Small-angle hypertropia in a case of inferior rectus aplasia: an intraoperative surprise.

Introduction: The clinical features of congenital inferior rectus (IR) aplasia are similar to IR paresis/palsy, and they include hypertropia, limitation of downgaze - especially in abduction, incyclotorsion, A-pattern strabismus, and abnormal head posture.Case report: A 22-year-old man presented with a right face turn, small-angle hypertropia (14 prism diopters) in the right eye, and limitation of downgaze that passed the midline; no diplopia was present and supraduction was normal in that eye. Double Maddox rod revealed 12 degrees of incyclotorsion for the right eye. The Titmus test revealed a stereo acuity of 400 seconds of arc. Based on the preoperative clinical findings, a presumptive diagnosis of IR paresis was made and a surgical plan for performing superior rectus muscle recession and IR muscle resection was devised. Intraoperatively, forced duction test was positive in depression. To our surprise, after performing a 4 mm recession of the superior rectus muscle, we found that the IR muscle was absent. We then revised the surgical plan and performed a 6 mm resection and anterior transposition of the inferior oblique (IO) muscle. Post-operatively, the patient was orthophoric in the primary position without diplopia, and his abnormal head posture was significantly improved.Conclusion: The amount of primary position hypertropia and severity of downgaze limitation are not adequate for differentiating congenital IR aplasia from IR paresis/palsy. Orbital imaging including CT, MRI or anterior segment optical coherence tomography (AS-OCT, UBM) is helpful to distinguish these two entities preoperatively.