19 - Cortical circuits, synchronization and seizures

Summary

The epilepsies as a group are amongst the most common of neurological disorders, so that about 1% of the population experience more than one seizure during their lifetime. They provide a particularly fertile area for the interaction of basic neurophysiology and clinical neurology. Epileptic activity can be considered a pathological extreme of normal neuronal synchronization. Studies of experimental epilepsies both help us understand the fundamentals of the underlying disease processes and provide the impetus for basic research on brain function. They do indeed provide a ‘Window to Brain Mechanisms’ to borrow the title of an earlier monograph (Lockard & Ward, 1980).

A key feature of epileptic foci is the abrupt transition from apparently normal neuronal activity to the intense and synchronous discharge of many, if not all, of the neurones in the focus. The extent and duration of this synchrony varies. For present purposes we can divide discharges from epileptic foci into: brief and localized ‘interictal’ bursts, which last <200 ms; ‘polyspikes’ or ‘after-discharges’, which last several seconds but which also tend to remain localized at the focus; and ‘seizures’, which last tens of seconds up to 2 minutes, and which normally involve large areas of the brain, and may generalize into most of the neocortex, resulting in motor fits (Fig. 19.1). Normally epileptic seizures are self-limiting, but when these constraints fail status epilepticus results, which is a life-threatening medical emergency, and which is likely to cause substantial brain damage.