The following two different modulatory procedures to control intractable epileptic seizures are presented: (1) chronic electrical stimulation of the centromedian–thalamic nucleus (ESCM) for control of generalized tonic–clonic seizures and atypical absences, and (2) subacute hippocampal stimulation (SAHCS) and chronic hippocampal stimulation for control of nonlesional temporal lobe seizures. The ESCM antiepileptic effect seems to be the result of activation of a nonspecific reticulothalamocortical system responsible for generalized electrocortical responses (recruiting, desynchronization, negative direct current shifts, and three spike–wave complexes per second). The success of the ESCM procedure depends on the following predictor factors: case selection (primary and secondary tonic–clonic seizures and atypical absences of the Lennox Gastaut syndrome), ventriculographic and electrophysiologic definition of the optimal stereotactic targets (based on the anterior commissure, posterior commissure, and the vertical line perpendicular to the posterior commissure and electrocortical recruiting responses), periodic electrophysiologic monitoring of the reliability of ESCM in the absence of the patient’s subjective sensations and with totally internalized subcutaneous stimulation systems (by recording scalp electrocortical recruiting, desynchronizing, and direct current responses), quantitative evaluation of clinical and EEG improvement, and analysis of the ON and OFF effects, taking into account a long-lasting (possibly plastic) effect of ESCM. SAHCS blocks clinical and EEG signs of temporal lobe epileptogenesis with no additional damage of the stimulated hippocampal tissue. Preliminary results suggest that this antiepileptic effect is, at least in part, the result of a physiologic inhibition of the stimulated hippocampal tissue, because after SAHCS the authors found the following: (1) increased threshold and decreased duration, propagation, and blockage of the clinical signs accompanied with the hippocampal afterdischarge; (2) flattening of the hippocampal-evoked response recovery cycles; (3) single photon emission computed tomographic hypoperfusion; and (4) increased concentration of benzodiazepine receptor binding at the stimulated hippocampal region. Chronic hippocampal stimulation persistently blocked temporal lobe epileptogenesis in one patient under open protocols during 24 months with no apparent additional alterations in recent memory.
Units for Stereotactic and Functional Neurosurgery, General and Children’s Hospitals of Mexico SS and UMR, Neurophysiology, National Medical Center, IMSS, Mexico City, Mexico
Supported in part by grants F348-A9301 and 2843M from the National Council of Science and Technology (CONACYT), Mexico, and a special grant from Medtronic Inc., Neurologic Division, Minneapolis, Minnesota, U.S.A.
Address correspondence to Dr. Marcos Velasco, UMR, Neurophysiology–Neurosurgery, National Medical Center IMSS, PO Box 73-032, Mexico City, Mexico.