Evaluating the Effect of Retinal Defocus (Blur) on Lateral Heterophoria
Purpose: This study was conducted to ascertain the effect of retinal defocusing or retinal blur on lateral heterophoria at far and near.
Methods: This was a quasi-experimental study involving 57 participants aged 5 to 35 years, who were selected based on the inclusion criteria of being myopes and emmetropes who had a normal general and ocular health. The Von Graefe technique was used for measuring phoria under standard testing conditions.
Results: The mean baseline lateral phoria for emmetropes were 0.53 ± 1.14∆ exophoria and 2.47 ± 2.08∆ exophoria at far and near respectively, while after defocusing, it was 2.80 ± 0.88∆ exophoria and 4.34 ± 1.70∆ exophoria at far and near respectively. For myopes, the baseline lateral phoria was 1.52 ± 1.09∆ exophoria and 4.76 ± 1.13∆ exophoria at far and near respectively while after defocussing, it was 3.39 ± 0.92∆ exophoria at far and 7.60 ± 1.00∆ exophoria at near. The mean difference in lateral phoria was found to be significant at the 95% confidence level (P = 0.000). There was no statistically significant difference in lateral phoria change at far (P = 0.072), while the near phoria difference was significant (P = 0.002).
Conclusion: Gas Retinal defocusing with +2.50D resulted in a shift towards exophoria at both far and near, with myopes experiencing more exophoric shift. This is because they are less sensitive to the presence of blur. This will enable practitioners to be cautious of the inhibition of accommodation due to higher exophoria at near when prescribing lenses.
Keywords: Retinal Defocus, Myopia, Emmetropia, Near lateral phoria, Distance lateral phoria.
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