Comparison of Overnight Orthokeratology in RGP and Non-RGP Wearers

Abstract

Abstract: : Purpose: Extended wear of RGP lenses has been associated with minor corneal shape changes, particularly a reduction in corneal eccentricity. This raises a question whether RGP lens wear affects the outcome of orthokeratology (OK), the process in which specially designed rigid lenses are used to correct low myopia by flattening the cornea. The objective of the study is to compare the refractive outcome of overnight OK in RGP and non-RGP wearers and identify the predictors. Methods: The RGP and non-RGP wearing groups consisted of seven and eight subjects respectively, a total of 15 subjects (30 eyes) in the age range of 18 to 42 years with myopia ranging from –0.50 to –4.50D. The average baseline myopia was –2.33D in non-RGP group and –2.71D in RGP group. RGP wearers were asked to discontinue lens wear 3 weeks prior to the study. Each subject was fit with BE OK lenses (Precision Optics) using Mountford's sag fitting approach. Apical radius, eccentricity and sag were measured via the Medmont corneal topographer and entered in the BE software. The suggested lens parameters were then calculated for each eye. The diagnostic lens fit was initially assessed one night after wearing the lens before the final lens was ordered. The goal was to achieve a Bull's eye fluorescein pattern was ensured. The subjects were advised to wear the lenses every night for about 8-10 hours. Visual acuity, subjective refraction, topography and slit lamp examination were performed at each weekly visit for a period of one month. Results: The average decrease in myopia was –2.64D in the non-RGP group and –2.34D in the RGP group. Since the attempted flattening of curvature depends on the baseline error, the myopia reduction in the two groups cannot be directly compared. Regression analysis shows an R² of 87% for the non-RGP group and 14.4% for the RGP group. This shows that greater degree of variance in myopia reduction is explained by baseline error in the non-RGP group. In order to identify more predictors to explain the difference, multiple regression was performed with baseline myopia and eccentricity as variables. Eccentricity results in a significant R² change in the RGP group accounting for 61.8% of the variance in myopia reduction. Conclusions: For the range of refractive errors considered in the study, eccentricity seems to be a better predictor for the RGP group compared to baseline error while in the non-RGP group, greater reduction was achieved in higher refractive errors.

Keywords: contact lens • myopia • visual acuity