Binocular function changes produced in response to overnight orthokeratology

To analyze the binocular function changes produced on subjects undergoing overnight orthokeratology (OK) treatment over short-term (3 months) and long-term (3 years) wear. A prospective, longitudinal study on young adult subjects with low to moderate myopia was carried out. Binocular function was assessed by the following sequence of tests: Distance and near horizontal phoria (Von Graefe technique), distance and near horizontal vergence ranges (Risley rotary prisms), accommodative convergence/accommodation (AC/A) ratio (gradient method) and the near point of convergence (standard push-up technique). The short-term sample consisted of: 21 subjects in the control group, 26 in a corneal refractive therapy (CRT) treatment lenses group and 25 in a Seefree treatment lenses group. Those subjects were evaluated at baseline and at a 3-month follow-up visit. Twenty one subjects were old CRT wearers that attended a 3-year follow-up visit (long-term group). A statistically significant difference over the 3-month treatment was found for divergence at distance: the break point decreased 1.4 Delta (p = 0.0006) in the CRT group and the recovery point increased 1.2 Delta (p = 0.001) in the Seefree group. Also, the Seefree group had an exophoric trend of 2.3 Delta at near (p = 0.02) and a base-out break decrease of 2.3 Delta (p = 0.03). For the long-term group, only the base-out break point at distant vision showed a statistically significant difference of 4.9 Delta (p = 0.02). OK induces minimal changes in the binocular function for either short-term or long-term periods, apart from a near exophoric trend over the short-term period.