Inferior Oblique Muscle "Overaction" Caused by Inferior Oblique Muscle Shortening, Not by Hypertonicity

Purpose: To determine whether fusional vergence adaptation in patients who can fuse in at least some gaze positions can cause curvature of the non-fixing eye movement paths of patients with apparent "overaction" or "underaction" of the oblique muscles, yielding possibly erroneous evidence of hypertonicity as the cause of the overaction or underaction. Methods: The authors retrospectively studied Lancaster red-green plots of patients with oblique muscle "overaction/underaction." If fusion was present, the plot had usually been repeated after monocular occlusion for at least 30 minutes. Fundus torsion and fusion status were also recorded. Results: After a patch test in patients displaying fusion, the non-fixing eye's movement path became more linear. Conclusions: Although it has been argued that true overaction of the oblique muscles would show curved eye movement paths on side gazes, in the study patients displaying fusion in at least some directions of gaze, the eye movement paths became more linear after patch testing, favoring the more mechanical explanation. Illustrated cases were consistent with the hypothesis that short inferior oblique muscles simply hold the globes in extorted positions, and the appearance of inferior oblique muscle "overaction" arises from the eyes' following their extorted movement paths on side gazes, not from hypertonicity of the inferior oblique muscle in the adducting eye.