Horizontal pursuit defect nystagmus.

A teenage girl with a chronic large posterior fossa tumor had primary position right-beating nystagmus with a constant-velocity slow phase. Smooth pursuit was normal to the left but absent to the right. Vestibuloocular move­ ments were present in both directions. The findings are analogous to downbeat and upbeat nystagmus, in which unidirectional pursuit palsies have been postulated as the pathogenic mechanism. This is the first reported case of a horizontal pursuit defect causing nystagmus.