Observation Versus Occlusion Therapy for Intermittent Exotropia

Inclusion Criteria: * Age 12 months to \< 11 years * Intermittent exotropia (manifest deviation) meeting all of the following criteria: * Intermittent exotropia at distance OR constant exotropia at distance and either intermittent exotropia or exophoria at near * Exodeviation at least 15PD at distance OR near measured by prism and alternate cover test (PACT) * Exodeviation at least 10PD at distance measured by PACT * No previous surgical or non-surgical treatment for IXT (other than refractive correction) * Visual acuity in the worse eye at least 0.3 logMAR (20/40 on ATS HOTV or 70 letters on E-ETDRS) for children ≥ 3 years of age * No interocular difference of visual acuity more than 0.2 logMAR (2 lines on ATS HOTV or 10 letters on E-ETDRS) for children ≥ 3 years of age * No hyperopia greater than +3.50 D spherical equivalent in either eye * No myopia greater than -6.00 D spherical equivalent in either eye * No prior strabismus, intraocular, or refractive surgery * No abnormality of the cornea, lens, or central retina * Investigator willing to observe the IXT untreated for 3 years unless specific criteria for deterioration are met Exclusion Criteria: * Pure phoria at both distance and near * Prior non-surgical treatment for IXT other than refractive correction (e.g., vergence therapy, occlusion, vision therapy/orthoptics, or deliberate over-minus with spectacles more than 0.50D) * Previous amblyopia treatment other than refractive correction within 1 year * Vision …