An 18-year-old right-handed male with a history of refractory epilepsy, global developmental delay, and AHC was admitted for video electroencephalographic (vEEG) monitoring. His epilepsy began in childhood, presenting with multiple seizure types: tonic seizures, myoclonic seizures, and bilateral tonic-clonic seizures. Whole exome sequencing identified a c.971A>G (p. E324G) mutation in the ATP1A3 gene. Interictal EEG showed sharp waves over the left temporal-parietal-occipital region. Ictal EEG revealed paroxysmal fast activity over the left occipital region at seizure onset (Figure 1). On video, downward tonic eye deviation of the right eye was observed, followed by multidirectional, dysconjugate saccadic movements of the right eye (Video). Both eyes then moved to the left side and began jerking with the fast phase towards the right side (Video). Brain MRI indicated severe cerebellar atrophy and hyperintense signals in both hippocampi (Figure 2).

This case underscores the existence of monocular opsoclonus as seizure semiology. The seizure-onset zone of monocular opsoclonus in our patient is the left temporal-occipital area. Previous reports indicate that binocular and monocular epileptic nystagmus lateralize to the contralateral hemisphere. (2,3) The tendency of these symptoms to occur together suggests that the respective symptomatogenic zones may be in close spatial proximity. Therefore, it may suggest that monocular epileptic opsoclonus lateralizes to the contralateral hemisphere. The detailed mechanisms by which ATP1A3 mutations lead to epilepsy and this semiology remain incompletely understood.