Dissociated palsy of vertical saccades: loss of voluntary and visually guided saccades with preservation of reflexive vestibular quick phases.

A patient with a diencephalic infarct displayed a persistent palsy of voluntary and visually guided vertical saccades with preserved vertical quick phases of vestibular nystagmus on magnetic search coil oculography. Vertical smooth pursuit had very low velocity in both directions without catch-up saccades. Vertical and torsional vestibulo-ocular reflex gains were normal. Preservation of vertical and torsional quick phases signifies integrity of the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF). This case is the first to provide evidence that disruption of descending cerebral corticofugal pathways to the riMLF with preserved ascending projections from the paramedian pontine reticular formation to the riMLF can cause a dissociated palsy of vertical fast eye movements.