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Down syndrome (DS) is associated with ocular and cognitive sequelae, which both have the potential to influence clinical measures of refractive error. This study compares variability of autorefraction among subjects with and without DS.Grand Seiko autorefraction was performed on 139 subjects with DS (age: 8–55, mean: 25 ± 9 yrs) and 138 controls (age: 7–59, mean: 25 ± 10 yrs). Subjects with three refraction measures per eye (DS: 113, control: 136) were included for analysis. Each refraction was converted to power vector notation (M, J0, J45) and a difference in each component (ΔM, ΔJ0, ΔJ45) was calculated for each refraction pairing. From these quantities, average dioptric strength (: square root of the sum of the squares of M, J0, and J45) and average dioptric difference (: square root of the sum of the squares of ΔM, ΔJ0, and ΔJ45) were calculated.The DS group exhibited a greater median (1Q: 1.38D M: 2.38D 3Q: 3.41D) than control eyes (1Q: 0.47D M: 0.96D 3Q: 2.75D) (P < .001). Likewise, the DS group exhibited a greater median in refraction (1Q: 0.27D M: 0.42D 3Q: 0.78D) than control eyes (1Q: 0.11D M: 0.15D 3Q: 0.23D) (P < .001) with 97.1% of control eyes exhibiting ≤0.50D, compared to 59.3% of DS eyes. An effect of on was not detected (P = .3009) nor was a significant interaction between and group detected (P = .49).In the current study, comparing three autorefraction readings, median total dioptric difference with autorefraction in DS was 2.8 times the levels observed in controls, indicating greater potential uncertainty in objective measures of refraction for this population. The analysis demonstrates that J45 is highly contributory to the observed variability.