Abstract
Background
To investigate changes in the corneal thickness profile in juvenile myopia after overnight wear of orthokeratology lenses (OK).
Methods
A total of 53 juveniles (53 right eyes) successfully wore OK between January 2016 and July 2017 and they were reviewed one day, one week and one month after first wearing it. Epithelial and corneal data were obtained by optical coherence tomography (OCT). Changes in uncorrected visual acuity, refractive error, corneal refractive power, and epithelial and corneal thickness were analyzed before and after wear of OK.
Results
The corneal epithelium was reshaped after 1 day of OK; the myopia degree was reduced, and uncorrected visual acuity reached 0 logMAR at 1 week. The central (2 mm) average epithelial thickness was 52.04 ± 2.35 μm, 49.25 ± 2.67 μm, 45.91 ± 2.80 μm, and 47.53 ± 3.44 μm before and after 1 day, 1 week, and 1 month of OK, respectively (t = 4.497, 9.741, and 7.340, respectively, P <0.001). The central epithelium was thinnest at 1 week, when the average thinning of 6.13 ± 1.67 μm accounted for approximately 11.78 % ± 3.21 % of the total epithelium thickness at baseline. The epithelial thickness of the reverse curve zone was 51.83 ± 2.49 μm, 57.62 ± 3.01 μm, 59.43 ± 3.19 μm, and 60.22 ± 2.75 μm before and after 1 day, 1 week, and 1 month of OK, respectively, showing a significant increase over time (t=-4.752, -6.208, and -6.848, respectively, P < 0.001).
Conclusion
In the early stage of OK, the corneal epithelium was redistributed very quickly. The central epithelium became thin, while the epithelium of the reverse curve zone correspondingly thickened. Effectively reduced the refractive power of the cornea equivalent to the effect of adding a concave spectacle lens, which ensures uncorrected vision after removal of the lenses.
1
Introduction
The corneal epithelium, which is the most superficial layer of the cornea, not only has a good protective function but is also involved in corneal refractive function; in particular, the central corneal epithelium has an especially important function for corneal refraction [ , ]. Overnight wear of OK is used to intentionally manipulate corneal shape, reduce refractive error and central corneal thickness, and effectively reduce and control myopia [ , ]. Current research shows that OK mainly caused global epithelial and former stromal changes [ , ]. However, how the corneal epithelium is redistributed in this process is less reported.
The advent of several imaging modalities, such as high frequency scanning ultrasound biomicroscopy, confocal microscopy through focusing, and optical coherence tomography (OCT), have facilitated measurement of corneal epithelial and corneal thickness. Among them, OCT has been reported as a repeatable and reproducible method with its advantages such as higher scanning speed and resolution [ , ], it creates good conditions for further research on the cornea and epithelium. Alharbi et al. [ ] found that repeatability of optical pachometry measurements was very good and reported a maximum standard deviation for three repeated measurements at the central cornea of 2 μm and at the midperiphery of 4.3 μm. The maximum difference between the average baseline measurements obtained was 3.2 ± 2.2 μm on two consecutive days. The recent corneal epithelial imaging by RTVue OCT with a corneal anterior module provides a practical tool for clinical in vivo epithelial mapping [ , ], and the good repeatability in normal, keratoconus, and post-LASIK corneas was demonstrated [ , ]. Ge et al. [ ] noted that the results of corneal epithelial thickness may not be affected by the axial resolution of different OCT, but higher optical resolution could yield higher precision. However, there are few reports on the study of corneal epithelium before and after OK wear, which is the focus and innovation of this paper.
This study continuously monitored the changes in the epithelial and corneal thickness in the central 6 mm area before and after lens wear by Fourier-domain OCT, monitoring measured including uncorrected visual acuity, refractive error, corneal refractive power, and the epithelial Min-Max value (Difference between the thinnest and thickest corneal epithelium thickness within 5 mm of central cornea). The results are reported as follows.
2
Materials and methods
2.1
Clinical data
This study is an observational study. Fifty-three patients (53 right eyes) were enrolled in this study from January 2016 to July 2017, wearing OK lenses overnight once the risks and benefits of OK wear had been explained. Fifty-three right eyes of all subjects were analyzed. All subjects had low to moderate myopia and with-the-rule refractive and corneal astigmatism of 1.50 diopters (D) or less. There were 31 males and 22 females, with an age range of 8–17 years and an average age of 12.3 ± 2.7 years. The average spherical refractive error was -3.35 ± 1.09 D, ranging from -0.75 D to -5.00 D. Standard subjective refraction was used to determine the manifest refractive error at baseline and subsequent visits. The inclusion criteria were as follows: age 8 to 17years, stable manifest refraction with an unaided distance visual acuity of 0 logMAR or better after OK treatment, and all subjects were reviewed on time. The exclusion criteria included a history of using hard contact lenses and contraindications to ocular and systemic OK determined by routine examination. This research conforms to the Declaration of Helsinki and was approved by the ethics committee of the Shandong Eye Hospital. All subjects’ guardians signed an informed consent form.
Orthokeratology lenses were dispensed to be worn on an overnight basis only, with lenses removed soon after eye opening. Subjects received instruction on lens handling and care before they received the lenses and were issued lens care solutions. OCT was conducted on both eyes at baseline and on days 1, 7, 30, in the morning 2 h after lens removal. Because of diurnal variations in corneal thickness, all measurements were conducted between 8 AM and 10 AM. All subjects also underwent a thorough contact lens aftercare examination at every visit, including visual acuity, refraction, and slit lamp biomicroscopy.
2.2
Lenses material and design
The AUTEK CHINA, INC. (Hefei, China) production of the rigid permeability OK lenses was adopted in this study. These lenses are made of Boston XO material, with a nominal total diameter of 10.4–10.8 mm, an optical zone diameter of 5.9–6.0 mm, and a reverse curve zone of 0.6 mm, giving a nominal oxygen transmissibility (Dk/t) of 100 × 10 −11 (cm 2 /s) (mlO 2 /ml × mmHg).
2.3
Lens fitting and wearing methods
Dynamic requirements included lenses that fit above and below the cornea and moved approximately 1−2 mm with blinking and eye movement. When not blinking, the lenses would automatically return to the central cornea. Static requirements included clear fluorescein staining of the lenses corresponding to each segment and a 3−5 mm contact area in the central region of the cornea. Wearing methods included 8−10 h of OK wear during nightly sleep. Upon opening the eyes in the morning, artificial tears without preservatives were applied, and the lenses were removed after 10 min.
2.4
Examinations
Patients wearing lenses were regularly examined at 1 day, 1 week, and 1 month following the start of OK use. Routine examinations included location of OK lenses, uncorrected vision, corrected visual acuity, intraocular pressure, slit lamp microscope, conjunctiva, cornea, lens fitting, lens cleanliness and corneal tomography OCT (RTVue100-2OCT, Optovue companies in the United States). All examinations were done 2 h after the removal of the lenses in the morning. Running on built-in software, version 6.11.0.12 (Optovue, Inc.), the OCT system automatically generated a total corneal thickness map and a corneal epithelial thickness map for each pachymetry scan. The data for epithelial and corneal thickness in the central 6 mm range for 17 regions were collected (see Figure 1 ). Average values in 17 areas were obtained: 1) one central zone within 0−2 mm diameter, 2) eight paracentral zones from 2−5 mm diameter, and 3) eight peripheral zones from 5−6 mm diameter. To reduce measurement error, OCT examinations were carried out by the same skilled physician. Each eye was scanned 3 times. The reverse curve zone on the corneal OCT image was marked and the thickness of the corneal epithelium and cornea at any one point in 8 regions was manually measured (see Figure 2 ). In the process, this study compared the corneal topography with the OCT image of the corneal epithelium, manually marking the annular area where the curvature was significantly steeper and the corneal epithelium was significantly thickened, defined as the position of the reverse curve zone.
2.5
Statistical analysis
In this study, the average values of the epithelial and corneal thickness of the eight regions in the circle were used in the paracentral, peripheral and reverse curve zones. The research data is a normal distribution through the (Kolmogorov-Smirnov) test. One-Way Repeated Measures ANOVA and LSD test was used by the SPSS 18 statistical software. This study shows that P < 0.05 has significant difference.
3
Results
3.1
The location of OK, visual acuity, refractive error and corneal surface parameters on OCT pachymetry map before and after lenses wear
During lenses wear, 60.4 % (32/53) of the patients had the lens centered, 39.6 % of them had different degrees of decentration that were less than 1.0 mm, according to the classification method of Tsai et al. [ ]. Uncorrected visual acuity improved significantly after 1 day of lens wear ( P < 0.001). The uncorrected visual acuity tended to stabilize at 1 week. There were significant differences in uncorrected visual acuity, refractive error, and corneal refractive power between before and after lens wear. The corneal refractive power of the anterior surface decreased after 1 day of lens wear and gradually stabilized at 1 week. The corneal refractive power of the posterior surface of the cornea exhibited little change at each point time, and compared to before lens wear, there was no significant difference (see Table 1 ).
| Time | n | UCVA | P | Refractive error (D) | P | Anterior surface (D) | P | Posterior surface (D) | P | total refractive power (D) | P |
|---|---|---|---|---|---|---|---|---|---|---|---|
| before | 53 | 0.51 ± 0.36 | −3.35 ± 1.09 | 48.42 ± 1.24 | −6.08 ± 0.21 | 42.46 ± 1.11 | |||||
| 1d | 53 | 0.36 ± 0.27 | <0.001 | −2.06 ± 0.85 | <0.001 | 46.58 ± 1.83 | <0.001 | −6.12 ± 0.13 | 0.421 | 40.48 ± 1.60 | <0.001 |
| 1w | 53 | 0 ± 0.00 | <0.001 | −1.05 ± 0.76 | <0.001 | 45.33 ± 1.35 | <0.001 | −6.00 ± 0.68 | 0.407 | 39.44 ± 1.32 | <0.001 |
| 1m | 53 | 0 ± 0.00 | <0.001 | −0.75 ± 0.60 | <0.001 | 45.21 ± 1.17 | <0.001 | −6.05 ± 0.14 | 0.593 | 39.20 ± 1.27 | <0.001 |
| F P * | 21.316 <0.001 | 19.532 <0.001 | 20.401 <0.001 | 0.527 0.864 | 15.825 <0.001 |
3.2
Epithelial thickness comparison in different areas before and after lens wear
The corneal epithelial thickness of the central 2 mm became obviously thinner after 1 day of lens wear and achieved the thinnest points at 1 week ( P < 0.001). The central epithelial thickness became thin, with a range of 5-13μm and average thinning of 6.13±1.67μm. The paracentral 2–5 mm and peripheral 5–6 mm average corneal epithelial thickness changes varied over time but not significantly ( P > 0.05). After lens wear, the reverse curve zone epithelium thickened after 1 day and gradually became stable at 1 week. Compared with the thickness before lens wear, the thickness at each time point after lens wear was significant ( P < 0.001). The min-max value (Data for automatic calculation of RTVue100-2OCT, the difference between the thinnest and thickest corneal epithelial thickness within 5 mm of the central cornea.) obviously increased after lens wear and gradually stabilized at 1 week. There were significant differences between before and after lens wear ( P < 0.001) (see Table 2 ).
