EPILEPSY – WHEN CHAOS FAILS

Temporal lobe epilepsy is characterized by episodic paroxysmal electrical discharges (ictal activity) originating in mesial structures of the temporal lobe. These discharges consist of organized synchronous activity of mesial temporal neurons, particularly those of the hippocampus. This activity is seen as rhythmic medium to high amplitude slow waves or spike and slow wave discharges on the electroencephalogram (EEG). The ictal discharges (seizures) often spread to involve widespread regions of the ipsilateral then the contralateral cerebral hemispheres. These diffuse discharges often persist for approximately 1 to 5 minutes and are followed by a postictal pattern of asynchronous low amplitude slow waves in the EEG. It is a widely held view that seizures arise from mesial temporal structures because of damage to hippocampal circuitry. The characteristic circuit abnormalities include drop out of neurons, simplification of the dendritic tree (reduced synaptic input), sprouting of dentate granule cell axons (increasing the number of excitatory-excitatory feedback connections), and increase in glial cell elements (sclerosis). There is a concomitant loss in neurotransmitter receptors in the hippocampus. Physiologic studies in epileptogenic hippocampi have demonstrated loss of neuronal inhibition. It is generally believed that loss of inhibition is, at least in part, responsible for the occurrence of epileptic seizures. The central questions as to why seizures occur intermittently, and begin and end when they do, remain unanswered. The structural abnormalities of the temporal lobe are relatively stable, yet they exhibit dramatically variable behavior, as characterized by the EEG. For example, during the interictal state, the EEG pattern is described by electroencephalographers as low to medium voltage, " irregular " and " arrhythmic ". This contrasts with the " organized " , " rhythmic " , and self-sustained characteristics of ictal EEG pattern. We have postulated that epileptic brains, being chaotic nonlinear systems, repeatedly make the abrupt transitions into and out of the ictal state because the epileptogenic focus drives them into self-organizing phase transitions from chaos to order. Further, we postulated that the seizure serves to reset the system. Our hypotheses have been supported by the following findings: (1) positive