Nearwork - a risk factor for overcorrectingpatients?

Nearwork - a risk factor for overcorrecting

patients?

Patrick Vall

Degree project work in optometry

Level: C

Degree project works made at the University of Kalmar, School of Pure and Applied Natural Sciences, can be ordered from: www.hik.se/student

or

University of Kalmar

School of Pure and Applied Natural Sciences SE-391 82 KALMAR

SWEDEN

Phone + 46 480-44 62 00 Fax + 46 480-44 73 05 e-mail: info@nv.hik.se

Nearwork – a risk factor for overcorrecting patients?

Patrick Vall

Optikerprogrammet 180 points

University of Kalmar, School of pure and applied sciences Examination Project Work 15 credits

Supervisor:

Baskar Theagarayan BS Optom

Lecturer in Optometry

Divison of Optometry University of Kalmar School of pure and applied Sciences

SE-391 82, Kalmar Sweden Examiner:

Peter Gierow Professor Divison of Optometry

University of Kalmar School of pure and applied Sciences

SE-391 82, Kalmar Sweden Abstract

Introduction: Research has shown that people become more myopic when performing

nearwork. This phenomenon is called nearwork-induced transient myopia (NITM). Although this nearsightedness is temporary it is possible that it has an effect on individuals over time, causing permanent myopia, but from a more short-term and clinical point of view; can NITM have implications on the ordinary visual examination?

Aim: The purpose of this study was to investigate if prolonged nearwork can produce a

change in the distance refraction significant enough to be measured with commonly occurring instrumentation used in clinics and if patients can be overcorrected because of this.

Methods: 23 emmetropic and myopic subjects with a mean age of 24 years were examined at

two occasions; prior to and after approximately 2 hours of nearwork. The refraction was determined using a Topcon VT-10 manual phoropter and the end-point of refraction was defined using the duo-chrome test.

Results: When the data was averaged across all test subjects it revealed a statistically

significant myopic refractive shift of 0.16 D. A subgroup analysis revealed an average myopic shift of 0.28 D in the myopic subjects (Student’s paired t-test; P < 0.05) and an average myopic shift of 0.05 D in the emmetropic subjects (Student’s paired t-test; P > 0.05).

Conclusion: Based on the data and results presented in this study and the results from earlier

studies it is reasonable to say that there is a small risk that some patients could be overcorrected due to prolonged nearwork being performed prior to a visual examination.

Summering

Forskning har visat att personer blir mer närsynta när de utför närarbete. Detta fenomen kallas närarbetesinducerad övergående myopi (förkortas NITM på engelska) och är resultatet av en förändring i ackommodationen. Fastän denna närsynthet är temporär finns det en möjlighet att den orsakar permanent myopi på lång sikt, men ur ett kortsiktigare och mer kliniskt perspektiv; kan NITM ha konsekvenser för den vanliga synundersökningen?

Målet med denna studie var att undersöka om ihållande närarbete kan producera en förändring i avståndsrefraktionen stor nog att uppmätas med vanligt förekommande instrument som används i kliniker, samt om patienter kan bli överkorrigerade på grund av detta.

23 emmetroper och myoper undersöktes vid två tillfällen; före och efter ungefär 2 timmars närarbete. När den insamlade informationen analyserades visade den en statistiskt signifikant genomsnittlig myopisk förändring på 0.16D. En undergruppsanalys visade ett genomsnittligt myopiskt skifte på 0.28D hos de närsynta försökspersonerna och ett genomsnittligt myopiskt skifte på 0.05D hos emmetroperna.

Baserat på informationen och resultaten från denna studie samt resultat från tidigare studier kan man rimligen säga att det finns en liten risk att vissa patienter skulle kunna bli överkorrigerade på grund av att de utfört ihållande närarbete före en synundersökning.

Table of contents

Introduction

NITM

NITM & permanent myopia Pseudomyopia & NITM Accommodation

Lag & lead of accommodation Emmetropia

Myopia

Etiology of myopia & myopia progression Factors & Conditions affecting Accommodation & Refraction

Aim

Methods

Results

Discussion

Conclusion

Acknowledgments

References

Appendix 1

Appendix 2

Appendix 3

Appendix 4

Appendix 5

1

1 5 6 6 7 7 7 8 10

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12

15

19

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27

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30

Introduction

Genetic inheritance and nearwork have been known factors in myopia development for many years; but what are the mechanisms causing myopia? Research has shown that people become more myopic when performing nearwork (with the exception of hyperopes) (Ong & Ciuffreda 1995; Ciuffreda & Wallis 1998; Ciuffreda & Lee 2002). This is called NITM (nearwork-induced transient myopia). It has been speculated that this transient nearsightedness can result in permanent myopia over time and a recently proposed theory has been able to explain why this occurs (Hung & Ciuffreda 2007). Most of the studies up to date have been conducted under experimental conditions, but from a clinical perspective; is it possible that NITM can have implications on the ordinary visual examination?

NITM –

what it is, what causes it, the ways it can be measured and what it can cause

NITM is a temporary nearsightedness caused by sustained nearwork. It is the outcome of an inward shift of the far point of accommodation and is observed as a myopic shift in the far point refractive state. Furthermore, NITM is defined as the difference in distance refraction before and immediately after nearwork (Ong & Ciuffreda 1995; Hung & Ciuffreda 2007). The first study to observe the existence of NITM was that of Lancaster and Williams in 1914 where a Badal optical system was used to assess the subjects’ far point before and after a near task. Subjective assessments were made at near and far distances. The end-point of the far distance refraction was defined as the maximum plus or minimum minus lens producing maximum visual acuity. A small target was placed at the test subjects’ NPA (near point of accommodation) and they had to maintain focus on the target for 45 minutes. Myopic shifts up to 1.30D were observed and also that the effects of the task decayed within 15 minutes. According to Ong & Ciuffreda, Lancaster & Williams stated that this was the result of “retarded relaxation” (1995, p. 64) (Ong & Ciuffreda 1995; Ciuffreda & Vasudevan 2008).

NITM has been studied under varying conditions, both experimental and natural. In a review from 1995 Ong & Ciuffreda listed twenty-four studies conducted on NITM. Eighteen of those studies demonstrated NITM and six did not. NITM has been observed after shorter periods of nearwork as well as after longer periods of nearwork (e.g. the length of a work-day). The studies presented in the review showed a magnitude of NITM ranging from 0.12D to 1.30D, with a mean of 0.40D. NITM subsides after a period of time and the myopic shift induced returns to what is called the pre-task baseline refraction (i.e. the refraction measured before nearwork) within a matter of seconds or minutes but, it has been reported that the decay can take up to over an hour (Ong & Ciuffreda 1995; Ciuffreda & Vasudevan 2008).

Different methods have been used to quantify NITM (visual acuity, contrast sensitivity functions and far point refraction). Nearwork may cause blurred distance vision if the depth of focus is exceeded and NITM can therefore be measured as a decrease in visual acuity but according to Ong & Ciuffreda using such a method to quantify NITM is “clearly indirect” (1995, p. 62). Another issue with using visual acuity is that it is a subjective measurement and the results are therefore very much dependent on the consistency in subjects’ response (Ong & Ciuffreda 1995).

The most common chart used in Sweden when measuring visual acuity is probably the Snellen chart. Due to the fact that NITM often is small in its magnitude visual acuity can seem inadequate or even inappropriate to use when measuring NITM. This was at least Jaschinski-Kruza’s view on the matter when he in 1984 used contrast sensitivity to quantify NITM. He observed a myopic shift corresponding to approximately 0.50D and recovery within 15 minutes. Ong & Ciuffreda states that although contrast sensitivity can detect smaller differences than visual acuity it is difficult to translate the findings into “standard clinical terms” (1995, p. 63) (Ong & Ciuffreda 1995).

The most appropriate method to use when measuring NITM and also the most common is to directly assess any change in the far point refraction (Ong & Ciuffreda 1995).

Three of the 24 studies presented in the review from 1995 are of special interest.

1. In 1980 Ostberg conducted a study on video display terminal users. He divided his test subjects into 2 groups (air traffic controllers in the first group and office workers and telephone directory service operators in the second group). The accommodative demand for the individuals in the first group was greater than the accommodative demand for the people in the second group. In the first group Ostberg measured the test subjects’ steady-state of accommodation for a target at 6 meters prior to and after two hours of nearwork and in the second group prior to and at the end of a work day. A NITM of approximately 0.25D was observed in group one but no NITM was observed in group two. This field study was conducted under normal working conditions and therefore it suggests that NITM is present not only under experimental conditions were the accommodative demand perhaps is somewhat greater than in normal situations. However, Ong & Ciuffreda pointed out in their review that a comparison between the groups was difficult due to the varying test conditions and the fact that refraction was not measured at equal intervals (Ong & Ciuffreda 1995).

2. In 1987 Ehrlich conducted a study where he measured the refractive state in 15 young adult subjects whose near-task consisted of continuously viewing a test paradigm at 20 cm. Ehrlich found that a myopic shift of approximately 0.29D was induced. The most interesting finding though was that the NITM did not decay during the 1 hour post-task assessment implying that NITM in some cases can be extremely persistent (Ong & Ciuffreda 1995: Ciuffreda & Vasudevan 2008).

3. Lastly, in 1995 Ciuffreda & Ordonez conducted a study investigating NITM in symptomatic individuals. They used similar methods as previous studies. A small sample size was used but the fact that these individuals exhibited a myopic shift of 0.93D after only a 10 minute near-task could perhaps imply that symptomatic individuals are more susceptible to NITM (Ong & Ciuffreda 1995).

The only factor Ong & Ciuffreda (1995) found that directly affects the magnitude of NITM is task duration (i.e. the longer the task duration the greater the NITM). A relationship between the magnitude of NITM and other variables such as accommodative demand and viewing condition (binocular or monocular) could not be found, although it should be said that there is a possibility that they too affect NITM directly (i.e. the greater the accommodative demand the greater the NITM). The decay of NITM also appears to depend more on the task duration than other variables. Tasks with durations over 40 minutes generally result in a slower decay (e.g. minutes instead of seconds) (Ong & Ciuffreda 1995).

As mentioned earlier, six of the twenty-four studies presented in the review from 1995 did not demonstrate NITM. According to the authors of the review the reason for this was probably because of methodological errors.

The following has been stated about NITM:

Susceptibility between individuals might differ

It is possible that not all individuals are affected

NITM is small in its magnitude

NITM subsides quickly (generally within seconds or minutes)

In 1998 Ciuffreda & Wallis conducted a study with the purpose of determining the relative accommodative susceptibility of the different refractive groups, i.e. myopes (late-onset and early-onset), emmetropes and hyperopes. The distance refractive state was assessed before and immediately after a 10 minute near task. It was concluded that:

Hyperopes are especially insusceptible to NITM.

Myopes are more susceptible to the effects of nearwork than the other refractive

groups.

And it was suggested that:

The hyperopic after-effect in some of the hyperopes was related to their refractive

Is it possible that NITM can have long-term consequences? It has been suggested that this might be the case. If NITM does not subside fully then it could be possible that a more or less constant defocused retinal image produces an increase in axial length and thereby causing myopia (Ong & Ciuffreda 1995).

Is there a link between NITM and permanent myopia?

Ciuffreda & Vasudevan (2008) concluded the following in their review:

NITM can affect the accommodative accuracy and the related residual retinal defocus.

Myopes are especially susceptible to NITM.

Progressing myopes are more susceptible to NITM than stable myopes.

Myopes have a prolonged NITM decay.

It is possible that retinal defocus can result in myopia.

If NITM can lead to permanent myopia then shouldn’t measures be taken to minimize its occurrence? Acquired myopia seems to be quite a problem in today’s society and according to several studies the unified response is to prevent or minimize the occurrence of NITM by; prescribing full distance correction in conjunction with a low plus add, taking periodic breaks during prolonged nearwork and undergoing accommodative vision therapy if needed (Ciuffreda & Lee 2002; Hung & Ciuffreda 2007; Ciuffreda & Vasudevan 2008).

Are pseudomyopia & NITM the same?

Pseudomyopia is a reversible form of myopia, a condition, where a person appears to be myopic but in fact suffers an abnormal refractive condition due to accommodative hysteresis. Grosvenor & Goss (1999) states that “either the manifest refraction is minus while the cycloplegic refraction is plano or plus, or the manifest refraction is more minus than the cycloplegic refraction” (1999, p. 93).

Young individuals who do a considerable amount of near work are the ones most prone to develop pseudomyopia. Patients can say that their distance vision feels blurry after near work and clinical findings can consist of; reduced visual acuity, fluctuations in the subjective refraction, low PRA and high NRA values (Grosvenor 2007; Grosvenor & Goss 1999). There are differences between pseudomyopia and NITM. Pseudomyopia is a more discernible condition than NITM which is milder in its nature. When a person suffers from pseudomyopia the ciliary spasm will, in most cases, not dissipate until the patient receives cycloplegic agents and, or, vision therapy. NITM is described and in most cases documented as a small and temporary nearsightedness which subsides within seconds or minutes (Ciuffreda & Ong 1995; Grosvenor & Goss 1999).

Accommodation

Accommodation is mainly a response to blur, a mechanism which allow near objects to be focused on the retina. It is a shape shifting process which the crystalline lens undertakes. According to Helmholtz theory, when an emmetropic eye is looking at far the ciliary muscle is relaxed, the zonular fibers stretched, and the crystalline lens has a flattened shape.

When that same eye has to focus light rays from a near object onto the retina the lens undergoes a change in shape. The ciliary muscle contracts which causes the zonular fibers to loosen their tension and the crystalline lens thickens. This shape change increases the power of the lens and allow near objects to be seen clearly (Borish’s Clinical Refraction 2006; Grosvenor T. 2007).

Lag & lead of accommodation

“Lag of accommodation” can be considered as an under-accommodative response to a certain stimulus. According to Borish’s Clinical Refraction (2006) the accommodative system changes by the minimum amount necessary in order to obtain a clear retinal image and that, although there is an inaccuracy in the accommodative response, if the retinal image already is subjectively clear then further increase of accommodation will be unnecessary. “Lead of accommodation” is the opposite of “lag of accommodation” (i.e. and over-accommodative response) (Borish’s Clinical Refraction 2006).

Emmetropia

Emmetropia is defined as the “normal” refractive condition of the eye where, if accommodation is relaxed, parallel light-rays are converged and focused on the retina (Grosvenor T. 2007).

Myopia

Myopia, or nearsightedness, is a condition where parallel light-rays focus in front of the retina. Myopia can divided into refractive myopia, where the refractive power is too great for the axial length, and axial myopia, where the eye’s axial length is too long for its refractive power. Myopia is a common condition although it is not yet fully understood. Throughout the years there have been many studies conducted on myopia, on its progression and etiology and various classifications have been devised (including the one above).

Etiology & progression of myopia

Myopia is considered to be multifactorial (i.e. it depends on many factors and one cannot point to a single factor as the cause for the condition). Although there have been many studies conducted it seems there is still no consensus as to what causes myopia, in fact, none of the numerous hypotheses has been universally accepted. According to Grosvenor (2007) studies which imply that myopia is genetic in its origin seldom rule out environmental factors. On the other hand, studies which imply that myopia is caused by environmental factors seldom rule out the possibility of genetic inheritance.

According to Grosvenor & Goss (1999) and Grosvenor (2007) many reviews show that the prevalence of myopia is greater among those who perform near work on a daily basis. One third or as many as two thirds of the population in an industrialized society develops myopia after years of schooling or during their adult years.

As mentioned earlier, many hypotheses and theories on why myopia develops have been devised and the one thing that most of them seem to have in common is that they suggest that myopia is the result eye growth. A recent theory called the incremental retinal-defocus theory (IRDT), whose fundamental principle was first suggested by Gwiazda et al. (1993), describes the mechanisms causing myopia. This theory is supposedly able to explain results which previous hypotheses and theories have not been able to explain in a satisfactorily manner. Earlier studies show that one can manipulate the growth of the eye inducing either myopia or hyperopia an also that the retina is the site controlling axial length growth (Grosvenor & Goss 1999; Grosvenor T. 2007; Hung & Ciuffreda 2007).

Two things fundamental to IRDT are that retinal defocus area (i.e. the size of the blur circle) alone does not provide directional information, which means that the response for an under-focused and over-under-focused retinal image will be the same. Secondly, the visual environment can be manipulated to produce refractive errors but only within a certain time window of susceptibility during a certain time period (e.g. ocular growth). According to the theory, and based on previous research, a decrease in retinal-defocus area lowers the rate of neuromodulator release, decreases the protheoglycan synthesis, weakens the sclera structural integrity and increases the axial growth rate causing myopia (Hung & Ciuffreda 2007).

Schematical testing under eight different conditions showed in the end that the theory was capable of explaining all of the findings. One of the conditions tested was prolonged nearwork. A simplified explanation is that nearwork affects the accommodative state which affects retinal-defocus area which in turn can result in myopia.

Normally, nearwork does not consist entirely of continuous viewing at near. Although the time spent looking at near usually dominates, once and a while one look at distant objects. According to Hung & Ciuffreda (2007) the critical factor is that NITM which does not decay fully carries over to the subsequent period of near viewing. This leads to a cumulative effect which decreases the accommodative response. Repeated transient decreases in retinal-defocus area at near finally, according to IRDT, induce axial growth (Hung & Ciuffreda 2007).

There are studies showing supporting evidence. According to Borish’s Clinical Refraction (2006) studies conducted on chickens showed that defocusing the retinal image with minus and plus lenses induces a posterior segment growth which results in myopia or hyperopia depending on the direction of the defocus. It became clear that it was the defocus itself causing the effect without the involvement of accommodation as the chickens were incapable of accommodating. According to Borish’s Clinical Refraction (2006) Sivak stated that one have to keep in mind that ocular optics and morphology differs between species. Furthermore, Allen & O’Leary (2006) conducted a 12 month long study on 64 young-adult humans and they concluded that lag of accommodation and accommodative facility affect retinal defocus and that these accommodative functions are probably involved in myopia progression. The study also concluded that lag of accommodation correlates to both the amount of myopia and myopia progression which, according to Allen & O’Leary suggests that lag of accommodation may be “a particularly important risk factor in the progression of higher amounts of myopia” (2006, p. 503) (Grosvenor & Goss 1999; Allen & O’Leary 2006; Borish’s Clinical Refraction 2006; Grosvenor T. 2007).

Factors & Conditions affecting Accommodation & Refraction

Many different factors can affect accommodation. In Borish’s Clinical Refraction (2006) these factors are divided into stimulus, cues and influences. As stated earlier, blur is the main stimulus to accommodation. Cues play an important role in inducing the right amount of accommodative response and without them the accommodative response would be random. They are divided into optical and non-optical (e.g. spherical aberration and object size). The main function of cues is that they provide necessary information on the direction of the blur. Voluntary effort, contrast, retinal image eccentricity and spatial frequency are all factors which can influence accommodation although they do not give rise to equal amounts of accommodative response. Target contrast and spatial frequency for example do not affect the accommodative response as much as retinal eccentricity (i.e. target contrast can be significantly varied without inducing a change in the accommodative response) (Borish’s Clinical Refraction 2006).

A variety of drugs can affect accommodation and they produce different results. Drugs that affect the parasympathetic and sympathetic nervous system are categorized into four groups (i.e. parasympathomimetics, parasympatholytics, sympathomimetics and sympatholytics). These drugs can for example cause miosis, mydriasis and loss of accommodation. There are also systemic drugs that can affect accommodation (e.g. alcohol, marijuana, antihistamines, digitalis etc.), most often causing reduced and variable responses (Borish’s Clinical Refraction 2006).

Diseases can also cause accommodative abnormalities. The list can be made long but examples of conditions affecting accommodation in some way are diabetes mellitus, HIV, malaria, neuro-opthtalmic lesions and meningitis (Borish’s Clinical Refraction 2006).

The World Health Organization (WHO) presents diabetic retinopathy, glaucoma and AMD as the three eye diseases posing the greatest threat to public eye health in middle income and industrialized countries. Furthermore, although not always considered as a disease, cataract which is a common and visually impairing condition mostly related to the normal human ageing process can also be a result of for example head trauma or overexposure to UV-light.

Aim

The purpose of this study was to investigate the following:

1. Can prolonged nearwork produce a change in the distance refraction?

2. If so, is the change significant enough to be measured with commonly occurring instrumentation used in clinics?

Methods

The study group

34 individuals from the University of Kalmar (mainly optometrist students and nurse students) were recruited for the study through advertisement and personal appearances.

The final sample size consisted of 23 individuals of whom 12 were emmetropic and 11 were myopic. Their ages ranged from 21 to 28 (mean = 24.17) and their spherical equivalent, distance refractions, ranged from +0.75 D to -9.25 D. Hyperopes were excluded from the study as previous research has shown that hyperopes are insusceptible to NITM (Ciuffreda & Wallis 1998). The test subjects were selected using similar criteria as previous studies (Ciuffreda & Wallis 1998; Ciuffreda & Lee 2002) and they were grouped based on the refraction of the left and the right eye. Refractive values between +0.75 DS and -0.25 DS were considered as emmetropia and refractive values equal to, or greater than -0.50 DS to -10.00 DS were considered as myopia. The cylindrical component did not exceed -0.50 D in the emmetropic subjects and a 4:1 ratio specified the greatest amount of astigmatism allowed in the myopic subjects.

Although no anisometropes were included, both asymptomatic and symptomatic individuals were included in the study. All the subjects had ≥ 6/6 corrected visual acuity in both eyes. Out of the original 34 subjects recruited, three dropped out and eight individuals were not examined due to lack of time.

Procedure

The subjects were examined at two occasions. The distance refraction was determined before and after a near task and these two refractions were then compared to see if there had been any changes. The test subjects were examined in the morning between 07.45 am and 11.00 am and again later the same day.

At the first examination, which was more detailed than the re-check, the subjects first received verbal and written information about the study whereupon they had to sign an

A brief questionnaire concerning patient name, age and gender was filled out. Patient history was taken and the subjects were asked questions concerning their ocular and systemic health, if any medications were taken, known allergies, if they had been ill recently and if they had astenopic problems (e.g. trouble when reading and if they had experienced blurred distance vision after nearwork).

Both the right eye and the left eye were then examined using a Topcon slit lamp (SL-4F). The pupil-distance and uncorrected visual acuity were determined whereupon the subjective refraction was performed monocularly. The end-point of refraction was defined with the duo-chrome test were the subjects were not supposed to see a difference in sharpness between the symbols in the red and the green field. If equality was not obtained the end-point was defined as the last point at which the symbols appeared sharper in the red field. Normally this is how the duo-chrome test is performed and one can even note the letter s, r or g behind the spherical power (i.e. s = same, r = red, g = green) (Grosvenor T. 2007).

A small modification was made and when the test subjects stated that the symbols appeared equally sharp in both the red and the green field they were asked if the symbols still appeared somewhat sharper in one of the fields. If this was the case then the following was noted s(r) or s(g), implying that the symbols were very much alike though they appeared slightly sharper in either the red or the green field. This, slightly modified, duo-chrome test was used to provide for an easier comparison between the pre-task and post-task distance refraction and for detecting small, non-measureable, changes. If the end-point of refraction couldn’t be obtained with the duo-chrome test then the end-point was defined as the maximum amount of plus producing the maximum visual acuity.

In most of the previous NITM-studies the refractive state has been assessed objectively with the intention to prove the existence of, and to quantify the induced NITM. And it has been proven that people, especially myopes, become more nearsighted when performing nearwork. However, as clinicians we are more interested in the subjective measurements as they are the basis for the final prescription. This was the main reason why the above described methods were chosen for the study.

Before the second examination (or re-check) the subjects were asked to perform prolonged nearwork at 50 cm (or closer) for 2 hours with a minimum amount of breaks. At the end of the first examination a note containing things to “keep in mind” for the re-check was given to the subjects. No controls were made during the nearwork to ensure that the subjects followed the instructions that had been given.

At the re-check the subjects were asked the following questions:

What type of nearwork has been performed?

What was the working distance?

For how long did you perform nearwork?

Did you take any breaks?

How much time has passed between the completion of the nearwork and the re-check?

The subjects’ uncorrected visual acuity was checked and the subjective refraction recorded earlier was double-checked (the subjects were dimmed with +0,75 DS and then, as before, the end-point was defined with the duo-chrome test or as the maximum amount of plus producing the maximum visual acuity).

To calculate the difference between the pre-task and post-task refraction the following equation was used:

Refractive shift = (Post-task: Sphere + ½ cylinder) - (Pre-task: Sphere + ½ cylinder)

Statistical methods

Microsoft Excel was used to analyze the collected data and Student’s paired t-tests were performed in order to obtain the p-values. The graphs presented in the results section were made using Microsoft Excel.

Results

Due to the fact that emmetropes and myopes show differences in their response to sustained nearwork (Ciuffreda & Lee 2002) it is more interesting to analyze the results in the respective subgroup (i.e. myopes and emmetropes). Still, a group analysis will also be presented. Furthermore, only the results for the right eye will be presented since a t-test conducted on the difference between the pre-task and post-task distance refraction on both the right eye and the left eye showed similar results. The spherical equivalents for the test subjects’ corrections (determined at the first and the second examination) were calculated and these are the values which where used in the analysis. Only data from 19 of the subjects was included in the analysis. Data from 4 test subjects was excluded from the analysis due to the following reasons:

▪ Two of the test subjects did nearwork intermittently.

▪ One test subject performed nearwork at a distance of more than 50 cm.

▪ One test subject did not spend enough time reading.

Group analysis

For the study group as a whole; the average time spent with nearwork was approximately 1.99 h (SD = 0.43 h) and the working distance was approximately 37 cm (SD = 8.00 cm). The time that passed between the end of the nearwork and the re-check ranged from 2 to 12 minutes with the exception for one subject whom was re-checked after 75 min (for more information, see appendix 4). When the data was averaged across all individuals it revealed a myopic post-task change in the distance refraction of approximately 0.16 D (Student’s paired t-test; P < 0.01). In several subjects no myopic post-task distance refractive change could be measured, yet there were signs that a small change had occurred in some of these subjects (see discussion).

All test subjects

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 M E M E M M M M E M E E E M E E E E M D is ta n c e r e fr a c ti o n ( d io p te rs ) Before After

Subgroup analysis

To further investigate whether this change was dependent on refractive error the data was analyzed in the respective subgroup. It revealed that 6 out of 9 myopes had a post-task myopic shift in the distance refraction (Student’s paired t-test; P < 0.01). When averaged across all myopic subjects, the data revealed a post-task myopic shift of approximately 0.28 D.

Myopes

-11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 D is ta n c e r e fr a c ti o n ( d io p te rs ) Before After

Above: Figure 2. The spherical equivalent distance refraction before and after nearwork in myopic subjects.

-1 -0,9 -0,8 -0,7 -0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0 1 2 3 4 5 6 7 8 9 R e fr a c ti v e s h if t (d io p te rs )

Figure 3. The post-task refractive shift in the myopic subgroup. Each number along the x-axis represents a test

The results from the emmetropic group showed more variability. 3 out of 10 emmetropes had a myopic post-task shift in the distance refraction and one subject had a surprising hyperopic post-task distance refractive shift (Student’s paired t-test; P > 0.05). The remaining subjects had no change in the post-task distance refraction. When averaged across all emmetropic subjects, the data revealed a post-task myopic shift of approximately 0.05 D.

Emmetropes

-0,75 -0,5 -0,25 0 0,25 0,5 0,75 1 1,25 1 2 3 4 5 6 7 8 9 10 D is ta n c e r e fr a c ti o n ( d io p te rs ) Before After

Above: Figure 4. The spherical equivalent distance refraction before and after nearwork in emmetropic subjects.

-0,6 -0,5 -0,4 -0,3 -0,2 -0,1 0 0,1 0,2 0,3 0,4 0,5 0,6 1 2 3 4 5 6 7 8 9 10 R e fr a c ti v e s h if t (d io p te rs )

A comparison between the subgroups showed that the myopic subgroup had a greater average refractive shift than the emmetropic subgroup.

Figure 6. The graph illustrates the average refractive shift in all subjects and the subgroups. Plotted is the +SD.

Average refractive shift

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

All subjects Myopes Emmetropes

R e fr a c ti v e s h if t (d io p te rs )

Discussion

1. The results from this study show that there was a significant shift in the distance refraction after prolonged nearwork.

2. NITM occurs and the distance refractive shift induced is measurable with ordinary instrumentation.

3. Myopes are more susceptible to the effects of nearwork than emmetropes.

4. The analysis show that the induced shift in the distance refraction was measurable several minutes after the subjects had ended their nearwork-task which indicate some persistence in NITM decay.

The results in this study concur with results from previous studies (Ciuffreda & Wallis 1998; Ciuffreda & Lee 2002). Susceptibility to the effects of nearwork is highly individual. Although susceptibility is greater within certain subgroups there can still be differences between individuals in the respective subgroup.

The fact that 3 out of 9 myopes did not have a measurable refractive shift could either have been due to a small refractive change and therefore the target image was still within the eye’s depth of focus (Ciuffreda & Wallis 1998) or the aftereffect could have already subsided and returned to the pre-task baseline distance refraction before any change was measured. It is known that NITM generally subsides within seconds or minutes (Ong & Ciuffreda 1995). Furthermore, as no controls were made to ensure that the subjects did prolonged nearwork any induced myopic shift could have subsided prior to the re-check due to intermittent nearwork (Ong & Ciuffreda 1995).

The above given explanations can also be applied on the emmetropic subgroup results. The emmetropic individuals affected could perhaps have a predisposition to NITM. It could be hereditary factors and, or, accommodative abnormalities (Ong & Ciuffreda 1995). The hyperopic shift measured in one subject could have been due to the fact that the test subject had a spherical equivalent distance refraction of +0.75D and was therefore “close to hyperopic” or perhaps the subject had latent hyperopia. Ciuffreda & Lee (2002) found that hyperopes become more hyperopic when reading.

The inconsistency in the results from the emmetropic subgroup (i.e. 3 test subjects had a myopic change in the refraction, 1 subject had a hyperopic shift and 6 subjects had no change) is probably the cause for the t-test result. As mentioned earlier it is not impossible that the 3 test subjects had a predisposition to NITM (genetic factors and, or, accommodative abnormalities). If 4 other emmetropic subjects had been included instead, then perhaps the results would have been different.

As mentioned earlier the end-point of refraction was determined with a slightly modified duo-chrome test were a note was made behind the spherical power (s, r, g, s(r) or s(g)). Furthermore, several subjects showed signs implying that a small change, although not measurable, might have occurred. This was concluded due to the fact that there was a change in the subjects’ response to the duo-chrome test.

It is known that light of shorter wavelength intersect in front of the retina and that light of longer wavelength intersect behind the retina. If a myopic shift occurs then the point of intersection for light-rays of longer wavelength will be pushed forward (toward the retina) which means that the subjects’ response will be that the symbols appear sharper in the red field (if there is a myopic shift) and vice versa if there is a slight hyperopic shift.

Figure 7. Image 1 (pre-task) and image 2 (post-task) illustrates a possible explanation to the duo-chrome test

results. The horizontal broken line represents the optical axis and the vertical (light blue) broken line represents the retina.

It could be interesting to conduct further investigations on this topic in the future; perhaps including hyperopes in the study group, enlarging the sample size and comparing if different types of nearwork yield different results.

Conclusion

Based on the data and results presented in this study and the results from earlier studies it is reasonable to say that there is a small risk that some patients could be overcorrected due to prolonged nearwork being performed prior to a visual examination. It is important to be able to identify these individuals whom are at risk, which mainly are; myopes and, to a certain extent, emmetropes. Especially those with symptoms of blurred distance vision after reading and those individuals with accommodative dysfunctions.

It will be useful to include questions in the history taking concerning what the patient has been occupied with during the day and to ask whether the patient has ever experienced blurred distance vision after reading or computer work.

Acknowledgments

I would like to thank all the test subjects, because without their participation this study could not have been conducted.

I would like to thank my supervisor; Baskar Theagarayan, for his help, support and dedication.

I would also like to thank some of my classmates for helping me with various problems. Last but not least I would like to thank Carolina for her patience with me during these weeks...

References

Books

Benjamin, W. J. (2006) Borish’s Clinical Refraction. Butterworth-Heinemann 2nd edition.

Chapter 1, 3 and 4

Grosvenor, T. & Goss, D. A. (1999) Clinical management of myopia. Butterworth-Heinemann. Page 3, 49-62, 92-95

Grosvenor, T. (2007) Primary Care Optometry. Butterworth-Heinemann 5th edition. Page 7,

13, 15, 47, 216, 267

Remington, L. A. (2005) Clinical anatomy of the visual system. Butterworth Heinemann. Page 97

Articles

Allen, P. M. & O’Leary, D. J. (2006) Accommodation functions: Co-dependency and relationship to refractive error. Vision Research 46: 491-505

Ciuffreda, K. J. & Lee, M. (2002) Differential refractive susceptibility to sustained nearwork. Ophthal. Physiol. Opt. 22: 372-379

Ciuffreda, K. J. & Ordonez, X. (1995) Abnormal transient myopia in symptomatic individuals after sustained nearwork. Optom. Vis. Sci. 72: 506-10

Ciuffreda, K. J. & Vasudevan, B. (2008) Nearwork-induced transient myopia (NITM) and permanent myopia – is there a link? Ophthal. Physiol. 28: 103-114

Ciuffreda, K. J. & Wallis, D. (1998) Myopes show increased susceptibility to nearwork aftereffects. Investigate Ophthalmology & Visual Science vol. 39, No 10. Page 1797-1803

Ehrlich, D. L. (1987) Near vision stress: Vergence adaption and accommodative fatigue. Ophthal. Physiol. Opt. 7: 353-7

Gwiazda, J. Thorn, F. Bauer, J. & Held, R. (1993) Myopic children show insufficient accommodative response to blur. Investigate Ophthalmology & Visual Science Vol. 34, No. 3. Page 690-4

Hung, G. K. & Ciuffreda, K. J. (2007) Incremental retinal-defocus theory of myopia development – Schematic analysis and computer simulation. Computers in Biology and Medicine 37: 930-946

Irving, E. L. Callender, M. G. & Sivak, J. G. (1991) Inducing myopia, hyperopia, and astigmatism. Optom. Vis. Sci. 68: 364-368

Jaschinski-Kruza, W. (1984) Transient myopia after visual work. Ergonomics 27: 1181-9 Ong, E. & Ciuffreda, K. J. (1995) Nearwork-induced transient myopia - A critical review. Documenta Ophthalmologica 91: 57-85

Ostberg O. (1980) Accommodation and visual fatigue in display work. In: Grandjean, E. Vigliani, E. (Eds), Ergonomic Aspects of Visual Display Terminals. London: Taylor & Francis: 41-52

Schaeffel, F. Glasser, A. & Howland, H. C. (1988) Accommodation, refractive error, and eye growth in chickens. Vision Res. 28: 639-657

Schaeffel, F. & Howland, H. C. (1991) Properties of the feedback loops controlling eye growth and refractive state in the chicken. Vision Res. 31: 717-734

Schaeffel, F. Troilo, D. Wallman, J. & Howland, H. C. (1990) Developing eyes that lack accommodation grow to compensate for imposed defocus. Vis. Neurosci. 4: 177-183

Sivak, J. G. (1991) Optical adaptations of the vertebrate eye. In: Grosvenor T. & Flom M. C. (Eds), Refractive Anomalies: Research and Clinical Applications, pp 219-234. Boston: Butterworth/ Heinemann.

Internet

World Health Organization, retrieved 30 April 2009 at 10.41 am,

Är du inne i en intensiv pluggperiod?

Sitter du framför en datorskärm hela dagarna?

– vi utför en studie om hur synen påverkas av närarbete

Om DU är mellan 18 och 35 år och villig att bidra

till vår studie,

RING!

070-443 87 43 eller maila pv22at@student.hik.se

Handledare: Baskar Theagarayan; Baskar.Theagarayan@hik.se

Närarbete – en riskfaktor för överkorrektion av patienter?

Information:

Det är känt att närarbete kan påverka framåtskridandet av närsynthet. I den här studien ska vi undersöka om närarbete kan leda till styrkeförändringar (i patientens glasögonstyrkor) som då skulle utgöra en chans att patienten föreskrivs för starka glas.

Du kommer att undersökas vid två (2) tillfällen. Den första undersökningen kommer att ske på en morgon innan du hunnit anstränga synsystemet alltför mycket. Den andra undersökningen kommer att ske senare samma dag så tätt inpå som möjligt efter att du utfört två (2) timmars närarbete (läsning på ett avstånd som är 50 cm eller mindre).

Vid undersökningstillfällena kommer du att få fylla i ett frågeformulär och vi kommer att mäta din syn och fastställa dina ögons brytfel.

Undersökningarna i denna studie kommer inte utgöra någon risk för din personliga hälsa.
Att delta i denna studie är helt frivilligt vilket innebär att du kan avbryta din

medverkan när som helst och utan att ange skäl till varför du avbryter din medverkan.
Resultaten kommer att behandlas konfidentiellt.

Ingen enskild individ kommer att kunna identifieras då resultaten presenteras.

Medverkandens samtycke:

Jag har tagit del av informationen, både muntligt och skriftligt, förstått syftet med studien samt vad medverkan innebär och samtycker till att delta i denna studie.

Namnteckning Namnförtydligande Datum och ort

___________ ________________ ____________

Frågeformulär

Datum: Patient-kod:____

1. Namn: _____________________________________________________

2. Man □ Kvinna □

3. Ålder:______________________________________________________

4. Typ av närarbete:_____________________________________________

5. Arbetsavstånd:_______________________________________________

6. Tid spenderad, med närarbete:___________________________________

7. Tid förfluten mellan närabetets avslut och undersökning:______________

Anamnes

Systemisk och okulär hälsa:

Medicinering:

Astenopi:

Protocol: Nearwork – a risk factor for overcorrecting patients?

Examination 1:

Date:

Auto refraction:
Time:______________________ OD:_______________________________________________________________ OS:_______________________________________________________________ Subjective refraction: Room illumination:_________

PD Fri visus Sph. Cyl. Axis Visus BKC

40cm

OD

OS

Inför nästa undersökning ska Patienten läsa eller utföra någon annan form av närarbete på ett avstånd som är ≤ 50 cm i 2 timmar med minsta möjliga avbrott.

Examination 2:

Auto refraction:
Time:______________________ OD:_______________________________________________________________ OS:_______________________________________________________________ Subjective refraction: Room illumination:_________

PD Fri visus Sph. Cyl. Axis Visus BKC

40cm

OD

OS

Myopes

_{Age Working} distance

(cm)

Time spent with nearwork (approx.)

(h)

Time between end of nearwork and re-check

(min) Diff. in distance refraction after nearwork (D) Myope 1 21 45 1.8333 5 -0.5 Myope 2 21 40 1.8333 2 0 Myope 3 22 40 2.0 2 0 Myope 4 21 40 2.0 3 -0.5 Myope 5 23 30 2.0 10 -0.25 Myope 6 25 25 2.0 3 -0.25 Myope 7 24 30 2.0 2 -0.5 Myope 8 24 20 2.0 3 0 Myope 9 24 45 1,8333 7.5 -0.5

Emm.

_{Age Working}

distance (cm)

Time spent with nearwork (approx.)

(h)

Time between end of nearwork and re-check

(min) Diff. in distance refraction after nearwork (D) Emmetrope 1 25 45 2 7 -0.25 Emmetrope 2 23 50 2 5 0 Emmetrope 3 23 35 3.5 2 -0.25 Emmetrope 4 23 40 1.25 2 0 Emmetrope 5 23 40 1.6666 4 0 Emmetrope 6 23 40 2.5 75 -0.25 Emmetrope 7 27 40 1.8333 10 0 Emmetrope 8 27 35 1.75 10 0.25 Emmetrope 9 28 25 1.8333 12 0 Emmetrope 10 27 40 2.0833 7 0