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.

Prospective observational case-control series.

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.

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).

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.