To describe an unusual case of upbeat nystagmus (UBN) with internuclear ophthalmoplegia (INO) from a midbrain infarction suggesting involvement of ascending vestibular pathway in the ventral tegmental tract.

Internuclear ophthalmoplegia (INO) occurs in a lesion to the medial longitudinal fasciculus (MLF). It features ipsilateral adduction deficits with contralateral beating horizontal nystagmus of the unaffected abducting eye, exotropia, and sometimes spared convergence.

We present a 43-year-old male with history of hypertension admitted with sudden onset of vertical diplopia and dysarthria. There was no motor weakness, dysesthesias, visual loss, or dysphagia. General examination revealed hypertensive urgency with systolic blood pressure above 200 mmHg. Cranial nerve exam revealed adductor deficit in the right eye with horizontal nystagmus on leftward gaze. Convergence was spared. There was also bilateral UBN on primary gaze that is accentuated with upward gaze. The rest of neurologic examination was unremarkable.

The diffusion weighted image and fluid-attenuated inversion recovery image demonstrated a lesion at the midline of the midbrain and upper pons suggestion infarction. Of note, the following day, the patient developed exotropia of the right eye and adductor deficit in the left eye and deficit in convergence. This suggests bilateral INO with the ponto-mesencephalic lesion likely affecting bilateral MLF.

There are occasional reports of vascular compromise to the MLF and interspersed oculomotor nuclei to cause unilateral or bilateral INO with exotropia. However an association with UBN is rare. These circumstances usually occur with demyelination or neoplasm rather than ischemic compromise. Shaikh et al (2013) reported two cases of acute onset of upbeat nystagmus, exotropia, and INO in the setting of acute ponto-mesencephalic junction infarction. Zhao et al (2015) reported a case of unilateral INO with UBN with infarction at dorsal upper pons. They suggest that the paramedian tract neurons may act as a vertical neural integrator causing UBN.