Monocular elevation paresis caused by an ipsilateral lesion

A 52-year-old woman presented with a paresis of elevation of the right eye, equally severe in abduction and adduction (monocular elevation paresis). CT demonstrated a small, right-sided tumor of the mesodiencephalic junction. Monocular elevation paresis has been attributed to lesions of the contralateral pretectum, although proof has been lacking. Now, with documentation of an ipsilateral brainstem etiology, we can postulate a lesion affecting the upgaze efferents from the ipsilateral rostral interstitial nucleus of the medial longitudinal fasciculus.