Evaluation Of Heterophoria After Sustained Near Work Under Varying Illuminances.

Background: Since reading and studying are major activities in the life of an optometry student, it would be relevant to explore the mechanism through which reading under
different levels of illumination causes fatigue. Heterophoria is the latent deviation of the eyes from the orthovergence, straight-ahead, position. Determining the level of illumination that causes a dissociated heterophoria to increase the least may mean that that specific level of illumination is best to use for reading and decreasing symptoms of asthenopia after prolonged near work. Methods: This controlled experiment examined whether eye fatigue after completion of a near task under different levels of illumination caused different increases in the
magnitude of a patient's dissociated heterophoria. The amount of heterophoria present in the 30 patients, aged 22 - 35 years, was measured with the modified Thorington phoria test. Results: The study demonstrated no consistent or predictable change in a subject's heterophoric posturewhen reading under varying illuminance levels. Conclusion: With the tested subject population, change in heterophoria does not seem to be the mechanism by which asthenopia occurs.