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Relationship between objective and subjective refraction measurements in patients with mild keratoconus
Masoud Khorrami-Nejad1,2, Ahmed Kamil Dakhil1, Hesam Hashemian3
1Optometry Department, School of Rehabilitation, Tehran University of Medical Sciences, Tehran 1148965111, Iran.
Aim:
To compare objective dry retinoscopy and subjective refraction measurements in patients with mild keratoconus (KCN) and quantify any differences.
Methods:
This cross-sectional study was done on 68 eyes of 68 patients diagnosed with mild KCN. Objective dry retinoscopy using autorefractometer and subjective refraction measurements were performed. Sphere, cylinder, J0, J45, and spherical equivalent values were compared between the two techniques.
Results:
The mean age of 68 patients with mild KCN was 21.32±5.03y (12-35y). There were 37 (54.4%) males. Objective refraction yielded significantly more myopic sphere (-1.44 D vs -0.57 D), higher cylinder magnitude (-2.24 D vs -1.48 D), and more myopic spherical equivalent (-2.56 D vs -1.31 D) compared to subjective refraction (all P<0.05). The mean differences were -0.87 D for sphere, -0.76 D for cylinder, and -1.25 D for spherical equivalent. No significant differences were found for J0 and J45 values, indicating agreement in astigmatism axis (P>0.05).
Conclusion:
In patients with mild KCN, objective dry retinoscopy overestimates the degree of myopia and astigmatism compared to subjective refraction. The irregular cornea in KCN likely impacts objective measurements. Subjective refraction allows compensation for irregularity, providing a more accurate correction. When determining refractive targets, the tendency of objective methods to overcorrect should be considered.
