Abstract
INTRODUCTION: Distance heterophoria has been reported to be different from tonic vergence due to the divergence accommodation. Meanwhile, near heterophoria varies with the control of accommodation and the accommodation has been closely related to heterophoria measurement. OBJECTIVES: The purposes of this study are to determine the viewing distance with minimum heterophoria and its relationship with refractive error and the resting position of accommodation. METHODOLOGY: Thirty-six optometry students (25 emmetropes and 11 myopes) were tested. Heterophoria was measured with the Free-Space Heterophoria Card at five different viewing distances (25cm, 33cm, 50cm, 100cm, 300cm). The dioptric viewing distance with minimum heterophoria was estimated from the graph where heterophoria in prism diopters was plotted against viewing distance in centimeters. The Canon R-1 autorefractor was used to determine the accommodation response at six different viewing distances (25cm, 33cm, 50cm, 100cm, 300cm, 600cm). The resting position of accommodation was estimated from the graph where the accommodative stimulus in diopters was plotted against the accommodative response in diopters. RESULTS: The dioptric viewing distance with minimum heterophoria ranges from 0.003D to 0.65D in emmetropes and ranges from 0.03D to 2.36D in myopes. There is no difference between myopes and emmetropes. Our results show a weak correlation between the dioptric viewing distance with minimum heterophoria and the resting position of accommodation. CONCLUSIONS: The viewing distance with minimum heterophoria is not affected by refractive error (stable early-onset myopia) and is poorly correlated with the resting position of accommodation.