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Mehmet Naci Agaoglu, Myhtri Pullela, Anand C Joshi, Sevda Agaoglu, David K Coats, Vallabh E Das; Surgical correction of strabismus in monkeys: III. Longitudinal evaluation of neuronal responses in the Abducens nucleus. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):5222. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Strabismus correction surgery is well documented in both the literature and in practice with varying levels of success and permanence. Potentially, muscle remodeling and/or central neural adaptation affects the final state of misalignment after treatment. Our goal was to assess central adaptation by examining Abducens neuron (ABN) responses in strabismic monkeys following surgical correction.
The study included one rhesus monkey with an exotropia (strabismus angle: OD: ~30°, OS: ~15°) that was induced in infancy using an optical prism-viewing paradigm. Surgical treatment when animal was ~6years old involved recession of the lateral rectus (LR) and resection of the medial rectus (MR) of the left eye only. We recorded from 75 ABNs prior to treatment and from 92 ABNs over the first 6 months following treatment. ABN firing rates (FR) and horizontal eye position and velocity acquired during a horizontal smooth pursuit task (0.3Hz, ±15°) were used to identify regression coefficients in a first-order model (FR = K*Epos + R*Evel + C). K and C coefficients were then used to compute the population LR neuronal drive (ND) necessary to produce static deviation of the non-fixating eye before surgery (pre), <1 month after surgery (post1), ~6 months after surgery (post6).
Strabismus angle (SA) was reduced by ~35% at post1. SA during OS view gradually increased back to its pre-surgery value while SA during OD view was still reduced by ~28% of its pre-surgery value at post6. Analysis of Left ABN cells showed that the ND to the LR of the treated left eye did not change at post1 although SA was reduced significantly (pre: 177±81 sp/s, post1: 173±79 sp/s). Analysis of Right ABN cells indicated that the ND to the LR of the untreated right eye was reduced at post1 (pre: 115±78 sp/s, post1: 80±57). At post6, the ND from Left ABN showed a significant drop (138±47 sp/s); the ND from Right ABN reverted to pre-surgery levels (107±26 sp/s).
The unchanged ND to the treated eye immediately after surgery suggests that alterations in muscle strength of the treated eye determined the improvement in strabismus angle; post1 reduction in ND to the untreated eye simply reflects Hering’s law. The post6 changes in the NDs to both treated and untreated eyes suggest a significant role of neural adaptation in addition to muscle remodeling in setting the steady-state strabismus angle.
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