Possible underlying mechanism of epileptic nystagmus as studied by stereoelectroencephalography.

Background Epileptic Nystagmus (EN) is a rare ictal phenomenon that is frequently reported in children rather than adults. EN is characterized by quick, repetitive eye movements. The direction of the nystagmus is defined based on the fast component of eye movements. EN can be a helpful lateralizing sign, and prior reports indicated that its direction is contralateral to the epileptogenic zone. EN is generated through inter-action between an ictal discharge frequency and cortical regions implicated in generating the sign. EN can be classified as either monocular or binocular. Monocular EN has been reported in occipital lobe lesions contralateral to the involved eye. Pendular nystagmus is induced by early-onset visual deprivation. It is associated with a wide range of disorders including demyelination, oculopalatal myoclonus, and Whipple's disease. This report addresses the anatomo-electro-clinical (AEC) correlations of pendular EN using stereoelectroencephalography (SEEG), which have not been reported previously.Methods A 14-year-old female with a history of tuberous sclerosis complex (TSC) and associated refractory epilepsy underwent presurgical evalu-ation with stereoelectroencephalography (SEEG). Her seizure semiology consisted of a feeling of "fuzziness" in the head with bilateral nystagmusand retained awareness. The frequency of the nystagmus was up to several times per day, and it usually lasted for up to one minute. There was no associated vertigo, blindness, or visual hallucination. MRI brain showed multiple tubers bilaterally including in the oc-cipital cortices. SEEG implantation targeted cortical areas involved in eye movement control including the frontal eye field (FEF), the dorsolateral prefrontal cortex (DLPFC), the parietal eye field (PEF), the temporo-parieto-occiptal junction (TPO), the primary visual cortex, middle temporal area (MT) and the cingulate cortex.Results The ictal onset by SEEG revealed the emergence of fast activity in the right cuneus, bilateral lingual, without symptoms, which then transi-tioned to an alpha frequency in those structures in addition to bilateral middle occipital gyri (Fig. 1). Epileptic nystagmus was captured during SEEG recording with the addition of an eye channel (Figs. 1 and 2). Synchronized 9-11 Hz ac-tivity in the right cuneus, bilateral lingual, and bilateral middle occipital gyri correlated with 3-4 Hz pendular horizontal nystagmus, which had the same velocity in both directions. There were no notable electro-graphic changes in the electrodes sampling the frontal eye fields. Elec-trooculogram (EOG) electrodes were placed 1 cm lateral to the left and right outer canthi. EOG showed horizontal pendular nystagmus where