Rectus Extraocular Muscle Size and Pulley Location in Concomitant and Pattern Exotropia

Abstract

Purpose To determine if rectus extraocular muscle (EOM) sizes and pulley locations contribute to exotropia, we used magnetic resonance imaging (MRI) to measure these factors in normal control subjects, and subjects with concomitant and pattern exotropia. Design Prospective, case control study. Participants Nine subjects with concomitant exotropia, 6 patients with pattern exotropia, and 21 orthotropic normal controls. Methods High-resolution, surface coil MRI was obtained in contiguous, quasi-coronal planes. Rectus pulley locations were determined in oculocentric coordinates for central gaze, supraduction, and infraduction. Cross sections in four contiguous image planes were summed and multiplied by the 2-mm slice thickness to obtain horizontal rectus posterior partial volumes (PPVs). Main outcome measures Rectus pulley locations, and horizontal rectus PPVs. Results Rectus pulleys were differently located in subjects with A, versus V and Y, pattern exotropia. The LR pulleys were significantly displaced superiorly, the MR pulleys were displaced inferiorly, and the IR pulleys were displaced laterally in A pattern exotropia. In opposite fashion, the array of all rectus pulleys was excyclorotated in V and Y pattern exotropia. The PPV of the medial rectus muscle was statistically subnormal by about 29% in concomitant but not pattern exotropia (P<0.05). The ratio of the PPV of the LR relative to the MR in concomitant exotropia was significantly greater than in normal subjects and pattern exotropia (P<0.05). Conclusions Abnormalities of EOMs and pulleys contribute differently in pattern versus concomitant exotropia. Abnormal rectus pulley locations derange EOM pulling directions that contribute to pattern exotropia, but in concomitant exotropia, pulley locations are normal and relatively small medial rectus size reduces relative adducting force.

Neural mechanisms of oculomotor abnormalities in the infantile strabismus syndrome

“…A subsequent MRI study showed that these pulleys could be mislocated in some patients with pattern strabismus (Clark et al 1998). A more recent MRI study showed that rectus muscle pulleys are misplaced in human subjects with pattern exotropia, but not in patients with concomitant exotropia (Hao et al 2016). Another recent MRI study showed that rectus muscle pulleys can also be misplaced in patients with superior oblique palsy (Suh et al 2016).…”

“…As noted early in this review, pattern strabismus is often clinically described in terms of oblique muscle dysfunction. MRI studies of rectus muscle pulleys (Hao et al 2016; demonstrate that other peripheral factors can contribute to incomitance. However, the neurophysiological evidence discussed above indicates that pattern strabismus is also associated with fundamental abnormalities in the neural signals controlling eye movement.…”

Normal correspondence of tectal maps for saccadic eye movements in strabismus

“…It is debated whether incomitant strabismus patterns are due to orbital mechanical forces, such as oblique muscle dysfunction, altered rectus muscle action, or displaced pulleys (Ghasia and Shaikh 2013;Ghasia et al 2015;Hao et al 2016;Kushner 2010;Narasimhan et al 2007;Oh et al 2002). The surgery performed to induce strabismus in monkeys 2 and 3 caused changes in eye muscle mechanics that contributed to the incomitance of their ocular deviation.…”

ACR Appropriateness Criteria® Orbits Vision and Visual Loss

“…MRI of the orbits without and with contrast is preferred [67] if ophthalmoplegia is felt to be related to a primary disease process within the orbit affecting the extraocular muscles or if there is history of trauma, enophthalmos, proptosis, orbital inflammation, or chemosis. An MRI of the orbits with the globes imaged during different gaze positions may aid in identifying a potential muscular slip or pulley abnormality [68].…”