Purpose: The aim of the study was to identify possible risk factors for new onset diplopia in 20° of primary position (NOD PP) after orbital decompression. A predisposition for NOD has been established for patients with pre-existing diplopia in secondary gaze; therefore, the authors focused on patients without preoperative diplopia.
Methods: Retrospective chart review of patients who underwent balanced orbital decompression between 2012 and 2019 due to Graves orbitopathy at the authors' institution. Exclusion criteria were incomplete clinical data set, revision surgery, and medial or lateral decompression only. The following clinical parameters were evaluated preoperatively and postoperatively: Hertel exophthalmometry, objective measurement of misalignment using the prism-cover-test, assessment of the field of binocular single vision, and measurement of monocular excursions. In addition, the diameter of the extraocular eye muscles was measured in all preoperative CT scans.
Results: We included 327 patients (612 orbits), 126 patients (242 orbits) had no preoperative diplopia. In patients with NOD PP (34%, n = 43/126), enlargement of the medial rectus muscle and restriction of abduction and elevation were significantly more frequent than in patients with no NOD PP. The degree of exophthalmos decrease positively correlated with postoperative squint angle.
Conclusion: We were able to identify the diameter of the medial rectus muscle, restriction of abduction, and elevation as well as an extensive reduction of exophthalmos as risk factors for NOD PP in patients with no preoperative diplopia.
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