"Inverted Brown pattern": A tight inferior oblique muscle masquerading as a superior oblique muscle underaction-clinical characteristics and surgical management

Purpose: To characterize, and evaluate the surgical management of, patients with unilateral deficiency of depression in adduction, suggesting superior oblique muscle underaction, without significant ipsilateral inferior oblique muscle overaction. Methods: Such patients were identified who also had received either ipsilateral inferior oblique (IO) muscle weakening or contralateral inferior rectus muscle recession. Their histories, motility patterns, intraoperative findings, types of strabismus surgery, and postoperative results were analyzed. Results: Twelve patients were identified with unilateral deficiency of depression in adduction, with no or minimal ipsilateral IO muscle overaction. Three of these patients (25%) had previously had surgery for Brown syndrome. Four (33%) had prior orbital floor trauma. On exaggerated forced duction testing recorded for nine patients, a tight IO muscle was recorded in 78%, with no laxity of the superior oblique tendon. Four patients (33%) underwent contralateral inferior rectus muscle recession, but in all four the deficiency of depression in adduction recurred. The other eight (67%) had an IO muscle weakening procedure and achieved overall improvement of ocular alignment. Nine subsequent patients with a similar pattern of misalignment were each managed with an IO weakening procedure, with good results. Conclusions: This motility pattern, which we are calling an "inverted Brown pattern," is caused by a tight or inelastic IO muscle. In such cases, IO muscle weakening yields better results than contralateral inferior rectus muscle recession, even though there is no significant IO muscle overaction preoperatively.