Ординатура / Офтальмология / Английские материалы / Advances in Understanding Mechanisms and Treatment of Infantile Forms of Nystagmus_Leigh, Devereaux_2008

Risley prism device. The introduction of computerized motion processing, running in Simulink, created the potential to explore more sophisticated tracking algorithms that would have been difficult to implement in analog electronics. Nevertheless, the secondgeneration system had some important shortcomings that would need to be resolved before the system could be useful to patients or clinicians. The control electronics consisted of three desktop computers (one for video oculography and two others to act, respectively, as the host and target for the Simulink-based motion processor) and assorted breadboarded electronics, and experiments usually required the offices of two engineers. The amplitude range of the stabilization plant was also limited to approximately ±1° of image deflection, too small to fully compensate nystagmus in 20% to 50% of APN patients.16 Finally, the field of view of the device was quite limited, and it did not permit spectacle-wearing subjects to be tested with their optical correction.

THIRD-GENERATION DEVICE: A WORK IN PROGRESS

The main goals for the latest-generation system include developing a stabilization plant capable of nullifying nystagmus up to ±5° amplitude; replacing the desktop computers with dedicated, self-starting microcontrollers integrated in a single, portable package; creating simplified controls suitable for use by a clinician or

patient; and creating head-mounted optics that can be donned and doffed without the need for repeated optical alignments.

Creating a stabilization plant that retained the fidelity of our modified Canon image-stabilizing device while quintupling its amplitude range has proven to be particularly challenging. A key feature of the Canon design was a pair of “floating” voice coil electromagnets, each of which could drive the lens in one axis while allowing its actuator to be carried (under the influence of the second electromagnet) in the orthogonal direction. Since the coils used in our second-gener- ation device were too weak to support a larger lens or lens excursion, and since no floating voice coils were commercially available, it was necessary to design and fabricate custom units. The oscillation fidelity of the design, however, proved disappointing; owing to the large distances between the centers of the two voice coils and the other bearing points, tiny shifts in the angle of the lens plane caused large excursions of components at the device periphery, causing unpredictable variations in friction and ultimately distorting the lens motion. Additionally, the variations in lens plane caused fluctuations in the optical feedback signals, due to changes in the distance between the optical emitter/sensor mounted on the frame and the optical target mounted on the lens carrier.

A major factor in our decision to use floating voice coils had been their compactness. As it became clear that a device capable of 5° oscillation would be large enough to necessitate some form of auxiliary support (in

addition to the patient’s head), the goal of compactness became a lesser concern, behind oscillation amplitude, fidelity, and field of view. We designed a radically different device (Fig. 8.3A) that dispensed with the floating voice coils in favor of commercially available, single-axis voice coils. The fact that each motor would move along a single axis also allowed us to replace the temperamental optical position sensors with precise linear variable displacement transducers (LVDTs). In the new design, the lens is held between the actuators of the voice coils and LVDTs. Each voice coil and its yoked LVDT are staggered to resist torsion motions of the lens. The number of bearing points was minimized, and friction at those points was minimized by using jewels and precision linear bearings. Figure 8.3B shows a plot of root mean square (RMS) error of the image deflection versus oscillation frequency for the prototype device during circular oscillation at 75% of full excursion. Distortion was held to below 10% of full deflection range for oscillation frequencies up to 10 Hz, comparable to our second-generation device over most of the frequency range.

APPLICATION OF SELECTIVE IMAGE STABILIZATION IN IN

In patients with IN, retinal image stabilization produces oscillopsia.3,4 Previous studies were conducted with “direct-drive” type motion processing. The question of how a patient with IN would respond to selective nullification of their nystagmus was germane to the theme of this volume. Although the interference cancellation filter was originally selected to accommodate the sinusoidal waveform of APN, it seemed likely that the filter could also lock to a quasi-sinusoidal IN waveform. We investigated these questions in a single patient with IN of the pseudo-pendular with foveating saccades (PPfs) waveform.

Eye movements were recorded with an EyeLink II (SR Research, Osgoode, Ontario, Canada) videooculography system. Motion processing was accomplished using the interference cancellation filter running on desktop computers. As our newest stabilization plant had not been completed at the time of these experiments, we accomplished stabilization by oscillating the projected optotype patterns via mirror galvanometers. The overall delay between an actual eye movement and motion of the optotypes on the screen measured 6 milliseconds. We used the same eightposition Landolt C optotype patterns and method of defining acuity described previously.15 At the beginning of the experiment, we adjusted the nullification gain by a forced-choice procedure, seeking the highest gain level at which the optotypes were perceived

as clear without our engendering oscillopsia. Following the gain determination, we tested acuity with the stabilization on and then off. All procedures were performed with monocular viewing and with the head free. During recording, the subject’s head position was periodically adjusted to maintain a roughly constant average eye-in-head position, sufficiently removed from the patient’s “null” position to evoke a clear pendular nystagmus. To determine foveation durations, we first removed any gradual variations of average eye position (mostly related to the gradual variations in average head position) by subtracting a polynomial that was fit to the foveation positions of no fewer than 10 cycles of nystagmus. Eye positions were then converted to retinal image position by subtracting the image deflection imparted by the mirror galvanometers. (Prior to this subtraction, the galvanometer deflection signal was shifted 6 milliseconds in the lagging direction to account for the total system delay.) Finally, foveation times were calculated as the percentage of time that image position was within ±0.5° and velocity was within ±4 deg/s.18

The patient selected a nullification gain of 0.3. At this gain, clarity of the optotypes was subjectively improved, and any higher level caused oscillopsia and reduced clarity. He also preferred a relatively high value of the adaptation constant ( = 0.1, 500 filter adaptations/s), producing a relatively nonselective gain-versus-frequency function. At a lower value of the adaptation constant ( = 0.025), nullification gain could not be raised above approximately 0.2. The filter was able to maintain lock to the subject’s nystagmus, as demonstrated in Figure 8.4A. Note how the galvanometer motion was nonsinusoidal, due to the low degree of the filter’s frequency selectivity. Measured acuity was unchanged by the stabilization, as demonstrated by the essentially identical extinction curves in Figure 8.4B.

The lack of any effect on acuity may be explained by a consideration of the effects on foveation duration in plots of eye velocity versus position (not shown). Although image stabilization shrunk the envelope of the “phase plane” trajectories, the time spent in the foveation region was essentially unchanged (1.8% for the original waveform versus 2.6% after subtracting the effect of visual stabilization, as determined from a representative data record spanning several seconds). There are multiple potential explanations for the patient’s failure to tolerate higher nullification gains, which might otherwise have resulted in a larger percentage of time in the foveation region. Previous studies demonstrating oscillopsia during image stabilization in IN patients speculated that the usual perception of stability in these patients reflects a match between actual image velocity and an internal eye

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movement signal (“efference copy”), and that oscillopsia may emerge when the artificial reduction of image velocity disrupts the match.3,4 Notably, our patient’s limiting nullification gain of 0.3 is essentially identical to the average limit reported in one of these referenced studies3 for their “global motion” condition, the condition most closely approximating the visual characteristics of our oscillating optotype test patterns.

Close inspection of the eye position and residual position in Figure 8.4A suggests an additional explanation for visual degradation above a nullification gain of 0.3. The residual image motion was actually greater than the original eye motion at the beginning of the foveation periods (arrows), probably because the filter is destabilized by the foveating saccades. Higher nullification would amplify the residual error and actually degrade stability early in the foveation period. Similarly, lower adaptation constants, while rendering the filter more frequency-selective, would make it less able to follow any departures of the nystagmus from sinusoidal behavior and more disruptive of the foveation period, which may explain why the patient preferred fast adaptation constants.

This pilot experiment agrees with a previous report that entirely nonselective electronic visual stabilization fails to improve acuity in patients with IN.4 Our results do not exclude the possibility that other IN patients might behave differently, or that a filtering algorithm that does not destabilize the patient’s own foveation period might be more effective. The experiment does

demonstrate that the adaptive interference cancellation algorithm we used to selectively nullify APN is also able to lock to an IN waveform. Thus, even if it is ineffective for the purposes of improving acuity, the algorithm could find uses in the laboratory setting as a method of synchronizing visual stimuli to specific points in the cycle of a patient’s IN waveform.

ACKNOWLEDGMENTS This study was supported by a Merit Review award from the Department of Veterans Affairs. We thank Louis F. Dell’Osso and Jonathan B. Jacobs for providing expertise and raw material, without which the IN experiment would not have been possible.

References

1.Leigh RJ, Rushton DN, Thurston SE, Hertle RW, Yaniglos SS. Effects of retinal image stabilization in acquired nystagmus due to neurologic disease. Neurology. 1988;38(1):122–127.

2.Rushton D, Cox N. A new optical treatment for oscillopsia. J Neurol Neurosurg Psychiatry. 1987;50:411–415.

3.Abadi RV, Whittle JP, Worfolk R. Oscillopsia and tolerance to retinal image movement in con-

genital nystagmus. Invest Ophthalmol Vis Sci. 1999;40:339–345.

4. Leigh RJ, Dell’Osso LF, Yaniglos SS, Thurston SE. Oscillopsia, retinal image stabilization and

congenital nystagmus. Invest Ophthalmol Vis Sci. 1988;29:279–282.

5.Averbuch-Heller L, Tusa RJ, Fuhry L, et al. A double-blind controlled study of gabapentin and baclofen as treatment for acquired nystagmus. Ann Neurol. 1997;41:818–825.

6.Leigh RJ, Averbuch-Heller L, Tomsak RL, Remler BF, Yaniglos SS, Dell’Osso LF. Treatment of abnormal eye movements that impair vision: strategies based on current concepts of physiology and pharmacology. Ann Neurol. 1994;36:129–141.

7.Stahl J, Averbuch-Heller L, Leigh R. Acquired nystagmus. Arch Ophthalmol. 2000;118:544–549.

8.Wang ZI, Dell’Osso LF, Tomsak RL, Jacobs JB. Combining recessions (nystagmus and strabismus) with tenotomy improved visual function and decreased oscillopsia and diplopia in acquired downbeat nystagmus and in horizontal infantile nystagmus syndrome. JAAPOS. 2007;11: 135–141.

9.Dell’Osso LF, Tomsak RL, Rucker JC, Leigh RJ, Bienfang DC, Jacobs JB. Dual-mode (surgical + drug) treatment of acquired pendular nystagmus and oscillopsia in MS. Invest Ophthalmol Vis Sci. 2005;46:E-abstract 2403.

10.Gresty MA, Ell JJ, Findley LJ. Acquired pendular nystagmus: its characteristics, localising value and pathophysiology. J Neurol Neurosurg Psychiatry. 1982;45:431–439.

11.Abadi RV, Bjerre A. Motor and sensory characteristics of infantile nystagmus. Br J Ophthalmol. 2002;86:1152–1160.

12.Stahl JS, Lehmkuhle M, Wu K, Burke B, Saghafi D, Pesh-Imam S. Prospects for treating acquired pendular nystagmus with servo-controlled optics.

Invest Ophthalmol Vis Sci. 2000;41:1084–1090.

13.Yaniglos SS, Leigh RJ. Refinement of an optical device that stabilizes vision in patients with nystagmus. Optom Vis Sci. 1992;69:447–450.

14.Leigh RJ, Rushton DN, Thurston SE, Hertle RW. Optical treatment of oscillopsia due to acquired nystagmus. Neurology. 1986;36(suppl):252.

15.Smith RM, Oommen BS, Stahl JS. Application of adaptive filters to visual testing and treatment in acquired pendular nystagmus. J Rehabil Res Dev. 2004;41:313–324.

16.Smith RM, Oommen BS, Stahl JS. Image-shifting optics for a nystagmus treatment device. J Rehabil Res Dev. 2004;41:325–336.

17.Widrow B, Stearns SD. Adaptive Signal Processing. Upper Saddle River, NJ: Prentice-Hall; 1985:474.

18.Dell’Osso LF, Jacobs JB. An expanded nystagmus acuity function: intraand intersubject prediction of best-corrected visual acuity. Doc Ophthalmol. 2002;104:249–276.

ABSTRACT

The term visual display terminal (VDT) syndrome has been used to describe the complex of eye and vision problems, along with systemic symptoms, reported by individuals with prolonged exposures to electromagnetic displays such as computer monitors, television screens, or even cellular phone graphic displays. Currently, millions of children use VDTs in school and at home for education and recreation. Children may experience many of the same symptoms related to VDT use as adults. In healthy eyes, the pupils naturally change size as focus shifts from far to near objects during VDT use. We measured convergence, accommodation, and pupillary size using a TriIRIS C9000 (Hamamatsu Photonics, Hamamatsu, Japan) in a group of 129 children ranging in age from 12 to 15 years, all of whom used some form of VDT. Pupillary responses were abnormal in 21.7% of subjects. Abnormal pupillary responses associated with prolonged usage of VDTs in children may be due to autonomic disorder induced by sustained near work.

Although recent advancements in information technology (IT) have contributed substantially to the quality of leisure time, a range of mental and physical problems have been attributed to sustained use of a range of visual displays, to which the terms IT syndrome1 and visual display terminal (VDT) syndrome2,3 have been applied. This disorder comprises a complex of eye and vision problems and systemic symptoms in individuals who

experience sustained and prolonged proximity to visual displays with electromagnetic fields (such as computer screens, televisions, and cellular phone screens). Use of computers in the workplace has increased greatly, and has been accompanied by the development of a number of health concerns. Thus, many individuals who work with VDT, television, television games, personal computers (PCs), mobile video games, and mobile phones have complained of ocular discomfort and eye muscle strain and stress. Several studies reported that the majority of VDT workers experience some eye or visual symptoms,1-3 including eyestrain; headaches; blurred vision; dry, red, or irritated eyes; and double vision.

Although there are several reports concerning the effects of prolonged VDT use by adults on eyes and vision, there are few such reports about children. Currently, millions of children use VDT in school4,5 and at home for education and recreation. Thus it might be expected that children would have many of the same symptoms related to VDT use as adults do. However, it seems that most children never make such complaints and, possibly because of their great adaptability, children may ignore problems that would be reported by adults. Children also may have different needs in order to comfortably use a VDT, and we postulate that, with a small amount of effort, it should be possible to reinforce appropriate viewing habits and ensure comfortable and enjoyable VDT use. In healthy eyes, the pupils naturally change size as focus shifts from far to near objects, such as during VDT use. In this study, we investigated the pupil responses during VDT use in children.

Figure 9.1 TriIRIS C9000. When the subject looks at the inside target moving from far (50 cm) to near, visual target display 1 shows pupil size and pupil movement, and display 2 shows waveforms of target, vergence, and pupillary change. Target position 3 is indicated by a sideboard ruler.

METHODS

This study was carried out on two groups of subjects: normal adults, who served as controls, and children. In the study of children, 378 students at Sho Kurashiki Municipal Junior High School agreed to complete a questionnaire about their use of VDT items in daily life. Eye examinations and near-response tests were performed on 213 students.

Testing was conducted by certified orthoptists and an ophthalmologist. We measured accommodation, convergence, and miosis responses to near stimuli using the TriIRIS C9000 (Hamamatsu Photonics, Japan) (Fig. 9.1). Reliable measurements were dependent on the ability of subjects to concentrate on the visual target. Subjects were required to place their face on chin and forehead rests on the apparatus and were encouraged to look binocularly at three colored targets inside the device at a distance of 50 cm. We adjusted the detection level of the pupil, which was signaled in a white square, and then reset to the center position of display. The tracking function of the white square to the pupil was turned on. Before pushing the start button, we needed to determine the subjective near point, which differs between subjects. Accordingly, we asked the subject to indicate this by pushing a button when the inner accommodative target, which moved progressively closer, first became blurred. We selected a speed of 0.3 diopters per second (D/s).

As the target moved forward and backward, pupil size and target position were continuously monitored horizontally by a pair of infrared charge-coupled

device (CCD) cameras. The images of the pupils were recorded for about 200 seconds, depending on the subject; the field of view was illuminated by a white-light- emitting diode (LED). We asked subjects to hold focus as the target approached; this was done three times. Movements of the center of the pupil of both eyes were measured and converted into vergence eye movement (convergence and divergence), and pupil diameter simultaneously measured at 30 Hz. Stimuli moved 7.5 to 50 cm (corresponding to accommodative stimulation of 13 to 2 D) in children, and 9 to 50 cm (11 to 2 D) in adults. Measurement using TriIRIS C9000 was performed under complete refractive correction. When the display was viewed binocularly, the pupil responses were measured simultaneously in each eye.

Measurements and Data Analysis

Normal Adults

We studied 36 adults (age 21.6 ± 1.0 years, 8 male), all of whom gave informed consent. All had normal near visual acuity, accommodation, and orthophoria. Although some subjects had a refractive error, none complained of asthenopia. Figure 9.2 shows normal and abnormal waveforms of convergence–divergence and miosis–mydriasis (also see color insert).

Three items were analyzed (Fig. 9.3; also see color insert): (1) a pupil-constriction ratio (PCR), expressed as a percentage according to the formula PCR = ([initial pupil size − maximum constricted pupil size] / initial pupil size) × 100; (2) amount of convergence (AOC) in millimeters, AOC = center of the pupil at farthest target − center of the pupil at the nearest target; and (3) a pupil-asthenia ratio (PAR), expressed as a percentage, PAR = ([initial pupil size − final pupil size] / initial pupil size) × 100. The dominant eye, decided by a hole- in-card test, was selected for analysis. In each trial, PCR and PAR were subjected to analysis of variance.

In addition, we determined the change in PCR, AOC, and PAR with change of the amount of near accommodative stimulation in 5 normal adults (age 22.2 ± 1.2 years). The far target was positioned at 50 cm (2 D), and the near target was set at four different positions: 14 cm (7 D), 11 cm (9 D), 9 cm (11 D), and 8 cm (12.5 D). We also measured responses during the viewing of near stimuli at 5 D, 7 D, 9 D, and 10.5 D in the same subjects.

Children

Questionnaire The participants for this study were 378 students (both girls and boys) from three grades in middle school (grade 1, 132; grade 2, 116; grade 3, 130 students) with ages ranging from 12 to 15 years.

All students and their parents agreed to fill out a questionnaire that requested age, gender, and five VDT items (PC, television, television games, mobile games, and phones), which provided indices of the nature and duration of VDT usage. We analyzed answers of all subjects concerning the five VDT items and divided them into three levels of frequency: low, middle, and

Figure 9.3 Methods of analysis. Pupil constriction ratio (PCR) of the three responses (1) are measured according to the formula PCR = [(Initial pupil size − Maximum constricted pupil size) / Initial pupil size]

×100. Amount of convergence (AOC) of the three

responses (2) are measured according to the formula AOC = Center of the pupil at farthest target – Center

of the pupil at the nearest target. Pupil asthenia ratio (PAR) (3) is measured according to the formula PAR = [(Initial pupil size − final pupil size) / Initial pupil size]

×100 (Also see color insert.)

high (Table 9.1). We also measured near visual acuity at 33 cm; performed autorefractometry; examined ductions, versions, and vergence eye movements; had subjects perform the alternate prism cover test, near point of convergence test, and stereo acuity (Titmus stereo) test; and conducted general ocular examinations.

RESULTS

Adults

PCR for the first trial was 47.4 ± 10.6%. For the second trial it was 49.0 ± 8.2%, and for the third trial, 51.0

±8.1%. There was no significant difference between the trials. AOC for the first trial was 2.4 ± 0.7 mm; it was 2.4 ± 0.4 mm in the second and 2.4 ± 0.4 mm in the third. There was no significant difference between

the trials. The final pupil size of the third trial was reduced by 9.0 ± 6.4% from the initial pupil size of the first trial.

The change in each item, with change in the amount of near stimulation, PCR for near stimuli of 5 D, 7 D, 9 D, and 10.5 D, is shown in Figure 9.4. There was a

gradually increasing value with increase of near stimulation: 28.2 ± 1.0% at 5 D, 43.2 ± 2.6% at 7 D, 51.5

±0.7% at 9 D, and, at the highest near stimulation of

10.5D, 50.9 ± 1.1%. AOC at near stimulation of 5 D, 7 D, 9 D, and 10.5 D is summarized in Figure 9.5. This

shows gradually increasing values with increase of near stimulation: 1.3 ± 0.1 mm at 5 D, 2.0 ± 0.1 mm at 7 D,

2.9± 0.2 mm at 9 D, and 2.8 ± 0.2 mm at 10.5 D. PAR at near stimulation of 5 D, 7 D, 9 D, and 10.5 D is summarized in Figure 9.6. At 5 D the pupil size returned

almost to the initial size, and PAR was 2.4 ± 6.1%. PAR increased gradually with increasing near stimulation: 7.5% ± 6.8% at 7 D, 10.9% ± 6.2% at 9 D, and 15.7% ± 5.5% at 10.5 D.

Children

Usage of VDT Items and Frequency

The total usage of VDT items by students in all three grades is shown in Figure 9.7. Among 378 students, the PC was listed by the highest percentage of students in all three grades. There was no difference between male and female students concerning usage of any particular type of VDT. All students used some type of VDT item, and at least one item each day. Usage of VDT (Fig. 9.8) was categorized into three levels: low (1 to 5 instances), middle (6 to 12), and high (13 to 23). Of the 378 subjects, 100 students (26.5%) were at the low level, 229 (60.5%) at the middle level, and 49 students (13%) at the high level of frequency.

Pupil-Asthenia Ratio (PAR)

Out of 213 students in whom eye examinations were performed, 84 students (42 with strabismus, 36 with anisocoria, and 6 who could not be recorded) were excluded from the PAR analysis. Based on the remaining group of 129 students, PAR was 12.4% ± 13.2% (mean ± standard deviation). We defined normal pupillary responses when PAR measurements were within the range of 0.8% to 25.6%, and abnormal responses when PAR measurement lay outside that range. Among 129 students, 101 (78.3%) had normal pupillary responses and 28 students (21.7%) had abnormal responses (Fig. 9.9).

We attempted to relate pupillary near response and frequency of use of any kind of VDT item in the 129 students studied. The percentage of students with normal and abnormal pupillary near responses was similar for each of the levels of usage. For students with normal pupillary response, there were 78.8% in the lowlevel category of usage, 78.2% at the middle level, and 82.4% at the high level. For students with abnormal

Figure 9.4 Pupil constriction ratio (PCR) of four different near stimulations. PCR is a gradually increasing value with increase of near stimulation: at 5 D, 7 D, 9 D, and the highest near stimulation of 10.5 D.

Figure 9.5 Amount of convergence (AOC) of four different near stimulations. AOC shows gradually increasing values with increase of near stimulation: at 5 D, 7 D, 9 D, and the highest near stimulation of 10.5 D.

Figure 9.6 Pupil asthenia ratio (PAR) of four different near stimulations. PAR increases gradually with increasing near stimulation.

Figure 9.8 Frequency of usage of any kind of visual display terminal (VDT) items. Proportion of low frequency is 26.5%, middle frequency 60.5%, and high frequency 13%. Low level = 1 to 5 points; middle level = 6 to 12 points; high level = 13 to 23 points (see Table 9.1).

pupillary responses: low level, 21.2%; middle level, 21.8%; and high level, 17.6%.

DISCUSSION

We have demonstrated that children who frequently use VDT items show pupil abnormalities as part of the near response. Prior studies have only reported the VDT syndrome in adults.4,5 In the study presently under discussion, we selected junior high school

Figure 9.7 Usage of visual display terminal (VDT) items in total students. There was no difference between males and females in all three grades concerning usage of any particular type of VDT. G1, G2, G3, grade 1, 2, 3 of junior high school students; MG, mobile game; PC, personal computer; phone, cellular telephone; TVG, terminal velocity game.

students. There were three main findings: (1) there was no difference in VDT usage between males and females from all three grades represented in the sample;

(2) the pupillary component of the near response in

129 students, measured by the TriIRIS C9000 system, was abnormal in 21.7%; and (3) there was no relationship between the frequency of use of VDT items and abnormalities of the pupillary near response.

Together, these findings suggest potentially important effects of VDT devices on near responses in children. The high prevalence (over one in five)

Figure 9.9 Proportion of pupil asthenia ratio (PAR). Among 129 students, 101 students (78.3%) had normal pupillary responses and 28 students (21.7%) had abnormal responses.

of pupillary responses to near stimuli in junior high school children should be considered seriously, since it could be interpreted as an abnormal behavior of the autonomic nervous system. When a target that is illuminated constantly approaches the eyes, parasympathetic innervation constricts the pupil (miosis). When the target recedes from the eyes, sympathetic innervation dilates the pupil (mydriasis). Continuous use of VDT items could pose excessive demands on components of the near response. In our study, we used the TriIRIS C9000 device, which was convenient for children and allowed simultaneous measurement of the near response components: convergence, divergence, and accommodation. Prior studies have used different instruments. Within a small range of ages, our data appear to be reliable, and we believe that our study is unique and points to the need for further research into the relationship between ocular responses and VDT items.

CONCLUSION

Prolonged and sustained use of VDT in children may lead to abnormalities of the pupillary near response. Abnormal pupillary responses to near stimulation may reflect changes in the autonomic nervous system and deserve further study.

ACKNOWLEDGMENTS We gratefully acknowledge our subjects and their great interest in this study, and we wish to thank the junior high school health teacher, Ms. Hayashi, and Hamamatsu Photonics Company for its help with the equipment, and also orthoptists and students of Kawasaki University of Medical Welfare were gratefully appreaciated. This study was supported by Mishima Sai-chi Foundation for the International Research Student of Ophthalmology (Mahmoodi Khadija).

References

1.Hiraoka M. [IT eye strain]. Gannka. 2005:47(1); 63–70.

2.Sheedy JE. Vision problems at video display terminals: a survey of optometrists. Am Optom Assoc. 1992;63(10):687–92.

3.Bergqvist UO, Knave BG. Eye discomfort and work with visual display terminals. Scand J Work Environ Health. 1994;20(1):27–33.

4.Yamamoto M, Nakabashi K. [Influence of video game in children from view point of visual function]. In: Ishikawa S, ed. [VDT Medicine Manual]. Tokyo: Kanehara; 1989:99–100.

5.Sano H. [Influence of video games in children from viewpoint of school health care]. In Ishikawa S, ed. [VDT Medicine Manual]. Tokyo: Kanehara; 1989:101–102.