Abstracts

Alternation of intracortical excitatory and inhibitory system in focal cortical dysplasia has not been well elucidated in vivo in humans. We here report the ictal alternation of the cortical excitability and intracortical inhibition. A 31-year-old man with intractable partial epilepsy who underwent invasive monitoring with subdural electrodes was investigated. The seizures started with somatosensory auras of the left foot, which evolved into either left foot clonic seizures or bilateral asymmetric tonic seizures. Invasive evaluation revealed a seizure onset zone in the primary sensorimotor area of the left foot. The pathology was cortical dysplasia. The subject gave a written consent to this protocol (IRB No. 443).
Repetitive single pulse electrical stimuli (1Hz, alternating polarity, duration of 0.3 msec) were first delivered to the left foot primary sensory area (SI) (focus) as well as the left hand SI (control). Cortico-cortical evoked potentials (CCEPs) were recorded by averaging electrocorticograms recorded from the adjacent areas time-locked to the stimulus (bandpass 0.5-1500 Hz, a total of 40-60 trials). The threshold intensity (TH) to elicit CCEPs was determined in this single pulse stimulation study. Then, the paired pulse stimulation was performed by conditioning (TH x 50%) and testing (TH+1 mA) stimuli with interstimulus interval (ISI) of 1-100 ms. Intracortical inhibition (ICI) at the stimulated site was investigated by analyzing amplitude change of CCEPs. 1) Single pulse stimulation at the foot and hand SI elicited CCEPs from the surrounding areas with the maximum CCEP at the primary motor area (MI) of the foot and hand, respectively. Compared with the control stimulation (hand SI), ICI by paired pulse stimulation of the focus (foot SI) was more intense (7-31% of decrease vs. 17-18%) in a wider range of ISI (1-10 ms vs. 1-2 ms). 2) Incidental recording of CCEPs during a somatosensory aura revealed the increased amplitude of CCEP (141% of interictally recorded CCEP) at the foot MI in response to single pulse stimulation of the foot SI. Different from the aforementioned interictal finding, ICI by paired pulse stimulation of the focus (foot SI) disappeared during the aura. No seizure pattern was seen at the foot SI or MI during the somatosensory aura before its evolution into the left foot clonic seizure. Increased cortical excitability and decreased intracortical inhibition during the aura is likely the underlying pathophysiology of seizure generation in this particular patient with focal cortical dysplasia. (Supported by The research grant from the Japan Epilepsy Research Foundation.)