Strabismus Surgery in a Case of Inferior Rectus Aplasia with a Relatively Small Amount of Hypertropia

To report the result of strabismus surgery in a case of inferior rectus aplasia with a relatively small amount of hypertropia.

A 22-year-old man was referred to the strabismus clinic for evaluation of an abnormal head posture and right eye deviation. Eyes exam revealed a best-corrected visual acuity of 10/10 in both eyes; the patient had a right head tilt along with a right hypertropia and exotropia. The hypertropia measured 14 PD in the primary gaze position and increased during the right gaze and left head tilt. The exotropia had an A-pattern. Ocular motility revealed downgaze limitation of the right eye in the abduction; however, in straight downgaze, the right eye passed the midline with a moderate overaction of the left superior oblique muscle. Based on the initial clinical findings, a diagnosis of right inferior rectus palsy was made. Intraoperatively, a positive force duction test indicating superior rectus restriction was noted; therefore, we planned to perform a right superior rectus recession along with a right inferior rectus resection. However, after recessing the right superior rectus, we were surprised to find that the IR muscle was not present, and there were only a few strands of fibrovascular tissue in that region. We changed our surgical plan and performed transposition of inferior oblique muscle to the expected site of insertion of aplastic IR muscle 6.5 mm from the limbus after a 6 mm resection. After surgery, the absence of IR muscle was confirmed by an orbital CT scan (Figure 2). Post-operatively, the patient’s eyes were orthophoric in the primary position, and his abnormal head posture was significantly improved. The exam findings remained stable until his last visit at the sixth postoperative month.

Inferior rectus aplasia should be kept in mind as a differential diagnosis of congenital inferior rectus muscle paresis/palsy. This condition may manifest with a modest amount of hypertropia in the primary position, and the ipsilateral superior oblique muscle may be over-active, thus generating enough force to move the eye downward past the midline. Orbital imaging is the only way to arrive at a definitive diagnosis preoperatively.