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FM John, NR Bromham, TR Candy, JM Woodhouse; Steady-State Vep and Behavioural Measures of Visual Acuity in Children With Down Syndrome; The Effect of Defocus . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1812.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Children with Down syndrome (DS) have reduced visual acuity (VA) when tested using behavioural techniques. Many children with DS also show accommodative inaccuracy at the viewing distances used with behavioural tests. This study investigated optical defocus as a candidate for reduced VA in children with DS. Methods:The subject group consisted of 34 children with DS and 35 controls, aged 3 months to 14 years. VA was measured using steady-state, swept VEP with vertical sine wave gratings at 5Hz phase reversal (software provided by A.M. Norcia; Norcia and Tyler, 1985). VA was also measured using conventional behavioural techniques. Dynamic retinoscopy was used to predict defocus at the testing distances. The modulation transfer function was calculated for each defocused child using geometrical optics. The first zero of this function was to estimate the upper spatial frequency limit on VA due to defocus. Results:VA was analysed using a subject group (DS vs. controls) x test type (behavioural vs. VEP) ANOVA with subject age as a covariate. A significant effect of subject group was observed (F(1,59)=8.632,p<0.001) and a significant interaction between subject group and test type (F(1,59)=5.169,p=0.027). The DS group showed reduced VA compared to the controls in both VEP and behavioural acuity tests, but the deficit was more pronounced with behavioural testing. Reduced VA in the DS group was still seen when the analysis was restricted to those children who accommodate accurately (F(1,26)=8.047,p=0.009). The DS group showed more defocus than controls (F(1,55)=5.978,p=0.018). VEP and behavioural VA thresholds approached the limit predicted by geometrical optics only in children whose defocus was greater than 0.3 diopters. Conclusion:Optical defocus due to accommodative inaccuracy does not fully explain reduced VA in DS. Children with DS who do not have significant levels of defocus still exhibit reduced VA when compared to controls suggesting an underlying neural sensory deficit in the DS visual system. Inaccurate control of accommodation in children with DS may reflect such a deficit.
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