Participants will be randomly assigned to treatment with a BFL or SVL spectacle correction for 3 years. Generally, participants will be followed at 3-month intervals for the first year and every 6 months thereafter until the primary outcome visit at 36 months. At each follow-up visit between 3 months and 36 months, ocular alignment and stereoacuity will be assessed to determine if any of the three study-specified failure criteria have been met (worsening of distance esotropia (ET), reduction in near stereoacuity, or frequent diplopia (more than 2 times per day over the last week). The primary outcome is failure at or before 36 months.~If a failure criterion is met between 3 to 30 months, participants randomized to SVLs will be prescribed BFLs and those in BFLs will be prescribed continued BFLs. Participants in both groups will return in BFLs 2 months after failure for a Post-failure Secondary Outcome Visit to determine the child's binocular function, after which the child will be released to treatment at investigator discretion. Participants with confirmed failure who complete the Post-failure Secondary Outcome Exam will return for the 12- and 24-month follow-up visits (abbreviated testing) as well as the 38-month Secondary Outcome Visit (they will not return for the 36-month Primary Outcome visit).~If a failure criterion is not met between 3 to 30 months, participants without confirmed failure will complete the 36-month Primary Outcome Visit, After the 36-month visit, SVL group participants will be prescribed BFLs and BFL participants will continue using BFLs.~All participants (i.e., regardless of failure status) will return for a 38-month Secondary Outcome Visit to assess binocular function.
Compare the proportion of BFL and SVL participants with treatment failure by 36 months, compare binocular function test scores between the BFL and SVL groups with both groups in BFL at 38 months after randomization (or 2 months after treatment failure), and evaluate treatment failure by 36 months according to baseline factor subgroups of: duration of constant esotropia pre-enrollment, presence of near stereoacuity on the Randot Preschool Stereoacuity test, in-office response of near alignment with +3.00 D lenses, and gradient AC/A ratio