MRI identifies 2 distinct types of superior oblique palsy

This is the first study to investigate the shape of the atrophic superior oblique (SO) belly using high-resolution MRI, revealing that MRI may be more reliable than conventional clinical methods for distinguishing actual SO palsy from masquerading conditions.

Comparing MRI images from patients with SO atrophy and healthy controls, the authors show 2 distinctive morphologies that correlate with different clinical features:

• Anisotropic, elongate atrophy
• Isotropic, round atrophy

Subjects exhibiting anisotropic atrophy had greater hypertropia in infraversion than central gaze, as well as greater excyclotorsion, than did subjects exhibiting isotropic atrophy.

Applying this new anatomic understanding in the context of compartmentalization, the authors conclude that the elongate, oval-shaped SO belly in the anisotropic atrophy group is consistent with palsy of only 1 compartment, while the round, uniform SO belly in the isotropic atrophy group is consistent with palsy of both compartments.

The anisotropic atrophy group would presumably maintain function in part of the SO. However, the authors could not differentiate medial compartment palsy from lateral compartment palsy based on SO imaging using current MRI techniques. The authors propose that patients with anisotropic atrophy could include both cases of selective medial and other cases of selective lateral compartment palsy. The clinical presentation of the resulting group would be mixed, unless 1 compartment were selectively vulnerable to pathology.

While previous authors have described various types of superior oblique palsy, classifying the condition based on the shape of the superior oblique cross section is novel. The authors call for additional study to determine if these 2 morphologic patterns of SO palsy respond differently to surgical treatment.