Late-onset Myopia Research Articles

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.

Objective blur threshold for ocular accommodation in emmetropia and late-onset myopia

Objective blur threshold for ocular accommodation in emmetropia and late-onset myopia

Dynamic accommodation and myopia: §Culhane HM, Winn B. Invest Ophthalmol Vis Sci 1999;40:1968-74

Accommodative effort during nearwork is thought to be a causative factor in the development of myopia. It has been proposed that an anomaly in autonomic control may be a precursor to the development of myopia. In the present study the closed-loop accommodation response after variations in fixation period was investigated in emmetropes, early-onset myopes and late-onset myopes to determine characteristics of reflex accommodation for each refractive group.Closed-loop accommodation responses were measured in a group of emmetropes (n = 7), early-onset myopes (n = 7), and late-onset myopes (n = 7) by use of a dynamic tracking infrared optometer. A variation in fixation period (10 seconds, 60 seconds, and 180 seconds) before an accommodative step was used to stimulate the accommodation control mechanism differentially.Group results of accommodative response times showed that late-onset myopes were significantly affected by the duration of fixation before the change in stimulus vergence. Accommodative response times after 3 minutes of sustained near vision were significantly longer than those observed for other groups for the near-to-far condition. Reaction time appears to be independent of refractive grouping, prior fixation period, and direction of step change.Late-onset myopes showed significantly extended accommodation response times after a sustained near vision task that was demonstrable under well-controlled experimental conditions. The extended response times observed in the present study were consistent with previous reports of refractive shifts in late-onset myopes and early-onset myopes and provide a corollary between reflex and adaptive components of the accommodation response. Potential mechanisms are discussed in an attempt to explain the resultant hysteresis under closed-loop viewing conditions.

Open-loop accommodation in emmetropia and myopia

Purpose. To investigate the influence of method of measurement and refractive error on the open-loop accommodation response. Methods. Open-loop accommodation was measured in darkness (dark accommodation, DA) and using a pinhole pupil (pinhole accommodation, PA) in emmetropic subjects (EMMs, n = 63), subjects with late-onset myopia (LOMs, n = 50) and subjects with early onset myopia (EOMs, n = 51). Further a control experiment examined the differences between DA and bright-field accommodation (BA) conditions in a subset of subjects. All measurements of open-loop accommodation were carried out monocularly using a Canon R1 infra-red optometer in static recording mode. All myopic subjects were fully corrected using soft contact lenses. Results. A significant variation (p < 0.001) in open-loop accommodation was found between DA and PA, but no variation in open-loop level was observed between the three refractive groups. There was no interaction between these two factors. No significant difference was found between the BA level and DA level in any of the refractive groups. Conclusions. Open-loop accommodation response positions vary according to the experimental conditions employed during measurement. No refractive group differences in the open-loop response were apparent.

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.

Oculomotor functions and late-onset myopia

In order to elucidate the role of optical defocus in the development of late-onset myopia (LOM), we employed both theoretical and experimental approaches. In the theoretical study, which has been reported previously, we suggested a model in which an accommodative sensory operator was added to simulate the sensory part of the accommodation system. Results derived from the model showed that the sensory part might increase the system’s threshold to the accommodative error (or defocus) signal. In this study, we measured the perceptual depth-of-focus and the system’s threshold to the defocus signal for three refractive error groups: emmetropic (Emm), stable myopic (S.M.), and progressing myopic (P.M.). Results show that there are no significant differences in the perceptual depth-of-focus among the three groups. However, the defocus threshold values of the P.M. group are significantly higher than the values of the other two groups. This result in combination with our previous findings, leads us to suggest that individuals susceptible of developing myopia from sustained near-work have a specific oculomotor risk profile. Although we are still trying to determine the specific sequence of changes among dark-focus, AC/A ratio, the accommodative system’s defocus threshold, and refractive error, we are convinced that the changes of oculomotor parameters underlie the type of myopia associated with near-work.

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.

Oculomotor functions and late-onset myopia.

In order to elucidate the role of optical defocus in the development of late-onset myopia (LOM), we employed both theoretical and experimental approaches. In the theoretical study, which has been reported previously, we suggested a model in which an accommodative sensory operator was added to simulate the sensory part of the accommodation system. Results derived from the model showed that the sensory part might increase the system's threshold to the accommodative error (or defocus) signal. In this study, we measured the perceptual depth-of-focus and the system's threshold to the defocus signal for three refractive error groups: emmetropic (Emm), stable myopic (S.M.), and progressing myopic (P.M.). Results show that there are no significant differences in the perceptual depth-of-focus among the three groups. However, the defocus threshold values of the P.M. group are significantly higher than the values of the other two groups. This result in combination with our previous findings, leads us to suggest that individuals susceptible of developing myopia from sustained near-work have a specific oculomotor risk profile. Although we are still trying to determine the specific sequence of changes among dark-focus, AC/A ratio, the accommodative system's defocus threshold, and refractive error, we are convinced that the changes of oculomotor parameters underlie the type of myopia associated with near-work.

Sensitivity analysis of the stimulus-response function of a static nonlinear accommodation model.

The effect of parameter variation of a nonlinear static feedback control model of the accommodation system was investigated. Simulations of a MATLAB/SIMULINK model showed a nonlinear relationship between stimulus and response in which the response curve was above the 1:1 line in the region to the left of the crossover and below the 1:1 line in the region to the right of the crossover. At the crossover, the response curve exhibited an inflection that was constant and equal to the tonic accommodation value (ABIAS). Sensitivity analysis showed that increasing depth of focus (deadspace range between +/- DSP) increased the separation between the boundary lines of the deadspace region, with a larger separation associated with late-onset myopia and congenital nystagmus. Increasing accommodative controller gain (ACG) increased the slope of the function on either side of the deadspace, with lower ACG values corresponding to lower slopes that indicated an amblyopic deficit. Increasing ABIAS increased the accommodative level at the inflection region. In addition, the saturation level of the accommodative response decreased with increasing age, while the slope remained the same, which was consistent with the Hess-Gullstrand theory of presbyopia. The accuracy and relative simplicity of the model indicated that it could serve as a basis for further comprehensive investigation of the basic and clinical aspects of the accommodation system.

Differences in the accommodation stimulus response curves of adult myopes and emmetropes

While the accommodation system has been implicated in myopia development, the nature of this relationship remains obscure. This study investigated the differences in accommodation stimulus response curves between adult myopes and emmetropes. Myopic subjects were classified according to age of onset and stability of their myopia. Accommodation stimulus response curves were measured using three different methods: (i) real targets presented at viewing distances of 4 m to 0.25 m, (ii) a target at 4 m viewed through negative lenses of increasing power, and (iii) a target at 0.25 m viewed through positive lenses of decreasing power. A Canon Autoref R-1 measured the accommodation responses at 5 levels of demand (increasing from 0 D to 4 D in 1 D steps). We found significant differences between the three methods used to stimulate an accommodation response in all subject groups, for example, accommodation lags at high accommodative demands were greatest for the negative lens series and least for the positive lens series. In addition, while differences between early-onset myopes, late-onset myopes and emmetropes were not observed, we did observe differences when myopic subjects were reclassified according to whether their myopia was progressing or stable. A reduced accommodation response to negative lens-induced accommodative demand was found in progressing myopes but not in stable myopes. These results provide further evidence for the link between myopia progression and inaccurate accommodation. The data also suggest that adult myopes with stable refractive errors show accommodation responses similar to that of emmetropes.

Vitreous Chamber Elongation is Responsible for Myopia Development in a Young Adult

In this case report, we present 3 years of longitudinal, refractive component data for a young adult who became myopic and progressed in myopia (late-onset myopia, LOM) during that period. We found a high correlation between refractive error and axial length (AL). The axial elongation was a consequence of the change in the vitreous chamber depth (VCD). Furthermore, the change in AL and VCD corresponded quantitatively to the change in refractive error. Corneal curvature change was not correlated with refractive error change. These findings support the suggestion that VCD is the refractive component that changes in LOM development.

The effect of topical β-adrenoceptor antagonists on accommodation in emmetropia and myopia

Using topical instillation of non-selective and selective beta-adrenoceptor antagonists we examine the proposal that a deficit in inhibitory sympathetic innervation of ciliary smooth muscle may be a specific precursor to the onset and development of late-onset myopia (LOM). Post-task accommodative hysteresis, a possible consequence of such a deficit, was assessed by measuring the time-course of regression of accommodation when open-loop (darkness) conditions were immediately imposed following far and near tasks. For the proposal to be feasible only LOMs should exhibit post-task responses which fail to differentiate the type of beta-antagonist employed. As the overall profile of responses to beta-adrenoceptor antagonism was equivalent for each of the three different refractive groups (emmetropes N = 6; early-onset myopes N = 5; LOMs N = 5) we conclude that a propensity to LOM is not associated with a deficit in sympathetic inhibition.

Repeatability of post-task regression of accommodation in emmetropia and late-onset myopia.

The characteristics of post-task regression of accommodation to pre-task tonic accommodation (TA) levels have been examined in a number of studies to clarify the nature of the within-task facility for accommodative adaptation. Of special interest is the recent observation that significant attenuation of post-task regression occurs in late-onset myopes (LOMs) when compared with emmetropes (EMMs). These findings have led to speculation that such attenuation may reflect a deficit in inhibitory sympathetic innervation to ciliary smooth muscle in late-onset myopia and hence a predisposition to sustained accommodative adaptation which then acts as a precursor to the induced myopia. A consequence of this study was that post-task regression may have some value in predicting those individuals who may be susceptible to post-task accommodative hysteresis. A pre-requisite for such a predictive value is that for a given individual the variation in inter-trial regression patterns is not significant. The aim of this study is principally to investigate the inter-trial variability of post-task regression for individual subjects following a sustained near vision task, and to confirm further differences that have been reported between EMMs and LOMs with respect to the time course of post-task regressions. A modified Canon R1 infrared optometer was used to measure accommodation objectively throughout a near task and for 2 min post-task. Accommodative level was measured following 3 min fixation of a high contrast photopic Maltese cross target placed 3 D above the subject's baseline TA. Repeatability of post-task regression in 10 EMMs and 10 LOMs was assessed by taking measurements on three separate occasions.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodative adaptation and age of onset of myopia

Tonic accommodation in an empty field and accommodative adaptation, defined as accommodation shift after 2 min viewing of a 4 D near task, were measured using an infrared optometer. Subjects were classified into three types: emmetropes (n = 18), early onset myopes (n = 18) and late onset myopes (n = 15). The relationship between accommodative adaptation and the onset age of myopia were compared. There was no statistical difference in the pre-task tonic accommodation among the three subject groups. However, the accommodative adaptation differed in the three groups during the 5 min post-task period. The average accommodative adaptation of emmetropia, early onset myopia and late onset myopia were 0.66, 0.27 and 1.19 D, respectively. Accommodative adaptation in the late onset myopia group was significantly higher than in the emmetropia group. Furthermore, the accommodative adaptation in the early onset myopia group was significantly lower than in the emmetropia group. Two possible aetiologies of myopia are discussed: one for the group which shows low accommodative adaptation that has no relationship with the age of onset and another for the group which shows high accommodative adaptation that has a relationship to an onset age of 15 years or later. The aetiology is also considered with regard to a potentiating chemical effect on the synapses of the ciliary muscle.

Steady-state accommodation and ocular biometry in late-onset myopia

The steady-state accommodative responses of emmetropes and late-onset myopes was measured for an array of numbers located at -1, -3 and -5 dioptres using an objective infra-red optometer. Responses were compared for passive (reading numbers) and active (adding numbers) conditions. For the passive condition, the late-onset myopes showed a significantly lower accommodative response than the emmetropic group. No significant differences were found between the two groups for the active condition. Ocular biometric characteristics were also measured in emmetropes, late-onset myopes and early-onset myopes using keratometry and ultrasonography. No significant differences in corneal curvature, anterior chamber depth and crystalline lens thickness were found between the groups. Late-onset myopes exhibited significantly deeper vitreous chambers than emmetropes, which more than accounted for the difference in refractive error between the two refractive groups. We conclude that, while significant differences exist in the accommodative responses of late-onset myopes and emmetropes, late-onset myopia is due predominantly to elongation of the vitreous chamber.

The influence of method on open-loop measures of accommodation in emmetropia and late-onset myopia

The influence of method on open-loop measures of accommodation in emmetropia and late-onset myopia

Adaptation of Tonic Accommodation to Sustained Visual Tasks in Emmetropia and Late-Onset Myopia

The study complements and extends our previous investigation and reports that late-onset myopes (LOM's, N = 15) exhibit differences in adaptation of tonic accommodation (TA) to sustained visual tasks when compared to emmetropes (EMM's, N = 15). Pre- and post-task TA was measured with a modified Canon R-1 objective infra-red optometer under darkroom conditions. Post-task TA was measured immediately after a 10-min counting task and at 1-s intervals over a period of 90 s; the task was located at distances equivalent to 1, 3, and 5 D. The pattern of regression back to the pre-task TA level was used to assess the degree of adaptation generated by the task. The data indicate substantial differences between regression patterns for EMM's and for LOM's which were enhanced as accommodation stimulus levels increased. Using a two-factor split-plot analysis of variance, statistically significant differences between EMM's and LOM's could be attributed to the rates of change of post-task accommodative levels to pre-task TA levels for both the 3 and 5 D tasks.

Effect of Target Proximity on the Open-Loop Accommodative Response

Proximal stimuli to accommodation were assessed by comparing the open-loop accommodative responses (AR's) to stimuli placed at viewing distances of 5 m (0.2 D) and 0.33 m (3D) in 10 late-onset myopes (LOM's) and 10 emmetropes. The accommodation and vergence loops were opened by subjects viewing targets monocularly through 0.5-mm pinholes. Pre- and post-task levels of tonic accommodation (TA) were used to assess the accommodative adaptation induced by the task. For the 0.2 D condition there was no significant difference between pre-task TA, within-task AR, or post-task TA, indicating that accommodation was open-loop throughout this distal condition. When viewing the 3 D target both refractive groups exhibited significant increases in AR, emmetropes showing higher levels of proximally induced accommodation (PIA) when compared with LOM's. The data indicate that target proximity will influence AR even when both blur and vergence cues have been stabilized.

Myopia in optometry students: family history, age of onset and personality.

The relationship between refractive error and family history, age of onset and personality was investigated in 189 optometry students. Subjects completed a questionnaire requesting details of their refractive history, type of correction and their family history. Subjects also completed an Eysenck Personality Inventory (EPI). Subjects were categorized as hyperopes, emmetropes or myopes on the basis of their questionnaire responses. No significant relationship was found between refractive group and the prevalence of myopia in parents. Myopic subjects, however, showed a significantly higher prevalence of myopic siblings. Furthermore, late-onset myopes showed a higher prevalence of myopic siblings than early-onset myopes. No significant personality differences were found between the refractive groups.

Disparity-induced accommodation in late-onset myopia

The effect of disparity-vergence on the accommodation response (AR) was measured in 10 late-onset myopes (LOMs) and 10 emmetropic subjects during the course of a 10 minute near-vision task (working distance 33cm). Disparity was induced by the introduction of 3 and 6Δ base-out prisms placed before the right eye, and the resulting AR for the left eye was measured at 1 min intervals using an objective infr-red optometer. LOMs showed a significant increase in AR with increasing disparity whereas emmetropes showed no significant differnce in AR for all disparity conditions. The implications of these findings are discussed with reference to the aetiology of late-onset myopia.