Identifying masked superior oblique tightness in thyroid eye disease to avoid postoperative A-pattern exotropia and intorsion

Purpose: To report masked superior oblique muscle tightness as a possible mechanism causing A-pattern exotropia with intorsion after inferior rectus muscle recession in the context of thyroid eye disease. Methods: Three patients with thyroid eye disease and involvement of the superior oblique muscle are presented, along with a fourth comparison case without superior oblique muscle involvement. Intraoperative torsion assessment and exaggerated traction testing were performed after detachment of the involved rectus muscles. A surgical procedure involving recession of tight superior oblique muscle(s) when recessing inferior rectus muscle(s) is presented, along with surgical results. Results: The first case illustrated the problem of A-pattern exotropia and intorsion after inferior rectus muscle recessions and subsequent treatment with superior oblique tendon recessions. Patients 2 and 3 demonstrated signs of coexisting inferior rectus muscle involvement and superior oblique muscle involvement both preoperatively and intraoperatively, with a tight superior oblique muscle and marked intorsion, suggesting the need for superior oblique tendon recession at the time of inferior rectus recession. Postoperatively there was no symptomatic intorsion or A-pattern exotropia and both patients were heterophoric distance and near with only rare diplopia. The fourth case, without superior oblique involvement, illustrated management with inferior rectus muscle recessions alone. Conclusions: Superior oblique muscle involvement may be masked by coexistent inferior rectus muscle involvement and if not identified and addressed at the time of the first surgery may result in symptomatic intorsion and A-pattern exotropia. The clinical finding of minimal extorsion, or frank intorsion, in the presence of a tight inferior rectus muscle, may be an important sign of masked superior oblique muscle tightness. Intraoperative assessment of torsion and superior oblique tension may also help identify patients at risk. Superior oblique tendon recession, at the time of inferior rectus muscle recession, prevented development of a postoperative A-pattern exotropia and intorsion.