We report data from two patients who developed reversible downbeat nystagmus (DBN) while using AEDs within the therapeutic range. All previous reported cases of epilepsy with drug-induced DBN related to toxic levels of AEDs were summarized, and DBN was found mostly occurring in those using a sodium channel blocking AED. We propose that in our cases, the DBN with therapeutic AED levels may be explained by additive effects of sodium channel blockers. Adverse drug effects should be considered as a cause of DBN in people with epilepsy treated with multiple AEDs.
Keywords: Adverse effects; Anticonvulsants; Downbeat nystagmus; Epilepsy; Toxicity.