Lateral rectus superior compartment palsy

Abstract

Purpose: To employ magnetic resonance imaging (MRI) to seek evidence of compartmental lateral rectus atrophy consistent with a lesion involving selective denervation of only 1 of the 2 neuromuscular compartments of the lateral rectus.

Design: Prospective observational case-control series.

Methods: At a single institution, surface coil coronal MRI was obtained at 312 μm resolution in quasi-coronal planes 2 mm thick throughout the orbit in 20 normal volunteers and 18 patients with unilateral lateral rectus palsy fixated monocularly on a target placed in central gaze. Maximum cross sections and posterior volumes of the superior and inferior lateral rectus compartments were computed and correlated with clinical findings.

Results: Twelve patients with lateral rectus palsy demonstrated symmetric, highly significant 40% reductions in maximum cross sections and 50% reductions in posterior volumes from normal for both compartments (P < 10(-6) for all comparisons). Six patients with lateral rectus palsy had similar significant but asymmetric reductions in those measures only for the superior compartment of the affected lateral rectus (P < 10(-4) for all comparisons), with insignificant 20%-30% reductions for the inferior compartment (P > 0.2 for all comparisons).

Conclusions: A subset of patients with clinical lateral rectus palsy may have palsy limited to the superior compartment. Paralytic esotropia may be caused by lateral rectus superior compartment palsy despite an intact lateral rectus inferior compartment. This finding is consistent with evidence supporting independent innervation of the 2 lateral rectus neuromuscular compartments.

FIGURE 1

Lateral rectus cross section. A computer program automatically calculated morphology of each cross section of the lateral rectus (LR) muscle belly. (Left) A line was best fit along the greatest dimension, and the image was then rotated to align this line with scanner vertical. (Right) The area was computed within the superior 40% and inferior 40% of the vertical extent of each rotated cross section, excluding the middle 20% because this region might contain mixed innervation from both the superior and inferior abducens nerve branches.

FIGURE 2

Quasi-coronal magnetic resonance images of normal and paretic (white asterisk) orbits. Patient 1 has a right lateral rectus complete palsy and Patient 16 has a right lateral rectus superior compartment palsy. In both patients, the paretic lateral rectus was obviously smaller than the uninvolved lateral rectus. SR = superior rectus muscle; SO = superior oblique muscle; MR = medial rectus muscle; IR = inferior rectus muscle; LR = lateral rectus; OD = right eye; ON = optic nerve; OS = left eye.

FIGURE 3

Magnetic resonance images of normal and completely paretic (white asterisk) lateral rectus muscles. All patients shown have unilateral complete lateral rectus atrophy. OD = right eye; OS = left eye.

FIGURE 4

Magnetic resonance images of normal and partially paretic (white asterisk) lateral rectus muscles. All patients shown have unilateral lateral rectus superior compartment atrophy. OD = right eye; OS = left eye.

FIGURE 5

Relative proportion of lateral rectus within superior vs inferior compartment. (Left) Maximum cross-sectional area. Controls and patients with complete lateral rectus palsy all had a less than 0.14 (14%) difference between superior vs inferior maximum cross sections, so data points for these muscles cluster to the upper left. The 6 affected muscles with superior compartment palsy (gray triangles) had inferior compartment maximum cross sections at least 0.16 (16%) greater than the inferior compartment maximum cross sections, so these data points clustered toward the lower right in the region demarcated by the gray lines. (Right) Posterior partial volume. Controls and patients with symmetric lateral rectus palsy all had less than 0.16 (16%) difference between the superior and inferior compartments. Six superior compartment lateral rectus palsies (gray triangles) had inferior compartment posterior partial volumes at least 0.16 (16%) greater than inferior compartment values, so that their data points clustered toward the bottom right of the graph in the region demarcated by the gray lines.