The comments of Pandey and associates regarding our novel description of superior lateral rectus (LR) compartmental palsy highlight the clinical importance of extraocular muscle (EOM) compartmentalization. Our histologic finding in human, monkey, cow, and rabbit of robust segregation of the LR and medial rectus (MR) muscles into superior and inferior neuromuscular compartments is remarkable and represents a fundamental anatomic feature of EOMs only recently recognized, but also extended to the inferior rectus and inferior and superior oblique (SO) muscles (Le A, Demer JL, Poukens V. Compartmental innervation scheme for the mammalian superior oblique (SO) and inferior oblique (IO) muscles. Program No. 62.22. Neuroscience Meeting Planner. Washington, DC: Society for Neuroscience, 2014. Online.). There is now magnetic resonance imaging demonstrating differential function in normal humans: for the MR during convergence, for the LR during ocular counter-rolling, and for the LR, inferior rectus, and SO during vertical fusional vergence (VFV). The unexpected robust differential participation of the LR superior compartment, and paradoxical compartmental role of the SO, in VFV are noteworthy. These findings challenge many longstanding assumptions about how EOMs function in health and disease.

Pandey and associates theorize that misdirection of compartmental innervation might create misinnervation within a single LR, causing some features analogous to Duane syndrome; such pathology remains undemonstrated. The typical intracavernous division and subsequent reunion of the abducens nerve into widely dispersed, parallel branches offers ample opportunity for pathology in that location to selectively target the superior division.

The surprising differential compartmental behavior of the LR makes it difficult to predict the effect of superior compartment palsy on A or V pattern incomitance. However, the fundus extorsion we observed in LR superior compartment palsy is consistent with known differential compartmental activity of the LR during incycloduction induced by head tilt and the effect of inferior transposition of the LR insertion. In our experience, subjective binocular torsion seldom correlates with objective cycloposition, especially in the setting of concurrent large-angle esotropia.

Of course, concurrent SO palsy and other pathologies complicated the clinical presentation of the resulting strabismus in the cases of Subjects 3, 5, 6, 10, and 11. The purpose of the paper was to demonstrate anatomically the existence of superior compartment LR palsy as a clinical entity. We do not imagine that clinical motility findings could suffice to diagnose LR superior compartment palsy in the setting of concurrent cranial nerve palsies or skew; orbital imaging is necessary.

Some clinical features of superior compartment LR palsy probably do overlap with those of complete palsy. Possible reasons for overlap could include secondary factors, such as MR contracture, that remain to be clarified for all variants of LR palsy.

With the exception of neuromuscular disorders and mitochondrial myopathies not included in this study, EOM bulk correlates well with strength, and changes in cross section correlate with function. EOM denervation is rapidly followed by atrophy.

Pandey and associates implied a need for the fellow eye as a control. Tables 1 and 2 included this essential comparison, since the reported atrophy was relative to the fellow eye, but in addition we included comparison with healthy volunteers.

Future studies of LR palsy should distinguish the compartmental from the complete variants.