Nearwork-induced Transient Myopia Research Articles

A Method for Profiling Biometric Changes During Disaccommodation

To demonstrate the application of low-coherence reflectometry to the study of biometric changes during disaccommodation responses in human eyes after cessation of a near task and to evaluate the effect of contact lenses on low-coherence reflectometry biometric measurements. Ocular biometric parameters of crystalline lens thickness (LT) and anterior chamber depth (ACD) were measured with the LenStar device during and immediately after a 5 D accommodative task in 10 participants. In a separate trial, accommodation responses were recorded with a Shin-Nippon WAM-5500 optometer in a subset of two participants. Biometric data were interleaved to form a profile of post-task anterior segment changes. In a further experiment, the effect of soft contact lenses on LenStar measurements was evaluated in 15 participants. In 10 adult participants, increased LT and reduced ACD was seen during the 5 D task. Post-task, during fixation of a 0 D target, a profile of the change in LT and ACD against time was observed. In the two participants with accommodation data (one a sufferer of nearwork-induced transient myopia and other a non-sufferer), the post-task changes in refraction compared favorably with the interleaved LenStar biometry data. The insertion of soft contact lenses did not have a significant effect on LenStar measures of ACD or LT (mean change: -0.007 mm, p = 0.265 and + 0.001 mm, p = 0.875, respectively). With the addition of a relatively simple stimulus modification, the LenStar instrument can be used to produce a profile of post-task changes in LT and ACD. The spatial and temporal resolution of the system is sufficient for the investigation of nearwork-induced transient myopia from a biometric viewpoint. LenStar measurements of ACD and LT remain valid after the fitting of soft contact lenses.

Reproducibility of nearwork-induced transient myopia measurements using the WAM-5500 autorefractor in its dynamic mode

To evaluate the reproducibility of nearwork-induced transient myopia (NITM) measurements obtained objectively using an open-field autorefractor (WAM-5500) in its dynamic mode. NITM was assessed in 22, visually-normal, teen-aged and young-adult subjects using an infrared autorefractor (WAM-5500) in the dynamic mode. Measurements were obtained from the right eye in two test sessions separated by either 30minutes or 2days. Initial NITM and its decay were assessed monocularly by the same experimenter immediately after binocularly viewing and performing a detailed near task (5D) for 5minutes incorporating a cognitive demand, with habitual distance refractive correction in place. Data were averaged over 10-second bins for 3minutes (180seconds; 18 bins) for the decay analysis. The NITM post- minus pre-task difference and its limits of agreement (LOA), as well as intraclass correlation coefficient (ICC), were calculated to evaluate reproducibility over the two test sessions for the initial NITM magnitude and its decay. The group mean (±SE) initial NITM and its decay duration were 0.33 ± 0.09 D and 0.28 ± 0.08 D, and 118.6 ± 14.3seconds and 132.3 ± 12.2seconds respectively, for each test session, which were not significantly different (p > 0.05). The difference (range), LOA, and ICC (95% confidence interval [CI]) were 0.06D (-0.15, 0.64), -0.29 to 0.40D, and 0.90D (0.77, 0.96) for the initial NITM; they were -13.6 (-150.0, 140.0) seconds, -174.5 to 147.3seconds, and 0.14 (0.00, 0.52) for decay duration, respectively, for each test session. The ICC range for the first 50secs of the NITM response/decay was 0.90 to 0.96. The initial NITM was highly repeatable. The initial decay phase was moderately repeatable, with the later decay phase being more variable, yet still yielding acceptable reproducibility in many cases. Both of these key parameters, namely initial NITM and its early decay, can be assessed reliably and with good reproducibility. This is important in future longitudinal studies of NITM, and its possible relation to refractive development.

Myopia: its prevalence, origins and control

Myopia: its prevalence, origins and control

Nearwork-induced transient myopia (NITM) following marked and sustained, but interrupted, accommodation at near

The purpose of the present investigation was to determine objectively the initial magnitude and decay of nearwork-induced transient myopia (NITM), and its potential additivity, following successive but interrupted periods of marked, sustained accommodation at near in asymptomatic young-adult myopic subjects. Fifteen visually-normal, asymptomatic young adults (ages 18-28 years) were tested with full distance refractive correction. They included nine early-onset (EOM) and six late-onset (LOM) myopic subjects. Accommodation was assessed objectively with a Canon R-1, open-field, infrared auto-refractor under monocular viewing conditions (RE). The distance refractive state was measured immediately before and after a 10 min period of focusing upon a moderate contrast (50%), very near target (12 cm; 8 D) subtending a visual angle of 1°. The task was repeated twice with a 5-min inter-trial rest period of imposed far viewing. NITM was defined as the post-task minus pre-task change in distance refractive state and was determined for each of the three trials. Significant amounts of NITM were generated following nearly each trial in each subject. These ranged from 0.11 to 0.71 D, with a mean of 0.31 D. The group mean NITM was 0.32, 0.29, and 0.31 D for trials 1, 2, and 3, respectively. For the EOMs subgroup, NITM was 0.28, 0.30, and 0.34 D, while for the LOMs subgroup, it was 0.38, 0.29, and 0.26 D, for trials 1, 2, and 3, respectively. Decay of NITM was prolonged in many of the subjects (67%). However, additivity of NITM was not found following the sequences of interrupted near tasks. There was no evidence of NITM additivity following a marked and sustained, but interrupted, near task. Although NITM has been reported to be additive following long periods of uninterrupted and sustained reading at lower dioptric levels, providing rest periods between each near task trial appears to prevent a cumulative effect (i.e. additivity effect).

Effect of customized near addition lenses on transient myopia induced by near-work

Correspondence: Bai-Chuan Jiang College of Optometry, Nova Southeastern University 3200 S. University Drive, Fort Lauderdale, FL 33328, USA Email bjiang@nova.edu Purpose: To verify whether optimal near add power can reduce near-work induced transient myopia (NITM). Methods: Eighteen subjects including nine progressive myopes (PMs) and nine stable myopes (SMs) were involved in this study. We measured the accommodative error and the near phoria at 40 cm under fully corrected conditions for each subject. These findings were then used to determine optimal near-add power for each subject based on formulae derived from a previous study. Under both with and without near-add conditions, NITM was obtained by comparing the refractions immediately before and after performing continuous computer work at 40 cm for an hour. The refraction was determined by a COAS aberrometer. Results: Using customized near-addition lenses (C-ADD) we found significantly decreased retinal defocus from 0.49 ± 0.12 (SD) D to 0.08 ± 0.11 D. The near phoria showed a significant exophoric shift from -1.84 ± 1.95 prism diopter (pd) to -5.37 ± 1.63 pd, but was still in the normal range. We also found that the differences in NITM between the two groups and between the two conditions (ie, with and without C-ADD) were both significant. Further comparisons indicated that the NITM in the SM group was significantly reduced after using C-ADD; in the PM group the reduction of NITM was not significant. Conclusions: The results suggest that C-ADD balances the load of accommodation and vergence, significantly decreasing retinal blur during the near-work, and may be able to reduce the NITM.

Effect of nearwork-induced transient myopia on distance retinal defocus patterns

The purpose of the current study was to assess the effect of nearwork-induced transient myopia (NITM) on retinal defocus patterns during distance viewing. An empirically derived, conceptual model of human blur perception and related retinal defocus patterns has been extended to determine the effect of NITM on the relative contributions of myopic and hyperopic retinal defocus during distance viewing. Under the normal hyperfocal refractive condition during distance viewing with isolated stimulus conditions, there is very small myopic defocus (∼0.25 diopters), and no hyperopic defocus, present. After sustained nearwork generating NITM, a moderate increase in this myopic defocus contribution takes place. In the normal isolated distance viewing situation with only very small myopic defocus present, as would occur with many far outdoor activities, the paucity of overall retinal defocus may provide a “protected” condition against myopia development. In contrast, with the addition of NITM producing increased myopic retinal defocus only, there is an imbalance of retinal defocus that may be myopigenic, especially in the context of foveal and peripheral retinal interactions.

Accommodative Training to Reduce Nearwork-Induced Transient Myopia

To assess changes in the nearwork-induced transient myopia parameters of initial magnitude and its decay duration, as well as accuracy of the near accommodative steady-state response and clinically based accommodative facility, after 6 weeks of home-based accommodative training in asymptomatic myopes. Ten young adult, progressing myopes participated in the study. The experimental paradigm consisted of a baseline session and two follow-up sessions at the end of the third and sixth weeks of training. At the first session, baseline refractive state and selected accommodative functions were assessed. Measurements were repeated at the two follow-up sessions. Home-based vision training included accommodative flippers (+/-2 D) at near, Hart chart at distance (6 m) and near (40 cm), and prism flipper (6 pd) training at near (40 cm), for a total of 20 minutes a day performed 5 days a week for 6 weeks. Several dynamic accommodative response functions improved significantly with training. Lens flipper rate increased significantly from 11 to 16 cpm in the OD (p = 0.04), 11 to 19 cpm in the OS (p = 0.03), and 8 to 11 cpm in the OU (p = 0.03). Hart chart rate increased significantly from 22 to 33 cpm in the OD (p = 0.01) and from 22 to 31 cpm in the OS (p = 0.02). There was a significant negative correlation between lens flipper rate and nearwork-induced transient myopia decay duration after training (p = 0.02) with binocular viewing. Training of the accommodative system in these progressing myopes resulted in improved dynamics in both laboratory and clinical measures. This is consistent with earlier reports in the literature of improvement in symptomatic myopic subjects.

Visual field does not affect steady-state accommodative response and near-work induced transient myopia

To investigate whether or not peripheral retinal defocus contributes to the refractive development of myopia by influencing the overall accommodative function. The steady-state accommodative stimulus response curve (ASRC) and the near-work induced transient myopia (NITM) after a near visual task were compared between different visual field conditions in emmetropes (EMMs), stable myopes (SMs) and progressing myopes (PMs). Results showed that visual field had no effect on the ASRC and NITM but PMs exhibited greater NITM than EMMs and SMs. The results of this study suggest that peripheral defocus does not influence the overall accommodative system so its probable contribution to myopia development is not via the accommodative system.

Sympathetic Inhibition of Accommodation after Sustained Nearwork in Subjects with Myopia and Emmetropia

The purposes of the present study were to assess the effect of a sympathetic inhibitory pharmacologic agent, timolol maleate, on the magnitude of nearwork-induced transient myopia (NITM) and its decay in different refractive groups for an extended near task duration and to determine the proportion of the young adult population manifesting effective sympathetic access under naturalistic closed-loop viewing conditions. Ten subjects with emmetropia and 10 with myopia were tested. They read binocularly for 1 hour at a distance of 35 to 40 cm. NITM was calculated as the difference in distance refractive state after task as compared with before task immediately after reading. All subjects received timolol maleate to block the sympathetic nervous system and betaxolol as a control agent in independent test sessions separated by at least 3 days. Forty minutes after drug instillation, the NITM measurement procedure was repeated. Initial NITM magnitude was larger in subjects with myopia than in subjects with emmetropia before and after timolol instillation. Furthermore, NITM magnitude in subjects with sympathetic access was increased after timolol instillation. In contrast, with the control agent betaxolol, there was no increase. NITM decay duration to baseline was increased after timolol instillation in the subjects with myopia only. Only 15% of the subjects (n = 3 subjects with myopia) demonstrated effective and significant access to sympathetic facility. Subjects with myopia demonstrated an increase in decay duration with timolol, thus suggesting impaired sympathetic inhibition of accommodation. This may be a precursor for myopia progression in some persons.

Nearwork‐induced transient myopia (NITM) and permanent myopia – is there a link?

Myopia is a worldwide public health problem. However, its understanding is incomplete, and many of its preventative and therapeutic aspects remain controversial. Nearwork is a primary, environmentally based factor in the aetiology of permanent myopia (PM), with nearwork-induced transient myopia (NITM) being a possible contributory component. A relationship between PM and NITM has been suggested, but that connection has remained somewhat indirect and elusive. However, based on recent converging evidence from clinical, laboratory and modelling studies, a five-fold argument will be advanced for a possible link between PM and NITM.

Incremental retinal-defocus theory of myopia development—Schematic analysis and computer simulation

Previous theories of myopia development involved subtle and complex processes such as the sensing and analyzing of chromatic aberration, spherical aberration, spatial gradient of blur, or spatial frequency content of the retinal image, but they have not been able to explain satisfactorily the diverse experimental results reported in the literature. On the other hand, our newly proposed incremental retinal-defocus theory (IRDT) has been able to explain all of these results. This theory is based on a relatively simple and direct mechanism for the regulation of ocular growth. It states that a time-averaged decrease in retinal-image defocus area decreases the rate of release of retinal neuromodulators, which decreases the rate of retinal proteoglycan synthesis with an associated decrease in scleral structural integrity. This increases the rate of scleral growth, and in turn the eye's axial length, which leads to myopia. Our schematic analysis has provided a clear explanation for the eye's ability to grow in the appropriate direction under a wide range of experimental conditions. In addition, the theory has been able to explain how repeated cycles of nearwork-induced transient myopia leads to repeated periods of decreased retinal-image defocus, whose cumulative effect over an extended period of time results in an increase in axial growth that leads to permanent myopia. Thus, this unifying theory forms the basis for understanding the underlying retinal and scleral mechanisms of myopia development.

Accommodation functions: Co-dependency and relationship to refractive error

We assessed the extent to which different accommodative functions are correlated and whether accommodative functions predict the refractive error or the progression of myopia over a 12 month period in 64 young adults (30 myopes and 34 non-myopes). The functions were: amplitude of accommodation; monocular and binocular accommodative facility (6 m and 40 cm); monocular and binocular accommodative response to target distance; AC/ A and CA/ C ratios, tonic accommodation (dark focus and pinhole), accommodative hysteresis, and nearwork-induced transient myopia. Within groups of related accommodative functions (such as facility measures or open-loop measures) measurements on individuals were generally significantly correlated, however correlations between functions from different groups were generally not significant. Although accommodative amplitude and pinhole (open loop) accommodation were significantly different in myopes than in non-myopes, these functions were unrelated to myopia progression. Facility of accommodation and accommodative lag was independent predictors of myopia progression.

The Effect of Common Reductions in Letter Size and Contrast on Accommodation Responses in Young Adult Myopes and Emmetropes

Accommodation errors during reading and the subsequent near work-induced transient myopia (NITM) that occurs have been implicated in the development and progression of myopia. This study investigated the effects of two letter variables, size and contrast, on accommodation accuracy during the near task and on NITM and its subsequent decay. These were varied so as to mimic what might occur when students photocopy and reduce reading materials. Based on their refractive errors, young adult subjects (18-25 years) were classified into three groups: emmetropes (n = 19), stable myopes (n = 17), and progressing myopes (n = 17). Three print sizes (N4, N6, and N8) and two print contrasts (90% and 60%) were used to give six different reading targets. Targets were presented in random order at 25 cm (4 D demand) and the text read for comprehension for 3 minutes. For each target, accommodation accuracy and NITM and its decay were measured using the free space Shin-Nippon SRW-5000 autorefractor. When data for all subjects were pooled, there was a significant effect of letter size (p = 0.030) but not contrast (p = 0.898) on accommodation accuracy; however, differences were small and unlikely to be clinically relevant. NITM (p = 0.033) and its decay (p = 0.012) also varied with letter size. NITM was greater and decay longer for larger letters. We found no effect of refractive error group on accommodation accuracy. In contrast, there was a significant difference in the magnitude of NITM and its decay for emmetropic and myopic subjects (although no effect of progression status); myopes had larger NITM values and longer decay times to baseline than emmetropes (NITM myopes: 0.37 +/- 0.14D vs. emmetropes: 0.19 +/- 0.17 D, p = 0.005; decay time myopes: 15.12 +/- 6.58 seconds vs. emmetropes 7.10 +/- 4.82 seconds, p = 0.0045). The differences in NITM and its decay between the two refractive groups were of similar magnitude for all six combinations of letter size and contrast. Our data do not support the suggestion that common reductions in letter size or contrast of reading material (as might occur for photocopied reading materials) cause greater accommodation inaccuracy and greater near work-induced adaptation effects that would exacerbate myopia development in young adults.

An Incremental Retinal-Defocus Theory of the Development of Myopia

Previous theories of myopia development involved subtle and complex processes such as the sensing and analyzing of chromatic aberration, spherical aberration, spatial gradient of blur, and spatial frequency content of the retinal image. However, these theories have not been able to explain all the diverse experimental results, which has been accomplished by our newly proposed incremental retinal-defocus theory. Our theory is based on a relatively simple and direct mechanism for the regulation of ocular growth. It states that a time-averaged decrease in retinal-image defocus decreases the rate of release of retinal neuromodulators, which decreases the rate of retinal proteoglycan synthesis, with an associated decrease in scleral structural integrity. This increases the rate of scleral growth, and in turn the eye’s axial length, which produces permanent myopia. Schematic analysis of the theory has provided a clear explanation for the eye’s ability to grow in the appropriate direction under a wide range of experimental conditions. In addition, the theory has been able to explain how repeated cycles of nearwork-induced transient myopia leads to repeated periods of decrease in retinal-image defocus, whose cumulative effect over an extended period of time also results in an increase in axial growth that produces permanent myopia. Thus, this unifying theory forms the basis for understanding the underlying retinal and scleral mechanisms of myopia development.

Refractive error, cognitive demand and nearwork-induced transient myopia

Purpose. Whereas many previous studies have identified the association between sustained near work and myopia, few have assessed the influence of concomitant levels of cognitive effort. This study investigates the effect of cognitive effort on near-work induced transient myopia (NITM). Methods. Subjects comprised of six early onset myopes (EOM; mean age 23.7 yrs; mean onset 10.8 yrs), six late-onset myopes (LOM; mean age 23.2 yrs; mean onset 20.0 yrs) and six emmetropes (EMM; mean age 23.8 yrs). Dynamic, monocular, ocular accommodation was measured with the Shin-Nippon SRW-5000 autorefractor. Subjects engaged passively or actively in a 5 minute arithmetic sum checking task presented monocularly on an LCD monitor via a Badal optical system. In all conditions the task was initially located at near (4.50 D) and immediately following the task instantaneously changed to far (0.00 D) for a further 5 minutes. The combinations of active (A) and passive (P) cognition were randomly allocated as P:P; A:P; A:A; P:A. Results. For the initial near task, LOMs were shown to have a significantly less accurate accommodative response than either EOMs or EMMs (p < 0.001). For the far task, post hoc analyses for refraction identified EOMs as demonstrating significant NITM compared to LOMs (p < 0.05), who in turn showed greater NITM than EMMs (p < 0.001). The data show that for EOMs the level of cognitive activity operating during the near and far tasks determines the persistence of NITM; persistence being maximal when active cognition at near is followed by passive cognition at far. Conclusions. Compared with EMMs, EOMs and LOMs are particularly susceptible to NITM such that sustained near vision reduces subsequent accommodative accuracy for far vision. It is speculated that the marked NITM found in EOM may be a consequence of the crystalline lens thinning shown to be a developmental feature of EOM. Whereas the role of small amounts of retinal defocus in myopigenesis remains equivocal, the results show that account needs to be taken of cognitive demand in assessing phenomena such as NITM.

Differential refractive susceptibility to sustained nearwork.

A possible role for nearwork-induced transient myopia (NITM) (i.e. transient myopia at distance following sustained nearwork) in the development and/or progression of permanent myopia has been suggested. If certain refractive groups are more susceptible to NITM than others, then measures may be implemented to prevent or retard the occurrence of myopia in these individuals. In a recent study (Ciuffreda and Wallis, 1998), it was found that young-adult late-onset myopes (LOM) and early-onset myopes (EOM) were highly susceptible to this NITM accommodative after-effect, while emmetropes (EMMs) were only moderately susceptible, and hyperopes (HYP) were particularly insusceptible, following short periods (10 min) of sustained nearwork at a very close distance (20 cm, 5 D). In the current study, this was extended to encompass a much longer period of nearwork under more naturalistic conditions. Young-adult subjects (n = 16) performed a reading task for a continuous period of 4 h at their habitual working distance under naturalistic conditions, i.e. binocular viewing with blur-related visual feedback present. Distance refractive state (binocular viewing) was assessed objectively using a Canon R-1 autorefractor immediately before (pre-task), every hour during the task, and immediately after (post-task) to assess NITM and its subsequent decay to pre-task refractive levels. Subjects were divided into the four refractive subgroups as above, which were each of equal size (n = 4). The NITM was induced in all subjects, except for the hyperopes in which there was a hyperopic refractive shift. The LOMs (0.12 D) and EOMs (0.13 D) were highly susceptible, while the HYPs were especially insusceptible (0.44 D hyperopia), and the EMMs were only moderately susceptible (0.09 D). The present study extended our earlier investigation by demonstrating the differential refractive susceptibility over a longer period of sustained nearwork, which may be the result of reduced sympathetic activation. It has been speculated that NITM may be a factor in myopigenesis, with a possible mechanism being time-integrated increased retinal defocus from NITM at both distance and near, in conjunction with the normal lag of accommodation during sustained nearwork, thereby causing axial elongation. Hence, a future longitudinal investigation specifically targeting this retinal defocus hypothesis is warranted.

Nearwork induced transient myopia during myopia progression

Purpose. Recent work has shown that differences exist in the accommodation response characteristics of myopes and emmetropes, although the exact nature of the differences has yet to be ascertained. This may in part be due to the classification of the refractive error employed in many studies. Previously it has been suggested that inaccuracies in the accommodation response occur predominantly during the progression of myopia. To test this hypothesis we measure nearwork induced transient myopia (NITM) in progressing myopes (PMs), stable myopes (SMs) and emmetropes (EMMs). Methods. A total of 41 young (18–27 years) subjects participated in the study (13 PMs, 14 SMs, 14 EMMs). Following a 10 minute near task (4D) the change in accommodation response back to distance viewing was measured over a two minute time period. Results. Grouped mean data revealed a significant nearwork after-effect in PMs in comparison to SMs and EMMs after 10 seconds (p < 0.01) and 30 seconds (p < 0.01) post-task. Significantly longer time constants were also found in the PMs group. Conclusions. Our results suggest that NITM is manifest during the progressive phase of myopia development.

Adaptation model of nearwork-induced transient myopia

A nearwork model was developed to determine whether variation in accommodative adaptation gain, KA, can account for the differences in the dynamic decay timecourse following near work in hyperopes (HYP), emmetropes (EMM), early-onset myopes (EOM), and late-onset myopes (LOM). The model incorporated a proximal component into a previously-developed adaptation model of accommodation and vergence. It was used to simulate the nearwork-induced transient myopia (NITM) response following 10min of congruent binocular near viewing (5 D, 5 MA). The accommodative adaptation gain, KA, value was varied from 1.0 to 6.0 in increments of 0.5. For the hyperopes, an additional constraint was imposed wherein the accommodative response was biased on the under-accommodated side of the deadspace operator (i.e., depth-of-focus). In addition, the effect of prolonged nearwork was simulated by alternating between 1hr of congruent near viewing (3 D, 3 MA) and 5min of congruent far viewing (0.25 D, 0.25 MA) over a 160hr period representing one work-month with 40 hours of nearwork per week. The steady-state rms value of the accommodative error was measured as a function of KA. It was found that the NITM timecourses for HYP, EMM, EOM, and LOM could be simulated accurately using KA values of 2.0, 2.5, 4.0 and 5.5, respectively. The long-term final steady-state rms of the accommodative error was found to increase from 0.182 D to 0.188 D as KA increased from 1 to 6. This indicated a small and progressive increase in residual accommodative error for higher KA values, which was associated with EOM and LOM. Thus, NITM for the different refractive groups could be quantified by the accommodative adaptation gain element, with KA for the HYP, EMM, and EOM and LOM groups having lower, intermediate, and higher values, respectively. The larger rms for higher KA values suggests that a myopic individual may have a predisposition to exhibit a slightly larger long-term accommodative error, which may stimulate axial elongation and in turn promote the progression of axial myopia.

Adaptation model of nearwork-induced transient myopia.

A nearwork model was developed to determine whether variation in accommodative adaptation gain, KA, can account for the differences in the dynamic decay timecourse following near work in hyperopes (HYP), emmetropes (EMM), early-onset myopes (EOM), and late-onset myopes (LOM). The model incorporated a proximal component into a previously-developed adaptation model of accommodation and vergence. It was used to simulate the nearwork-induced transient myopia (NITM) response following 10 min of congruent binocular near viewing (5 D, 5 MA). The accommodative adaptation gain, KA, value was varied from 1.0 to 6.0 in increments of 0.5. For the hyperopes, an additional constraint was imposed wherein the accommodative response was biased on the under-accommodated side of the deadspace operator (i.e., depth-of-focus). In addition, the effect of prolonged nearwork was simulated by alternating between 1 hr of congruent near viewing (3 D, 3 MA) and 5 min of congruent far viewing (0.25 D, 0.25 MA) over a 160 hr period representing one work-month with 40 hours of nearwork per week. The steady-state rms value of the accommodative error was measured as a function of KA. It was found that the NITM timecourses for HYP, EMM, EOM, and LOM could be simulated accurately using KA values of 2.0, 2.5, 4.0 and 5.5, respectively. The long-term final steady-state rms of the accommodative error was found to increase from 0.182 D to 0.188 D as KA increased from 1 to 6. This indicated a small and progressive increase in residual accommodative error for higher KA values, which was associated with EOM and LOM. Thus, NITM for the different refractive groups could be quantified by the accommodative adaptation gain element, with KA for the HYP, EMM, and EOM and LOM groups having lower, intermediate, and higher values, respectively. The larger rms for higher KA values suggests that a myopic individual may have a predisposition to exhibit a slightly larger long-term accommodative error, which may stimulate axial elongation and in turn promote the progression of axial myopia.

Near-vision lens effects on nearwork-induced transient myopia

Nearwork-induced transient myopia (NITM), a lenticular-based pseudomyopia, is commonly found in myopic teenagers and young adults. It has been speculated that its presence may be a factor in nearwork-related symptoms and in the development of permanent myopia. Thus, the differential effect of various powered near-vision lenses on NITM was investigated. Nine young adults monocularly-viewed a near target (25 cm) for 20 min through either a +4, +2, or 0 diopter (D) near lens, or a 0.5-mm pinhole, over their habitual distance prescription. Monocular distance refraction was objectively assessed immediately before and after the near task. Decreasing the accommodative stimulus reduced the blur-driven adaptive aftereffect. Only with the 0 D accommodative stimulus condition was NITM not produced. These results suggest that the full blur-driven monocular stimulus to accommodation must be negated to prevent the occurrence of NITM. With the pinhole providing blur-free viewing conditions, a substantial proximal accommodative adaptive component was present.