It has recently been shown that corneal curvature can be steepened with overnight wear of orthokeratology (OK) lenses to provide correction for hyperopia.1–5 Significant changes in corneal topographic and refractive effects are found at lens removal after the first night of lens wear, with regression of effect during the day without lenses but greater effect at lens removal and longer retention of effect during the day after 7 nights of lens wear.1 In this regard, the time course of hyperopic OK has been described as being analogous to myopic OK. However, the maximum range of reliable refractive correction was found to be reduced by a factor of 3 relative to the accepted upper correction limit of −4.50 D from myopic OK.6
Issues identified as limiting refractive outcomes with hyperopic OK are a lack of change in central corneal thickness after lens wear, with anterior corneal steepening created by paracentral epithelial thinning alone.3 In myopic OK, changes in epithelial thickness are instead reported at both central and paracentral locations.7–10 A further limiting factor is that the central zone of corneal steepening from hyperopic OK decreases in diameter with longer periods of lens wear.2 This is opposite to changes seen in the zone of central corneal flattening from myopic OK, which instead increases in diameter with longer periods of lens wear.11–13 Distance best-corrected visual acuity (VA) was also found to reduce with the decreasing central steepened zone diameter in hyperopic OK, leading to a reduced rating of satisfaction with distance vision, although the rating of satisfaction with near vision was found to be less affected.2
The purpose of this study was to assess whether the same OK lens design used for hyperopic OK could be used to provide near vision correction for presbyopia. Fitting the lenses to one eye only, in presbyopes who did not require habitual optical correction at distance, allowed a monovision type effect to be targeted while simultaneously providing a non–lens-wearing control eye. In this manner, the time course of refractive and corneal topographic effects over 1 week of lens wear could be investigated while assessing the success of OK in providing functional visual correction for presbyopia.
Sixteen subjects were enrolled. Three subjects discontinued wear after the first night because of intolerance to lens discomfort. Thirteen subjects completed 7 nights of lens wear (mean [SD] age, 51.5 [4.0] years; range, 43.2–58.7 years; 11 female, 2 male). Only data from the thirteen subjects who completed the study were analyzed. All subjects had best unaided distance VA of at least 6/9 (20/30) and did not habitually wear optical correction at distance, required optical correction at near, and had no evidence of ocular disease or previous ocular surgery. Approval from the institutional human research ethics committee was obtained before the study began. All subjects gave informed written consent and were screened before enrolment to ensure that they met study eligibility criteria. All subjects were treated in accordance with the tenets of the Declaration of Helsinki.
Study lens design was based on the BE hyperopic OK biaspheric design (BE Enterprises, Brisbane, Australia) manufactured by Capricornia Contact Lens Pty Ltd. (Slacks Creek, Australia). The individual lens specifications are based on corneal topography to provide the same tear film profiles in all subjects. The lenses were designed to induce +2.00 D of hyperopic correction and had an overall diameter of 10.5 mm. Details of the lens design and fitting protocols can be found in an earlier publication.1 Lenses were worn in the nondominant eye to provide a near monovision effect, with the dominant eye for distance acting as a non–lens-wearing control. Ocular dominance was determined by asking subjects to sight a distance target with both eyes open and then determining which eye was most closely aligned to the target. This process was repeated three times, and the eye most consistently found to be aligned to the target was designated as the dominant eye for distance viewing.
All subjects attended an initial visit to assess eligibility and to capture corneal topography data for lens ordering. Once lenses had been received, subjects returned for baseline measurements followed by lens fitting assessment. Lens wear was then scheduled with a total of 7 continuous nights of lens wear, with lenses removed during the day. Subjects returned for study measurements after the first (Day1) and seventh (Day7) night of lens wear, in the morning, within 1 hr of waking (A.M.), and in the afternoon, 8 hr after lens removal (P.M.). Lens wear was ceased at Day7, and all subjects returned 1 week later (Day14) when study measurements were repeated to assess for regression of effect. At all visits, corneal topography was measured first followed by refraction. Slit-lamp observations were also recorded and will be reported elsewhere.
The E300 videokeratoscope (Medmont Pty Ltd., Melbourne, Australia) was used to capture corneal topography, with data analyzed using Medmont Studio 4, v. 18.104.22.168 (Medmont Pty Ltd.). Three images of each eye were obtained at each visit, and data were averaged. Axial curvature along the horizontal chord at 0.5-mm intervals over a chord of 8 mm was plotted from the averaged values.
Best vision sphere (BVS) refraction was recorded at all visits. Because it proved difficult to obtain a consistent endpoint with spherocylindrical refraction for some subjects, particularly in respect to cylindrical power and axis, the results for BVS refraction were used for analysis in this article. Additional spherical plus power required at near was measured over the distance BVS refraction as the minimum plus power required to enable each subject to reach their best corrected near VA. Uncorrected and BVS-corrected distance VAs were measured using computer-displayed logMAR test charts at a distance of 6 m. Uncorrected and BVS plus near add–corrected near VA were measured in Jaeger format using hand-held near test charts at 30 cm.
Three-way repeated measures analysis of variance was conducted on the refraction and corneal curvature data, with planned comparisons to compare effect over time relative to baseline. Post hoc Student t tests with Bonferroni correction were used to compare effects between selected visits (IBM SPSS 20; IBM Corp., Armonk, NY). The p values were adjusted by the SPSS software according to the Bonferroni correction, such that a reported P value of less than 0.05 denotes statistical significance.
The corneal curvature analysis was carried out at central (0.5-mm temporal to corneal apex), and paracentral nasal (2.5-mm nasal to apex) and temporal (3.5-mm temporal) corneal locations. These locations were chosen to compensate for the 0.5-mm temporal lens decentration relative to the pupil center, which was evident on lens fitting. A similar degree of temporal decentration of lens fitting and corneal topographic effect was noted in previous studies using this lens design.1 Axial curvature over the horizontal 8-mm chord was also plotted at 0.5-mm intervals for each target group to allow comparison between A.M. and P.M. results on Day1 and Day7 by inspection.
At baseline, there was no difference between test and control eyes for uncorrected distance and near VA, refraction, near add, best corrected distance and near VA, and keratometry (Table 1).
During lens wear, there was a statistically significant difference in BVS refraction between lens-wearing and non–lens-wearing eyes (F1,12 = 77.48, p < 0.001), and over time (F2.67,32.15 = 41.06, p < 0.001). In lens-wearing eyes, planned within-subject contrasts showed that there was a significant change from baseline at A.M. and P.M. visits on Days 1 and 7 (all, p < 0.001; Fig. 1). On lens removal, BVS refraction had changed by −1.00 (0.33) D after the first night of lens wear and by −1.11 (0.61) D after the seventh night of lens wear (mean [SD]). Retention of effect 8 hr after lens removal increased from Day1 to Day7, with BVS change in refraction increasing from −0.50 (0.32) D at Day1 P.M. to −0.91 (0.41) D at Day7 P.M. (mean [SD]; p < 0.05). There was no significant difference in BVS refraction 1 week after lens wear ceased compared with baseline (p > 0.99), indicating full regression of refractive effect once lens wear ceased.
During lens wear, there was a statistically significant difference in monocular unaided distance VA between lens-wearing and non–lens-wearing eyes (F1,12 = 6.04, p < 0.05), and over time (F2.81,33.74 = 6.74, p = 0.001). In lens-wearing eyes, planned within-subject contrasts showed that there was a significant change from baseline on Day1 at the A.M. visit only and on Day7 at A.M. and P.M. visits (all, p < 0.05; Fig. 2). On lens removal, monocular unaided distance VA in lens-wearing eyes reduced to logMAR 0.25 (0.16) and 0.21 (0.17) after 1 and 7 nights of lens wear, respectively (mean [SD]). There was regression of effect during the day to logMAR 0.09 (0.10) (mean [SD]) on Day1 (p < 0.05), but no loss of effect during the day on Day7.
Binocular unaided distance VA did not change from baseline at any post–lens-wearing visit (all, p > 0.05) and was significantly better at all measurement visits when compared with monocular unaided distance VA (all, p < 0.05). There was no significant difference in unaided distance VA monocularly in either eye or binocularly 1 week after lens wear ceased when compared with baseline (p > 0.99).
There was a statistically significant difference in monocular unaided near VA between lens-wearing and non–lens-wearing eyes (F1,12 = 127.61, p < 0.001), and over time (F2.52,30.27 = 24.36, p < 0.001). In lens-wearing eyes, planned within-subject contrasts showed that there was a significant change from baseline at A.M. and P.M. visits on Days 1 and 7 (all, p < 0.01; Fig. 3). On lens removal, monocular unaided near VA improved to Jaeger 4.6 (2.5) and 3.2 (2.3) after 1 and 7 nights of lens wear, respectively (mean [SD]). There was regression of effect during the day to Jaeger 6.5 (3.3) on Day1, but a greater retention of effect during the day on Day7 to Jaeger 3.9 (3.0) measured 8 hr after lens removal (mean [SD]; both, p < 0.05). Although binocular unaided near VA was significantly better at baseline when compared with monocular unaided near VA in the lens-wearing eye (p = 0.001), there were no significant differences between monocular and binocular near VAs at any measurement interval during overnight OK lens wear. There was no significant difference from baseline in unaided near VA monocularly in either eye or binocularly 1 week after lens wear ceased (p > 0.99).
There was a greater change in corneal axial curvature after lens removal in the lens-wearing eyes compared with control eyes at both central (F1,12 = 22.55, P < 0.001) and paracentral nasal (F1,12 = 5.61, p < 0.05) locations. Corneal curvature also changed over time in the lens-wearing eyes at central (F2.94,35.29 = 11.74, p < 0.001) and paracentral nasal (F3.14,37.67 = 42.69, p < 0.001) locations. There was no difference in corneal curvature at paracentral temporal locations between eyes (F1,12 = 0.25, p = 0.63) or over time (F1.81,21.75 = 1.28, p = 0.30). Fig. 4 demonstrates a typical corneal topography axial power difference map at Day7 A.M.
Planned within-subject contrasts revealed that overnight OK lens wear induced steepening of the central cornea and flattening of paracentral nasal corneal curvature at A.M. and P.M. visits on Days 1 and 7 (all, p < 0.05, Fig. 5). Central corneal curvature steepened by 1.10 (0.62) D at lens removal on Day1, with regression of effect during the day to steepening of 0.62 (0.34) D 8 hr after lens removal (mean [SD]; p < 0.01). At lens removal on Day7, the central cornea steepened by 1.29 (0.68) D, which was not significantly different from Day1 A.M. (p = 0.58), but there was greater retention of effect during the day to steepening of 1.01 (0.55) D at Day7 P.M. (mean [SD]; p < 0.05). Paracentral nasal corneal curvature flattened by −0.60 (0.31) D at lens removal on Day1, with regression of effect during the day to flattening of −0.20 (0.22) D 8 hr after lens removal (mean [SD]; p < 0.001). Increasing nights of lens wear led to a greater paracentral flattening effect at lens removal on Day7 to −0.72 (0.30) D (mean [SD]; p < 0.05). There was regression of effect during the day to flattening of −0.50 (0.28) D 8 hr after lens removal (mean [SD]; p < 0.001), but a greater retention of effect relative to Day1 P.M. (p < 0.001). There was no significant difference in central or paracentral corneal curvature 1 week after lens wear ceased when compared with baseline (p > 0.99).
Orthokeratology (OK) has become a well-established vision correction modality for low to moderate degrees of myopia. Yet, despite first being advocated as a method for correcting hyperopia,14 it is only in recent years, through advances in lens material and manufacturing technology, that success in reliable and repeatable OK correction of hyperopia has been achieved. In this study, we have shown that optical correction of presbyopia can also be achieved with hyperopic OK lenses, bringing the advantages of waking hour visual correction without the need for glasses or contact lenses to a rapidly growing sector of the optical market.
In presbyopic subjects, hyperopic OK lenses induced a myopic shift in refraction of −1.00 (0.33) D after the first night of lens wear, which is consistent with our previous report of −1.03 (0.25) D of refractive change after the first night of lens wear using a similar lens and fitting protocol in a younger population (mean [SD]).1 Although the same lens design was used in the two studies, +2.00 D of refractive change was targeted in the current study, compared with +1.50 D in the earlier study. Targeted refraction change is derived using the Jessen factor, whereby the back optic zone radius (BOZR) of the lens is fitted steeper than the corneal curvature in diopters by the amount of correction required.14 In the current study, the BOZR was fitted 2.00 D steeper than flat K, which should have led to a greater refractive effect than reported for the earlier study that used a lens fitted +1.50 D steeper than flat K. Furthermore, reported outcomes from lenses fitted steeper still, at +2.50 D3 and +3.50 D1 steeper than flat K, are again similar to the current study. The similarity between refractive outcomes, despite differing steepness ratio of BOZR relative to corneal curvature, suggests that the Jessen factor is not an accurate predictor of refractive change with hyperopic OK lens designs despite its successful use in myopic OK.
Similar to previous reports of hyperopic1 and myopic15–18 OK, refractive effect reduced during the day without lens wear, but there was a greater retention of effect at the end of the day with increased nights of lens wear. This indicates that the time course of hyperopic OK is not altered when fitted to an older presbyopic population, at least up to the 1 week of overnight lens wear that we monitored. In a study investigating the influence of age on outcomes from 1-hr wear of myopic OK, a subject cohort aged 36 years and older (mean [SD], 43.9 [6.1] years) was found to have reduced refractive, corneal topographic, and central epithelial thinning response relative to younger subjects.19 This led the authors to suggest a reduced corneal epithelial response to OK with increasing age. The current study shows that this does not appear to be the case over longer periods of lens wear, at least for hyperopic OK lenses over up to 1 week of wear.
Binocular uncorrected distance VA did not change from baseline at any post–lens-wearing study measurement, or between visits, indicating that distance vision remained functional despite induction of a near correction effect in the nondominant eye. These outcomes are similar to monovision using daily wear contact lenses where binocular distance VA has been shown to reduce only slightly when compared with spectacle correction and to not change over periods of 3 weeks20 and 8 weeks21 of monovision lens wear. When measured monocularly, hyperopic OK-induced changes to BVS refraction led to a reduction in uncorrected distance VA in lens-wearing eyes, and followed the same trends as BVS refraction, with regression of effect during the day but greater retention of effect at the end of the day with increased nights of lens wear. An interesting observation is that, at all post–lens-wearing visits, the reduction in uncorrected distance VA in lens-wearing eyes was less than expected from changes to BVS refraction. It is generally accepted that 0.25 D of refractive change results in a one-line (logMAR 0.1) change in VA. This ratio was found at lens removal after 1 night of lens wear, with a mean (SD) BVS refraction of −0.75 (0.46) D, resulting in an unaided distance VA of logMAR 0.25 (0.16). However, after 7 nights of lens wear, despite mean (SD) BVS refraction increasing to −1.04 (0.59) D, unaided distance VA remained similar to Day1 A.M. levels at logMAR 0.21 (0.17), half of the reduction expected from the residual refractive error.
Improvements in monocular VA over time despite increasing ametropia raises the possibility that the visual system may adapt to hyperopic OK-induced distance blur during the period of lens wear. Multifocal refractive intraocular lenses lead to a decrease in contrast sensitivity that has been shown to return toward presurgical levels over time,22 with a neuroadaptation response suggested as a possible explanation.23 However, recovery of contrast sensitivity took up to 1 year, making the same adaptation process an unlikely explanation for the changes reported here for just 1 week of hyperopic OK lens wear. Orthokeratology does offer similar advantages to intraocular lenses in that the refractive effect is relatively stable over time and is certainly more consistent when compared with traditional daily contact lens–wearing modalities. Studies for longer periods of lens wear are required to investigate whether similar neuroadaptation responses occur with hyperopic OK when used for the correction of presbyopia.
An alternative and more likely explanation for the increasing discrepancy between induced ametropia and monocular VA over time is that hyperopic OK creates an effect similar to a center near multifocal contact lens design in which the center of the lens is dedicated to near focus, blending toward distance-focussed zones in the periphery. Although there was no significant difference in central corneal steepening between Day1 A.M. and Day7 A.M., visual inspection of Fig. 5 reveals the appearance of a sharper transition between the area of central corneal steepening and paracentral flattening. The central corneal steepening responsible for the measured myopic shift in refraction is still present at Day7 A.M., but the sharper transition allows a quicker return through areas of less corneal steepening into the flattened paracentral zone, with these zones better suited to providing a clearer focus for distant objects, hence explaining the reduced effect of central steepening on unaided distance VA.
Changes to unaided near VA also followed the same time course as BVS refraction with a maximum effect of Jaeger 3.2 (2.3) at lens removal after 7 nights of wear and retention of effect throughout the day to Jaeger 3.9 (3.0) 8 hr after lens removal (mean [SD]). Similar levels were retained binocularly, indicating that improvement in near VA was retained despite rivalry from the uncorrected fellow eye. Jaeger 4 is equivalent to 6.5-pt text, and with normal newspaper print in the region of 8 pt (Jaeger 5), this means that OK-induced monovision provides functional near vision correction for most normal reading tasks.
Orthokeratology is described as a temporary treatment with changes to refractive error returning to baseline pre–lens-wearing values in myopic24,25 and hyperopic5 OK. In the current study, measurements were repeated 1 week after cessation of lens wear to reveal that all measured parameters returned to baseline values. This gives confidence that the hyperopic OK effect is reversible in presbyopic eyes, though further studies are needed to verify that the same regression to baseline occurs after longer periods of lens wear. Some consideration does need to be given to the relatively small sample size; however, the statistical significance that was achieved gives confidence that similar outcomes can be expected across a larger population.
We have shown that hyperopic OK lenses offer a viable option for providing monovision correction of emmetropic presbyopia. Although refractive change did not reach the level predicted by the degree of BOZR steepening relative to corneal curvature, the change was sufficient to provide functional near vision correction while retaining good distance VA. Similar to the time course of changes reported for myopic OK lenses, the refractive and corneal topographic effects reduced during the day but retained a greater effect during the day with increased nights of lens wear. The effect on binocular VA was consistent with previous reports for traditional contact lens monovision correction. All measured parameters returned to baseline values 1 week after cessation of lens wear, indicating that hyperopic OK-induced changes in presbyopic eyes are reversible, at least after the first week of overnight lens wear. Studies are currently underway to evaluate refractive and corneal topographic changes over longer periods of overnight lens wear and to investigate ways to reliably increase refractive effect.
School of Optometry and Vision Science
University of New South Wales
Sydney, NSW 2052
e-mail: [email protected]
This research was supported by an Australian Research Council Linkage Project grant, with industry partners Bausch & Lomb Boston, Capricornia Contact Lens Pty Ltd., and BE Enterprises Pty Ltd.
The authors thank Dr Pauline Kang for her assistance with some of the data collection.
Received September 26, 2012; accepted December 19, 2012
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