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William R Bobier, Ian Erkelens; Motor adaptation of reflexive fusional vergence is impaired in Convergence Insufficiency. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1018. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Patients with convergence insufficiency (CI) exhibit reduced reflexive fusional convergence response dynamics and reduced tonic (phoria) adaptation to crossed disparities. Reflexive vergence, like saccades, exhibits a rapid adaptive response to double-step paradigms; however, tonic and reflexive adaptations are likely rooted in different neural pathways. The effects of CI on the latter form of oculomotor adaptation in vergence is currently unknown. We contrast the adaptive capacities of both reflexive fusional convergence and divergence in patients with CI to aged match controls.
10 adults with CI (26±3.8y/o) and 10 aged-matched controls completed the study. At 2 separate visits, reflexive convergence or divergence was measured to a 2° disparity step and then adaptively lengthened via a repeated double-step disparity stimulus (2° ± 1.5°, 175ms inter-step delay). Stimuli were presented dichoptically from 40cm. Eye movements were monitored binocularly at 250Hz using video-based infrared oculography.
The results confirm that reflexive convergence responses are significantly slower in CI; mean and SE are given (CI: 7.4°/s ± 3 vs Control: 12.0°/s ± 2, p<0.001). Baseline reflexive divergence response velocities were similar between groups (CI: 8.3°/s ± 2 vs Control: 7.5°/s ± 3, p= 0.81). Importantly, individuals with CI exhibited a limited capacity to adaptively lengthen their reflexive convergence response gain (CI: -11.1% ±22 vs Control: 30.1% ±11, p<0.001). Reflexive divergence gain lengthening was similar between groups (CI: 9.1% ±21 vs Control: 10.3% ±9, p>0.99) and significantly less robust when compared to the degree of convergence gain adaptation in controls (p<0.01)
The condition of CI leads to weakened vergence adaptive capacities. This now includes reflexive adaptive lengthening as well as tonic vergence adaptation. Symptomology for CI is then expected to reflect more than a reduced ability to converge.The results support the hypothesis that the adaptive capacities of vergence are related to the strength of the underlying reflexive fusional response mechanisms and that vergence control is directionally asymmetric. The evidence suggests the neurophysiological under-pinning of CI is rooted in an underdeveloped or perturbated reflexive fusional vergence mechanism.
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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