphone was placed on the table so that it did not interfere with any readings or distract the subject. Proper alignment was maintained by using the target on the display on the WR-5100 Grand Seiko and the number of letters correct per line was kept track of by the researcher on a replica of the corresponding Bailey-Lovie chart next to the autorefractor out of the subject’s view.

Figure 2.1 The Badal track system mounted on the autorefractor without pinhole.

Figure 2.2 Photograph of the system without pinhole from the subjects’ point of view (not aligned in order to show the chart inversion).

A) Upright Image of Chart EVZDR HNPEV

UPNFH

Inverted Chart +6.25D lens +6.25D lens Subject at 6m

B) Upright Image of Chart

EVZDR

HNPEV

UPNFH

Inverted Chart Pinhole +6.25D lens +6.25D lens Subject at 6m

Figure 2.3 A) Schematic of the system without the pinhole. B) Schematic of

system with pinhole.

2.5 Calibration of Instrumentation Vergence Level

To calibrate the instrumentation a CCD video camera module was used as if it were an emmetrope. It was focused for infinity while looking outside at very distant objects and then placed behind the autorefractor in the appropriate position (subject height and vertex distance). A +6.25D lens was put onto the Badal Track and the image of the Bailey-Lovie acuity chart at six meters was brought into a sharp, well-

defined focus on a TV monitor adjacent to the examiner. The lens position was marked with a pencil on the Badal Track. This was repeated a total of ten times after moving the +6.25D lens to the back of the Badal Track so that the chart was blurred and then brought back into focus. These lines were averaged into one line and this place on the track was noted as the 0D setting. This process was repeated for the -1D,

-2D, -3D, of myopic blur and +4D hyperopic blur settings using the appropriate trial lenses taped in the spectacle plane of the camera to create the desired blur stimulus.

The positions on the track were then observed subjectively with a ruler and determined to be equidistant from one another. This further confirmed the accuracy of the Badal nature of the calibration of the blur levels.

2.6 Magnification of the Badal System

First, black, outlined squares with a hollow center, all of uniform size, were printed off on a laser printer and hung where the Bailey-Lovie chart was going to be placed. They were constructed using Microsoft Word. They were taped to the wall as if they were an upside down triangle to represent the Bailey-Lovie chart converging more towards the bottom. These images were recorded on video tape with and without the Badal lens in place. However, Image Analyst 8.23 hard a hard time differentiating the borders of these squares. Next, solid black squares of the same size were printed off and taped to the wall in the same manner. These squares were easily distinguishable by the Image Analyst program. They were analyzed in terms of

height, width, and area both with and without the Badal lens in place. These values were recorded in Microsoft Excel. This was done to determine constancy across the expanse of the chart as well as the magnification properties of the Badal lens.

The program was then asked to compute the distance between the 20/63 row and the

20/20 row of the Bailey-Lovie chart with and without the Badal lens in place. For this procedure we used the Bailey-Lovie chart and captured the solid lines (one double line and one single line) next to these rows to analyze the vertical magnification. All of this information was used to compute the total magnification of the Badal system.

The Badal lens magnified the image by 1.208 times at the 0D defocus level.

2.7 Magnification at each Blur Level

Magnification of the +6.25D lens was also accounted for at each blur stimulus using Image Analyst 8.23 to ensure that the setup was truly Badal. Thirty standard outlined squares with a hollow center were constructed with Microsoft Word and then were taped onto the wall at the six meter test distance. They were taped to the wall after it was ensured they were level, equidistant from one another, and covered the same amount of space as the acuity chart. An image of this was captured on videotape with and without the Badal lens in place and then analyzed with the Image Analyst program. Once again the squares could not be differentiated by the software so thirty solid black squares were used and hung in the same manner. The Image Analyst software was again unable to differentiate the squares because of the blur. An array of

LED lights was used and imaged at every blur stimulus. The distance between the

centers of these blurred images was measured and charted in Microsoft Excel. These

were analyzed at every blue stimulus with and without the +6.25D lens in place.

Blur Level Minification/Magnification

-4D 0.985

0D 1.000

+1D 1.005

+2D 1.011

+3D 1.013

Table 2.1 The first column represents the trial lens used, the second is the total magnification or minification, as a total, of the entire system.

2.8 Protocol for Data Analysis

Every stimulus level was both audio and video recorded. Threshold visual acuity was determined by the total number of letters correct at each stimulus level. A replica of the chart was printed out and if the subject correctly identified a letter on the chart it was circled on the sheet; if the letter was incorrectly identified it was marked with a slash on the sheet. Each number of letters per row and the total number of letters

correct at each stimulus was recorded on the sheet as well as in Microsoft Excel. This

was done while watching the tape playback of the study session.

Each stimulus response was recorded in a Microsoft Excel spreadsheet. The tape

was replayed and it was paused after each autorefractor reading. The sphere, cylinder

and axis values were recorded in the spreadsheet. The number of responses entered

into the spreadsheet was then compared with the number of letters read by the subject

to ensure that no readings had been skipped over on the tape. This process was done

for all fifteen subjects.

2.9 Procedure for Data Collection under Cycloplegia

The same subject pool was used for the second visit of the study. The setup remained the same except that the blur stimulus levels of +1D, +2D and +3D of hyperopic blur had to be added to the Badal Track. This was done in the same fashion as the non-cycloplegic data where we used the appropriate trial lenses to determine the correct placement for the lens on the Badal Track. The stimuli levels that the subject was going to be tests on now included: -3D, -2D, -1D, 0D, +1D, +2D, +3D and +4D. They were also randomized using Microsoft Excel so that there was no set order in which they were presented. Each was presented one time and only right eyes were tested again. The subject read the chart starting in the upper left corner and proceeded until they could not identify three of five letters in a row correctly. While this was being done an autorefractor reading was taken simultaneously with each

letter read. Also an artificial pupil was attached to the +6.25D extra lens 155 millimeters behind in Maxwellian view so that all subjects experienced controlled, constant pupil conditions. The size of the artificial pupil was 4.2 millimeters. The pupil was positioned so that the subject could see the entire chart before each stimulus level was tested. These sessions were audio and video recorded.

2.10 Cycloplegia

Prior to drop instillation all subjects were examined with slit lamp biomicroscopy to ensure that the angle structures were open. This was done using Von Herrick angle estimation technique. Cycloplegia was then accomplished using two drops of tropicamide 1% instilled into the right eye five minutes apart. Twenty five minutes was allowed to pass after the second drop had been instilled to start the experiment.

2.11 Protocol for Cycloplegic Data Collection

This was done in the same manner as the non-cycloplegic data. It was gathered from the recorded sessions and charted in Microsoft Excel.

2.12 Data Analysis

Data were analyzed using SPSS (v. 16.0) repeated measures ANOVA with multiple pair wise comparisons with Bonferroni correction.

Name Last: First: MI: Date: Date of Birth: Gender: Male (0) Female (1) Subject ID: Visual Acuity: General Health/History/Problems:

______

______Meds:______

______

Ocular Health/History/Problems:

______

Figure 2.4 This is the preexam form which was used to make sure subjects were eligible to participate in the study.

Score Sheet Letter Chart HEFPU Given by: Brian Landrum

Subject ID: Date: Diopter

Setting:

Circle=letter identified correctly Crossed out=letter not identified correctly # Letter correct per line H E F P U

E P U R Z

H N R Z D

**F N H V D

N D Z R U

V D E H P

N F V H D

N R E H U

*R Z V D E

D H E V P

E P N R Z

H P V D U

N U P F H

Z P E H R

Score:

Figure 2.5 This is the first of two score sheets used in the scoring process of the study. 31

Score Sheet Letter Chart DVNZR Given by: Brian Landrum

Subject ID: Date: Diopter

Setting:

Circle=letter identified correctly Crossed out=letter not identified correctly Letter # correct per line D V N Z R

H N F D V

F U P V E

**P E R Z U

F H P V E

Z R F N U

P R Z E U

F V P Z D

*U P N F H

R Z U F N

F H U V D

N E F Z R

Z D R V E

U D F V N

Score: Figure 2.6 This is the second sheet used to score subjects as they completed the study. 32

CHAPTER THREE

RESULTS

3.1 Non-cycloplegic Visual Acuity Results

Myopic blur produced a decrease in visual acuity as one would expect (Figure

3.1). This decrease in acuity was observed to progress in a monotonic fashion as the myopic blur was increased. This trend has previously been documented by several investigators 36 37 38 39. The data from the current study are very similar when

compared with the results from these previous authors (Figure 3.2).

Hyperopic blur produced no decrease in visual acuity without cycloplegia as

subjects were free to accommodate as needed. All subjects accommodated reasonably

accurately as they were all able to read lines as small as three minutes of arc at the

four diopter hyperopic blur vergence level. The accommodative response noted at the

four diopter hyperopic blur stimulus was centered over +0.50 to +0.75D. One would

expect that the response would be centered over zero diopters; however, the autorefractor is calibrated to mimic a subjective refraction where clinicians use

maximum plus. This half diopter plus that is observed may be representative of the depth of field (see Discussion).

1.0 0D 0.8 -1D 0.6 -2D -3D 0.4 +4D 0.2

0.0

-0.2 Threshold Acuity Threshold Acuity (LogMAR) -0.4 -5 -4 -3 -2 -1 0 1 2

Defocus (D)

Myopic Blur Hyperopic Blur

Figure 3.1 Threshold Visual Acuity vs. Focus Error

This graph shows the relationship between threshold visual acuity and defocus in subjects without cycloplegia who participated in the study. It is noted that as myopic blur is introduced visual acuity decreases. When hyperopic blur is introduced subjects accommodated accurately and were able to able to see letters as small as –0.20LogMAR.

1.40 0D 1.20 Raasch 1995 .... 1.00 Akutsu 2000 ‐‐‐ -1D Thorn 1990 0.80 -2D 0.60 -3D

0.40 +4D 0.20 0.00 -0.20 Threshold Acuity Threshold Acuity (LogMAR) -0.40 -5 -4 -3 -2 -1 0 1 2 Defocus (D)

Myopic Blur Hyperopic Blur

Figure 3.2 This graph depicts the data collected under non-cycloplegic conditions compared to three other studies 36 37 39. The trend lines are very comparable to that which was collected in the current study.

3.2 Non-cycloplegic Lag Results

Mauchly’s test of sphericity was run and deemed significant at p<0.0001. This required use of the Greenhouse-Geisser adjustment in degrees of freedom for within- subject effects. Subjects exhibited a larger lag, less accurate accommodative response, when reading large letters. The converse was also noted; a smaller lag, 35

more accurate accommodative response occurred, when reading smaller letters. At

letter sizes of 37.8 minutes of arc the average accommodative response was 2.97 ±

0.36D at the four diopter accommodative demand. Therefore, subjects lagged about one diopter with these sized letters when presented with a four diopter stimulus. At smaller letter sizes, three minutes of arc, the average accommodative response was significantly larger at 3.44 ± 0.24D (F3.2,45.3 = 22.4, p<0.0001, repeated measures

ANOVA). On these small letters the amount of lag was only about 0.50D. As subjects

read from large to small letters a significant increase in the accommodative response

was noted. Compared to the response at 37.8 minutes of arc, the first significant

increase in accommodative response occurred at 9.7 minutes of arc (Figure 3.3); 3.27

± 0.22D (t14 = 5.0, p=0.014).

Large hyperopic lag values were not observed in this part of the study due to its

non-cycloplegic nature. This warranted a further cycloplegic investigation.

3.3 Non-cycloplegic pairwise comparisons letter to letter

Adjustments were made for multiple comparisons using Bonferroni

correction.

0.4 c d * e d e h e g 0.6 a a f a a b e f b b f d h c c e g f 0.8 a b h g Lag (D) b h Accommodative

1.0

1.2 38302419151210865432 Letter Angular Subtense (min)

Figure 3.3 depicts the data collected for non-cycloplegic lag findings. Letters in common represent values which are not statistically different. This graph is only depicting the four diopter accommodative stimulus.

* n=9: 6 subjects unable to read line successfully, not included in comparisons.

This graph depicts the change in accommodative lag as a function of letter size in non-cyclopleged subjects. One can note that as letter size is decreased the accommodative response becomes more accurate (less lag). As subjects read from large to small letter sizes the first significant increase in accommodative response was noted at 9.7 minutes of are (3.27 ± 0.22D; p=0.014).

Table 3.1 Pairwise Comparisons

Letter Size Letter Size Mean (min) (min) Difference Std. Error Sig.(a)

38 30 .063 .044 1.000 24 .122 .050 1.000 19 .170 .056 .613 15 .196 .074 1.000 12 .253 .069 .176 10 .300(*) .060 .014 7 .335(*) .066 .012 6 .368(*) .066 .005 5 .398(*) .065 .002 4 .457(*) .073 .001 3 .473(*) .073 .001

30 38 -.063 .044 1.000 24 .058 .031 1.000

19 .107 .044 1.000 15 .133 .051 1.000 12 .189(*) .043 .041 10 .237(*) .038 .001 7 .271(*) .047 .003 6 .305(*) .054 .004 5 .335(*) .053 .001 4 .394(*) .065 .002 3 .410(*) .058 .000 24 38 -.122 .050 1.000 30 -.058 .031 1.000

19 .049 .037 1.000 15 .074 .046 1.000 12 .131 .050 1.000

10 .178(*) .037 .019 7 .213 .053 .080 6 .247(*) .049 .012

5 .277(*) .053 .009 4 .335(*) .069 .016

3 .352(*) .065 .006

(continued) Table 3.1 Pairwise comparisons for the non-cycloplegic data set based on estimated marginal means, * The mean difference is significant at the .05 level. a Adjustment for multiple comparisons: Bonferroni 38

Table 3.1 Pairwise comparisons for the non-cycloplegic data set

Letter Letter Size Size Mean (min) (min) Difference Std. Error Sig.(a) 19 38 -.170 .056 .613 30 -.107 .044 1.000 24 -.049 .037 1.000 15 .026 .033 1.000 12 .083 .037 1.000 10 .130(*) .029 .038 7 .165 .042 .108 6 .198(*) .035 .004 5 .228(*) .037 .002 4 .287(*) .054 .007 3 .303(*) .050 .002 15 38 -.196 .074 1.000 30 -.133 .051 1.000 24 -.074 .046 1.000 19 -.026 .033 1.000 12 .057 .029 1.000 10 .104 .027 .105 7 .139 .035 .093 6 .172(*) .038 .029 5 .203(*) .033 .002 4 .261(*) .049 .007 3 .277(*) .041 .001

12 38 -.253 .069 .176 30 -.189(*) .043 .041

24 -.131 .050 1.000 19 -.083 .037 1.000

15 -.057 .029 1.000 10 .047 .024 1.000

7 .082 .027 .553 6 .115 .034 .271 5 .146 .036 .072 4 .204(*) .045 .029 3 .221(*) .038 .003 (continued)

Table 3.1 Pairwise comparisons for the non-cycloplegic data set

Letter Letter Size Size Mean

(min) (min) Difference Std. Error Sig.(a) 10 38 -.300(*) .060 .014 30 -.237(*) .038 .001 24 -.178(*) .037 .019 19 -.130(*) .029 .038 15 -.104 .027 .105 12 -.047 .024 1.000 7 .035 .027 1.000 6 .068 .028 1.000 5 .099 .033 .679 4 .157 .047 .328 3 .173 .041 .060 7 38 -.335(*) .066 .012 30 -.271(*) .047 .003 24 -.213 .053 .080 19 -.165 .042 .108 15 -.139 .035 .093 12 -.082 .027 .553 10 -.035 .027 1.000 6 .033 .030 1.000 5 .064 .029 1.000 4 .122 .037 .366 3 .138(*) .032 .041 6 38 -.368(*) .066 .005 30 -.305(*) .054 .004 24 -.247(*) .049 .012 19 -.198(*) .035 .004 15 -.172(*) .038 .029 12 -.115 .034 .271

10 -.068 .028 1.000 7 -.033 .030 1.000

5 .030 .030 1.000 4 .089 .038 1.000

3 .105 .039 1.000 (continued)

Table 3.1 Pairwise comparisons for the non-cycloplegic data set

Letter Letter Size Size Mean (min) (min) Difference Std. Error Sig.(a) 5 38 -.398(*) .065 .002

30 -.335(*) .053 .001 24 -.277(*) .053 .009 19 -.228(*) .037 .002 15 -.203(*) .033 .002 12 -.146 .036 .072 10 -.099 .033 .679 7 -.064 .029 1.000 6 -.030 .030 1.000 4 .059 .025 1.000 3 .075 .022 .305 4 38 -.457(*) .073 .001 30 -.394(*) .065 .002 24 -.335(*) .069 .016 19 -.287(*) .054 .007 15 -.261(*) .049 .007 12 -.204(*) .045 .029 10 -.157 .047 .328 7 -.122 .037 .366 6 -.089 .038 1.000 5 -.059 .025 1.000 3 .016 .018 1.000 3 38 -.473(*) .073 .001 30 -.410(*) .058 .000 24 -.352(*) .065 .006 19 -.303(*) .050 .002 15 -.277(*) .041 .001 12 -.221(*) .038 .003 10 -.173 .041 .060 7 -.138(*) .032 .041 6 -.105 .039 1.000 5 -.075 .022 .305 4 -.016 .018 1.000

(continued)

3.4 Acuity, Defocus: Myopic vs. Hyperopic Blur during Cycloplegia

For the subjects in the current study visual acuity showed a linear deterioration as the blur level was increased whether or not the blur is myopic or hyperopic. The slopes of these two lines appears to be similar demonstrating that defocus has the same affect on visual acuity no matter its sign. As in the non-cycloplegic data, where these two lines intersect is centered above +0.50 diopters. This value is representative of depth-of-focus. The autorefractor is calibrated to mimic the refraction of a skilled clinician where maximum plus is pushed. This value is typically +0.50D.

1.0

0.8

(LogMAR) 0DIL 0.6 ‐1DIL 0.4 ‐2DIL Acuity ‐3DIL 0.2 +1DIL

Visual +2DIL

0.0 +3DIL +4DIL ‐0.2

Threshold ‐5 ‐4 ‐3 ‐2 ‐10123456

Defocus (D)

Figure 3.4 A depiction of threshold acuity vs. defocus for the cycloplegic hyperopic and myopic stimuli.

1.2 OUND 1.0 ‐1UND 0.8 ‐2UND (LogMAR)

0.6 ‐3UND 0.4 +4UND 0DIL Acuity

0.2 ‐1DIL 0.0 ‐2DIL Visual ‐0.2 ‐3DIL ‐0.4 +1DIL +2DIL ‐0.6 +3DIL

Threshold ‐5 ‐4 ‐3 ‐2 ‐10123456 +4DIL Defocus (D)

Figure 3.5 A depiction of the entire data set, cycloplegic and non-cycloplegic threshold acuity vs. defocus both myopic and hyperopic.

3.5 Cycloplegic visual acuity results

Orthogonal linear regression was performed for the cyclopleged myopic data and

the cycloplegic hyperopic data to determine if the slopes were statistically different.

The bivariate slope for the myopic blur data was -0.24 with a 95% confidence interval from -0.27 to -0.21 (Figure 3.6). The bivariate slope for the hyperopic blur data was

0.22 with a 95% confidence interval from 0.18 to 0.26 (Figure 3.7). It is noted that the

confidence levels overlap for the two scenarios (after allowing for the signs to be

opposite). It can be stated with the current sample size that myopic blur and

hyperopic blur have the same effect on degradation on visual acuity with increasing blur levels, as would be predicted from basic geometric optics assuming complete cycloplegia.

1.0

0.8 y=‐0.24x+0.11 (LogMAR)

0.6 Acuity

0.4

0.2 Visual

0.0

‐0.2 Threshold ‐4 ‐3 ‐2 ‐10 1 Defocus (D)

Figure 3.6 A depiction of myopic threshold acuity vs. myopic blur under cycloplegia.

1.0

0.8 y=0.22x+ (‐0.18) (LogMAR) 0.6

Acuity 0.4

0.2 Visual

0.0

‐0.2 Threshold 012345 Defocus (D)

Figure 3.7 A depiction hyperopic threshold visual acuity vs. hyperopic blur under cycloplegia.

3.6 Myopic comparisons cyclopleged and non-cyclopleged

The slope of the orthogonal regression for non-cycloplegic data was more

negative, -0.31(95% confidence interval -0.34 to -0.28), than it was under

cycloplegia, -0.24 (95% confidence interval -0.27 to -0.21). The 95% confidence bounds also do not overlap; therefore, myopic blur under cycloplegia has a different effect on the visual acuity measured at each stimulus level than myopic blur not under cycloplegia. This occurred whether or not the extreme stimuli, 0D or -3D, were

included in the computation (Figure 3.9). This change in visual acuity could be due to

aberrations, pupil size, or other factors. Another factor analyzed was whether acuity with no defocus was worse under cycloplegia versus without cycloplegia. A paired t- test showed that acuity was worse under cycloplegia by an average of -0.27 ± 0.13

LogMAR (t11=-7.0 p<0.0001).

1.0

0.8 y=‐0.31+(‐0.20) (LogMAR) 0.6

0.4 Acuity

0.2 Visual 0.0

‐0.2

‐0.4 Threshold ‐5 ‐4 ‐3 ‐2 ‐10 1 Defocus (D)

Figure 3.8 A depiction myopic threshold visual acuity vs. myopic blur under non- cycloplegic conditions.

1.0

0.8 Dilated Myopic

0.6

0.4

Dilated 0.2 y=‐0.24x+0.11 Undilated 0.0 y=‐0.31+ (‐0.20)

‐0.2

Theshold Visual Acuity (LogMAR) ‐0.4 ‐5 ‐4 ‐3 ‐2 ‐10 1 Defocus (D)

Figure 3.9 compares the myopic data both with and without cycloplegia.

CHAPTER FOUR

DISCUSSION

The subjects in this study had normal ocular health, visual acuity, systemic health, and were taking no medicines that could interfere with accommodation determined by standard clinical testing (acuity, slit lamp, pre-exam questioning). During the first phase of the study subjects were asked to read letters of various letter sizes at varying myopic demands as well as plano and a +4 diopter hyperopic stimulus under normal conditions (no cycloplegia). When comparing threshold visual acuity with defocus, between -3 and -1 diopters of defocus, the same trend is observed in the current study pool as has been observed with past research (see Figure 3.2). As one increases the myopic defocus, threshold visual acuity decreases as well. This relationship is nearly linear. This concept is widely accepted and well known throughout the professions of optometry and ophthalmology. Due to this relationship, a reduction in vision due to uncorrected myopia warrants glasses to be prescribed in most patients.

Recommendations for prescribing criterion are on the order of 0.50 to 0.75 diopters of myopia. In Figure 3.8, this level of defocus corresponds to a logMAR of 0.10. While

this acuity might be considered to be very good, it represents a loss of about one line

of acuity compared to no defocus. The equivalent criterion for hyperopic blur was not

able to be evaluated at this point due to the subjects’ ability to accommodate throughout testing with the four diopter hyperopic stimulus. All subjects accommodated accurately and were able to read small letters and near blur seemed to have little effect on visual acuity.

During the second phase of this study results were obtained under cycloplegia for the same myopic stimuli as well as for the opposite hyperopic stimuli so that comparisons could be gathered to note if similar effects were found. Hyperopic defocus produced the same degradation on threshold visual acuity as myopic defocus for the current sample size. Nearly identical slopes were found when comparing the effects of hyperopic defocus on threshold visual acuity with myopic defocus on threshold visual acuity. In Figure 3.6, 0.75D of myopic defocus corresponds to a logMAR of 0.20. As with non-cycloplegic conditions, this acuity represents a loss of two lines compared to no defocus. Where these lines intersect is above +0.50D (see

Figure 3.4) which is indicative of one’s depth-of-focus. One’s depth of focus is the amount of, or distance an image can be moved away from the retina and remain in subjective sharp focus. The autorefractor is calibrated to mimic a clinical refraction where maximum plus is pushed while not degrading threshold visual acuity. This value often pushed at the end of a clinical refraction is +0.50 to obtain a final binocular endpoint. There is also some variation on the x-axis of Figure 3.1 due to

possible residual refractive error, or the accuracy of the accommodative response if

considering the non-cycloplegic data.

This similarity between hyperopic and myopic blur on threshold visual acuity that

was found in the current study is not consistent with the findings of a past study by

Radhakrishnan and Charman. The previous study also found that myopes have a reduced accommodative response to negative lenses compared with nonmyopes.

Mathematical models predict that the reduced accommodative response is due to a decrease in sensitivity to blur in myopes. It was determined in the previous study that the myopic group showed more acuity loss with positive lenses compared with negative lenses. The magnitude of visual acuity loss was lower with negative lenses in myopes compared with nonmyopes. Residual accommodation after cycloplegia

was about 0.20 D in both myopes and nonmyopes and was too small to explain the

relatively good visual acuity through minus lenses in the myopic group. The reduced

accommodative response known to occur in myopes may be due to the relatively

small effect that hyperopic negative lens blur, such as from accommodative lag, has

on their visual acuity 31. The different findings in the current study could be due to

higher order aberrations not having as big of an effect on the current subject pool

compared to that of the previous study. Also, a 6mm artificial pupil was used in the

previous study, where the current study used a 4.2mm artificial pupil perhaps

reducing the effect of aberrations. Another possibility is that the differences are

extremely small.

When analyzing the current data set one may note that the slopes of the

cycloplegic and non-cycloplegic myopic defocus differ significantly. This difference could exist due to a variety of factors. One of which is the loss of subjects. Fifteen

subjects completed the initial non-cycloplegic trial where only twelve subjects from

the original sample completed the cycloplegic trial. Although all subjects had 20/20

vision to enter the study some subjects might have had better acuity and this is more

evident as the affect the average more with the smaller sample. This difference could

also exist simply due to guessing ability on a given day. Subjects were encouraged to

guess as long as they were able to correctly identify three of five letters on a given

line. The willingness of someone to guess well may play a part in the change in slope.

Under cycloplegic condition subjects did not see as well when compared to the same

non-cycloplegic defocus, confirmed by the paired t-test. This could be due to the

effect of higher order aberrations. An artificial pupil was not used for the non-

cycloplegic trial so subjects were free to accommodate or attempt to accommodate to

any stimulus decreasing pupil size, which decreases the aberrations. Under

cycloplegic conditions an artificial pupil was used to cut down on these aberrations,

however, perhaps not to the extent of the eye with a natural pupil without cycloplegia.

Also, the first time the study was run without cycloplegia a random order was not

used. It was setup so that patients started out reading the most blurry stimulus and

proceeded through the appropriate protocol accordingly. Therefore; blur adaptation

could have played a part difference in acuity level. The source of the difference in the

slopes of the two trials may be difficult to pinpoint, but the data show the same trend for both conditions, namely that myopic blur has a predictable effect on visual acuity.

In the current study letter size had a measurable effect on the accommodative response. Subjects exhibited a larger lag when reading large letters. The converse was also noted; a smaller lag, more accurate accommodative response occurred when reading smaller letters. When measuring lag or accommodation, a clinician is typically taught to use a fixation target subtending 10 minutes or arc at near or smaller (20/40 equivalent). However the pediatric population uses reading material that typically consists of larger print. Several different children’s books were analyzed at a book store to determine the “normal” letter size that a child would encounter. Books were chosen at random from a preschool children’s section and were measured. Letter size varied between 0.3-0.7cm. A total of twenty-five books were sampled. A PD stick was used to make all measurements to the nearest 0.01cm.

Assuming that a habitual working distance for a child is nine inches; it was determined that normal letter sizes range from 45-75 minutes of arc 35.

The current study found that subjects lagged more on larger letters than on smaller letters. This finding is not consistent with the previous work of Lovasik et. al

(1987) where no relationship was found between letter size and the accommodative response. In fact, the differences in accommodative response in Lovasik’s work did not exceed 0.50 diopters from the largest to the smallest letter sizes. The aforementioned study used a laser speckle optometer which projected an image onto

the subject’s retina and the subject had to read letters and observe a “boiling” of the speckles. Accommodation versus letter size was measured in the previous study by using seven rows of letters ranging from letters subtending five minutes of arc to those subtending twenty-five minutes of arc. As mentioned in Methods, it is unclear what sort of letter quality the smallest letters had in Lovasik et al. (1987). Perhaps accommodative response did not improve with small letters because letter quality was poor with small letters. In the current study an autorefractor was used to take all measurements. This allowed the experimenter to be in complete control of when the measurements were taken and if the subject was in proper alignment or not. Also, accommodation was able to be measured when the subject read a letter rather than over the course of a minute where a subject might lose interest and fatigue.

The clinical implication of a relationship between accommodative response and letter size is that if clinicians are measuring accommodation and lag with letters much smaller than what a child is actually reading, clinicians may be missing the true picture between the amount of lag and poor reading and school performance. The clinician might find a smaller lag than that which would occur when children read their usual large letters. If a clinician would test lag with an appropriate letter size, a larger more valid lag may be elicited that may give a proper indication of the need for glasses or not for these children.

A meaningful amount of lag that would have a profound effect on visual acuity with near work is 1.75-2.25 diopters. This is derived by the following: adopting a

normal criterion for hyperopia similar to that for myopic children of 0.75-1.00 diopters of farsightedness because myopic blur and hyperopic blur have similar effects on acuity, then adding an assumed normal lag for letter subtending 45 minutes of arc to 75 minutes of arc of 1.00-1.25 diopters.

Unacceptable Lag = Normal referral criterion for blur + normal lag

Unacceptable Lag = +0.75-1.00D + 1.00-1.25D = 1.75-2.25D

If lag were to be measured using an autorefractor normal depth of field would not have to be added to this value as it is automatically factored in by the machine.

However, if a clinician were to measure lag using the appropriate letter size, the

+0.50D depth of field would not be necessary to add in as it is not for a factor when using dynamic MEM. MEM is an objective technique that gives the absolute defocus.

It does not contain the depth of field adjustment because it is not based on maximum plus for maximum visual acuity. If dynamic MEM were to be used to measure the amount of lag, +0.50D would need to be subtracted from the 1.75D to 2.25D criterion above. With MEM, 1.25D to 1.75D might be a reasonable referral criterion.

For school age children who are reading a large font at near a clinically significant amount of lag is 1.75-2.25D when measured by autorefraction or 1.25-1.75D when measured by dynamic retinoscopy. This amount of lag might prompt a clinician to prescribe full prescription distance glasses to a hyperopic child to reduce the amount of lag present when the child is reading at near. The amount of plus to be given should be the maximum plus the child will accept without degrading the distance

visual acuity. If the child accepts the full cycloplegic refraction it should be given. If

the child does not accept the full cycloplegic distance refraction, additional plus

should be pushed from the manifest until a decrease in acuity is noted. It should also

be noted that this may be pushed more at the next visit as the child might adapt and

become more accepting of the prescription. If the child is myopic, or after the full

hyperopic prescription is given and the lag is still undesirable, a bifocal may be the next step. Another option may be vision therapy to train the accommodative system to work more efficiently in focusing and maintaining this focus. This may help to improve the child’s reading ability, thus improving school work and performance overall.

BIBLIOGRAPHY

1. American Optometric Association: Care of the Patient with Hyperopia. http://www.aoa.org/documents/CPG-16.pdf 2. Vitale S, Ellwein L, Cotch MF, Ferris FL, 3rd, Sperduto R. Prevalence of refractive error in the United States, 1999-2004. Arch Ophthalmol 2008;126:1111-9. 3. Williams C, Northstone K, Howard M, Harvey I, Harrad RA, Sparrow JM. Prevalence and risk factors for common vision problems in children: data from the ALSPAC study. Br J Ophthalmol 2008;92:959-64. 4. Bolinovska S. Hyperopia in preschool and school children. Med Pregl 2007;60:115-21. 5. Luyckx J. Measurement of the Optical Components of the Eye of the Newborn using Ultrasound Echography. Archive of Ophthalmology (Paris) 1966;26:156-170. 6. Verma M, Chhatwal J, Jaison S, Thomas S, Daniel R. Refractive errors in preterm babies. Indian Pediatr 1994;31:1183-6. 7. Mutti DO, Mitchell GL, Jones LA, Friedman NE, Frane SL, Lin WK, et al. Axial growth and changes in lenticular and corneal power during emmetropization in infants. Invest Ophthalmol Vis Sci 2005;46:3074-80. 8. Wallman J, McFadden S. Monkey eyes grow into focus. Nat Med 1995;1:737-9. 9. Mutti DO, Bullimore MA. Myopia: an epidemic of possibilities? Optom Vis Sci 1999;76:257-8. 10. Zadnik K, Mutti DO, Mitchell GL, Jones LA, Burr D, Moeschberger ML. Normal eye growth in emmetropic schoolchildren. Optom Vis Sci 2004;81:819-28. 11. Lee KE, Klein BE, Klein R, Wong TY. Changes in refraction over 10 years in an adult population: the Beaver Dam Eye study. Invest Ophthalmol Vis Sci 2002;43:2566-71. 12. Atkinson J, Braddick O, Nardini M, Anker S. Infant hyperopia: detection, distribution, changes and correlates-outcomes from the cambridge infant screening programs. Optom Vis Sci 2007;84:84-96. 13. Anker S, Atkinson J, Braddick O, Nardini M, Ehrlich D. Non-cycloplegic refractive screening can identify infants whose visual outcome at 4 years is improved by spectacle correction. Strabismus 2004;12:227-45. 14. Shankar S, Evans MA, Bobier WR. Hyperopia and emergent literacy of young children: pilot study. Optom Vis Sci 2007;84:1031-8. 15. Grisham JD, Simons HD. Refractive error and the reading process: a literature analysis. J Am Optom Assoc 1986;57:44-55. 16. Atkinson J, Nardini M, Anker S, Braddick O, Hughes C, Rae S. Refractive errors in infancy predict reduced performance on the movement assessment battery for children at 3 1/2 and 5 1/2 years. Dev Med Child Neurol 2005;47:243-51. 17. Rosner J. The relationship between moderate hyperopia and academic achievement: how much plus is enough? J Am Optom Assoc 1997;68:648-50. 18. Wutthiphan S. Guidelines for prescribing optical correction in children. J Med Assoc Thai 2005;88 Suppl 9:S163-9. 57

19. Wildsoet CF. Active emmetropization--evidence for its existence and ramifications for clinical practice. Ophthalmic Physiol Opt 1997;17:279-90. 20. Atkinson J, Anker S, Bobier W, Braddick O, Durden K, Nardini M, et al. Normal emmetropization in infants with spectacle correction for hyperopia. Invest Ophthalmol Vis Sci 2000;41:3726-31. 21. Fern KD. Visual Acuity Outcome in Isometropic Hyperopia. Optometry and Vision Science 1989;10:649-658. 22. Tongue AC. Refractive errors in children. Pediatr Clin North Am 1987;34:1425-37. 23. Lyons SA, Jones LA, Walline JJ, Bartolone AG, Carlson NB, Kattouf V, et al. A survey of clinical prescribing philosophies for hyperopia. Optom Vis Sci 2004;81:233-7. 24. Donahue SP. Prescribing spectacles in children: a pediatric ophthalmologist's approach. Optom Vis Sci 2007;84:110-4. 25. Association AO. Care of the Patient with Myopia. 1997, Reviewed 2006; 26. Hunter DG. Dynamic retinoscopy: the missing data. Surv Ophthalmol 2001;46:269- 74. 27. Benjamin WJO, MS, PhD. Borish's Clinical Refraction. Second Edition. St. Louis, Missouri: Butterworth Heinemann Elsevier; 2006: 28. Atchison DA, Charman WN, Woods RL. Subjective depth-of-focus of the eye. Optom Vis Sci 1997;74:511-20. 29. Majaj NJ, Pelli DG, Kurshan P, Palomares M. The role of spatial frequency channels in letter identification. Vision Res 2002;42:1165-84. 30. Lovasik JV, Kergoat H, Kothe AC. The influence of letter size on the focusing response of the eye. J Am Optom Assoc 1987;58:631-9. 31. Radhakrishnan H, Pardhan S, Calver RI, O'Leary DJ. Unequal reduction in visual acuity with positive and negative defocusing lenses in myopes. Optom Vis Sci 2004;81:14-7. 32. Johnson CA. Effects of luminance and stimulus distance on accommodation and visual resolution. J Opt Soc Am 1976;66:138-42. 33. Heron G, Furby HP, Walker RJ, Lane CS, Judge OJ. Relationship between visual acuity and observation distance. Ophthalmic Physiol Opt 1995;15:23-30. 34. Subbaram MV, Bullimore MA. Visual acuity and the accuracy of the accommodative response. Ophthalmic Physiol Opt 2002;22:312-8. 35. Rouse MW, Hutter RF, Shiftlett R. A normative study of the accommodative lag in elementary school children. Am J Optom Physiol Opt 1984;61:693-7. 36. Akutsu H, Bedell HE, Patel SS. Recognition thresholds for letters with simulated dioptric blur. Optom Vis Sci 2000;77:524-30. 37. Thorn F, Schwartz F. Effects of dioptric blur on Snellen and grating acuity. Optom Vis Sci 1990;67:3-7. 38. Raasch TW, Bailey IL, Bullimore MA. Repeatability of visual acuity measurement. Optom Vis Sci 1998;75:342-8.

39. Raasch TW. Spherocylindrical refractive errors and visual acuity. Optom Vis Sci 1995;72:272-5.