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Apoorva Karsolia, Emily Burns, Mythri Pullela, Vallabh Das; Longitudinal evaluation of ocular alignment in normal and prism-reared infant monkeys. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1556.
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Rearing infant monkeys under prism-viewing conditions during the critical period of development disrupts binocular vision and induces strabismus. This study investigates the longitudinal change in horizontal and vertical ocular alignment in normal and prism-reared infant monkeys.
We examined ocular alignment in six infant monkeys reared under prism-viewing (right eye: 20PD base-down; left eye: 20PD base-in) starting from day one after birth, and two infant monkeys reared with normal visual experience. Photographs were taken bi-weekly using a Nikon D300 camera with an attached ring light placed at a distance of 60 cm from the animal in primary gaze. ImageJ software was used to identify the pupil center (PC) and the first Purkinje image (PI) in each photograph. Horizontal and vertical position of each eye was calculated by taking the difference between PC and PI in mm and converting to degrees using a published Hirschberg ratio of 14°/mm. Strabismus angle was calculated as the difference in position of the left and right eyes. A correction of 5° was incorporated to compensate for angle kappa.
At birth, all monkeys presented with small exotropia (XT) of <5°. At 3 weeks after birth, horizontal alignment was significantly different between normal and strabismic monkeys (NM: mean 1.2° esotropia (ET); SM: mean 10.3° XT, range 2.1° to 17.7° XT; Mann-Whitney p<=0.001), but no significant difference was seen in vertical alignment (NM: mean 1.8°; SM: mean 2.4°, range 1.3 to 5.2°; p>0.05). A gradual change in alignment was seen in all strabismic monkeys stabilizing at 10 weeks, at which time five monkeys were exotropic (mean 18.2° XT, range 12.5° to 23.9° XT) and one monkey was esotropic (6.5° ET). Horizontal alignment was significantly different from the normal monkeys (NM: mean 1.6° ET; p<=0.001). Vertical alignment in strabismic monkeys was small but significantly different from the normal monkeys (NM: mean 0.6°; SM: mean 3.4°, range 0.2 to 8.1°; p<=0.001).
Prism-rearing disrupts binocular fusion mechanisms and strabismus is seen to develop as early as 3 weeks of age in prism-reared monkeys, equivalent to about 3 months in humans. Strabismus angle was stable by about 10 weeks after birth. The time course of change in alignment overlaps with disruption in various visual sensory functions suggesting a causal temporal link between sensory and motor mechanisms for alignment.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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