Downbeat nystagmus with phenytoin.

Downbeat nystagmus is generally the result of a structural lesion at the craniocervical junction. It has rarely been reported as a manifestation of metabolic disease or drug intoxication. We observed two patients with downbeat nystagmus secondary to phenytoin (Dilantin) intoxication. Both individuals had other features of phenytoin-induced central nervous system dysfunction with toxic blood levels of the drug (greater than 20 micrograms/ml). Complete resolution of the downbeat nystagmus followed the return of phenytoin levels to the therapeutic range.