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.