Abstracts

Rationale: Understanding the connectivity of malformations of cortical development is important for understanding their role in seizure networks, for surgical planning and to interpret intracranial studies.  Single pulse electrical stimulation (SPES) has recently become popular as a tool to explore oligo-synaptic cortico-cortical connectivity in patients undergoing intracranial monitoring (Matsumoto, Kunieda, Nair, 2017).  We employ this technique to examine the connectivity of malformations and clarify their role in seizure networks. Methods: In patients undergoing stereoelectroencephalographic study for pre-surgical planning, we included 1 Hz SPES during extra-operative mapping. Mapping was performed in one patient with periventricular heterotopia and another with a frontal cortical malformation after restarting medications and obtaining an adequate number of seizures. For electrode pairs of interest, we performed multiple trains of 20-30 pulses of 1 ms/phase that were either at subjective threshold or near maximal current (8 mA). Consequent cortico-cortical evoked potentials (CCEPs) were then averaged to remove stochastic activity. The CCEPs elicited from contacts within hypothesized regions of seizure onset were then compared to the pattern of early ictal involvement. 50 Hz mapping of non-limbic contacts was also performed for comparison to 1 Hz stimulation. Results: In all cases, evoked potentials were elicited in cortex outside of the malformation. Short-latency responses (< 15 ms) were elicited in radially adjacent tissue, but sometimes involved several electrode sites outside of adjacent cortex. In each patient, there was an electrode in the malformation that reproduced typical aura and, in the same contacts, showed evoked potentials in the same contacts that were involved at seizure onset. In an implicated heterotopion, clear ictal field potentials were absent during spontaneous seizures, but there was a significant degree of overlap of electrodes showing CCEPs and those that were involved at ictal onset (p=0.002). 50 Hz mapping resulted in more after-discharges or brief seizures possibly confounding the interpretation of mapping and does not permit study of evoked potentials. Conclusions: SPES is a useful approach to exploring the connectivity of the human brain, including of malformations of cortical development. Furthermore, this technique has advantages for mapping over 50 Hz stimulation. Here we show that this may be a useful adjunct in pre-surgical evaluation and raises the possibility that this may identify plausible candidate regions of seizure onset that may not have adequate tissue organization to produce clear ictal field potentials. Funding: NPP was supported by the American Academy of Neurology and American Brain Foundation (CRTF).