Stability of the Residual Higher–Order Optical Aberrations in Keratoconus

Abstract

Abstract: : Purpose:The corneal ectasia in keratoconus produces a massive increase in the higher–order optical aberrations versus the normal eye. These aberrations are currently best managed by fitting the keratoconic cornea with a rigid gas permeable contact lens. However, the residual aberrations, those measured with the rigid contact lens in place, are often visually significant in patients with moderate to severe keratoconus. If we are to work toward correcting these residual aberrations, we must understand how stable the aberrations are to correct them most effectively. Methods:Eleven moderate to severe keratoconic eyes in seven patients were measured with a Wavefront Sciences COAS aberrometer. Measurement visits occurred at monthly intervals for three months. Five measurements were taken on each non–dilated eye in standard room illumination at each visit. Patients were asked to remove their head from the chin and forehead rests between each measurement. Data from OSA Zernike Polynomials (3,–3), (3,–1), (3, 1), (3, 3), and (4, 0) were averaged across the five measurements at each visit. The average magnitude from each visit for each aberration was averaged with similar measurements taken during the other three visits. The overall mean and standard deviation for each aberration in each eye were compared to characterize temporal stability. Results:Within each eye, the standard deviation generally varied across aberrations by a factor of about five. The mean amplitude of the individual aberrations varied routinely by a factor greater than five and occasionally as high as 147 within an eye. Importantly, the standard deviation and the mean did not vary together...smaller means were routinely associated with larger standard deviations. Conclusions:The data from this study suggest that multiple measurements spread out over multiple visits may be needed to optimize the correction of higher–order aberrations over time. In addition, if only a single measurement visit is possible, it might be best to only correct the large amplitude aberrations; correcting the lower amplitude aberrations could actually leave the patient with poorer vision for significant periods of time.

Keywords: optical properties • keratoconus • contact lens