Simple febrile seizure pathogenesis clinical findings infection HHV6 influenza virus child hippocampus fever 38 exogenously elevated brain temperature GABA receptor mutation sodium channel fever increased production mediators physiologic familial sporadic ion channels elevated temperature glutamate GABA excitability and synchronization of activity generalized tonic-clonic activity no focal signs <15 minutes normal EEG MRI complex focal signs eye deviation head turning status epilepticus beyak Howard Klein