Pathophysiology of rapid eye movements in the horizontal, vertical and torsional directions.

Saccades and fast phases of optokinetic or vestibular nystagmus are rapid eye movements which are generated in the reticular formation of the brain stem. Palsies of rapid eye movement generation therefore always point towards an infratentorial lesion. Two cell assemblies are responsible for the generation of rapid eye movements: the PPRF, which triggers all rapid eye movements and in this sense plays an important coordinating role in addition to generating movements with a horizontal movement component, and the riMLF in the mesencephalon for movements which have a vertical or torsional direction component. As neurone populations are anatomically segregated according to their respective on-directions, focal lesions lead to the loss of rapid eye movements in particular directions (Table 2). Such palsies of rapid eye movement generation which are direction specific can be distinguished from dysmetria, which points towards a cerebellar lesion, or the difficulty to trigger a saccade to a particular target, which can involve cortical systems.