Masked bilateral superior oblique muscle paresis: A simple overcorrection phenomenon?

Objective: This study aimed to determine a mechanism by which the masked bilateral superior oblique muscle paresis phenomenon may be explained. Design: A retrospective study of the authors' patients with the preoperative diagnosis of a unilateral superior oblique muscle paresis was performed. Patients in whom an apparent contralateral superior oblique muscle paresis developed after surgery (masked bilateral superior oblique muscle paresis) were compared with those patients in whom this condition did not develop. Participants: One hundred eight patients participated. Results: Of the 108 patients studied, 30 (27.7%) patients had signs of an apparent superior oblique muscle paresis develop in the contralateral eye after surgery. In comparing those patients in whom an apparent contralateral superior oblique muscle paresis did develop after surgery with those patients in whom this finding did not develop, no significant differences were found in the age at surgery; etiology (traumatic vs. nontraumatic); average hyperdevations in primary gaze, ipsilateral and contralateral gazes, and ipsilateral and contralateral head tilts; average V pattern; inferior and superior oblique muscle function; extorsion on double Maddox rod testing; and objective fundus extorsion. Conclusion: Analysis of the authors' data showed that a surgical overcorrection of a unilateral superior oblique muscle paresis can masquerade as an apparent contralateral superior oblique muscle paresis. This is caused by a persistence of the head tilt and side gaze misalignment pattern from the original superior oblique muscle paresis.