Functional anatomy of extraocular muscles during human vergence compensation of horizontal heterophoria.

We employed magnetic resonance imaging to quantify human extraocular muscle (EOM) contractility during intermittent convergent and divergent strabismus with each eye viewing monocularly at 20 cm compared with centered target fusion. Contractility, indicated by posterior partial volume change, was analyzed in transverse rectus and in medial and lateral superior oblique (SO) muscle compartments. In five subjects with intermittent esotropia, abduction of the deviated eye to monocular target fixation was associated with significant whole lateral rectus (LR) contraction, but with medial rectus (MR) relaxation that was significantly greater in the superior than inferior compartment. Esotropic eye abduction to binocular fusion was associated with similar relaxation in the two MR compartments, but with greater contraction in the LR's superior than inferior compartment. The whole diverging eye SO muscle relaxed. In three subjects with intermittent exotropia, converging eye fusional adduction was associated with significant whole LR relaxation and with MR contraction attributable to significantly greater contraction in the superior than inferior compartment. In adduction of the exotropic eye to monocular target fixation but not fusional adduction, the whole SO exhibited significant relaxation. Rectus pulley positions were not significantly altered by fusion of either form of intermittent strabismus. Globe rotational axis was eccentric in intermittent strabismus, rolling the eye so that rectus EOM lever arms facilitated vergence. These results confirm, and extend to fusion of intermittent horizontal strabismus, differential compartmental function in horizontal rectus EOMs and suggest a novel role for the SO in compensation of both intermittent esotropia and exotropia.