Pathophysiology of epileptic encephalopathies

The application of metabolic imaging and genetic analysis, and now the development of appropriate animal models, has generated critical insights into the pathogenesis of epileptic encephalopathies. In this article we present ideas intended to move from the lesions associated with epileptic encephalopathies toward understanding the effects of these lesions on the functioning of the brain, specifically of the cortex. We argue that the effects of focal lesions may be magnified through the interaction between cortical and subcortical structures, and that disruption of subcortical arousal centers that regulate cortex early in life may lead to alterations of intracortical synapses that affect a critical period of cognitive development. Impairment of interneuronal function globally through the action of a genetic lesion similarly causes widespread cortical dysfunction manifesting as increased delta slow waves on electroencephalography (EEG) and as developmental delay or arrest clinically. Finally, prolonged focal epileptic activity during sleep (as occurring in the syndrome of continuous spike-wave in slow sleep, or CSWSS) might interfere with local slow wave activity at the site of the epileptic focus, thereby impairing the neural processes and, possibly, the local plastic changes associated with learning and other cognitive functions. Seizures may certainly add to these pathologic processes, but they are likely not necessary for the development of the cognitive pathology. Nevertheless, although seizures may be either a consequence or symptom of the underlying lesion, their effective treatment can improve outcomes as both clinical and experimental studies may suggest. Understanding their substrates may lead to novel, effective treatments for all aspects of the epileptic encephalopathy phenotype.