Lea symbol charts are useful in visual acuity assessment in children, but different test designs can lead to discrepancies in measured visual acuity as a result of differential effects of crowding, a new study finds.

The study compared habitual visual acuity in a sample of young children using two versions of the single Lea symbol charts with different crowding features.

Researchers measured monocular habitual visual acuity in a sample of 77 young children ages four to six using crowded Lea symbol charts with either flanking bars separated from the central symbol by 0.5 optotype width or flanking Lea optotypes separated from the central symbol by 1.0 optotype width.

Lea symbols with flanking optotypes resulted in higher visual acuity than the Lea symbols with flanking bars, believed to be a result of differences in the crowding effect. The logarithm of the minimum angle of resolution measured using the two chart versions with different flankers and flanker-target separations differed, on average, by a small amount of about 1.5 optotypes. The study notes that this difference is unlikely to be clinically significant.

Researchers expected their results to show the opposite effect, where the Lea symbols with flanking optotypes resulted in lower acuity values, not higher than the Lea symbols with flanking bars. 

The study concludes that flanker-target separation may be more important in determining the amount of crowding and may override the effect of flanker type when using single flanked optotypes for testing visual acuity in children. Still, the researchers recommend using the Lea symbols with flanking bars because of the closer flanker-target separation.