Quantitative Evaluation of Corneal Regular and Irregular Astigmatism in Subjects Undergoing Overnight Orthokeratology

Abstract

Abstract: : Purpose: Orthokeratology is defined as the temporary reduction in myopia by the programmed application of rigid gas-permeable contact lenses. Development of new reverse geometry contact lens designs has led to a renewed interest in this field. However, the influence of this procedure on corneal topography has not been studied in detail. We conducted the current prospective study to quantitatively assess changes in corneal regular and irregular astigmatism in subjects undergoing overnight orthokeratology. Methods: Fifty-nine eyes of 36 patients undergoing overnight orthokeratology for myopia were examined. Inclusion criteria were uncorrected visual acuity of 20/20 or better after treatment and minimum follow-up of 3 months. Mean age was 15.9 ± 6.3 years (range 11 to 37). Mean spherical equivalent before treatment was -3.15 ± 1.13D (range -1.25 to -5.75). Videokeratographic data were obtained with computerized videokeratography (TMS-2N, Tomey). Using Fourier series harmonic analysis, topography data were decomposed into spherical component, regular astigmatism, asymmetry (tilt or decentration), and higher order irregularity. These data were calculated for 3-mm and 6-mm pupils. Results: By orthokeratology, regular astigmatism significantly increased for both 3-mm (p=0.0177, paired t-test) and 6mm (p=0.0039) pupils. Similarly, asymmetry component increased significantly by the treatment for 3-mm (p<0.0001) and 6-mm (p<0.0001) pupils. No significant changes were observed in higher order irregularity (p=0.2772, 0.0811). The amount of myopic correction significantly correlated with asymmetry for 3-mm (Pearson r=0.44, p=0.0004) and 6-mm (r=0.65, p<0.0001) pupils, but not with regular astigmatism (r=0.15, p=0.2705) (r=0.14, p=0.2860). Conclusions: Even in patients who acquired good uncorrected visual acuity by means of orthokeratology, asymmetry as one of the irregular astigmatism components increased considerably. The magnitude of myopic correction correlated with the increases in asymmetry component. Future studies are needed to investigate the exact influence of corneal irregular astigmatism on patients’ daily vision after overnight orthokeratology.

Keywords: contact lens • astigmatism • myopia