Magnetic Resonance Imaging (MRI) of Differential Compartmental Function of Extraocular Muscles (EOMs) During Vertical Fusional Vergence

Abstract

 

Purpose

 

Differential compartmental activation of horizontal rectus EOMs has been suggested as a possible vertical vergence mechanism. We used MRI to evaluate EOM compartmental function during vertical fusional vergence.

 

Methods

 

Surface coil MRI of the orbits was performed at 312 mm pixel resolution in 2 mm thick quasi-coronal planes in 6 normal young adults who binocularly fused a target affixed to the scanner bore. MRI was repeated for each orbit with and without a 2 PD (1°) base up prism over each eye monocularly. Posterior partial volumes (PPV) were determined as indices of contractility for medial and lateral compartments of vertical rectus EOMs, for superior and inferior compartments of horizontal rectus EOMs, and for medial and lateral compartments of superior oblique (SO).

 

Results

 

There was no significant PPV change for either compartment of superior rectus in prism viewing or fellow eyes. There was a large and similar PPV increase in both inferior rectus (IR) compartments in the prism viewing orbit, (analyzed as IRl and IRm) and corresponding decrease in the fellow orbit as appropriate to the vertical vergence. There was no PPV change in either medial rectus compartment (analyzed as MRi and MRs) in the prism viewing or fellow eye. However, there was a large and similar PPV increase in the lateral rectus (LR) superior (LRs) compartment in both the prism viewing and fellow orbits (P<0.01), compared with an insignificant change in LR inferior (LRi) compartments. There was a greater PPV reduction in the medial than lateral SO of the prism viewing than fellow orbit, and a greater PPV increase in the lateral than medial SO in the fellow orbit (P<0.05).

 

Conclusions

 

Differential compartmental contractility of horizontal rectus EOMs does not implement the vertical component of vertical fusional vergence, which is mainly accomplished by the IR without much contribution from SR. While selective bilateral contractility of LRs and bilateral differential contractility of both SO compartments are antagonistic to the vertical component of vertical fusional vergence, these behaviors are appropriate to mediate the torsional component.

 

 

Both the lateral (IRl) and medial (IRm) inferior rectus sides contracted similarly in the orbit viewing through base up prism.

 

 

Superior LR compartment LRs contracted significantly more than the inferior compartment LRi in prism viewing orbit.

 

Keywords: 521 extraocular muscles: structure • 751 vergence • 522 eye movements