Additivity of Near Work–Induced Transient Myopia and Its Decay Characteristics in Different Refractive Groups

Abstract

purpose. To determine the additive effect, if any, of NITM after 1 and 2 hours of reading in different refractive groups.

methods. Fifteen early-onset myopes (EOMs), 14 late-onset myopes (LOMs), and 15 emmetropes (EMMs), as well as progressive myopes (PMs) and stable myopes (SMs), were tested. Subjects read binocularly for 2 hours at a distance of 35 to 40 cm. Distance refractive state of the right eye was assessed every 2 seconds for 30 seconds after the first hour of reading, and then every 2 seconds for 120 seconds after the second hour of reading. NITM was calculated as the difference in posttask distance refractive state compared with the pretask distance refractive state after each hour.

results. Initial NITM values (mean ± SE) recorded at the end of the near work tasks were 0.22 ± 0.03 D and 0.29 ± 0.03 D for the EOMs, 0.14 ± 0.02 D and 0.20 ± 0.03 D for the LOMs, 0.14 ± 0.02 D and 0.15 ± 0.02 D for the EMMs, 0.20 ± 0.03 D and 0.27 ± 0.03 D for the PMs, and 0.09 ± 0.04 D and 0.20 ± 0.05 D for the SMs, after the first and second hours of reading, respectively. After the second hour, only in the EOMs and LOMs was NITM significantly greater than that found after the first hour. Seventy percent of the myopes (EOMs and LOMs) but only 47% of the EMMs exhibited increased NITM in the second hour compared with the first hour. Only EOMs exhibited longer decay duration after the second hour of reading. NITM was increased in progressive myopes (PMs), but not in stable myopes (SMs), after the first hour of reading only. Within the PMs, NITM was increased after the second hour compared with the first hour. The time constant for decay was greater in the PMs than in the SMs. Lastly, many myopes (up to 46%) did not experience decay to baseline after the near task over the 120-second posttask period.

conclusions. EOMs and LOMs demonstrated larger NITM than the EMMs and exhibited NITM additivity, but the EOMs also exhibited prolonged decay of NITM compared with the EMMs and LOMs. PMs, but not SMs, exhibited additivity of NITM. These findings may be attributed to impaired sympathetic function in the subjects with myopia. It is speculated that with repeated cycles of near work, residual NITM may contribute to the progression of permanent myopia.

Two basic mechanisms underlie the development of human myopia. ^{1 2} The first one is genetic, which is related to preprogrammed growth of the eye. The second one is environmental, with near work an important factor. Based on a conventional classification of myopia, ³ early-onset myopia (EOM) manifests before the age of 13 years and results in relatively high degrees of myopia. This refractive group is presumably more susceptible to genetic aspects and family history, ⁴ with near work assumed to be a secondary influence. In contrast, late-onset myopia (LOM) manifests after the age of 15 years and results in relatively low degrees of myopia. LOM has a presumably predominant environmental component, such as near work, with genetic and family history assumed to be a secondary influence. Thus, the factor of near work in LOM is presumed to be increasingly important as the demands of school reading increase. Hence, an interplay appears to exist between genetics and near work that has different levels of expressivity within and between the EOM and LOM refractive groups. 

The relation between myopia and near work has been reasonably well established in humans ^{5 6 7 8 9} and in animal models. ^{10 11 12} This includes the environmental factor of near work-induced transient myopia (NITM), ^{5 13 14 15} which refers to the small, transient, pseudomyopic shift in the far point of the eye after a period of sustained near work. It reflects an inability of the crystalline lens to reduce its power appropriately and rapidly, thus reflecting an accommodative aftereffect/hysteresis phenomenon of presumed pharmacologic origin. ^{5 13} In the normal population, the mean magnitude of NITM is typically sufficiently small (approximately 0.3 D) and remains within the depth of focus of the eye, thus producing no perception of blur. ^{16 17} One of the first efforts to study NITM was by Lancaster and Williams. ¹⁸ They showed that when school-aged children and young adults were presented with a substantial and sustained near work demand, NITM magnitude was as large as 1.5 D. In young, asymptomatic adults, NITM values as large as 0.60 D have been recorded under more typical near-viewing conditions. ¹⁹ In contrast, the smallest amount of NITM reported was 0.12 D, which is equal to the resolution of conventional autorefractors. ²⁰  

NITM is influenced by a variety of stimulus-driven and physiological factors. ^{5 19} Near task duration is a primary factor. Studies on NITM have been performed with monocular and binocular tasks over a range of near work durations. NITM is sensitive to task duration; it is positively correlated with this parameter. ¹⁹ This has been demonstrated in studies involving a range of relatively short reading durations (15 seconds to 8 minutes), ¹⁹ and relatively long ones (4 hours). ¹⁷ A second primary factor is refractive group. With respect to NITM magnitude, several studies have reported NITM to be greater in LOMs and EOMs than in EMMs. ^{16 17} Furthermore, Vera-Diaz et al. ²¹ have reported that PMs were more susceptible to NITM than were SMs. With respect to NITM decay characteristics, some studies have reported complete decay, ^{16 17} whereas others have found only partial decay, to baseline after a period of near work. ^{13 19} For example, a recent study compared NITM decay in three refractive groups—EMMs, EOMs, and LOMs—after 10 minutes of near work. ¹⁶ The magnitude of NITM in the LOMs, EOMs, and EMMs was 0.36 D, 0.35 D, and 0.09 D, respectively. The decay was considerably more rapid in the EOMs (35 seconds) than in the LOMs (65 seconds). Hence, NITM amplitude and decay are dependent on refractive error. ^{19 22}  

One important aspect of NITM that has not been addressed is the notion of its additivity. In other words, is NITM additive over repeated and sequential closely spaced near work tasks? With this idea in mind, we hypothesized that NITM would be greater, exhibit temporal additivity, and at times manifest lack of complete decay in myopes. Thus, the aims of the present study were to assess the refractive error dependence of NITM induced by prolonged reading, its additivity, and its decay characteristics. 

Subjects and Methods

Subjects

Forty-four visually asymptomatic optometry and graduate students were recruited from the college. Ages ranged from 21 to 34 years; mean age was 23.7 years. The group consisted of EMMs (n = 15), EOMs (n = 15), and LOMs (n = 14). Myopes were categorized as EOMs or LOMs based on their ages at the time of refractive error onset, ^{3 16} as described in the Introduction. LOMs had a spherical equivalent refractive range of −0.5 D to −3.00 D, with a mean (±SE) of −1.87 ± 0.32 D. EOMs had a spherical equivalent refractive range of −2.25 D to −7.75 D, with a mean (±SE) of −4.12 ± 0.73 D. EMMs had a spherical equivalent refractive range of +0.5 DS to −0.25 DS, with a mean (±SE) of +0.15 ± 0.55 D. In addition, these same subjects were also categorized as stable myopes (SMs) or progressive myopes (PMs) based on their refractive history. ²¹ If they had a refractive error change of −0.5 D or more over the past 2 years, they were categorized as PMs. If not, they were categorized as SMs. Seven (all LOMs) were categorized as SMs, and 22 (15 EOMs + 7 LOMs) were categorized as PMs. PMs had a spherical equivalent refractive range of −1.75 D to −7.75 D, with a mean (±SE) of −3.67 ± 0.53 D. SMs had a spherical equivalent refractive range of −0.5 D to −3.00 D, with a mean (±SE) of −2.12 ± 0.38 D. In all 44 subjects, the cylindrical component was no greater than −1.50 D, with a mean of −0.67 D. All had 20/20 or better Snellen visual acuity in the distance and near in each eye with their current spectacles or contact lenses, which had been recently prescribed (less than 1 year) based on noncycloplegic subjective refraction. Three of the 44 subjects wore their spectacles, and the remainder wore their contact lenses throughout the experiment. Before the experiment was begun, overrefraction was performed on all subjects with their spectacles or contact lenses in place to ensure that the refraction was accurate. Informed consent was obtained from each subject after the nature and possible consequences of the study were explained. The research followed the tenets of the Declaration of Helsinki and was approved by the college’s internal review board. 

Instrumentation

All measurements of refractive state were obtained objectively using an infrared autorefractor (Canon R-1; Canon Inc., Lake Success, NY), which is a widely used infrared autorefractor for vision research. ²⁰ Measurements using this device have been found to be repeatable. ²⁰ Its noise level was determined using four sequential measurements performed on a custom schematic eye (Bausch & Lomb, Rochester, NY) and four similar measurements from an absolute presbyope, simulating different refractive errors (plano to 5 D myopia) with trial lenses. The noise level ranged from 0.06 to 0.075 D (for additional details of the device, see McBrien and Millodot ²⁰ ). 

Procedure

Pretask.

All subjects were seated in total darkness for 5 minutes to allow for the dissipation of potential transient accommodative aftereffects. ²³ Then the distance refractive state was assessed in the right eye while the subject binocularly viewed 20/30 Snellen letters at 6 m. Fifteen measurements were taken, and the mean spherical equivalent, representing the mean pretask baseline distance refractive state, was calculated. During these assessment periods, most subjects wore contact lenses correcting the distance refractive error to avoid spectacle reflections that might interfere with the measurements. However, three subjects wore spectacles, and their measurements were taken with very slight pantoscopic lens tilt to avoid reflections without inducing change in the accommodative measurements. 

Task.

Subjects read adult-level text consisting of optometric lecture notes during the 2-hour test period. They were instructed to maintain the text in focus at all times at a distance of 35 to 40 cm and not to gaze into the distance. Reading activity was periodically monitored, and reading distance was reassessed every 15 minutes. 

Posttask.

Immediately after the first hour of reading, the subject’s distance refractive state was assessed every 2 seconds for a period of 30 seconds. After this, subjects were instructed to read for another hour under the same test conditions. Immediately at the end of the second hour, the distance refractive state was reassessed every 2 seconds for 120 seconds. Data for each subject were divided into 10-second bin intervals and averaged across subjects within each specified refractive group. The posttask minus pretask refractive state difference represented the NITM dioptric magnitude. Data were analyzed with respect to initial NITM magnitude, NITM additivity, decay duration, and decay time constant (TC). Subjects were given only a brief 30-second distance viewing period after the first hour of reading to allow for possible NITM additivity. In contrast, subjects were given a relatively long 120-second distance viewing period after the second hour of reading, similar to previous investigations, to study the time course of NITM decay to baseline. In addition, the present 30-second/120-second protocol differed from that of Ciuffreda and Lee, ¹⁷ which had 120-second measurement periods throughout, allowing for additivity as described. 

Statistical Analysis

Measurements from the infrared autorefractor (Canon R-1; Canon Inc.) were obtained on a paper printout and manually entered into a spreadsheet (Excel; Microsoft, Redmond, WA) for analysis. First, any measurement taken during an involuntary blink was recorded as an error by the system and hence do not affect the data analysis. Second, any unusually high values (e.g., −8 D) were deleted from the data analysis. Furthermore, if the sphere and cylinder values were at least three times larger than the preceding and subsequent readings, they were considered noise and deleted. In addition, the video monitor continuously displayed the refractive measurements and eye image (including the pupil), which were viewed in real time to assess steadiness of fixation. 

Such blink and eye movement–related artifacts comprised less than 2% of the raw data. The spherical equivalent was calculated for each reading, which was then averaged for each subject and across the group and subgroups. 

Accommodation was assessed after the second hour of near work for 120 seconds. The time taken for the NITM magnitude to dissipate in each subject—in other words, the time to decay to the pretask baseline level—was calculated for each subject and then averaged in each refractive group to obtain mean decay duration. 

Exponential decay functions were fitted to the posttask accommodative response. From this function, the mean time constant for decay was calculated. The time constant was defined as the time elapsed between the end of the closed-loop task and the point at which the closed-loop accommodative response decayed to 63.2% of the pretask baseline. Exponential regression plots were obtained using a statistics and analytics software package (Statistica; StatSoft Inc., Tulsa, OK) that provided the time constant values for the group mean in each refractive group. 

NITM was determined after the first and second hour of near work. The difference between these two values was obtained in each subject, and the group mean was calculated in each refractive group, which reflected the NITM additivity. 

Results

Initial NITM Magnitude

Refractive Group Effect.

Group mean (±SE) initial NITM magnitudes for the EMMs after the first and second hours were 0.14 ± 0.02 D and 0.15 ± 0.02 D, respectively. In the LOMs, they were 0.14 ± 0.02 D and 0.20 ± 0.03 D after the first and second hours, respectively, whereas they were 0.22 ± 0.03 D and 0.29 ± 0.03 D after the first and second hours in the EOMs, respectively (Fig. 1) . 

Two-way, repeated-measures ANOVA with factors of reading duration and refractive group was performed using the initial NITM values. It revealed a significant effect for reading duration (F[1, 80] = 4.21; P = 0.043) and refractive group (F[2, 80] = 6.63; P = 0.002). Interaction effects were not significant (P > 0.05). Post hoc analysis was then performed using the Fisher LSD test. After the first hour of near work, EOMs exhibited greater NITM than did LOMs, and this difference approached significance (P = 0.06). However, after the second hour of reading, the NITMs of the EOMs were significantly greater than of the EMMs (P = 0.001). No other significant differences were observed (P > 0.05). Paired t-tests were also performed on the same data. This revealed that for the EOMs, NITM after the second hour was significantly greater than after the first hour (t[14] = −2.50; P = 0.02). A similar result was found in the LOMs (t[13] = −2.40; P = 0.03). In contrast, no significant difference was observed between the first and second hours for the EMMs (t[14] = −0.08; P > 0.05). Thus, there was an increase in NITM after the second hour, but only in the myopes. Interestingly, LOMs and EOMs recorded the same mean increase in NITM of 0.07 D between the first- and second-hour measurements (Fig. 1) , although mean NITM values were significantly different for both sessions. The number of subjects manifesting an increase in NITM after the second hour was also higher (approximately 70%) in the myopes. In contrast, only 45% of EMMs exhibited an increase in NITM after the second hour. 

Progressive versus Stable Myopes.

For the PMs, the initial NITM means (±SE) for the first- and second-hour measurements were 0.20 ± 0.03 D and 0.27 ± 0.03 D, respectively. Analogous values for the SMs were lower for the first hour and similar for the second hour (0.09 ± 0.04 D and 0.20 ± 0.05 D, respectively). 

Two-way, repeated-measures ANOVA for reading duration and refractive group was performed. It revealed a significant effect for reading duration (F[1,54] = 4.17; P = 0.045) and refractive group (F[1,54] = 4.32; P = 0.042). Interaction effects were not significant (P > 0.05). The Levene test indicated homogeneity of variance between the PM and SM subgroups despite the unequal sample sizes. Furthermore, although the ANOVA revealed a significant effect, post hoc analysis performed using the unequal N-test showed no significant differences for the relevant comparisons (P > 0.05). 

Further analyses were then conducted for reading duration and refractive group in the PMs and SMs. First, paired t-tests were performed within each subgroup between the first and second hour of reading. For the PMs, there was a significant increase in NITM after the second hour (t(21)=2.48; P = 0.02), whereas this was not true for the SMs (t(6) = −1.86; P = 0.14). Second, unpaired t-tests between subgroups after the first hour were significantly different (t(27)=1.97; P = 0.05) but not after the second hour (t(27)=1.05; P = 0.30). Hence, NITM in the PMs was larger after the second hour compared with the first hour, and NITM after the first hour was significantly different between the PMs and SMs. 

Decay of NITM

Refractive Group Effect.

Two-way, repeated-measures ANOVA for reading duration and refractive group for NITM decay to baseline was performed. It revealed a significant effect for reading duration (F[1, 78] = 51.01; P < 0.001) but only a trend for refractive group (F[2, 78] = 2.60; P = 0.080). Interaction effects were not significant (P > 0.05). Post hoc analysis was then performed using the Fisher-LSD test. It revealed that within each refractive group, decay duration after the second hour was significantly longer than that found after the first hour (P < 0.05). However, for the between-group comparisons, decay duration in the EOMs was longer than in the EMMs after the second hour, which approached significance (P = 0.06). In addition, the group mean NITM decay was assessed as follows: time to reach the zero pretask baseline and its related time constant (TC). In the EMMs, the group mean decay to baseline after the first and second hours of near work—20 seconds after the first hour and 50 seconds after the second hour, with the difference approaching significance (P = 0.06)—is shown in Figure 2A . The decay TC after the first and second hours for the EMMs was 4 and 12 seconds, respectively. Figure 2Ashows the group mean NITM decay to baseline after the first and second hours for the LOMs. It was 20 seconds after the first hour and 60 seconds after the second hour; these values were significantly different (P = 0.002). Decay TC after the first and second hours was 5 and 22 seconds, respectively. Figure 2Ashows the group mean NITM decay to baseline for the EOMs. It was 28 seconds after the first hour and 87 seconds after the second hour, which was significantly different (P < 0.001). Decay TC after the first and second hours in the EOMs was 8 and 34 seconds, respectively. 

Not all subjects exhibited complete decay to baseline. Table 1shows the number and percentage of subjects exhibiting incomplete decay in each of the refractive groups. Depending on the condition, incomplete decay was found in 0% to 60% of the subjects. This frequency was greater in the EOMs after the first and the second hours. Of the total EOMs tested (n = 15), seven did not decay fully after the first hour, and of these, four did not fully decay (for 120 seconds) after the second hour. Interestingly, three of these four subjects exhibited increased NITM after the second hour (for 120 seconds). Similar to the EOMs, of the total LOMs tested (n = 14), three did not decay fully after the first hour, and of these, two did not decay fully after the second hour (for 120 seconds). Only one of them exhibited an increase in initial NITM after the second hour (for 120 seconds). Lastly, more subjects exhibited residual NITM during the 30-second posttask interval after the second hour of reading than during the 120-second period (46%–60% vs 0%–27%; Table 1 ). 

Progressive versus Stable Myopes.

Figure 2Bpresents the NITM decay profiles in the PMs and SMs after the first and second hours of near work. Two-way, repeated-measures ANOVA for reading duration and refractive group was performed. It revealed a significant effect for reading duration (F[1, 54] = 20.96; P < 0.001) but not for refractive group (F[1,54] = 1.61; P = 0.20). Interaction effects were not significant (P > 0.05). An unequal N post hoc analysis was performed. The decay was longer after the second hour for the PMs (P < 0.001) only, but not for the SMs (P = 0.11), compared with the first hour. However, no difference was observed between refractive groups after the first or the second hour (P > 0.05). The group mean decay duration for the PMs after the first and second hours was 21 (TC = 7 seconds) and 69 seconds (TC = 21 seconds), respectively. In contrast, the SMs exhibited a relatively faster decay duration of 10 (TC = 4 seconds) and 36 seconds (TC = 15 seconds) after the first and second hours, respectively. 

Similar to the refractive group comparison, not all PMs and SMs decayed to baseline within 120 seconds after task. Depending on the condition, incomplete decay of NITM was found in 14% to 81% of the subjects (see Table 1 ). 

Discussion

There were several interesting and important findings in the present study. In general, NITM characteristics differed between the myopes and the EMMs. First, NITM was elevated in both myopic groups compared with the EMMs after the first and the second hours of reading. Similar results have been found for shorter (10 minutes) ¹⁶ and longer (4 hours) ¹⁷ durations of near work. Thus, the myopes exhibited increased susceptibility to near work; that is, they were prone to the NITM, accommodatively based aftereffect. Second, NITM increased after the second hour of reading; that is, it was additive, but only in the myopic subgroups. This is a new finding. Third, the decay of NITM was more prolonged in the EOMs than in the EMMs. This result confirms and extends recent findings in which a much shorter reading duration was used (10 minutes). ¹⁶ Thus, EOMs are exposed to more prolonged periods of retinal defocus than that found in the EMMs and LOMs during their relatively extended posttask recovery to baseline. ²⁴ Furthermore, and more generally, given the premise that myopic retinal defocus is also myopiagenic, ^{25 26} the increased initial NITM magnitude, the phenomenon of additivity, and the prolonged decays lend support to the notion of this subgroup as more susceptible to near work-induced myopia. 

Historically, LOMs have been thought to have a primarily environmental factor, whereas EOMs have been thought to have a primarily genetic factor. However, the findings of present study, and those of others, ^{16 17} suggest that NITM represents a more global myopic tendency. That is, both LOMs and EOMs appear to have a prominent near work-based environmental component. 

Only one previous study investigated NITM over a period of relatively prolonged reading to assess the possible additive aspects of NITM across refractive groups. Ciuffreda and Lee ¹⁷ assessed NITM in 16 subjects (four LOMs, four EOMs, four EMMs, and four hyperopes) after a near reading task of 4 hours, with only a brief interruption every hour for 120 seconds, to determine the magnitude and any subsequent decay of NITM. They found NITM to be increased after the first hour in LOMs and EOMs. However, after the second hour, only the LOMs increased, thus demonstrating an additive effect, whereas NITM decreased in the EOMs. Differences between the studies might be attributed to the shorter test interval, less variability of NITM, and larger sample size in the present study. However, for the first hour, the results of the Ciuffreda and Lee ¹⁷ report were similar to the present findings. The results were not compared after the second hour because both the measurement time interval and the experimental design were different in the two studies. 

Only one previous study ²¹ has compared NITM in PMs with those in SMs, and it was for a short reading duration (10 minutes). In that study, PMs exhibited greater susceptibility to NITM than did SMs; this was an important finding. PMs exhibited larger initial magnitude and incomplete decay of NITM than did SMs. Hence, NITM could be an important factor in myopiagenesis, at least for this group. ¹⁵ When subjects in the present study were categorized as they were in the previous study, ²¹ PMs exhibited NITM additivity but SMs did not. Both groups included LOMs and EOMs. Hence, the additive feature of NITM in PMs only was confirmed and extended for a longer duration of near work in the present study. Possible mechanisms for NITM influencing the development of permanent myopia will be discussed later. 

The group mean plots of NITM (Fig. 2)revealed transient myopia decay that went beyond the pretask baseline to produce a hyperopic aftereffect. This has been found in many other studies. ^{5 13 17 21} It has been attributed to a sympathetic rebound effect, as discussed by Woung et al., ²⁷ which acts to attenuate the posttask accommodative adaptive mechanism. This important phenomenon warrants further detailed investigation because it may provide pharmacologic insight into the development of myopia. 

In contrast, some subjects (approximately 30% of the myopes) in the present study exhibited NITM that did not decay to baseline during the 2-minute posttask measurement period. A similar feature has also been reported in two other studies in which NITM was assessed in children and in one recent investigation that included older children and young adults. Ciuffreda and Thunyalukul (IOVS 1999;40:ARVO E-Abstract 2365) compared NITM in myopes (n = 10) and emmetropes (n = 10) between the ages of 4.7 and 10 years. Approximately 25% of these subjects across both refractive groups exhibited increased NITM, and in all cases it was sustained without any decay over the 2-minute posttask measurement interval. Wolffsohn et al. ¹³ assessed NITM after a 5-minute near task in Hong Kong Chinese children (ages 6–12 years) with a known predisposition to myopia. They found the group mean NITM to exhibit incomplete decay over the 3-minute posttask test measurement interval. Finally, Vera-Diaz et al. ²¹ reported that after a 10-minute near task in a group of 13 PMs aged 14 to 27 years, the group mean NITM did not decay completely to baseline over the 120-second posttask test interval. Thus, incomplete decay of NITM appears to be a relatively common finding in subsets of children and in young adults with PMs. 

Based on animal and human models and on human-based computer simulations, one of the primary causes of myopic onset is thought to be increased and chronic retinal defocus. Hung and Ciuffreda ²⁸ simulated their findings based on the Ciuffreda and Wallis ¹⁶ experimental investigation. Consistent with the experimental findings, the model simulation revealed that NITM decay was delayed in myopes (EOMs and LOMs) compared with EMMs. Similar findings were observed in the present study. This slowed decay of NITM has been attributed to an increased output of the accommodative adaptive element of the model, which acts to increase the accommodative controller time constant. ²⁸ Given that NITM may be sustained above the pretask baseline for up to several minutes (Ciuffreda KJ, et al. IOVS 1999;40:ARVO E-Abstract 2365) ²¹ as a result of its additivity, the amount of accommodation necessary to focus on a new near target would be reduced, at least transiently. In other words, the residual NITM would function like the addition of a built-in, low-powered plus lens to reduce the effective accommodative stimulus; hence, the resultant accommodative error would be reduced according to the proportional control property of the system. ¹⁵ Thus, there would be short-term variation in the interaction of the tonic-phase relationship ¹⁵ as NITM progressively decays. The blur-driven accommodation necessary for accurate focus would gradually increase, and, once the NITM is fully dissipated, the normal interactive relationship would be restored. 

NITM is primarily influenced by the autonomic nervous system. It has two sources of innervation, namely the sympathetic and the parasympathetic components, based on their anatomic and functional differences. ²⁹ Two hypotheses attempt to explain this. First, a deficit in sympathetic input alone would result in enhanced accommodative adaptation and, therefore, a relatively prolonged period of decay to baseline. ²² This is the conventional explanation. Second, the slowness of the decay could be attributed to a deficit in sympathetic and parasympathetic innervation. ²² That is, because the level of sympathetic activation is positively correlated with the level of parasympathetic activity, a decrease in the latter would lead to a relative decrease in the former. Hence, a deficit in sympathetic function after parasympathetic activity for a high accommodative task demand would produce a relatively large and transient myopic aftereffect, ²² such as increased NITM. 

In contrast to previous findings, ²² one recent study suggested ²⁹ considerable intersubject variability of sympathetic inhibition. The decay of accommodative aftereffects under open- and closed-loop viewing conditions was assessed in different refractive groups, with and without sympathetic block. For the EMMs, LOMs, and EOMs, a deficit in sympathetic activation was uncovered in approximately 20% to 30% of the subjects. Related to this finding, in the present study approximately 30% of the myopic subjects did not exhibit complete decay to baseline (Table 1) . Furthermore, as mentioned, Ciuffreda et al. (Ciuffreda KJ, et al. IOVS 1999;40:ARVO E-Abstract 2365) reported that 25% of children did not experience decay to baseline after the 2-minute near task. This similar frequency of subjects (approximately 25%–30%) manifesting either lack or prolonged decay of NITM agrees well with the reported magnitude of intersubject variation (approximately 30%) in sympathetic function. This has been proposed as one of the models linking sympathetic inhibition of accommodation and myopia development. ²⁹  

One important controversy remains. Hung and Ciuffreda ^{15 30} developed the incremental retinal-defocus theory, which proposes that NITM may be involved in the etiology of permanent myopia, at least in some people. They speculated that repeated cycles of near far–near work over an extended period (days or months), in which residual NITM persisted after brief distance viewing, would result in a small but significant transient myopic shift at near. This residual NITM would function like a low-powered plus lens placed before the eye. It would reduce the accommodative stimulus and the accommodative error at near and hence would reduce the amount of hyperopic defocus (lag of accommodation). Hung and Ciuffreda ^{15 30} believed that this residual defocus promoted the emmetropisation process and therefore produced axial elongation. Results from the present study (summarized in Table 1 ) extend support to this hypothesis based on the relatively high frequency of subjects manifesting residual NITM after 30- and 120-second post task periods. 

Three mechanisms may be involved in the lenticular-based additivity of NITM. ³¹ One possibility is a biomechanical hysteresis effect that involves the crystalline lens. Ong et al. ³² used various combinations of prisms and lenses to modulate the overall accommodative drive in young adults. They found that the within-task accommodative responses obtained monocularly and binocularly at far (6 m) and near (40 cm) were approximately equivalent, whereas the resultant NITM was not. Furthermore, the NITM magnitude and time course of decay were different, which led them to assume that the task-induced adaptation was related specifically only to the blur drive and not to the overall lens biomechanical changes. Thus, its role was considered to be negligible. ⁵ Another possibility is that the mechanism could be neuromuscular in origin, possibly reflecting an inability to relax the ciliary muscle fibers after sustained near work. Based on an investigation by Suzuki ³³ using excised ciliary muscle from bovine eye, an increase in duration or frequency of electrical stimulation produced increased response magnitude without affecting the time course of decay. Suzuki ³³ proposed that ciliary muscle contraction was pharmacologically produced, not neuromuscularly mediated, through electric potential changes. The third and most likely possibility is a pharmacologically based mechanism that involves the autonomic nervous system. With sustained focus at near, both the parasympathetic and the sympathetic systems are activated, with the initial and immediate excitatory effect of the parasympathetic system followed 10 to 40 seconds later by the inhibitory and sustained effect of the sympathetic system. ²⁹ Subsequent to prolonged near work, the presence of a sympathetic inhibitory dysfunction would result in the increased activation of accommodation through the parasympathetic system, which would result in an increased myopic shift because of its heightened excitatory action. Thus, closely spaced repeated periods of near work may produce successive and transient increases in NITM that would be additive in nature and potentially myopiagenic. ²⁸ Further longitudinal laboratory investigations and clinical trials are needed to confirm these and related ideas. 

In conclusion, compared with the EMMs, myopes manifested greater magnitude of NITM, experienced longer decays to baseline (only EOMs), and exhibited NITM additivity. These results are consistent with impaired sympathetic function and with the speculation that NITM may play a role in the etiology of permanent myopia. 

 Supported by the College of Optometrists in Vision Development and the Minnie Flaura Turner Memorial Fund for Impaired Vision.

 Submitted for publication February 14, 2007; revised May 23, August 9, October 4, and October 31, 2007; accepted December 17, 2007.

 Disclosure: B. Vasudevan, None; K.J. Ciuffreda, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Figure 1.

 

Mean NITM plotted as a function of refractive group. (white bars) Values for the first hour. (black bars) Increases after the second hour.

Figure 2.

 

(A) Decay of NITM after the first and second hours of reading in the EMMs, LOMs, and EOMs. Plotted is the mean ± 1 SEM. (B) Decay of NITM after the first and second hours of reading in the PMs and SMs. Plotted is the mean ± 1 SEM.

Table 1.

 

Number (%) of Subjects Exhibiting Incomplete Decay in Each Refractive Group

Refractive Group	First Hour (30 s)	Second Hour (30 s)	Second Hour (120 s)
EMM	5/15 (33)	9/15 (60)	0/15 (0)
LOM	3/14 (21)	7/14 (50)	2/14 (14)
EOM	7/15 (46)	7/15 (46)	4/15 (27)
PM	9/22 (41)	18/22 (81)	5/22 (23)
SM	1/7 (14)	4/7 (57)	1/7 (14)

The authors thank the College of Optometrists in Vision Development and the Minnie Flaura Turner Memorial Fund for Impaired Vision for their financial support.