Contrast Sensitivity Function and Ocular Higher-Order Aberrations following Overnight Orthokeratology
Investigative Ophthalmology and Visual Science
PURPOSE. To evaluate relationships among contrast sensitivity function, ocular higher-order aberration, and myopic correction in eyes undergoing overnight orthokeratology for myopia. METHODS. A prospective study was conducted in 46 eyes of 23 patients undergoing orthokeratology. Inclusion criteria were spherical equivalent refraction between -1.00 and -4.00 diopters (D), refractive astigmatism up to 1.00 D, and best-corrected visual acuity of 20/20 or better. Ocular higher-order aberrations and
... der aberrations and contrast sensitivity function were determined before and 3 months after initiation of the procedure. We measured three indices of contrast sensitivity function: contrast sensitivity, low-contrast visual acuity, and letter contrast sensitivity with the CSV-1000 charts (Vector Vision Co., Greenville, OH). Area under the log contrast sensitivity function (AULCSF) was calculated from the contrast sensitivity data. RESULTS. Orthokeratology significantly improved logMAR uncorrected visual acuity (P Ͻ 0.0001; paired t-test) but significantly increased ocular higher-order aberrations (P Ͻ 0.0001) and decreased contrast sensitivity function, including AULCSF (P Ͻ 0.0001), low-contrast visual acuity (P ϭ 0.0025), and letter contrast sensitivity (P Ͻ 0.0001; Wilcoxon signed-rank test). The induced changes in AULCSF, low-contrast visual acuity, and letter contrast sensitivity by orthokeratology showed significant correlation with changes in third-order (Pearson r ϭ -0.430, P ϭ 0.0026; r ϭ 0.423, P ϭ 0.0031; and Spearman r s ϭ -0.351, P ϭ 0.0186, respectively), fourth-order (r ϭ -0.418, P ϭ 0.0035; r ϭ 0.425, P ϭ 0.0029; and r s ϭ -0.566, P ϭ 0.0001, respectively), and total higher-order (r ϭ -0.460, P ϭ 0.0011; r ϭ 0.471, P ϭ 0.0008; and r s ϭ -0.434, P ϭ 0.0036, respectively) aberrations. The induced changes in contrast sensitivity function and higher-order aberrations significantly correlated with the amount of myopic correction (P Ͻ 0.01). CONCLUSIONS. Orthokeratology significantly increases ocular higher-order aberrations and compromises contrast sensitivity function, depending on the amount of myopic correction. O rthokeratology, also known as corneal refractive therapy or corneal reshaping, is a method of temporarily changing refraction in myopic patients by the programmed application of specially designed rigid contact lenses. It was introduced in the early 1960s. 1 The central cornea is flattened and thinned, resulting in a reduction in myopia and an improvement in unaided vision.     In the 1980s, application of the orthokeratology lens during sleep became possible with the development of higher gas-permeable lens materials. This wearing modality, called overnight orthokeratology, allowed satisfactory daytime vision without contact lenses or eyeglasses. In addition, the advent of sophisticated lens designs (reverse geometry design) caused much faster, more accurate, and greater achievement of corneal and refractive changes. 4,6 -8 Overnight orthokeratology has come into greater use as a treatment modality to correct refractive errors, and its efficacy has been confirmed in terms of high-contrast visual acuity. 4,    In recent years, with the development of a wavefront sensing technique that can quantify optical aberration, increasing attention has been paid to changes in the optical quality of the eye after refractive procedures. Several studies have reported increases in ocular higher-order aberrations after photorefractive keratectomy (PRK) 10,11 and laser in situ keratomileusis (LASIK). 12-14 Increases in higher-order aberrations are among the main causes of reduced visual performance after PRK and LASIK. 10, 11,     These procedures are designed to correct defocus by surgical modification of the corneal curvature and to produce a nonphysiological oblate cornea with a flat central area and increasing power toward the periphery. Similarly, an oblate cornea is created by orthokeratology, 7 and previous studies have demonstrated that orthokeratology increases corneal and ocular higher-order aberrations. 19 -21 A paucity of information exists about contrast sensitivity function after orthokeratology. 21 Moreover, the impact of higher-order aberrations induced by orthokeratology on contrast sensitivity function has not been reported. We conducted the current prospective study to analyze the relationship between changes in contrast sensitivity function and changes in ocular higherorder aberrations in eyes undergoing overnight orthokeratology. SUBJECTS AND METHODS Subjects Forty-six eyes of 23 subjects (12 men, 11 women) were included in this prospective study. Subjects were selected based on the following inclusion criteria; age between 20 and 35 years, spherical equivalent refraction between -1.00 and -4.00 diopters (D), refractive astigmatism up to 1.00 D, mean keratometry readings between 40.00 and 46.25 D, best-corrected visual acuity of 20/20 or better, and no ocular or systemic disease. Subjects' ages ranged from 21 to 33 years (24.2 Ϯ 3.3 years [mean Ϯ SD]). Baseline uncorrected visual acuity (logMAR) ranged from 0.22 to 1.52 (0.77 Ϯ 0.31).