The prevalence of myopia in developed East Asian countries, such as Hong Kong, Taiwan, and Singapore1–7 is alarming. There is strong evidence that the rapid increase in the prevalence of myopia is linked to the environment; high myopia prevalence in children is associated with increased education demands and greater housing density.8 Many epidemiological studies report a correlation between high myopia prevalence and lengthy periods spent performing nearwork,9,10 particularly at very close distances.11 Recent works12,13 suggest that outdoor activity can ameliorate myopia.
There are different mechanisms by which long periods performing nearwork could result in myopia. One of the most researched aspects is the accommodation system.14 Two main characteristics of accommodation have been considered to be important: the accommodation error that occurs during nearwork (typically the response is less than the demand, that is, a lag of accommodation is measured) and the accommodation hysteresis that occurs after nearwork (accommodation takes time to relax for distance viewing after prolonged near tasks).15–17 The latter effect is termed “accommodative adaptation” or “nearwork-induced transient myopia” (NITM).
Both myopic children and adults exhibit lags of accommodation at near.18–21 The persistent lag of accommodation in prolonged nearwork will result in hyperopic retinal defocus that may lead to myopia development.18,20 Accommodative adaptation (NITM) is also suspected to play an important role in myopia development.22–25 After a sustained period of nearwork, a myopic shift for distance viewing is measured under close-looped conditions.22,24–27 Although the etiology of NITM remains unclear, the inability to relax accommodation rapidly and fully to the far point is probably caused by a neuropharmacological adaptation of innervations to the ciliary muscle (Lancaster and Williams,28 cited by Ong and Ciuffreda29). Myopic children and young adults show significantly greater posttask myopic shifts and prolonged NITM than emmetropic children,22,25 emmetropic young adults, and hyperopes.24,26
A common prolonged nearwork activity for children is reading. Reading involves continuous eye movements following the words and constant cognitive demand. Young children from East Asian countries read stories in both Chinese and English. Written Chinese characters consist of more strokes in a logographic system (where a single character can be a whole word), which are more closely packed and probably harder to resolve, than English letters. These unique characteristics imply that some of the neurocognitive mechanisms underlying Chinese logographic reading differ from those underlying alphabetic word reading.30 It is possible that accommodation adaptations are different for these two different types of text, and we propose that the type of characters a child is exposed to when learning to read may play an important role in the development of myopia.
The aim of this study was to measure the accuracy of accommodation responses during and NITM after reading of Chinese and English texts and to determine if there were any differences related to myopia. Our first hypothesis was that lags of accommodation would be lower (improved accuracy in accommodation because of smaller details) and NITM higher for Chinese text than for English text. Our second hypothesis was that both lags of accommodation and NITM would be greater in myopic children than in emmetropic children, irrespective of whether Chinese or English text was read.
Participants were recruited from the Singapore Polytechnic Optometry Centre and its satellite clinic in the West Coast Community Centre. The research followed the tenets of the Declaration of Helsinki and was approved by both Singapore Eye Research Institute Institutional Review Board and the Queensland University of Technology. Informed written consent was obtained from both the child and a parent or guardian before participation.
Some time before the study, a pilot study of lag of accommodation was conducted on a young adult group of 17 emmetropes and 33 myopes. The mean difference between groups was 0.15 diopters (D) (myopes > emmetropes), with a combined SD of 0.22 D. Using this mean as being clinically significant, a level of confidence of 95% and a power of 80%, the number of required participants per group was determined as 35.
Based on the power analysis, 83 children aged 7 to 12 years (mean ± SD, 10.1 ± 1.7 years) were recruited, composed of 40 emmetropes (spherical equivalent refraction range, +0.75 to −0.25 D; mean ± SD, +0.23 ± 0.25 D) and 43 myopes (−0.50 D or less spherical equivalent refraction; mean ± SD, −2.87 ± 1.30 D). The myopic children were all progressing myopes (myopia had increased by at least 0.50 D per year during the past 2 years31) who wore their prescriptions full time. Myopia progression status was determined from past optometric records or by comparing the subjective refraction with the spectacle prescription and reported age of the spectacles. The children completed a questionnaire on their refraction history and family ocular history with the aid of a parent or guardian.
Vision screening tests were performed to ensure that the children met the inclusion criteria of at least 6/6 monocular visual acuity; amplitude of accommodation of at least the minimum amplitude of the Duane-Hoffstetter formula of 15.0 – 0.25 × age, as measured with a RAF rule; cylinder, ≤0.50 D; myopia, ≤6.00 D; hyperopia, ≤0.75 D; anisometropia, ≤1.00 D; and absence of any ocular disease, including strabismus. Another criterion was that the children could read both Chinese and English texts. Generally, the right eye was tested, but the left eye was tested if it passed the inclusion criteria and the right eye failed (four children had >0.75 D of astigmatism in the right eye). Subjective refraction was performed using the maximum plus for best visual acuity principle.32 All participants including the emmetropes (with plano lenses) were corrected for distance with soft spherical contact lenses for the reading tasks and accommodation measures. Each child completed four trials involving two working distances and two languages on the same day.
A preliminary study was conducted on 20 children (10.2 ± 1.3 years) to determine typical reading distances for both Chinese and English books. This study was performed at a public library. The children were asked to choose their own reading materials. Measurements of reading distance were made after 5 minutes of reading. The reading distances adopted for both texts were 33 ± 6 cm, so 33 cm was chosen as a test distance. A second reading distance of 25 cm was included because some previous accommodation studies involving children have used this shorter reading distance.18,26,33
A free-space autorefractor (WAM-5500; Grand Seiko Co., Ltd, Japan) was used in static mode to measure refraction. This device provides a wide open field of view that includes the test stimuli and surrounding environment. The vertex distance for the autorefractor was set at zero. The autorefractor was connected to a computer using an RS-232C cable to allow collection of high-speed measurements, and the data output was converted to an Excel spreadsheet.
Reading materials were presented on a tablet PC, HP Pavilion TX 1000, with 90 cd/m2 screen luminance, as measured with a Minolta Luminance Meter LS-100. The tablet PC screen was 26 by 16 cm, subtending 26 degrees vertically by 38 degrees horizontally at 33 cm and 33 degrees vertically by 46 degrees horizontally at 25 cm. The tablet PC was placed in front of the autorefractor. The turntable screen could be quickly positioned and lowered. When participants were looking at the distance target, the screen was lowered. When participants read, the screen was raised.
The two reading tasks were composed of paragraphs of words in 10.5-point SimSun Chinese font and 12-point Times New Roman English font, with 120 cd/m2 background and 90% Michelson contrast. Both text sizes were used in newspapers distributed in Singapore: SimSun 10.5 points for Chinese text in My Paper and Times New Roman 12 points for English text in the national paper The Straits Times. The stroke frequencies of both texts and their respective logMAR notation are given in Table 1.34
The reading tasks were administered in random order in terms of type of text, but the lower accommodation demand distance of 33 cm was always tested first. The reading and viewing tasks were performed binocularly, whereas refraction measures were monocular. Refraction measures were performed before, during, and after the 10-minute reading period. A 10-minute reading duration was chosen because 15 minutes has been shown to induce a fatigue effect,35 with the potential to interact with the tasks and confound the data.
The distance fixation target was a 110-cd/m2 internally illuminated distance visual acuity chart at 4 m (LIGHTHOUSE-modified ETDRS chart, Lighthouse International, New York, NY). Participants were asked to fixate the middle letter of the 0.2 logMAR line (equivalent to Snellen 6/9). The room illuminance was kept relatively low at about 25 lux so that it would not interfere with the visibility of the targets.
Participants spent 5 minutes in darkness to open the accommodation loop and dissipate the effects of any previous near activity.24,35,36 The lights were switched on, and participants viewed the distant chart at 4 m. Autorefractor measurements commenced within 3 seconds, and 30 measurements at 2-second time intervals were made; the average of these gave the pretask close-looped refraction. Next, the participants performed one of the near reading tasks. The computer screen was raised, and the children were asked to read the text aloud. Most of the children were more proficient in English than Chinese, with only five more proficient in the Chinese language. This was addressed by choosing Chinese stories that were short and easy to understand. The children were told to skip any Chinese words that they could not pronounce. Autorefractor measures were taken at 2-minute intervals; 10 measurements were averaged at each of 2, 4, 6, and 8 minutes, and the 40 measurements during the whole period were also averaged. When measurements were performed, participants were asked to briefly stop reading and view a bolded word that aligned with the autorefractor measurement system (i.e., was centered within the autorefractor alignment ring). In Chinese text reading, the word “
Equation (Uncited)Image Tools
’ and a short word such as “there” in English were used for fixation. The short word used was unlikely to drive a different amount of accommodation response because the words were located within the center of the paragraph of text (i.e., they were not presented alone). Once the near task was completed, the computer screen was lowered and participants were instructed to view the distance fixation target. Autorefractor measurements were performed every 2 seconds during a 3-minute period; the average of the first three measurements gave the posttask 6-second NITM. The time taken for accommodation to return to pretask levels was taken as the regression time. This completed one trial of about 20-minute duration. In-between trials, children took at least 10 minutes of break.
Autorefractor readings were converted to spherical equivalent refraction by adding half the cylinder to the sphere. Invalid autorefractor readings, which occurred as a result of blinking or momentary loss of fixation, gave outputs with large cylinder components of more than 1.00 D and were disregarded. To determine accommodation accuracy, the average of near refraction measures was subtracted from the accommodation demand. The accommodation accuracy during the second, fourth, sixth and eighth minute of reading was analyzed to determine whether accommodation accuracy changed during this period.
Nearwork-induced transient myopia was calculated by subtracting the pretask close-looped accommodation response from the posttask value, with the myopic shift representing NITM expressed as a positive value.37 Data for each participant were divided into 6-second bin intervals, resulting in a total of 30 readings posttask. Nearwork-induced transient myopia of emmetropic and myopic children was compared at 6 seconds (an average of 2, 4, and 6 seconds) and 24 seconds (an average of 20, 22, and 24 seconds) posttask. As the accommodation responses posttask were measured at the interval of 2 seconds, NITM was obtained by the difference between the average of the first three distance refraction measures taken after reading (the first 6 seconds posttask) and the average pretask refraction. Regression times were expressed in terms of the time taken (seconds) for the posttask refraction to reach the average pretask refraction.36 If the refraction had not returned to pretask levels by 3 minutes, measurements were ceased and a value of 186 seconds was recorded. This occurred for 47 measures.
Analyses of variance using the general linear model were performed to determine the significance of the results. For dependent variables accommodation accuracy, NITM, and NITM regression time, the between-group factor was refraction group (emmetropes, myopes), and within-subject factors were text type (Chinese, English) and distance (25 cm, 33 cm). Post hoc analysis was performed using the Bonferroni 95% confidence interval.
Effect of Text Type and Reading Distance on Accommodation Accuracy and NITM: All Participants
Table 2 shows the effect of text type and reading distance on accommodative accuracy, NITM, and regression time. When data from all participants were pooled, there were significant effects of text type on accommodation accuracy during the 10-minute reading tasks (lag mean ± SD: Chinese text, 0.97 ± 0.32 D; English text, 1.00 ± 0.37 D; F1,1230 = 7.24, p = 0.007) and reading distance (lag 33 cm, 1.01 ± 0.30 D; lag 25 cm, 0.97 ± 0.39 D; F1,1230 = 7.74, p = 0.005). Although the differences were statistically significant, the effects were small and not likely to be physiologically relevant.
As has been shown previously for children of other ethnic groups,38 the Chinese children underaccommodated at near; this was by 0.99 ± 0.34 D across all tasks. The mean lags of accommodation reduced significantly as reading time increased (F3,1230 = 4.59, p = 0.003) from 1.01 ± 0.37 D at 2 minutes to 0.95 ± 0.37 D at 8 minutes. However, the lag reduction was small at 6%.
In contrast to accommodation accuracy, when data of all participants were pooled, text type and distance did not have significant effects on NITM (text: F1,164 = 0.05, p = 0.824; distance: F1,16 = 0.00, p = 0.988) nor its regression time (text: F1,246 = 0.00, p = 0.954; distance: F1,246 = 2.06, p = 0.152). This was also the case when data were analyzed with refraction group included as the between-group factor (NITM and text: F1,243 = 0.03, p = 0.874; NITM and distance: F1,243 = 0.00, p = 0.956; regression time and text: F1,243 = 0.00, p = 0.968; regression time and distance: F1,243 = 2.07, p = 0.151).
Accommodation Accuracy, NTIM, and Regression: Emmetropes versus Myopes
There was no significant difference in accommodation accuracy between emmetropic and myopic children (F1,81 = 0.51, p = 0.475) (Fig. 1). The average accommodation lags across both text types and distance for emmetropes and myopes were 0.96 ± 0.35 D and 1.01 ± 0.33 D, respectively, for the 8 minutes.
Refraction group had significant effects on both NITM (F1,81 = 5.05, p = 0.027) and its regression time (F1,81 = 31.08, p < 0.001). Nearwork-induced transient myopia was greater for myopes than for emmetropes (0.15 D vs. 0.08 D) (Fig. 2) and took 50 seconds longer to dissipate in the myopes (Fig. 3). Myopic children showed significantly larger NITM than emmetropic children at 6 seconds (F1,81 = 5.05, p = 0.027) and at 24 seconds (F1, 81 = 7.92, p = 0.006) posttask. At 24 seconds posttask, the mean NITM of emmetropic children fell beyond the baseline of 0 D (Fig. 4) but the mean NITM of myopic children was still 0.07 D above their baseline. When regression times were averaged across four tasks for each individual, nine emmetropic children, compared with 21 myopic children, displayed 120-second regression times or longer.
As the distance refractions were tested at 4 m instead of the conventional 6-m distance, the distance accommodation demand was 0.25 D. However, the mean accommodative responses during pretask distance viewing were 0.19 D lag and 0.09 D lead for emmetropic and myopic children, respectively, and this difference was statistically significant (F1,81 = 15.38, p < 0.001). This shows that myopic children accommodated 0.28 D more at distance than the emmetropic children before performing any near tasks.
Consistent with others,22,25–27,36 we found that nearwork induced greater NITM, which took longer to dissipate in myopic than in emmetropic children. Our new finding was that this occurs irrespective of whether the children read Chinese or English text. Unlike some studies,18,19 we found that refraction group had a minimal effect on accommodative accuracy, although this may be caused by accommodation being induced by a near target rather than by negative lenses.20,21 The data support the first hypothesis posed in the Introduction in part; lags of accommodation were less during reading of Chinese text but the NITM induced was not higher than the comparable measures for English text. Our second hypothesis was also partly supported in that myopic children presented with higher NITM and regression time but no difference in accommodation lag was found. The findings are discussed with reference to theories of myopia development.
Lags of accommodation were slightly less when reading Chinese text than when reading English text, but the difference was small and unlikely to be physiologically relevant. This slight difference could be caused by the different input units (letters vs. characters) between Chinese and English, different mapping functions, and different processing between letters and characters of both Chinese and English text.39 The square shape of the logography in the Chinese text, requiring an elaborated analysis of the spatial information and locations of the various strokes comprising the logographic character, may involve a greater scrutiny of the characters that leads to higher accommodation. However, the difference in accommodative accuracy observed when children read the texts was small. Charman and Tucker40 pointed out that at higher spatial frequencies, the accommodative response is more accurate. The logMAR notations of both the Chinese and English texts used were similar at about 0.8 logMAR (33 cm) and 0.9 logMAR (25 cm). As the fundamental spatial frequency is similar for both fonts (about 3.8 cpd at 25 cm) and as they are composed of sharp edges of the high contrast, the spatial frequency amplitude spectra should be similar.
Accommodation accuracy was similar for myopic and emmetropic children, with both groups having lags around 1 D. Other studies have found that the accommodation lag is greater in myopic children and myopic young adults, but this occurs typically when accommodation is stimulated with negative lenses20,21 and not by varying target distance.20,21,25,36 The availability of proximal cues, when accommodation is induced by a near target, has been shown to improve the accommodation responses of the myopes, reducing the difference in response between different refraction groups.37 Furthermore, in this study, the children read under binocular conditions to mimic daily reading tasks. This differentiates the current study from the study of Gwiazda et al.,18 where one eye was occluded. Accommodative lag is reduced for myopes under binocular viewing conditions because of asymmetry of the relative position of the target between tested and nontested eyes.41
NITM and Regression
The accommodative adaptation (NITM) was measured under close-looped conditions and the pretask and posttask measurements of accommodation were assessed while participants viewed a distant letter chart. Thus, normal blur-feedback mechanisms were allowed to operate.42 Although the pretask reading represents the far point of accommodation, the posttask reading represents the combined response of the far point of accommodation plus the output of the slow blur accommodative response.42
Nearwork-induced transient myopia and its regression were not affected by text type nor task distance. The lack of text type effect could be because the two texts were read at the same distances, although there is a report of NITM being similar for different reading distances (20 cm vs. 40 cm).25 Nearwork-induced transient myopia is reported to vary with task difficulty,43 and reading either Chinese or English text might be considered as low-cognitive tasks compared with doing arithmetic sums.43
The values of NITM reported in previous studies with young-adult participants, rather than children,24,26,27 ranged from 0.12 to 0.16 D for emmetropes and 0.17 to 0.70 for myopes. One reason for the lower effect in our study may be the younger age of our participants. There may be other reasons such as the differences in the nature (e.g., counting vs. reading) and the greater angular size of the material in this study. In previous studies, participants were instructed to look at numbers of size equivalent to 0.0 or 0.2 logMAR24,26,27 and performed simple arithmetic operations at distances of 20 to 25 cm. In this study, the targets were printed text used commonly in reading materials, with equivalent logMAR values of 0.8 to 0.9, and hence, the detail was four to eight times larger angular size than in the previous studies. We believe that our testing conditions emulate the real life environment better than those used previously.
Relevance to Myopia Development
Although we found that refraction group had a minimal effect on accommodative accuracy, its importance in myopia development cannot be disregarded. Although some studies observed a greater lag of accommodation in emmetropic children before they became myopic,44,45 others reported that the increased lag of accommodation accompanies the development of myopia rather than precedes it.46,47 A recent study measured axial length increases in young adults after a prolonged near task of 30-minute duration at 5-D demand.48 This elongation required 10 minutes of distance viewing to dissipate. The authors suggested that larger amounts of nearwork, performed at closer distances, might potentially be expected to lead to prolonged short-term eye length changes of greater magnitude that could predispose a patient to greater amounts of eye elongation in the longer term.
Nearwork-induced transient myopia of the myopic children was significantly higher than NITM of the emmetropic children, and the myopic children accommodated 0.28 D more than emmetropic children at pretask level. It has been suggested that NITM induces retinal image defocus, which over time may play a role in myopia development.29 The NITM of some children for a range of reading tasks lingered beyond the 3-minute posttask measurement period. This has been observed previously, where NITM was still evident 3 minutes after viewing a 5-D near task for 5 minutes.25 If NITM is a function of task duration,29 then task duration and the time course of NITM decay support the notion that repeated occurrences of transient myopia lead to permanent myopia.29 In a recent study when NITM was measured after the first hour of reading and measured again after the second hour of reading, progressing myopes showed a greater time constant of decay than stable myopes.49 Progressing myopes, but not stable myopes, also exhibited additivity of NITM.49 Our findings are consistent with this: myopic children showed significantly longer regression times (by 50 seconds) than emmetropic children, and 43% of the myopic children showed regression times 120 seconds or longer posttask compared with only 15% of the emmetropic ones. These differences, although small, could accumulate over time.
An adaptation model of NITM was proposed to link NITM with myopia development.50 The model was based on animal studies that found retinal image blur to be an important cue for regulating eye growth.51 In the model, if the adaptation component is large, the time constant for the accommodative controller is increased. This means that the decay of NITM toward the pretask baseline is delayed. During the long-term, there are typically alternating periods of prolonged nearwork and brief distance viewing, representative of everyday activities. Under this condition, both lag of accommodation at near and lead of accommodation at far occur.24 Therefore, the retinal defocus (with hyperopic defocus at near and myopic defocus at distance) accumulating for months or years when performing considerable amount of nearwork could be myopigenic in nature.
Neural Input to Accommodation
The accommodation system receives dual innervations from the autonomic nervous system. It is thought that the sympathetic division, with its known slow or adaptive system in a time course of 10 to 40 seconds and its inhibitory nature, may play an important role in accommodative adaptation and NITM.52,53 A deficit in sympathetic input leads to enhanced accommodative adaptation effects and a prolonged period of regression because of an unantagonized parasympathetic tone (reviewed by Chen et al.37). Many studies hypothesized that a susceptibility to accommodative hysteresis leads to cumulative retinal defocus from enhanced transient pseudomyopic changes in distance refraction, thus triggering an increase in axial length.15,23,24,54 However, there are others who believe that myopes could exhibit an overall reduction of both sympathetic and parasympathetic innervations.37,55–57 This is because sympathetic activation is positively correlated with parasympathetic activity, with a decrease in the latter leading to decrease of the former.23
Reading Chinese text caused smaller accommodative lags than reading English text, but the difference was small and unlikely to be of physiological relevance. Myopic children had significantly greater NITM and longer regressions than emmetropic children for both texts. Whether differences in NITM are a cause or consequence of myopia cannot be answered from this study.
Anna Chwee Hong Yeo
Applied and Health Sciences Cluster
Singapore Polytechnic 500 Dover Rd,
The first author thanks Queensland University Technology for the Queensland International Doctoral Scholarship, Singapore Polytechnic for the research grant, and Mr Lai Nai Shin for statistics guidance.
Received June 7, 2012; accepted October 30, 2012.
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