A case of highly myopic strabismus with large exotropia after Yokoyama procedure

PURPOSE: Highly myopic strabismus (heavy eye syndrome) is characterized by superotemporal prolapse of the elongated globe, resulting in nasal displacement of the superior rectus (SR) muscle and inferior displacement of the lateral rectus (LR) muscle. This abnormal extraocular muscle configuration can result in restrictive esotropia, limitation of abduction, and progressive binocular diplopia. The Yokoyama procedure restores the extraocular muscle paths by uniting the SR and LR muscle bellies and generally provides excellent ocular alignment. Persistent postoperative exotropia is uncommon and its management has rarely been described. We report a case of persistent large-angle exotropia following an uncomplicated bilateral Yokoyama procedure despite satisfactory postoperative muscle repositioning on orbital magnetic resonance imaging (MRI). METHODS: A 72-year-old woman with high myopia and progressive horizontal diplopia underwent ophthalmologic and strabismus evaluation, including orbital MRI before and after bilateral Yokoyama procedures. Ocular alignment, axial length, and SR-LR displacement angles were assessed. Following development of persistent postoperative exotropia, the patient was observed for six months. Because the deviation did not improve spontaneously and postoperative MRI demonstrated satisfactory SR and LR repositioning, bilateral medial rectus (MR) plication was subsequently performed while preserving the SR-LR union. RESULTS: Preoperatively, the patient had 35 prism diopters (PD) of esotropia at distance with limitation of bilateral abduction and elevation. Axial lengths were 27.19 mm in the right eye and 26.91 mm in the left eye. Orbital MRI demonstrated nasal displacement of the SR and inferior displacement of the LR, with SR-LR displacement angles of 142° and 144°, respectively. Following bilateral Yokoyama procedures, approximately 55 PD of exotropia developed on the first postoperative day and persisted with little spontaneous improvement over three months. Repeat MRI demonstrated restoration of the normal SR-LR relationship, with displacement angles improving to 97° and 95°, confirming successful anatomical correction despite persistent exotropia. After six months of observation, bilateral 7-mm MR plication was performed. One month later, ocular alignment improved to near orthotropia, with resolution of diplopia and recovery of stereopsis. CONCLUSION: Persistent large-angle exotropia may occur despite successful anatomical correction following the Yokoyama procedure. Postoperative orbital MRI can help distinguish persistent muscle malposition from successful SR and LR repositioning. When satisfactory muscle positioning is confirmed and spontaneous improvement does not occur, preserving the SR-LR union and addressing the residual horizontal deviation with MR plication may provide an effective secondary surgical strategy. This case highlights the value of postoperative imaging in guiding management of unexpected overcorrection after the Yokoyama procedure.