Abstract:

Objective To study symmetry patterns of axes (direct and mirror) in bilateral astigmatism of pre-school children. Design Cross-sectional study. Participants One hundred and sixty-four pre-school children aged 3~4 years old from new kindergartners being medically examined from June to August 2017 was collected. Methods Refractive status data were measured using hand held autorefractor (WelchAllyn). Descriptive statistics of symmetry patterns of axes data were analyzed through two symmetry models of axes (the mirror and direct symmetry models). The relationship between gender, spherical values, magnitude of astigmatism, type of astigmatism, anisoastigmatism and inter-ocular axis difference was analyzed respectively. Main Outcome Measures Inter-ocular axis difference. Results The median of the absolute inter-ocular axis difference from exact symmetry was 4° under the mirror symmetry model (mirror axes). Under the direct symmetry model (equal axes), the median of the absolute inter-ocular axis difference was significantly higher, at 8° (P<0.01). 6% and 0% had exact mirror and direct symmetry, respectively. In addition, 87% of cases with mirror symmetry model and 77% with direct symmetry model were within 15 degrees of exact symmetry. In direct and mirror symmetry model, the median of inter-ocular axis diffrence of with-the-rule(WTR)  astigmatism children were both the lowest, at 8° and 3°. In direct symmetry model, type of astigmatism and anisoastigmatism had a positive correlation with inter-ocular axis difference (R=0.42, P<0.01; R=0.22, P=0.01); while our data also shows a positive correlation between type of astigmatism (R=0.44, P<0.01), anisoastigmatism and inter-ocular axis difference(R=0.14, P=0.04), and a negative correlation between magnitude of astigmatism and inter-ocular axis difference in mirror symmetry model (R=-0.20, P=0.01). Conclusion The bilateral symmetry of the astigmatism axes was highly prevalent and mostly of the mirror pattern. Lower anisoastigmatism children and with-the-rule (WTR) astigmatism children showed better symmetry.

Key words: pre-school children；bilateral astigmatism；symmetry models of axes, symmetry patterns of axes