July 2018
Volume 59, Issue 9
Open Access
ARVO Annual Meeting Abstract  |   July 2018
Magnetic resonance imaging (MRI) demonstrates different patterns of horizontal extraocular muscle co-contraction in fusional divergence to near and distant targets
Author Affiliations & Notes
  • Joseph L Demer
    Ophthalmology, Stein Eye Inst, UCLA, Los Angeles, California, United States
  • Robert A Clark
    Ophthalmology, Stein Eye Inst, UCLA, Los Angeles, California, United States
  • Footnotes
    Commercial Relationships   Joseph Demer, None; Robert Clark, None
  • Footnotes
    Support  NIH Grant EY008313 and Unrestricted Grant from Research to Prevent Blindness
Investigative Ophthalmology & Visual Science July 2018, Vol.59, 1552. doi:
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      Joseph L Demer, Robert A Clark; Magnetic resonance imaging (MRI) demonstrates different patterns of horizontal extraocular muscle co-contraction in fusional divergence to near and distant targets. Invest. Ophthalmol. Vis. Sci. 2018;59(9):1552.

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      © ARVO (1962-2015); The Authors (2016-present)

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Abstract

Purpose : Divergence has lower amplitude than convergence and is critical to compensate esophoria, yet divergence muscle mechanisms are less studied than convergence. We employed MRI to quantify extraocular muscle contractility during prism-induced fusional divergence.

Methods : In 2 groups of adults, surface coil MRI was obtained in axial and coronal planes for each orbit during binocular fusion of a centered, accommodative target, and repeated during monocular viewing by each eye through a prism-inducing divergence. The target at 20cm was viewed through 8Δ and at 400cm through 4Δ base-in prism. Contractility, indicated by change in posterior partial volume (PPV), was analyzed by automated algorithm in the transverse halves of each rectus muscle. Contractility, indicated by change in posterior partial volume (PPV), was analyzed in medial (torsion) & lateral (vertical) superior oblique (SO) compartments and superior & inferior horizontal rectus compartments.

Results : At 20cm, 2.5±0.6° (SEM) diverging eye abduction in 10 subjects was associated with 4.2±1.5% whole lateral rectus (LR) PPV increase (P<0.05), with no asymmetry between inferior & superior LR compartments. During diverging eye abduction, there was 1.7±1.1% overall decrease in MR PPV attributable to 3.1% reduction in the MR superior compartment (P<0.025), without significant change (0.4±1.0%) in the inferior compartment, or in muscles of the non-prism viewing eye. In the medial SO compartment of the diverging, prism viewing eye there was 6.1±2.9 % PPV reduction, without significant change in lateral SO compartment.
At 400cm, 1.4±0.2° diverging eye abduction in 9 subjects was associated with PPV increase of 6.1±1.3% in whole LR (P<10-5) and 0.9±1.7% (P>0.5) increase in whole MR, without asymmetry between inferior & superior compartments or PPV change in muscles of the non-prism viewing eye. There was no contractile change in SO PPV of either eye.

Conclusions : Results confirm and extend to proximal target divergence the particularly active role of the superior MR compartment, yet show no MR role at all for far target divergence. Moreover, the failure of all or part of the MR to relax during divergence implies horizontal rectus co-contraction, consistent with motor neuron recordings in monkey (Miller et al, J. Neurophysiol. 105:2863, 2011) but undetected by direct force measurement.

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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