AIM： To investigate the influence of astigmatism on visual acuity in school-aged children, and to define a cutoff for clinically significant astigmatism. METHODS： This is a population-based, cross-sectional study. Among 5053 enumerated children aged 5-15 y in Guangzhou, 3729（73.8%） children aged 7-15 with successful cycloplegic auto-refraction（1% cyclopentolate） and a reliable visual acuity measurement were included. Ocular measurement included external eye, anterior segment, media and fundus and cycloplegic auto-refraction. Primary outcome measures included the relationship between severity and subtypes of astigmatism and the prevalence of visual impairment. Three criteria for visual impairment were adopted： best-corrected visual acuity（BCVA） ≤0.7, uncorrected visual acuity（UCVA） ≤0.5 or 〈0.7 in the right eye.RESULTS： Increases of cylinder power was significantly associated with worse visual acuity（UCVA： β=0.051, P〈0.01; BCVA： β=0.025, P〈0.001）. A substantial increase in UCVI and BCVI was seen with astigmatism of 1.00 diopter（D） or more. Astigmatism ≥1.00 D had a greater BCVI prevalence than cylinder power less than 1.00 D（OR=4.20, 95%CI： 3.08-5.74）, and this was also true for hyperopic, emmetropic and myopic refraction categories. Oblique astigmatism was associated with a higher risk of BCVI relative to with the rule astigmatism in myopic refractive category（OR=12.87, 95%CI： 2.20-75.38）.CONCLUSION： Both magnitude and subtypes of astigmatism influence the prevalence of visual impairment in school children. Cylinder ≥1.00 D may be useful as a cutoff for clinically significant astigmatism.
