[A case of non-paralytic pontine exotropia with vertical dissociated nystagmus in the contralateral eye].

We report a 74-year-old woman presenting non-paralytic pontine exotropia (NPPE) with vertical monocular nystagmus in her contralateral eye, due to a small infarction in the upper pons. On forward gaze, the ocular position of the right eye was fixed at the midline, while the left eye was abducted. On the leftward gaze, the left eye was abducted and monocular nystagmus was noted, but the right eye was not able to pass midline, which indicated NPPE. Neither a skew deviation nor an alternating exotropia was observed. In the acute phase of this case, disturbances in the vertical eye movement of her right eye and the monocular upper gaze-evoked nystagmus of her left eye were observed. On the 7th hospital day, the monocular nystagmus disappeared simultaneously with an improvement of vertical eye movement. This finding suggests that the patient's vertical monocular nystagmus occurred due to an adaptive increase in the innervation to the left eye according to Hering's law of equal innervation, which indicates either horizontal dissociated nystagmus in MLF syndrome or NPPE.