Micro-electrode in vivo signatures of human periventricular heterotopia

Periventricular nodular heterotopia (PNH) is a common cause of drug-resistant epilepsy, characterized by nodules of ectopic neurons adjacent to the lateral ventricles. It remains controversial whether neurons within the nodules generate epileptic activity. In contrast to other types of cortical malformations, no unique interictal patterns have been described in PNH. We report the first multi-level analysis of long-term microelectrode recordings of the interictal activities from three nodules in two epileptic patients during presurgical evaluation. In both patients, seizures originated from the ectopic nodules. Highly consistent interictal activities were identified in all three nodules 1) trains of periodic slow waves (n=2855), 2) isolated slow deflections (n=1631), both with superimposed fast activity, and 3) epileptic spikes (n=6986). Patterns were highly local and largely invisible on the adjacent macro-electrode contacts. Spike analyses showed that the vast majority of units (n=25) were strongly modulated during all interictal patterns. The same units were involved in all three patterns, while showing different patterns of firing rate modulation during the interictal events. These results are consistent with an altered regulation of cellular excitability and suggest that periodic patterns may result from fluctuation in inhibition and rebound excitation in the same neuronal network.