Crowding in children's visual acuity tests--effect of test design and age.

PURPOSE The aim of this study was to investigate the effect of test design (crowding) and age on visual acuity in a sample of young children. METHODS Vision was measured in 103 children aged between 4 and 9 years using five different visual acuity tests. The tests included three crowded tests: logMAR Crowded test, the Sonksen logMAR test, and the Crowded Kay Picture test, and two single optotype tests: the Single Kay Picture and Sheridan Gardiner tests. Tests were presented in a random order using standardized instructions and a defined end-point. Results were analyzed in two age groups, younger (4 to 6 years) and older (7 to 9 years). RESULTS In both groups, there was a significant main effect of test on acuity (younger: F = 63.92, dF = 4, p < 0.001; older: F = 63.59, dF = 4, p < 0.001). In the younger group, an effect of crowding was seen in all three crowded tests, but in the older group, an effect of crowding was seen only in the crowded letter tests. In both groups, mean acuity was lowest with the logMAR Crowded test, which has the closest interoptotype spacing (0.5), slightly higher with the Sonksen test (with interoptotype spacing of 1.0) and highest with the single optotype tests (no crowding). More crowding was seen in the younger children. CONCLUSIONS Our results show that maturation of line acuity is still taking place between the ages of 4 and 9 years. Measured acuity is affected by the amount of contour interaction induced by the type of optotype (letter or picture) and by the interoptotype separation. Another factor, probably a maturation of gaze control or selective attention is responsible for the reduction in crowding with age.