Different trajectories in the development of visual acuity with different levels of crowding: The Milan Eye Chart (MEC)

Eye charts are typically optimized to assess visual acuity (VA) with constant and controlled spacing, while close-to-acuity crowding limits letter identification in the normal fovea when adjacent letters are closely spaced. Here we developed a clinical tool that enables the assessment of acuity with different levels of crowding. In a cross-sectional study, we examined the developmental trajectories with our newly devised Milan Eye Chart (MEC). A total of 252 children of 1st, 3rd and 5th grade were assessed with the MEC using SLOAN letter optotypes with 100%, 50%, 25% and 12.5% inter-optotype spacing. Results show an interaction between spacing and grade. The performance to the 100% standard VA was not significantly different among grades, while the narrow spaced acuity (12.5% spacing) strongly improved with the grade. The different trajectories of acuity measured with high spaced and low spaced eye-charts suggest that the mechanisms able to reduce the crowding effects develops later than VA, and it is, at least in part, dissociated by the psychophysiological development of lower level visual mechanisms. The MEC charts are feasible and useful in assessing visual acuity with different level of crowding during the whole lifespan. The opportunity to assess crowding-limited acuity in early age is particularly relevant since it plays a significant role in amblyopia screening.