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Joseph L Demer, Robert A. Clark; Magnetic Resonance Imaging (MRI) Demonstrates Vergence Compensation of Strabismus By Differential Compartmental Mechanisms. Invest. Ophthalmol. Vis. Sci. 201657(12):.
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© 2017 Association for Research in Vision and Ophthalmology.
Differential compartmental function of extraocular muscles (EOMs) has been demonstrated by MRI during normal adduction, convergence, & vertical fusional vergence (VFV). We asked how vergence controls intermittent strabismus.
We studied 1 patient with 25° intermittent exotropia (IXT), and another with 3° intermittent hypertropia (IHT). Surface coil MRI was repeated in fusing & deviated conditions with 1 eye aligned, as confirmed by optic nerve position on MRI. Contractility, indicated by change in posterior partial volume (PPV), was analyzed in medial (equatorial insertion, torsion) & lateral (posterior insertion, vertical) SO compartments, superior & inferior horizontal rectus compartments, and medial & lateral inferior rectus (IR) compartments.
Distant target fusion in IXT was accomplished by 40% increase in PPV in the inferior compartment of the converging medial rectus (MRi), more than the 28% increase in the superior compartment, with uniform relaxation of both lateral rectus (LR) compartments. In the converging eye, PPV decreased 29% in the medial (torsional) SO compartment, without change in the lateral compartment. In the aligned eye, PPV decreased 20% in the lateral (vertical) SO, without change in the medial SO. There were no other significant PPV changes in other EOM compartments in either eye. In normal subjects, MRi has less contractility in adduction than superior compartment MRs in adduction, but similar in convergence.Near target fusion in IHT was accomplished in the infraducting eye by 11% PPV increase in the inferior rectus (IR) lateral compartment & 16% increase in the SO lateral compartment. In the aligned eye there was a 24% PPV decrease in the superior medial rectus (MR) compartment, and a 12% increase in the superior rectus. There were no other PPV changes in EOM compartments in either eye. This contractility is larger and different from normal VFV, where the LR superior compartment and IR medial compartment contract selectively in the infraducting eye.
Fusional vergence compensating for intermittent horizontal & vertical strabismus involves supranormal differential compartmental EOM patterns distinct from normal adduction, convergence, infraduction, & VFV (Demer and Clark, J. Neurophysiol. 113:2150-63, 2015). Surprisingly, SO participates differentially in compensation of both IHT & IXT, while MR does so in IHT.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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