Incidence of A pattern strabismus after inferior rectus recession in patients with Graves' orbitopathy: A retrospective multicentre study

Introduction and purpose: Patients with Graves' orbitopathy (GO) may develop restricted elevation; this can lead to hypotropia, sometimes in combination with an abnormal head posture. Recession of one or both inferior rectus muscles is the first line surgery to restore eye motility in these patients. However, this may result in A pattern strabismus. This study was performed to determine the rate of occurrence of this type of incomitant strabismus and potential predictive factors. Methods: All patients undergoing surgery on one or two inferior rectus muscles over a 10-year period were screened retrospectively for the A pattern, defined as a ≥5° difference in squint angle between the primary gaze and downgaze. The extraocular muscle thickness in patients with acquired A pattern was determined by computed tomography (CT) and compared with a control group consisting of patients randomly selected from the total cohort. Results: In a total of 590 patients, surgery was performed on the inferior rectus muscle(s) during the study period; the A pattern was identified in 59 patients. Simultaneous surgery was performed on one or both medial rectus muscles in 32% of the patients. This group had significant incyclotorsion (p = 0.000) and less depression (p = 0.000) postoperatively. The mean amount of recession was 4.38 ± 1.53 mm in the A pattern group and 3.91 ± 1.37 mm in the control group (p = 0.032). The amount of depression was 50.2° ± 7.4° in the A pattern group and 57.3° ± 4.4° in the control group (p = 0.045). The inferior rectus muscle was significantly thicker in the A pattern than in the control group (p = 0.027), while there was no significant difference in the thickness of the superior oblique muscle between the two groups (p = 0.870). Of all patients with the A pattern, 47% required further surgery to achieve adequate binocular single vision. Conclusion: Increased preoperative inferior rectus muscle thickness and relatively limited depression could be predictors of postoperative A pattern inferior rectus recession in patients with GO. Step-by-step procedures are preferable in this surgically challenging group of patients.