Optometry & Vision Science:
Repeatability of Relative Peripheral Refraction in Untreated and Orthokeratology-Treated Eyes
Lee, Tsui-Tsui*; Cho, Pauline†
†MEd, PhD, FAAO
School of Optometry, The Hong Polytechnic University, Hung Hom, Hong Kong, China (TTL, PC).
Received April 16, 2011; accepted June 5, 2012.
Tsui-Tsui Lee School of Optometry The Hong Polytechnic University Hung Hom, Hong Kong SAR China e-mail: firstname.lastname@example.org
Purpose. To determine the repeatability of relative peripheral refraction (RPR) measurements using an open-view automatic refractor in untreated and orthokeratology (ortho-k)-treated eyes of children.
Methods. Cycloplegic central (CR) and peripheral refraction (PR) across the central 60° horizontal field at 10° intervals were measured in children (6–9 years) using a Shin-Nippon NVision K5001 autorefractor. In experiment 1, measurements were repeated at the same visit for 59 untreated (right) eyes, whereas in experiment 2, measurements were repeated at the same visit and another visit (1–2 weeks apart) for 24 ortho-k–treated (right) eyes to determine the intra- and intervisit repeatability, respectively. Measurements were transposed into M, J0, and J45 vectors, and RPR were calculated by subtracting central refraction from PR values. Intra- and intervisit differences and coefficients of repeatability (COR) of RPR were determined.
Results. The mean intravisit differences in RPR for all three vectors were ≤0.05 D for untreated eyes, and both intra- and intervisit differences in RPR were ≤0.32 D for ortho-k–treated eyes. No significant intra- and intervisit differences in RPR were found at any field angle in this study. COR of measurements were <±0.71 D in the untreated eyes, and <±1.25 D (intravisit) and <±3.00 D (intervisit) in ortho-k–treated eyes. The COR increased with field angles in all situations and were greater in ortho-k–treated than in untreated eyes. In ortho-k–treated eyes, intervisit COR of measurements beyond central 40° field were larger than the intravisit COR.
Conclusions. The mean intra- and intervisit differences of RPR measurements were small in both untreated and ortho-k–treated eyes. Variability of PR measurements increased with field angle and with ortho-k treatment.
Relative peripheral hyperopic defocus has been recently suggested as a potential mechanism responsible for myopic progression.1–3 Studies have reported that the field curvatures of myopic and hyperopic eyes are different.4–8 Although individual variations exist, myopic eyes tend to have relative peripheral hyperopia, and hyperopic eyes tend to have relative peripheral myopia. A few studies have also investigated how relative peripheral refraction (RPR) changes before and during myopic development.9–11 In a recent study, Schmid12 found a significant correlation between baseline temporal relative peripheral eye length and central myopic shift in children, which suggests that retinal steepness may play a role in eye growth.
Based on the hypothesis that relative peripheral hyperopia may trigger foveal myopic progression, a number of myopic control studies have examined the possibility to arrest myopic progression in children using lens designs that produce various extents of peripheral defocus. The efficacy of these novel designs applied in contact and spectacle lenses has been reported recently.13–19 Shen et al.20 found that alignment-fit rigid gas-permeable (RGP) lenses can significantly reduce relative peripheral hyperopia in myopic eyes, although the field curvatures remained relatively hyperopic. However, RGP has been shown to be ineffective in arresting myopic progression.21,22 The reduction in relative peripheral hyperopia is more prominent with reverse-geometry design [i.e., orthokeratology (ortho-k) lens], which is more effective in reducing relative peripheral hyperopia of myopic eyes.18,23,24 Cho et al.,13 Walline et al.,15 and Kakita et al.25 have shown that ortho-k lenses can effectively retard myopic progression in kids by 36–56%. Ortho-k results in a relative peripheral myopia, and this may explain why myopic progression can be retarded by ortho-k lens wear but not by RGP lens wear.23 Using soft contact lenses of dual-focus design, which created simultaneous retinal myopic defocus to the eye, Anstice and Phillips17demonstrated that axial elongation in myopic teenagers was slowed by up to 50% when compared with soft contact lenses of conventional design. However, the extent of relative myopic defocus in the peripheral field caused by this design was not addressed in detail. Lopes-Ferreira et al.19 recently reported that higher add power (+3.00 D and +4.00 D) of Proclear Multifocal Dominant design (Coopervision, Pleasanton, CA) multifocal soft contact lens could significantly increase the relative peripheral myopia in a group of emmetropic young adults. They claimed that lenses with addition power of ≥+3.00 D could lead to an increase of around 3 D in peripheral myopia beyond 10° in the nasal field and 20° in the temporal field. Spectacle lenses for correcting myopia have been challenged for inducing relative peripheral hyperopia.26 However, Tabernero et al.14 found it possible to induce relative peripheral myopia in eyes with a novel spectacle lens design featuring a radially decreasing refractive gradient in ∼1 D per 10° change of field angle away from foveal fixation. In a 12-month investigation on another spectacle lens design based on similar principle, Sankaridurg et al.16 found that in children aged 6 to 12 years, those wearing lenses with a rotationally asymmetric lens type showed slower myopic progression than those wearing the symmetrical type or control lenses (i.e., single-vision lenses). However, the reduced relative hyperopic defocus could not adequately explain the reduced rate of axial elongation with this lens type. Despite these investigations, further studies are warranted to confirm whether RPR is indeed a risk factor for the onset and the progression of myopia.
Various methods have been used to define the field curvature of human eyes, including subjective refraction, retinoscopy, autorefraction, photorefraction, wavefront technology, double-pass technique, and so forth.27 Previous studies have validated these methods.28,29 Among these, autorefraction is the most accessible method for objective assessment. Because of unrestricted peripheral vision, the open-view autorefractors were the most commonly used. In the past 2 decades, instruments like the Canon Autoref R-1,9,10,30–34 Shin-Nippon SRW5000 [Ajinomoto Trading, Inc., Tokyo, Japan; also marketed as Grand Seiko WV-500 (Grand Seiko Co., Ltd., Hiroshima, Japan)],5,23,33,35–39 Shin-Nippon NVision K5001 [Ajinomoto Trading, Inc.; also marketed as Grand Seiko WR-5100K (Grand Seiko Co., Ltd.)],6,7,9,10,16,28,40–42 or the latest version Grand Seiko Auto Ref/Keratometer WAM-5500 (Grand Seiko Co., Ltd.)8,19,24,43 have been widely used for the objective assessment of peripheral refraction (PR). Refraction up to 40° in the temporal and the nasal fields and up to 15° to the superior and the inferior fields have been reported under different test conditions (e.g., different fixation distances, with or without the use of cycloplegia, corrective lenses, etc.) in various populations.27
To our knowledge, there has not been any work investigating the repeatability of PR or RPR in children, although there was a report on the repeatability of PR measurements using Shin-Nippon SRW5000 in adults undergoing ortho-k treatment.23 However, the sample size was relatively small, and the intra- and interset differences reported were combined data taken before and after ortho-k treatment in adults. A delineation of ortho-k effect on the PR of the eye may be more appropriate because ortho-k–treated eyes have different corneal shapes compared with non-ortho-k–treated eyes.
The repeatability and reproducibility of RPR measurements can interfere with the conclusions drawn from the observation on PR changes over time. Despite the increasing use of autorefractors for PR assessment, studies on the repeatability of RPR measurements undertaken using autorefraction are scarce. It is important to evaluate repeatability in children because the majority of research on myopic progression and retardation will recruit children as subjects. The current study aimed to determine the repeatability of RPR determined from measurements using Shin-Nippon NVision K5001 autorefractor in untreated and ortho-k–treated eyes of children.
The first experiment was designed to determine the repeatability of RPR in untreated eyes, whereas the second one was performed in ortho-k–treated eyes. The study complied with the tenets of the Declaration of Helsinki of 2002, and ethics clearance was approved by the Departmental Research Committee of the School of Optometry of The Hong Kong Polytechnic University. Written consent was obtained from the parents after a detailed explanation of the examination procedures and complete disclosure of the effects of the topical cycloplegic used. A detailed eye examination was performed in each subject to confirm normal ocular condition before the commencement of the study.
Experiment 1—Repeatability of RPR Measurements in Untreated Eyes
Children (6–9 years old) with normal ocular health and who were participating in an observational study on refractive development were invited to participate in this study. Subjects with previous contact lens experience and those who were undergoing or had taken part in interventional myopic control treatments were excluded. Cycloplegia was achieved by instilling 0.5% Alcaine, 1.0% tropicamide, and 1.0% cyclopentolate (1 drop of each into each eye) 5 min apart. After instillation of the 3 drops, the subject was allowed to rest for 45 min before performing subjective refraction. The amplitude of accommodation was assessed using an RAF rule to ensure that accommodation was paralyzed. Central refraction (CR) and PR were then performed using the open-view autorefractor (Shin-Nippon NVision K5001). RPR was determined by subtracting CR from each PR measurement after transposition (refer to “Treatment of data” later in the text). To investigate the repeatability of RPR, two sets of PR measurements (S1 and S2) were taken at the same visit by the same examiner. After the completion of the first set of PR data collection, the subject was asked to retreat from the instrument, which was reset before the second set of data was taken. On average, the two sets of data were taken about 10 min apart.
Experiment 2—Repeatability of RPR Measurements in Ortho-k–Treated Eyes
Children between the age of 6 and 9 years, who had no previous contact lens experience before enrolling into ortho-k treatment, were invited to participate. Measurements were performed only after ortho-k treatment had stabilized. Stabilization was confirmed when lens centration, topographical responses, and residual refraction were found to be consistent at two consecutive visits. The geometrical center of the treatment zone was required to be within a 0.5 mm distance from the pupil center, and the intervisit difference in subjective refraction was required to be not >0.50 D in either the spherical or cylindrical components. The cycloplegic refraction procedures were similar to those used in experiment 1. PR examinations were performed at two different visits separated by 1 to 2 weeks. Measurements were made at about the same time of day at each visit to minimize possible influence of diurnal variation in ocular biometrics44,45 and regression in ortho-k responses.46 In the first visit, two sets of PR measurements (S1 and S2) were obtained and used to determine the intravisit repeatability. To determine intervisit repeatability, a third set of PR data (S3) was obtained at a second visit, and RPR determined in S3 were compared with those in S1.
CR and PR across the central 60° horizontal field were measured using Shin-Nippon NVision K5001. A self-fabricated external fixation system was attached to the instrument as shown in Fig. 1. The fixation system consisted of two components: a swinging arm and an optical system. The swinging arm had its center of rotation 15 mm behind the head rest of the autorefractor to align with the center of eye rotation. The optical component was attached to the other end of the swinging arm. It comprised a 45°-inclined plane mirror and a condensing lens, which projected the image of a 3-mm flashing red LED (3 V) at 1 meter from the center of rotation of the swinging arm. The unit was swung to the corresponding angle to direct the fixation of the test eye for each measurement angle measured using a protractor.
The subject was seated behind the autorefractor, with the chin and forehead resting firmly against the chin and forehead rests, respectively. The subject's head was held firmly by the examiner during the measurement to avoid any head movement. The eye level was aligned to the canthus indicator of the instrument. Only the right eye of each subject was refracted, while the left eye was occluded. The instrument axis was carefully aligned to the center of pupil, and the peripheral measurements were made with eye turns. When the subject fixated temporally, refraction measurement was measured from the nasal field and vice versa.
Autorefraction was performed for CR when the subject looked straight ahead, with the LED at 0°. PR measurements were made sequentially, with the subjects turning their eyes and fixating the LED in the nasal fields at 10° (N10), 20° (N20), and 30° (N30) and then in the temporal fields at 10° (T10), 20° (T20), and 30° (T30). For each field angle, measurements were taken manually after the instrument axis (visualized as the reticule on the screen) was aligned with the pupil center, while the subject fixated at the flashing LED. Davies et al.47 reported that the average intrasession CR repeatability were 0.11 D and 0.13 D for spherical and cylindrical components, respectively, using six repeated readings. Based on their findings, and the allowance of ±0.25 D from the mean value, and with consideration of potentially larger variations in off-axis measurements, we set the selection criteria for the Shin-Nippon measurements during examination as listed in Table 1.
For each field angle, 5 to 10 measurements were made to include at least five readings that satisfy the aforementioned criteria. A complete set of data for each eye included CR and five PR located 10° apart, excluding T20 owing to its proximity to the optic nerve head. Two methods were used in the determination of repeatability of RPR. One was determined from the average of the first five consecutive PR measurements (Method 1), and the other used the average of the first five PR measurements that satisfied the preset criteria (Table 1; Method 2).
Limiting the difference between repeated measurements to a clinically acceptable level of 0.25 D and considering the standard deviation (SD) of 0.32 D for the mean differences in foveal refraction using the same instrument as reported by Davies et al.,47 a minimum of 20 subjects were required to achieve a power of 90% at the 5% level of statistical significance.
Treatment of Data
Each spherocylindrical refraction in negative cylinder expression [Sphere (S)/Cylinder (C) × Axis (θ)] was transposed into M, J0, and J45 according to the following formula48,49:
The vector components from each of the five readings were averaged to obtain the corresponding mean M, J0, and J45 at each fixation angle. RPR was obtained by subtracting CR from the PR. Positive RPR indicates a hyperopic defocus in the peripheral field, whereas negative RPR indicates a myopic defocus in the periphery. Because the distributions of RPR data were not significantly different from a normal distribution [Kolmogorov-Smirnov tests, M: p ≥ 0.07; J0: p ≥ 0.07; J45: p ≥ 0.06 in untreated eyes; M: p ≥ 0.07; J0: p ≥ 0.07; J45: p ≥ 0.12 in ortho-k–treated eyes (exceptions: in untreated eyes, second measurement of M along T10 and first measurement of J45 along T10; in ortho-k–treated eyes, second measurement of M along T30, second measurement of J0 along N10, and third measurement of J0 along N20)], parametric tests were used for statistical analysis. Paired t-tests (in untreated eyes) and repeated-measures analysis of variance (ANOVA; in ortho-k–treated eyes) were used to test for the presence of intra- and intervisit differences in RPR at each field angle measured. If there were no significant differences between or among the data sets, Pearson correlation tests were used to investigate the relationship between the differences and their means. If there was no significant correlation, coefficients of repeatability (±1.96 × SD of differences in RPR; COR) were determined. Comparison on measurement variability between CR and RPR was performed by analyzing the equality of variances (F tests) for M factor (i.e., spherical equivalent) in each group. Comparisons of measurement variability between the untreated and ortho-k–treated eyes (intravisit measurement only) and between the intra- and intervisit in ortho-k–treated eyes were performed in the same way for CR as well as RPR for each of the peripheral field angles. All statistical tests were performed using SPSS 18.0 for Windows (SPSS Inc., Chicago, IL) unless otherwise specified.
Fifty-nine children (8.0 ± 0.8 years old) were enrolled in experiment 1. All data were included in the statistical analysis. The mean (SD) spherical equivalent refractive error of the selected eyes was −1.86 (1.95) D. Eleven of these (right) eyes were hyperopic (spherical power ≥0.75 D), eight were emmetropic (−0.75 D < spherical power < 0.75 D), and 40 were myopic (spherical power ≤−0.75 D).
Twenty-eight subjects who had been wearing ortho-k lenses for ≥1 month were enrolled in experiment 2. Topographical responses in four of these subjects showed significant decentration [center of treatment zone being >0.5 mm from pupil center as shown on the topographical difference map using the Medmont Studio, version 126.96.36.199 (Medmont Int. Pty Ltd., Vermont, Australia)] in either visit; therefore, their data were not included in the analysis. The remaining 24 subjects (8.4 ± 0.8 years old) had worn ortho-k lenses for 4 to 14 weeks before the first measurement was taken. The mean (SD) spherical equivalent of the residual refractive errors of the right eyes of the remaining 24 eligible ortho-k–treated subjects was −0.53 (0.30) D.
Differences between the Two Methods Used in the Determination of RPR Repeatability
Paired t-tests showed no significant differences between the two methods in the determination of repeatability of RPR [p > 0.02 (critical value = 0.05/7, Bonferroni correction for 7 comparisons)]. Because there were no significant differences between the two methods, only the results from Method 2 are presented and discussed in the rest of this article. In Method 2, the five measurements fulfilling the preset criteria could usually be obtained from six to eight consecutive measurements made for both untreated and ortho-k–treated eyes.
Repeatability of CR in Untreated and Ortho-k–Treated Eyes
Objective CR measurements and the intravisit differences in untreated eyes are summarized in Table 2. There were no significant intravisit differences in central M, J0, or J45 (paired t-tests, p ≥ 0.19). Intravisit differences of the various power vectors were not significantly different in the untreated eyes among different refractive groups (two-way repeated-measures ANOVA, p = 0.93 and p = 0.83, respectively).
The objective CR measurements and the intra- and intervisit differences in ortho-k–treated eyes are summarized in Table 3. There were no significant differences in central M, J0, and J45 among the three sets of measurements (repeated-measures ANOVA, p ≥ 0.23). The intravisit differences in CR between untreated and ortho-k–treated eyes were not statistically significant (unpaired t-tests, p ≥ 0.34). The variability of mean measurement differences, as expressed by COR, is summarized in Table 4. The COR were <0.51 D in M and <0.37 D in J0 and J45 in both treated and untreated eyes. The COR for CR were not significantly different between the untreated and the ortho-k–treated eyes in M and J0 (M: F = 1.10, p = 0.75; J0: F = 2.14, p = 0.08). The COR for J45 were significantly different between untreated and ortho-k–treated eyes (F = 2.64, p < 0.01). No significant differences in the COR for M, J0, and J45 were found between intra- and intervisit measurements in ortho-k–treated eyes (1.17 ≤ F ≤ 2.14, 0.08 ≤ p ≤ 0.74). Only statistical results of M are presented in Table 5.
Repeatability of RPR Measurements in Untreated Eyes
RPR (M, J0, and J45) results in untreated eyes are also shown in Table 2. There were no significant intravisit differences in M, J0, and J45 at any of the field angles (paired t-tests, p ≥ 0.10). There were no significant differences found in the intravisit differences among different power vectors and among different refractive groups (two-way repeated measures ANOVA, p ≥ 0.49 and p ≥ 0.42, respectively) at all peripheral field angles. The mean difference between any pair of CR and RPR at any angle was <0.05 D. The intravisit differences in CR and RPR did not differ significantly across the horizontal 60° field (repeated-measures ANOVA, p ≥ 0.51).
No significant correlation was found between the differences and their means for each parameter (Pearson correlation, M: −0.16 ≤ r ≤ −0.01, J0: 0.03 ≤ r ≤ 0.31, J45: −0.21 ≤ r ≤ 0.18). The COR ranged from ±0.53 D (N10) to ±0.71 D (N30) for M, from ±0.37 D (N20) to ±0.49 D (T10 and T30) for J0, and from ±0.20 D (N10) to ±0.37 D (T30) for J45 (Table 4). The COR of RPR measurements were compared with those of CR measurements, and the statistical results are shown in Table 6. Except for N10, the COR of RPR measurements were significantly different from those of CR measurements in untreated eyes (1.778 ≤ F ≤ 2.939, p < 0.03).
Repeatability of RPR Measurements in Ortho-k–Treated Eyes
Table 3 summarizes the RPR results of the ortho-k–treated eyes (M, J0, and J45).
No intra- or intervisit RPR differences at any of the field angles were found (repeated-measures ANOVA, p ≥ 0.13). The mean intravisit difference was <0.15 D at any angle for M, J0, and J45 (Table 3). The intervisit differences were <0.17 D at any angle, except for M at T30. No significant differences in both intra- and intervisit differences were found between CR and RPR or across the horizontal 60° fields (repeated measures ANOVA, p ≥ 0.42).
No significant correlations (Pearson correlations, M: −0.49 ≤ r ≤ 0.32; J0: −0.38 ≤ r ≤ 0.31; J45: −0.42 ≤ r ≤ 0.55) were found between the differences and their means. The intra- and intervisit COR for ortho-k–treated eyes are summarized in Table 4. The intravisit COR of RPR (except for T10) were significantly different from those of CR (2.98 ≤ F ≤ 846, p < 0.01), whereas the intervisit COR of RPR were significantly different from those of CR beyond central 20° field (8.10 ≤ F ≤ 34.63, p < 0.01; Table 6). At the same field angle, the intravisit COR were generally smaller than the intervisit COR in ortho-k–treated eyes for any of the vectors (Table 4). Within the central 40° field, the scatter of intervisit measurements were roughly 20% wider than those of the intravisit measurements. Such differences were, however, shown to be insignificant (1.18 ≤ F ≤ 1.34, 0.42 ≤ p ≤ 0.69). Beyond the central 40°, the intervisit measurement variabilities were significantly greater than the intravisit ones (3.06 ≤ F ≤ 5.72, p ≤ 0.01) by a factor of 2.5 times at most (Tables 4 and 5).
The mean measurement differences in ortho-k–treated eyes were not significantly different when compared with the results from untreated eyes (unpaired t-tests, p ≥ 0.09). However, beyond the central 10°, significant differences in measurement variability were found between the two groups across the horizontal field (1.98 ≤ F ≤ 4.55, p ≤ 0.04; Table 5). The ortho-k–treated eyes show a twofold greater scatter of measurement bias when compared with the untreated eyes, toward the peripheral field (Table 4).
In the current study, we determined repeatability of RPR from five consecutive PR measurements (Method 1) as well as the first five PR measurements that satisfied the preset criteria limiting the maximum difference in sphere and cylinder (Method 2). During our pilot study on PR measurements, we found that the variations in autorefraction, particularly for the cylindrical component and in ortho-k–treated eyes, could be large. Measurements at more peripheral angles were more prone to such variations. This might be the result of larger values of cylinder being seen in ortho-k–treated eyes, and being seen more consistently between successive readings of each measurement. The current study showed that both methods gave similar RPR results and levels of repeatability in both untreated and ortho-k–treated eyes. Although Method 1 is the common method reported in published reports on PR, we prefer to restrict the internal variability of the five measurements by using Method 2.
Previous studies have investigated the validity and repeatability of non-cycloplegic CR made by the Shin-Nippon NVision K5001 in adults only.47,50 The mean (SD) intersession differences in M, J0, and J45 reported by Davies et al.47 were 0.03 (0.32) D, −0.02 (0.27) D, and −0.00 (0.11) D, respectively, for CR. Cleary et al.50 also reported good levels of repeatability, although measurements were not taken under cycloplegia. Both studies included adults aged from 18 to 60 years. The majority of their subject groups lied in the range of 20 to 40 years old. Cleary et al.50 reported a more scattered test-retest variability, although the measurements were made using a Badal optometer for better control of accommodation. The more scattered results may be due to monocular fixation during measurements. Davies et al.,47 in contrast, allowed their subjects to fixate binocularly. Monocular fixation may induce relatively more accommodative response compared with binocular fixation,51 and monocular viewing may lead to larger accommodation errors than in binocular viewing.52
Our results on CR in untreated eyes of children are in agreement with those reported by Davies et al.47 [unpaired t-tests, p ≥ 0.05 for all parameters, Instat v3.36 (GraphPad Software, Inc., La Jolla, CA,)] and Cleary et al.50 (unpaired t-tests, p ≥ 0.56 for all parameters, Instat v3.36) in adults. Our study also shows smaller scattering of the mean differences in children. Therefore, the repeatability of CR measurements is not worse in children, although their attention and fixation are believed to be more variable.
To our knowledge, there is no report on the repeatability of RPR determined from Shin-Nippon NVision K5001. Charman et al.23 reported the repeatability of PR in four ortho-k–treated eyes using Shin-Nippon NVision SRW5000 and commented that the interset differences in PR measurements increased with peripheral field angle. They performed two sets of PR measurements at the pretreatment and 7-day and 14-day post-treatment visits and combined the pre- and post-treatment data for the interset difference investigation. Because Charman et al. combined pre- and postortho-k data and NVision SRW5000 measures refractive errors over a wider area (∼3 mm) than the NVision K5001 does (∼2.3 mm), direct comparison of our results with theirs were not performed. In the current study, the mean intravisit differences in RPR were <0.05 D and <0.32 D in untreated and ortho-k–treated eyes, respectively. The mean measurement differences found for RPR were not significantly different from those found for CR in both treated (intravisit) and untreated eyes (intra- and intervisits).
Although the mean intra- and intervisit differences were insignificant among different field angles, COR of RPR beyond the central 20° field were significantly greater than those of CR in both treated and untreated eyes. Moreover, COR increased with field angles and were generally larger in the nasal than in the temporal fields (Table 4). This was observed for both untreated and treated eyes and is in agreement with the findings reported by Charman et al.23 COR were significantly larger in ortho-k–treated eyes than in untreated eyes for most field angles (except central and T10). The increase in COR toward the more peripheral angles was more prominent in ortho-k–treated eyes than in untreated eyes (Table 4). Ortho-k results in a flattening of the central cornea and a steepening of mid-peripheral cornea. The deviation of the anterior corneal profile from a normal prolate elliptical shape is a likely source of errors when the instrument is used to measure the modified corneal surface. Larger variations (COR) were observed for nasal than temporal fields in ortho-k–treated eyes. There might be asymmetrical corneal eccentricities (or shape factor) between the half fields, which results in differences in repeatability. Angle alpha has been suggested to be a potential source of asymmetry between the nasal and the temporal field in PR.53 Effectively, if we take angle alpha into account, we were actually assessing a wider nasal peripheral field, and this may be a reason for poorer repeatability of data in the nasal field angles because variability increased with field angle. However, the current study did not make further investigation on these factors.
It is unknown how the altered corneal profile, after corneal reshaping, affects PR measurements, particularly at more extreme angles (e.g., 30° to the periphery) where the retinal image reflects through a corneal area with abruptly changing profiles over the treatment zone. However, an analysis of such effect on PR measurement is beyond the scope of this study.
In ortho-k–treated eyes, intervisit COR were significantly greater than intravisit COR, when measurements were made beyond the central 40° field. As mentioned before, diurnal variation in ocular biometrics was minimized by taking measurements around the same time of the day for the intervisit repeatability (not more than 1 hour difference). All subjects were instructed to wear ortho-k lenses during sleep for a minimum of 8 hours every night. They were also required to mark the time of lens insertion and removal every day on a logbook. The larger intervisits variations in ortho-k–treated eyes are probably due to the regression of corneal shape and small day to day variation of topographical responses, although intervisit measurements were made at about the same time of day. Ortho-k lens wear induces transient corneal shape changes, which may affect the refractive state of the eye and thus are more prone to day to day changes such as under- or overcorrection, lens decentration, binding, and so forth. Although usually small, these changes may affect the repeatability of PR measurements. In the current study, measurements in ortho-k–treated eyes were only taken after confirmation of stabilization of ortho-k treatment.
Fedtke et al.41 have recently investigated the influence of lateral pupil misalignment on PR measurement. Their findings suggested that pupil alignment is very critical for the accuracy of PR measurements. They also observed a smaller tolerance on misalignment in the nasal field than in the temporal field. This may explain the larger COR in the nasal field than in the temporal field as observed in our study. At 30° nasal field, Fedtke et al.41 reported that a 0.20 mm misalignment of the instrument axis from the pupil center would give a clinically significant error of 0.25 D and 0.125 D for M and J0, respectively. Ehsaei et al.40 reported tolerance of translational misalignment from the center of pupil of ∼1.0 mm and ∼0.5 mm for the 10° and 20° field angles, respectively, which gave a ±0.50 D variability in M factors. Hence, pupil misalignment during PR measurement may also contribute to the variations observed particularly at more extreme angles in ortho-k–treated eyes. The current study, like most studies measuring PR using similar instruments,5–11,16,19,23,24,28,33,35–39,42,43 did not incorporate any measuring scale to assist pupil alignment.
Cycloplegia is not routinely used in PR measurements. Some studies measured PR without paralyzing the accommodation.5,7,14,24,35,41,54–56 It is generally accepted that accommodation of <2.00 D only had limited effect on peripheral astigmatism within the central 60° horizontal field.57,58 However, Lundström et al.54 reported that the change in RPR due to accommodation was not similar between emmetropic and myopic eyes. Without cycloplegia, the pupil size has to be controlled with ambient room lighting because measurements made by Shin-Nippon open-view autorefractors are valid for pupil size of 2.3 mm at minimum. If cycloplegia is not used, potential fluctuation in pupil size and crystalline lens thickness may result in more variability in the PR measurements in children. Further studies on PR measurements could include investigation of the influence of pupil size and accommodation.
The mean intravisit difference in RPR was <0.05 D in untreated eyes, whereas the mean intra- and intervisit differences were <0.32 D in ortho-k–treated eyes. Although mean measurement bias did not show significant differences in all three situations, a larger variability was observed for measurements made beyond the central 20° field in both untreated and ortho-k–treated eyes. Larger variations (larger COR) were found in ortho-k–treated eyes than in untreated eyes at most angles (except T10), and larger variations in intervisit measurements than in intravisit measurements beyond central 40° field were found for ortho-k–treated eyes. In addition, larger COR were found at more peripheral angles and in the nasal field than in the temporal field.
School of Optometry
The Hong Polytechnic University
Hung Hom, Hong Kong SAR
The study was supported by The Hong Kong Polytechnic University (PolyU) PhD studentship (RGVM) to TTL and a Collaborative Research Agreement between PolyU and Menicon Co, Japan. This work was conducted in facilities supported by a Niche Area Funding (J-BB7P) from PolyU.
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peripheral refraction; myopia; orthokeratology; repeatability; autorefraction
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Ophthalmic and Physiological OpticsRelative peripheral refraction in children: twelve-month changes in eyes with different ametropiasOphthalmic and Physiological Optics
© 2012 American Academy of Optometry
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