A Comparison between Subjective and Aberrometry–Derived Refractions in Keratoconic and Normal Eyes

PURPOSE: To compare the differences in subjective and three aberrometry–derived refractions, along with the visual acuity achieved with the refractions, in keratoconic (KC) and normal eyes. METHODS: Subjective refraction and Hartmann–Shack aberrometry were performed on 6 KC eyes and 12 normal eyes. Additionally, the logMAR acuity achieved using the subjective and aberrometry auto–refraction data was measured for the KC patients. RESULTS: The subjective and aberrometry–derived spherical equivalent (SE) refraction data were significantly different in the KC group (p=0.015) but not in the normal group (p=0.10). Neither the SE values derived from the 2nd–order Zernike terms, nor from a combination of the 2nd-, 3rd- and 4th–order terms, gave results equal to that of the subjective refraction SE data in either group. The mean difference between the subjective and aberrometry auto–refraction SE data was considerably smaller in the normal group (0.19 D) than in the KC group (2.35 D). In the KC group, the subjective refraction data gave better acuity than the aberrometry auto–refraction data. The magnitudes of vertical coma (R2=0.71, p=0.035) and higher–order root mean square (HORMS) error (R2=0.71, p=0.036) showed significant correlation with the subjective refraction acuities. The difference between the subjective and aberrometer auto–refraction SE data showed significant correlations with the magnitudes of vertical coma (R2 = 0.86, p = 0.007) and HORMS (R2 = 0.89, p = 0.004) CONCLUSIONS: The subjective and aberrometry–derived SE refraction data were significantly different in the KC group. The larger the magnitude of the higher–order aberrations in KC eyes, the poorer the subjective refraction acuity and the larger the difference between the subjective and aberrometry–derived SE terms. Further investigation into deriving objective refraction data from aberrometry measurements is warranted in keratoconus, as an objective method of accurately predicting refraction would be clinically useful when assessing KC patients.