Acute and chronic electrical stimulation of the centromedian thalamic nucleus: modulation of reticulo-cortical systems and predictor factors for generalized seizure control.

The present report recapitulates the clinical and electrophysiologic studies we have performed on patients with certain forms of medically intractable epilepsy to investigate the basic mechanisms and predictor factors for seizure control of the electrical stimulation of the thalamic centromedian nucleus (CM) procedure. Acute electrical stimulation of CM reveals that in humans, as in other animals, CM represents a thalamic relay of a reticulo-cortical system that participates crucially in wakefulness and attentive processes and in regulation of cortical excitability, as well as in the physiopathology of genuine generalized epileptic seizures. For example, unilateral, threshold, low-frequency (6/sec) stimulation of CM produced electrocortical incremental responses, while high-frequency (60/sec) stimulation of CM produced electroencephalogram (EEG) desynchronization and electronegative DC shifts with no behavioral counterparts. In contrast, combined suprathreshold low-frequency (3/sec) stimulation of CM on one side and of mesencephalic reticular stimulation on the other produced generalized spike-wave complex discharges accompanied by the symptoms of a typical absence attack, including motionless stare, eye blinking, and unresponsiveness of patients to a series of flashes under a simple response task. Chronic bilateral, threshold, high-frequency (60/sec) stimulation of CM significantly decreased the number of primary and secondary generalized tonic-clonic seizures and atypical absence attacks and the amount of interictal generalized EEG discharges in both. In addition, it improved the psychological performance of patients and normalized the EEG by increasing the frequency of background EEG activity. In contrast, chronic stimulation of CM reduced neither the number of complex partial seizures nor the epileptic EEG activities localized in the temporal region. Good outcomes of the chronic CM stimulation procedure were achieved depending on correct selection of patients and accuracy of ventriculographic stereotactic targets, as well as on periodic clinical and EEG evaluation and electrophysiologic monitoring of CM electrical stimulation reliability. However, the presence of 3- to 6-month long-lasting effects of CM stimulation made statistical evaluation of ON-OFF effects of CM stimulation under placebo, double-masked randomized experiments difficult.