Primary position upbeat nystagmus. Another central vestibular nystagmus?

Recent studies of the vestibulo-ocular reflex have revealed a distinct pathway from the anterior semicircular canal to the contralateral oculomotor nucleus via the superior vestibular nucleus. Axons of this pathway ascend in the brachium conjunctivum, while axons of the other semicircular canal pathways ascend in the medial longitudinal fasciculus (MLF). We report two cases of primary position upbeat nystagmus where lesions of the brachium conjunctivum were suggested by computed tomography (CT) scans. One of these lesions was confirmed at autopsy. We concluded that primary position upbeat nystagmus, like downbeat nystagmus, is a type of central vestibular nystagmus resulting from an imbalance of vertical vestibulo-ocular reflex activity.