Modern corneal refractive therapy (CRT) or orthokeratology (OK) aims to reshape the anterior corneal surface by the overnight application of reverse geometry contact lenses (CL). In the case of myopia correction, the central cornea is flattened to achieve the desired reduction in the power of the anterior corneal surface, while the midperipheral cornea steepens [
In clinical practice, OK subjects used to complain of dysphotopic phenomena in the form of haloes, ghosting, or glare. These phenomena are described as more intense at the beginning of the treatment period and decrease over time [
The purpose of this study was to evaluate the time-course variations in the size of light distortion phenomena measured with an experimental device and correlate them with the aberrations of the anterior corneal surface and contrast sensitivity function (CSF) over the first month of treatment.
A total of 29 neophyte subjects were recruited and fitted with OK lenses for myopia correction with reverse geometry rigid gas permeable CL. Inclusion criteria required that they be over 18 years of age, have less than 1.00 diopters (D) of refractive astigmatism, be free of ocular disease, have no contraindication for overnight CL wear, and present a best spectacle corrected monocular visual acuity of 0.90 decimal (20/25) or better. Subjects were followed up and they wore their lenses successfully for 1 month. Demographic and refractive data of subjects are presented in Table
Demographic, refractive, and keratometric data of subjects (mean ± SD) and range (minimum and maximum).
Age | 22.34 ± 8.08 years (18–43) |
Sample (male/female ratio) | 29 subjects (6/23) |
M (baseline) | −2.10 ± 0.93 D (−1.00 to −4.75) |
J0 (baseline) | −0.03 ± 0.14 D (−0.50 to 0.46) |
J45 (baseline) | 0.00 ± 0.08 D (−0.29 to 0.36) |
Flattest keratometric radius | 7.78 ± 0.28 mm (7.20 to 8.64) |
Steepest keratometric radius | 7.55 ± 0.29 mm (6.92 to 8.52) |
Decimal VA (monocular) | 1.16 ± 0.09 (0.90 to 1.50) |
Subjects were informed of the purpose of the study and signed a consent form after all their questions were answered following the tenets of the Declaration of Helsinki. Subjects underwent a comprehensive optometric examination prior to enrollment.
Visual acuity was measured under photopic conditions at 5 m using a decimal scale chart. Subjective baseline refraction and refraction at the time of data collection were determined as the spherocylindrical combination that rendered the best distance visual acuity with the highest positive power.
Light distortion was analysed with an experimental prototype consisting of a central light emitting diode (LED) surrounded by 240 small LED sources distributed in 24 semimeridians with an angular separation of 15°. For the purpose of the present experiment, an angular separation of 30° was considered. Figure
Distribution of main central source of light and peripheral stimuli.
The Best Fit Circle Radius (BFCr) is defined as the circle that best fits the distortion area resulting from the linear binding of all points in each meridian of the device. This parameter is expressed in mm and is linearly related to the Distortion Index (LDI) parameter.
LDI is calculated as the ratio of the area or points missed by the subject and the total area explored and is expressed as a percentage (%). The higher values of distortion (BFCr and LDI) are interpreted as a lower ability to discriminate small stimuli surrounding the central light source.
The irregularity of the distortion area is derived as the deviation of the actual polygonal shape obtained from the BFC fit and is called the BFC Irregularity (BFCirr). The standard deviation of BFCirr, called BFCsd, measures how asymmetric the departure of the actual limits of the distortion from the perfect circular shape of the BFC is. Together, BFCirr and BFCsd can be interpreted as the deviation of the actual distortion from a perfectly rotational symmetric shape. The higher the value of this parameter, the larger the deviation from a circular shape and it is expressed in mm. The device has been applied in several studies to different ocular conditions, showing consistency in its measurements. In a recent study, the system has been able to differentiate the light disturbance between monofocal, bifocal, and trifocal pseudophakic patients (Brito et al., J Cataract Refract Surg,
Corneal aberrations were derived from topography data obtained with the Oculus Easygraph (Oculus, Dudenhofen, Germany). Corneal higher order aberrations in the form of spherical-like, coma-like, and secondary astigmatism were calculated from the Zernike coefficients provided by the topographer for 3.0, 4.5, and 6.0 mm pupil sizes.
Measurements of CSF (CSF) were carried out using a 22′′ LCD screen (Topcon CC-100XP 75-1, Hasunuma-cho, Itabashi-ku, Tokyo 174-8580, Japan). The frequencies tested were 1.50, 2.12, 3.00, 4.24, 6.00, 8.49, 12.00, 16.97, and 24.00 cpd. Test distance was 5 m under photopic conditions. This consisted of a total of 81 presentations (9 spatial frequencies at 9 different contrast levels).
Light distortion and CSF parameters were recorded monocularly and binocularly while the subject was wearing the best spherocylindrical overrefraction. For monocular analysis, only the right eye of each subject has been considered. All measurements were obtained at baseline and after 1, 7, 15, and 30 days of treatment.
Statistical analysis was carried out using SPSS software v15.0 (SPSS Inc., Chicago, IL). Descriptive statistics of the variables measured in the study were produced. Normality of data distribution was assessed with Kolmogorov-Smirnov test. All parameters followed a normal distribution (
Size-related light distortion parameters (LDI and BFCr) and regularity-related parameters (BFCirr and BFCsd) showed a statistically significant change over time on ANOVA analysis (
Monocular and binocular Best Fit Circle Radius (BFCr) (a), Light Distortion Index (LDI) (b), and BFC Irregularity (BFC Irregularity) parameter (c) of light distortion. Bars represent the Standard Error of the Mean (SEM).
LDI index returns to baseline on day 7 and remains at baseline level afterwards. Conversely, irregularity parameter (BFCirr) shows a different path displaying a significant reduction after day 7 compared to baseline after a transient increase on day 1. Binocular analysis did not report any statistically significant change from baseline to day 1 or subsequent visits for LDI (
There have been statistically significant changes from baseline in all optical quality descriptors of the corneal front surface for the three pupil sizes under analysis (
Optical quality of the corneal front surface for different pupil sizes represented by the root mean square (RMS) of spherical-like aberrations (a), coma-like aberrations (b), and secondary astigmatism (c). Bars represent the Standard Error of the Mean (SEM).
Monocular CSF results presented statistically significant changes over time for frequencies between 3.00 and 24.00 cpd (ANOVA,
Monocular (a) and binocular (b) log contrast sensitivity (CS). Bars represent the Standard Error of the Mean (SEM) only for visits on days 1 and 30; the remaining error bars are omitted for clarity.
Correlations between the outcome measures (light distortion, corneal higher order aberrations, and CSF) were obtained after the first night of lens wear, after 7 days, after 15 days, and after 30 days. It was observed that spherical-like and coma-like aberrations were inversely correlated with the irregularity of the light distortion (BFCirr) and the monocular CSF, respectively, after first night. These correlations were statistically significant after the first night of treatment for coma-like aberration with 3 mm pupil (
At the 7 days’ follow-up visit, light distortion parameters and corneal higher order aberrations were inversely correlated with monocular CS. Size (LDI and BFCRad) and irregularity (BFCIrreg) were significantly correlated (
In the present study, BCVA was maintained at baseline level over the follow-up period. Ocular higher order aberrations significantly increased at 1 month after the procedure and remained stable thereafter. There was an initial loss in CS after overnight OK, and the loss persisted during the 1st-year follow-up. As a whole, all these parameters were stable throughout the posttreatment period from 1 month to 12 months. That is, posttreatment clinical parameters including refraction, visual acuity, corneal higher order aberrations, and CS were stable in the eyes that underwent overnight OK [
CS is a fundamental feature of vision, and its measurement provides useful information about visual function that may not be obtained by standard visual acuity testing [
The impact of OK on visual performance, however, has not been studied in detail. There are few long-term OK studies in which the variation of higher order aberrations and CSF are studied [
In the present longitudinal study, we investigated changes in perception of light distortion, changes in corneal higher order aberrations, and CSF as representative of visual quality in eyes undergoing overnight OK for one year.
The analysis of light distortion surrounding a light source has proved to be effective in determining the time-course of changes in visual quality from the subject’s perspective and irrespective of the corneal higher order aberrations pattern observed. A previous study by Villa et al. used a similar methodology but based on a software platform and a computer screen which is more limited in the presentation of bright stimuli. In their study, they reported an increase in light disturbances after corneal refractive surgery [
We observed an inverse correlation between coma-like aberrations and CSF for medium and higher spatial frequencies, which might be related to the relatively frequent decentration and irregularity of the treatment zone after the first night wearing CL. Our results showed no significant correlation between the different optical (corneal aberrations) and quality of vision (CSF and light distortion) parameters at the end of the month. The absence of significant correlations at this stage might be interpreted as a neural adaptation process that is independent of the optical quality of the cornea. A hypothetical component of this neural adaptation might even take place at the retinal level, with a change in the apodization mechanism of the photoreceptors such that they overcome the effects of deteriorated peripheral optics of the eye and take advantage of the relatively unchanged optical quality within the more central part of the pupil. In fact, our results agree with previous reports by our group [
Interestingly, we observed a decrease in the distortion parameters related to the irregularity of the light distortion. This might be explained by the spherical aberration induced that leads to a more uniform and more radially symmetric distortion pattern compared to baseline, which will result in a reduction of the irregularities in the form of spikes surrounding the light source. Regarding aberration changes in the anterior corneal surface, an average increase of 110% for 3.0 mm pupil to 233% for 6.0 mm pupil in the spherical-like RMS was observed. Changes in comatic aberration were less severe and more similar for different pupil sizes ranging from 108% to 150% for 3.0 and 6.0 mm pupil sizes. Secondary astigmatism showed a marked increase ranging from 62% to 195% for 3.0 and 6.0 mm pupil sizes. Similar values have been reported by other studies [
To our knowledge, this is the first study addressing the short-term changes in the optical quality of the anterior corneal surface and the visual quality determined by means of monocular and binocular CSF and light distortion measurement. Although the light distortion and CSF recovered to baseline values, the lack of correlation after 15 days between light distortion and CSF might confirm that both methods are measuring different aspects of visual quality.
In summary, we have shown that despite the reduction in optical quality of the anterior corneal surface, some of the parameters related to visual quality of the eye return to being within normal range values within a narrow period of time, and this might be related with a neural adaptation process.
The authors have no proprietary interest in any of the instruments or materials mentioned in this paper.
This research was supported in part by Projects PTDC/SAU-BEB/098391/2008//FCOMP-01-0124-FEDER-010897 and PTDC/SAU-BEB/098392/2008//FCOMP-01-0124-FEDER-010898. The present work was presented in part at the CIOCV2012 conference (Braga, April 9-10, 2011).