Effect of Spectacle Centration on Stereoacuity
To determine the percentage of decentered spectacles ,the possible relationship of decentration of spectacles on stereoacuity,the difference between interpupillary distance (IPD) and optical centration distance (OCD), the relationship between decentration of spectacles and asthenopic symptoms, magnitude of prismatic effect and the direction of base of prism in decentered spectacles. Methods:A hospital based cross-sectional study conducted on the 100 students, with the age range of 18 to 26 years.Horizontal decentartion of spectacles was assessed by taking the difference between the interpupillary distance and the optical centration distance. Vertical decentration was assessed by difference between the optical centre of the spectacles and the centre of the pupils. Direction of decentration determines the prism base direction and stereoacuity was assessed by titmus fly test.
Results: All individuals were using decentered spectacles in both horizontal and vertical direction. Decentration was highest (33%) in the range of 0 – 1.49 (mm) horizontally in both eyes. Vertical decentration in the range of 3 – 5 (mm) was highest, 39% in right eye and 48% in the left eye of the individuals. 56% of the individuals have horizontal prismatic effect in their spectacles in right and left eye within the range of 0 – 0.49 prism diopters. Vertical prismatic effect was within range of 0 – 0.99 prism diopters in maximum number of individuals. Majority (76%) had base-in prisms induced in their spectacles in both right and left eyes. Vertically induced prism was in the base-up direction in 87% and 90% of the individuals in their right and left eyes respectively and 8% and 7% base-down prisms in their right and left eyes respectively. Difference between IPD (inter-pupillary distance) and OCD (optical centration distance) of individuals was quite significant with p-value 0.000. The mean difference between IPD and OCD was -3.57000. Mean IPD was less than the mean OCD. Horizontal prismatic effect in right eye caused decline in the stereo-acuity with p-value 0.019. Highest number of individuals (42) had reduction of stereo-acuity within the range of 20 – 100 minutes of arc having horizontal prismatic effect in the range of 0 – 1.49 in their right eyes.
Conclusions: Due to improper dispensing of the spectacles, prismatic effect is induced in the spectacles that shifts the image position formed on the retina and results in the reduction of the stereoacuity of the individuals.
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