Tuberous sclerosis complex (TSC) and severe cortical dysplasia (CD), or CD type 
II according to Palmini classification, share histopathologic similarities, 
specifically the presence of cytomegalic neurons and balloon cells. In this 
study we examined the morphologic and electrophysiologic properties of cells in 
cortical tissue samples from pediatric patients with TSC and CD type II who 
underwent surgery for pharmacoresistant epilepsy. Normal-appearing pyramidal 
neurons from TSC and CD type II cases had similar passive membrane properties. 
However, the frequency of excitatory postsynaptic currents (EPSCs) was higher in 
neurons from TSC compared to severe CD cases, particularly the frequency of 
medium- and large-amplitude synaptic events. In addition, EPSCs rise and decay 
times were slower in normal cells from TSC compared to severe CD cases. Balloon 
cells were found more frequently in TSC cases, whereas cytomegalic pyramidal 
neurons occurred more often in CD type II cases. Both cell types were similar 
morphologically and electrophysiologically in TSC and severe CD. These results 
suggest that even though the histopathology in TSC and severe CD is similar, 
there are subtle differences in spontaneous synaptic activity and topographic 
distribution of abnormal cells. These differences may contribute to variable 
mechanisms of epileptogenesis in patients with TSC compared with CD type II.