Autorefraction versus manifest refraction in patients with keratoconus

PURPOSE: To compare visual performance using autorefraction and manifest refraction assessments in patients with keratoconus and investigate whether autorefraction measurements lead to suboptimal visual performance. METHODS: Corrected distance visual acuity (CDVA) was measured in 90 eyes of 61 patients with keratoconus with both autorefraction and manifest refraction, in a random order. Maximum keratometry (Kmax), cone location, and wavefront aberration were determined with Scheimpflug tomography. The difference between the autorefraction and manifest refraction outcomes was converted to vectors and a multivariable analysis was performed to identify potential underlying causes of this difference. RESULTS: A significantly better CDVA was achieved with manifest refraction (0.06 vs 0.29 logMAR [20/23 vs 20/38 Snellen], P <.001). After vector analysis, a mean difference of 4.83 diopters was found between autorefraction and manifest refraction. Increased Kmax was strongly and significantly associated with better visual performance of manifest refraction compared to autorefraction (B = 0.496, P =.002). CONCLUSIONS: This study showed that a superior CDVA is achieved with manifest refraction compared to autorefraction in patients with keratoconus. Furthermore, the difference between the two refraction methods increases as the cornea steepens. According to this study, autorefraction is unreliable in patients with keratoconus and should be avoided.