27 Pathophysiology of Neocortical Epileptic Seizures

Introduction and Definitions Neocortical seizures are seizures that are primarily generated within neocortex. Seizures for which primary foci are located in other brain structures and secondarily project to neocortex are not considered to be neocortical. The origin of epileptic seizures accompanying various types of epileptic fits is hotly debated. Neocortical seizures are primarily focal and often become secondarily generalized tonic-clonic seizures (Crunelli and Leresche 2002). Electrographically, these seizures are most oftenly composed of spike-wave/polyspike-wave (SW/ PSW) electroencephalographic (EEG) discharges at 1.0–2.5 Hz and runs of fast spikes at 7–16 Hz (> Fig. 27-1). However, on some occasions, neocortical seizures are characterized by SW complexes at approximately 3 Hz. Spontaneously occurring SW complexes at 1–2.5 Hz and fast runs at 7–16 Hz develop without discontinuity from slow (mainly 0.5–0.9 Hz) cortically generated oscillations (Steriade and Contreras 1995). At the focus, the onset of neocortical seizures is accompanied with generation of high frequency oscillations (>100 Hz) termed ripples. During seizure, ripples can be recorded at multiple cortical locations. Because of the high likelihood of seizure development from slow sleep oscillations, neocortical seizures very often develop during slowwave sleep or transition to or from this state of vigilance. Therefore most of neocortical seizures belong to a class of nocturnal seizures. Some neocortical epileptic seizures start with EEG flattening and changes in global synchrony. Clonic components of seizures are associated with SW/PSW complexes and runs of fast EEG spikes lasting longer than 2–3 sec with tonic component of seizures (Niedermeyer 2005). Both excitatory and inhibitory cortical neurons are depolarized and fire action potentials during EEG spikes and are hyperpolarized and silent during EEG waves (> Fig. 27-1). During EEG fast runs, the majority of cortical neurons is oscillating at approximately the same frequency as EEG but the synchrony between individual neurons and field potentials or between field potentials recorded in different locations is loose.