© 2008 Contact Lens Association of Ophthalmologists, Inc. Eye & Contact Lens 34(4): 224–228, 2008 Reversibility of Effects of Orthokeratology on Visual Acuity, Refractive Error, Corneal Topography, and Contrast Sensitivity Yasuko Kobayashi, M.S., Ryoji Yanai, M.D., Ph.D., Nobuhiko Chikamoto, M.D., Ph.D., Tai-ichiro Chikama, M.D., Ph.D., Kiichi Ueda, M.D., Ph.D., and Teruo Nishida, M.D., D.Sc. Objectives. To investigate the changes in corneal shape and optical performance during and after discontinuation of overnight orthokeratology for correction of myopia. Methods. Both eyes of 15 subjects were fitted with overnight reverse-geometry orthokeratology lenses, which were then worn for ⬎4 hr overnight for 52 weeks. Subjects were free of ocular disease and had a corrected visual acuity of ⱖ1.0. Refractive correction, uncorrected visual acuity, corneal topography, and contrast sensitivity (at 4 spatial frequencies) were measured under photopic conditions. Results. Refractive error (spherical equivalent) and contrast sensitivity were decreased, whereas uncorrected visual acuity, the surface asymmetry index, and the surface regularity index were increased, 1 week after the onset of overnight orthokeratology and remained so during the 52 weeks of treatment. These parameters had largely returned to baseline values by 8 weeks after treatment discontinuation. Conclusions. Overnight orthokeratology improved uncorrected visual acuity and reduced refractive error but increased corneal irregularity and impaired contrast sensitivity. However, these changes in visual function and corneal shape were reversed after discontinuation of orthokeratology lens wear. Key Words: Overnight orthokeratology—Contact lens—Corneal topography—Contrast sensitivity. images on the retina, with the cornea typically contributing 70% of the refractive power of the eye. Even minor changes in corneal shape can therefore have a substantial negative impact on vision.7 A corneal topographer measures corneal surface curvature, and several computer algorithms derived from contour maps are able to reconstruct a model of the shape of the corneal surface. The surface asymmetry index (SAI) reflects the asymmetry and local abnormal increases in corneal power, whereas the surface regularity index (SRI) reflects the optical quality of the central cornea and correlates with visual acuity. Clinically, the SAI is used as a quantitative indicator for monitoring changes in corneal topography, and the SRI is used to differentiate whether a reduced visual acuity is because of a change in corneal topography or to other factors.8 –10 Even in clinically successful cases, orthokeratology renders the cornea a nonphysiologic oblate shape and asymmetric, depending on the extent of myopic correction. Contrast sensitivity is an important clinical indicator of visual quality and function.11 Orthokeratology has been shown to increase higher-order ocular aberrations and to compromise contrast sensitivity function, again depending on the amount of myopic correction.12–14 Among the various types of correction methods available for refractive errors, however, orthokeratology has the advantage of being reversible. In contrast to refractive surgery, the cornea and, consequently, the refractive error return to baseline after cessation of the wearing of orthokeratology contact lenses. However, although optical performance after nonsurgical corneal reshaping for myopia has been monitored, changes in other optical characteristics over time associated with overnight orthokeratology or its discontinuation have not been determined. We have now investigated the dynamics of corneal shape and changes in optical characteristics during as well as after discontinuation of the wearing of overnight orthokeratology lenses for myopia. Orthokeratology is a method of temporarily changing refraction in myopic patients by the programmed application of rigid contact lenses.1– 4 The reverse-geometry lens designs used in modern orthokeratology are thought to apply positive pressure at the center of the cornea as well as negative pressure in the midperiphery under the steeper, secondary “reverse” curve.5,6 This differential pressure profile induces a flattened central treatment zone that corrects the myopic refractive error by reducing corneal power. The shape of the cornea is important for the generation of clear From the Department of Ophthalmology, Yamaguchi University Graduate School of Medicine, Ube City, Yamaguchi, Japan. K.U. and T.N. designed the study. Y.K., R.Y., N.C., and T.C. contributed to collection, management, analysis, and interpretation of the data. T.N. also contributed to management, analysis, and interpretation of the data and wrote the manuscript together with Y.K. and R.Y. All authors approved the final version of the manuscript. Address correspondence and reprint requests to Ryoji Yanai, MD, PhD, Department of Ophthalmology, Yamaguchi University Graduate School of Medicine, 1-1-1 Minami-Kogushi, Ube City, Yamaguchi 755-8505, Japan; e-mail: yanai@yamaguchi-u.ac.jp Accepted December 19, 2007. METHODS Subjects The study design was a prospective, interventional case series. Thirty eyes of 15 individuals (4 men, 11 women; mean age ⫾ SD, 27.3 ⫾ 5.0 years; age range, 21–37 years) undergoing overnight orthokeratology for correction of myopia were enrolled in the study. The subjects were selected from consecutive patients attending a contact lens clinic and underwent an initial ocular DOI: 10.1097/ICL.0b013e318165d501 224 EFFECTS OF ORTHOKERATOLOGY Criteria for Inclusion and Exclusion of Study Subjects Inclusion criteria Of legal age (20 yr) and able to volunteer Understand rights as a research subject Willing to sign a statement of informed consent Willing and able to follow the study protocol Best-corrected visual acuity of ⱖ1.0 Refractive spherical correction of between ⫺1.00 and ⫺4.00 D Refractive with-the-rule cylindrical correction of between 0.00 and 1.00 D Have realistic expectations of the outcome of treatment Exclusion criteria More than 38 yr of age (to avoid presbyopia) Ocular or systemic disorders that would normally contraindicate contact lens wear Refractive against-the-rule cylindrical correction of ⬎0.50 D Anterior eye clinical signs Use of topical ocular medication Previous corneal refractive surgery or keratoconus Pregnant, lactating, or planning pregnancy during the study period examination to determine whether they matched the criteria in Table 1. At this visit, the details of the study were explained, and, if the subject agreed to participate, an informed consent form was signed. The protocol and informed consent forms were approved by institutional review boards of Yamaguchi University Graduate School of Medicine. The subjects had mild-to-moderate myopia (mean refractive error ⫾ SD, ⫺2.54 ⫾ 0.96 D; range, ⫺1.00 to ⫺4.00 D), with-the-rule refractive and corneal astigmatism of ⬍1.00 D (⫺0.14 ⫾ 0.23 D), a spherical equivalent of ⫺2.54 ⫾ 0.97 D, a best-corrected visual acuity of 1.24 (range 1.0 –1.5), and an uncorrected visual acuity of 0.14 (range, 0.04 – 0.4). Study Protocol Both eyes of each subject were fitted with orthokeratology lenses of reverse-geometry design (BE lenses, designed and developed by Mountford and Noack and supplied by Eiko, Nagoya, Aichi, Japan). The lenses were supplied in Boston XO material (nominal Dk of 100 ⫻ 10⫺11 (cm2/sec) [mLO2/(mL 䡠 hPa)]). Initially, apical radius, eccentricity, refraction, and horizontal visible iris diameter were entered into proprietary computer software (BE Optimal Orthokeratology, version 4.0) to select a lens for use in an overnight trial. Subjects returned to the clinic the morning after the overnight trial (within 1 hr of waking) with the lenses in situ, and the topographic and refractive outcomes were entered into the software to select the appropriate lenses for dispensing. If the first overnight trial was unsuccessful, another BE lens was tried at least 1 week later. Subjects were instructed to wear the dispensed lenses for ⬎4 hr each night for 52 weeks and were examined at 1, 2, 4, 8, 12, 24, and 52 weeks. Subjects discontinued use of the lenses after 52 weeks and were examined at 56 and 60 weeks to check the recovery to baseline visual acuity, refraction, corneal shape, and contrast sensitivity. Measurements All subjects underwent comprehensive ocular examination, including subjective refraction, uncorrected and corrected visual acuity, autorefraction, autokeratometry, corneal topography, and slitlamp examination, at baseline and all subsequent clinic visits. Refractive error was determined by the same observer using the monocular refractive technique, in which the spherical end points for the 2 eyes are represented by maximum plus or minimum minus powers to achieve maximum visual acuity at distance. Standard subjective refraction techniques were used to determine the manifest refractive error, and results were converted to spher- ical equivalent for ease of analysis. Uncorrected and corrected visual acuity were assessed with Landolt C charts. Corneal topography was measured with an E300 instrument (Medmont, Camberwell, Victoria, Australia); corneal indexes (SAI, SRI) provided by the E300 Attributes summary were analyzed. Static contrast sensitivity was measured with the use of a contrast grating with internal illumination and a contrast luminance of 85 cd/m2 (CSV 1000; Vector Vision, Dayton, OH, USA). The test presents 4 rows of sine-wave gratings with spatial frequencies of 3, 6, 12, and 18 cycles/degree at 2.5 m; sensitivity levels range from 0.7 to 2.08, 0.91 to 2.29, 0.61 to 1.99, and 0.17 to 1.55 log units, respectively. Statistical Analysis Data are presented as means ⫾ SD. Visual acuity was calculated as the geometric mean from Landolt C charts.15,16 Results obtained at each clinic visit were compared with the baseline measurements with the use of Dunnett’s test. The extent of myopic correction was defined as the reduction in refractive spherical equivalent at each visit. A P value of ⬍0.05 was considered statistically significant. RESULTS The mean best-corrected visual acuity of eyes treated by overnight orthokeratology with a reverse-geometry lens was ⱖ1.0 during the study period. As shown in Figure 1, uncorrected visual acuity increased from 0.14 to 0.79 after treatment for 52 weeks (P ⬍ 0.001). Treatment also induced a significant reduction in spherical equivalent value (Fig. 2), with this effect being statistically significant after 1 week (from ⫺2.54 ⫾ 0.97 to ⫺0.95 ⫾ 0.96D, P ⬍ 0.001) and the value after 52 weeks being ⫺1.04 ⫾ 1.05D. Both uncorrected visual acuity and refractive error had largely returned to the baseline values by 4 weeks after discontinuation of orthokeratology and had done so completely at 8 weeks (Figs. 1 and 2). Both the SAI and SRI were increased significantly 1 week after the initiation of orthokeratology and remained inOrthokeratology * 1.0 * * * ** * Uncorrected visual acuity TABLE 1. 225 0.5 0.1 0 10 20 30 40 50 60 Time (weeks) FIG. 1. Change in mean uncorrected visual acuity during treatment of myopia by overnight orthokeratology with a reverse-geometry lens for 52 weeks and after treatment discontinuation. *P ⬍ 0.001 vs. baseline value. Eye & Contact Lens, Vol. 34, No. 4, 2008 226 Y. KOBAYASHI ET AL. Orthokeratology Orthokeratology 2.0 3 cycles/degree 0 * ** * * -1.0 * * 1.5 1.0 -2.0 -4.0 0 10 20 30 40 50 60 Time (weeks) FIG. 2. Change in refraction (spherical equivalent value) during treatment of myopia by overnight orthokeratology with a reversegeometry lens for 52 weeks and after treatment discontinuation. Data are means ⫾ SD. *P ⬍ 0.001 vs. baseline value. creased thereafter up to 52 weeks (Fig. 3). Discontinuation of orthokeratology was accompanied by a return of both SAI and SRI to baseline values. Log contrast sensitivity at the spatial frequencies of 12 and 18 cycles/degree decreased significantly during the treatment period but had fully recovered to baseline values by 8 weeks after discontinuation of orthokeratology (Fig. 4). Contrast 2.0 1.5 2.0 12 cycles/degree -3.0 6 cycles/degree 2.5 Log contrast sensitivity Spherical equivalent value (D) 1.0 ‡ † † 1.5 1.0 1.5 Orthokeratology 18 cycles/degree Surface asymmetry index 2.0 1.5 †* * † 1.0 † † ‡ † † 1.0 ‡ 0.5 0.5 0 10 20 30 Time (weeks) 40 50 60 0 0 10 20 30 Time (weeks) 40 50 60 Orthokeratology Surface regularity index 2.0 1.5 sensitivity at 3 or 6 cycles/degree was not significantly affected by the wearing of orthokeratology lenses. 1.0 ** * * * * * DISCUSSION 0.5 0 FIG. 4. Changes in contrast sensitivity at 3 (top), 6 (top middle), 12 (bottom middle), and 18 (bottom) cycles/degree during treatment of myopia by overnight orthokeratology with a reverse-geometry lens for 52 weeks and after treatment discontinuation. Data are means ⫾ SD. †P ⬍ 0.01, ‡P ⬍ 0.05 vs. baseline. 0 10 20 30 Time (weeks) 40 50 60 FIG. 3. Changes in the surface asymmetry index (top) and surface regularity index (bottom) during treatment of myopia by overnight orthokeratology with a reverse-geometry lens for 52 weeks and after treatment discontinuation. Data are means ⫾ SD. *P ⬍ 0.001, †P ⬍ 0.01, ‡P ⬍ 0.05 vs. baseline. Eye & Contact Lens, Vol. 34, No. 4, 2008 We have shown that overnight orthokeratology for correction of myopia resulted in a significant improvement in visual acuity and refractive error, but increased corneal irregularity and decreased contrast sensitivity. However, these changes in visual function and corneal regularity were fully reversed within 8 weeks after discontinuation of orthokeratology lens wear. Reversibility is a key feature of orthokeratology for the correction of myopia. The corrective effect of orthokeratology in the EFFECTS OF ORTHOKERATOLOGY present study was apparent within 1 week after the initiation of lens wearing and persisted for the duration of treatment. However, the corrective effect of orthokeratology had disappeared within 2 months after its discontinuation. On the one hand, the temporary nature of the correction might be considered a drawback of orthokeratology compared with surgical correction of refractive error. On the other hand, it may be considered an advantageous feature by patients concerned about the safety of corneal refractive surgery. Our present results on the reversibility of the effects of orthokeratology lens wearing are in good agreement with those of previous studies. In an analysis of corneal and refractive recovery after wearing of overnight orthokeratology lenses by subjects for 6 to 9 months, Barr et al.17 observed a 90% recovery of refractive error toward baseline values within 72 hr of lens wear discontinuation. These researchers examined only short-term recovery and did not evaluate complete recovery to baseline levels. Furthermore Soni et al.18 found that monocular uncorrected vision had not returned completely to baseline values within 2 weeks after cessation of orthokeratology lens wear. Our data demonstrate complete recovery of visual acuity and refractive error as well as of corneal regularity and contrast sensitivity function at 8 weeks after discontinuation of lens wear. We evaluated corneal topography and the quality of vision after orthokeratology on the basis of parameters including the SAI and SRI and contrast sensitivity. Orthokeratology applies positive and negative pressures in the central and midperipheral regions, respectively, of the cornea, and thereby reshapes the anterior corneal surface. The flattened central zone and steepened peripheral zone of the cornea achieved after orthokeratology correct myopic refractive error as a result of a reduction in corneal power. In all cases in the present study, orthokeratology resulted in a marked improvement in uncorrected visual acuity. Of the 30 eyes included in our study, an uncorrected visual acuity of ⱖ1.0 was achieved in 17 eyes after treatment for 52 weeks. However, orthokeratology increased both the SAI and SRI in the study subjects, even in eyes that achieved an uncorrected visual acuity of ⱖ1.0. As far as we are aware, the effect of orthokeratology on contrast sensitivity has not previously been described. We have now shown that orthokeratology induced a reversible decrease in contrast sensitivity in the eyes of our study subjects. Although orthokeratology is effective in improving uncorrected visual acuity, it also increases the incidence of higher-order corneal aberrations, even in clinically successful cases. Hiraoka et al. thus showed that the occurrence of higher-order corneal aberrations, such as third- or fourth-order root mean square aberrations, was significantly increased by orthokeratology.12,13 We have now shown that orthokeratology did not permanently affect contrast sensitivity at the high spatial frequencies of 12 or 18 cycles/degree even though it rendered the cornea oblate and asymmetric. These results suggest that contrast sensitivity at the high spatial frequencies of 12 and 18 cycles/degree is more sensitive to abnormalities of the optical transduction system induced by orthokeratology than that at the low spatial frequencies of 3 or 6 cycles/degree. Contrast sensitivity has been found to be increased after radial keratotomy.19 A significant correlation was also observed between ocular aberrations and the loss of low-contrast visual acuity after photorefractive keratectomy.20 In addition, both total and corneal aberrations were significantly increased and contrast sensitivity decreased after standard myopic laser in situ keratomileusis.21 227 Conventional laser in situ keratomileusis also significantly increased higher-order ocular aberrations, compromising postoperative contrast sensitivity function.22 However, orthokeratology affected contrast sensitivity only transiently in the present study, even though it increased both the SAI and SRI. The normal cornea is prolate in shape, appearing to flatten in curvature as the periphery is approached. However, after orthokeratology with a reversegeometry lens, the cornea appears flatter at the center and gradually increases in steepness in the midperiphery before tending to flatten again. These changes render the corneal surface oblate, especially over the central 6.00-mm chord.23 Visual acuity reflects the ability to resolve small details of high spatial frequencies at high contrast. It thus provides an incomplete assessment of overall visual function, given that the environment is composed of a variety of objects of differing spatial frequencies viewed under different levels of contrast. Depending on the test chart, contrast sensitivity testing is able to estimate the resolving ability of the visual system over a broader range of spatial frequencies and contrasts relative to those encountered during measurement of visual acuity. The statistically significant changes in contrast sensitivity apparent at certain time points during treatment with orthokeratolgy in the present study might represent normal fluctuations in this parameter exacerbated by treatment rather than a decrease in the quality of vision. Tang showed that patients subjected to orthokeratology did not differ in contrast sensitivity from control individuals under high-contrast photopic or mesopic conditions, but that they manifested a reduced contrast sensitivity under low-contrast conditions.24 Tang therefore suggested that orthokeratology subjects may experience a loss of contrast sensitivity function at intermediate spatial frequencies. This apparent discrepancy with our results warrants further investigation. REFERENCES 1. Kerns RL. Research in orthokeratology. Part II: Experimental design, protocol and method. J Am Optom Assoc 1976;47:1275–1285. 2. Kerns RL. Research in orthokeratology. Part III: Results and observations. J Am Optom Assoc 1976;47:1505–1515. 3. Kerns RL. Research in orthokeratology. Part VIII: Results, conclusions and discussion of techniques. J Am Optom Assoc 1978;49:308 –314. 4. Sorbara L, Fonn D, Simpson T, et al. Reduction of myopia from corneal refractive therapy. Optom Vis Sci 2005;82:512–518. 5. Mountford J. Design variables and fitting philosophies of reverse geometry lenses. In: Mountford J, Ruston D, Dave T, eds. Orthokeratology: Principles and Practice. Oxford, Butterworth Heinemann, 2004, pp. 69 –107. 6. Haque S, Fonn D, Simpson T, et al. Corneal refractive therapy with different lens materials. Part 1: Corneal, stromal, and epithelial thickness changes. Optom Vis Sci 2007;84:343–348. 7. Courville CB, Smolek MK, Klyce SD. Contribution of the ocular surface to visual optics. Exp Eye Res 2004;78:417– 425. 8. Dave T. Current developments in measurement of corneal topography. Cont Lens Anterior Eye 1998;21:S13–S30. 9. Dingledein SA, Klyce SD, Wilson SE. Quantitative descriptors of corneal shape derived from computer-assisted analysis of photokeratographs. Refract Corneal Surg 1989;5:372–378. 10. Wilson SE, Klyce SD. Quantitative descriptors of corneal topography. Arch Ophthalmol 1991;109:349 –353. 11. Ginsburg AP. Contrast sensitivity: determining the visual quality and function of cataract, intraocular lenses and refractive surgery. CurrOpin Ophthalmol 2006;17:19 –26. 12. Hiraoka T, Matsumoto Y, Okamoto F, et al. Corneal higher-order aberrations induced by overnight orthokeratology. Am J Ophthalmol 2006;139:429 – 436. 13. Hiraoka T, Okamoto C, Ishii Y, et al. Contrast sensitivity function and Eye & Contact Lens, Vol. 34, No. 4, 2008 228 14. 15. 16. 17. 18. 19. Y. KOBAYASHI ET AL. ocular higher-order aberrations following overnight orthokeratology. Invest Ophthalmol Vis Sci 2007;48:550 –556. Lu F, Simpson T, Sorbara L, et al. Corneal refractive therapy with different lens materials. Part 2: Effect of oxygen transmissibility on corneal shape and optical characteristics. Optom Vis Sci 2007;84:349 – 356. Holladay JT, Prager TC. Mean visual acuity. Am J Ophthalmol 1991;111:372–374. Moseley MJ, Jones HS. Visual acuity: Calculating appropriate averages. Acta Ophthalmol 1993;71:296 –300. Barr JT, Rah MJ, Jackson JM, et al. Orthokeratology and corneal refractive therapy: A review and recent findings. Eye Contact Lens 2003;29:S49 –S53. Soni PS, Nguyen TT, Bonanno JA. Overnight orthokeratology: Refractive and corneal recovery following discontinuation of reverse geometry lens. Eye Contact Lens 2004;30:254 –262. Applegate RA, Howland HC, Sharp RP, et al. Corneal aberrations and Eye & Contact Lens, Vol. 34, No. 4, 2008 20. 21. 22. 23. 24. visual performance after radial keratotomy. J Refract Surg 1998;14: 397– 407. Seiler T, Holschbach A, Derse M, et al. Complications of myopic photorefractive keratectomy with the excimer laser. Ophthalmology 1994;101:153–160. Marcos S, Barbero S, Llorente L, et al. Optical response to Lasik surgery for myopia from total and corneal aberration measurements. Invest Ophthalmol Vis Sci 2001;42:3349 –3356. Yamane N, Miyata K, Samejima T, et al. Ocular higher-order aberrations and contrast sensitivity after conventional laser in situ keratomileusis. Invest Ophthalmol Vis Sci 2004;45:3986 –3990. Mountford JA, Noack DB. A mathematical model for corneal shape changes associated with Ortho-K. Contact Lens Spectrum 1998;13: 21–25. Tang W. The relationship between corneal topography and visual performance. PhD Thesis. Centre for Eye Research, Queensland University of Technology, Brisbane, Australia, 2001.