Clinical characteristics of the five groups of children examined.

Mean values of: binocular vision (Stereoacuity test, TNO measured in seconds of arc); near point of convergence, NPC measured in cm; Heterophoria at near distance measured in prism diopters; negative values indicate exophoria and positive values indicate esophoria; Vergence fusional amplitudes (divergence and convergence) at near distance measured in prism diopters.