Reproducibility of manifest refraction in patients with keratoconus compared with healthy subjects: A prospective cohort study

Purpose To measure and compare the inter- examiner reproducibility of manifest refraction in patients with keratoconus. Design Prospective, inter-examiner reliability analysis comparing “cases and controls” Methods Patients with Keratoconus (KC) and healthy subjects had undergone manifest refraction by the same two skilled optometrists, each was masked to the refraction of the other, on the same day. The KC group was comprised of patients with KC, who were recruited from the cornea clinic. The control group consisted of healthy individuals who wore spectacles and did not have KC. Participants for the control group were recruited from the clinic's staff, including doctors, technicians, nurses, and medical students. The study took place in one tertiary medical center in Israel from April 2021 to May 2022. The results of the manifest refraction and achieved corrected distance visual acuity (CDVA) were compared between groups. Results A total of 120 eyes of 60 patients were enrolled in the study; Thirty in the keratoconus group and 30 in the control group. A difference of 0.67±0.83 D and 0.19±0.21 D in the absolute manifest cylinder was observed between the optometrists in the keratoconus (95% LoA, -0.96, 2.30) and control group (95% LoA, -0.22, 0.61), respectively (p<0.001). Multivariate analysis revealed a 22-fold higher likelihood of an error exceeding 0.75 D in cylinder measurements (OR, 22.24; 95% CI, 2.39–206.95) and a tenfold likelihood of a difference of at least one row on the Snellen chart for CDVA (OR,10.32; 95% CI,2.39-44.44) in the KC group. Conclusion when compared to healthy subjects, patients with KC exhibit greater variability in manifest refraction. This discrepancy has the potential to influence the decision-making process when managing patients with KC.