Trial of Overminus Spectacle Therapy for Intermittent Exotropia

The main objectives of this randomized trial comparing overminus lens treatment to non-overminus (spectacles without overminus or spectacles with plano lenses) are to determine: * The long-term on-treatment effect of overminus treatment on distance IXT control score. * The off-treatment effect of overminus treatment on distance IXT control score (following weaning and 3 months off treatment). The recently completed IXT3 pilot study (NCT02223650) addressed the question of whether overminus lens therapy has an initial short-term therapeutic effect for IXT while wearing overminus spectacles. There have been no rigorous studies that address the following important questions related to overminus lens therapy: * Does overminus lens therapy have a long-term therapeutic effect for IXT while wearing overminus spectacles (over many months or years)? * Does overminus lens therapy have a long-term therapeutic effect for IXT when overminus spectacles are discontinued? In November 2019, a protocol amendment discontinued overminus lens treatment and extended the study for an additional 18 months after the 18-month randomized trial has ended. The objective of the extension study is to compare long-term refractive error between subjects originally treated with either overminus spectacles or non-overminus spectacles as part of the 18-month randomized trial; treatment is at investigator discretion.