Intraoperative neurophysiological responses in epileptic patients submitted to hippocampal and thalamic deep brain stimulation.

PURPOSE: Deep brain stimulation (DBS) has been used in an increasing frequency for treatment of refractory epilepsy. Acute deep brain macrostimulation intraoperative findings were sparsely published in the literature. We report on our intraoperative macrostimulation findings during thalamic and hippocampal DBS implantation. METHODS: Eighteen patients were studied. All patients underwent routine pre-operative evaluation that included clinical history, neurological examination, interictal and ictal EEG, high resolution 1.5T MRI and neuropsychological testing. Six patients with temporal lobe epilepsy were submitted to hippocampal DBS (Hip-DBS); 6 patients with focal epilepsy were submitted to anterior thalamic nucleus DBS (AN-DBS) and 6 patients with generalized epilepsy were submitted to centro-median thalamic nucleus DBS (CM-DBS). Age ranged from 9 to 40 years (11 males). All patients were submitted to bilateral quadripolar DBS electrode implantation in a single procedure, under general anesthesia, and intraoperative scalp EEG monitoring. Final electrode's position was checked postoperatively using volumetric CT scanning. Bipolar stimulation using the more proximal and distal electrodes was performed. Final standard stimulation parameters were 6Hz, 4V, 300µs (low frequency range: LF) or 130Hz, 4V, 300µs (high frequency range: HF). KEY FINDINGS: Bilateral recruiting response (RR) was obtained after unilateral stimulation in all patients submitted to AN and CM-DBS using LF stimulation. RR was widespread but prevailed over the fronto-temporal region bilaterally, and over the stimulated hemisphere. HF stimulation led to background slowing and a DC shift. The mean voltage for the appearance of RR was 4V (CM) and 3V (AN). CM and AN-DBS did not alter inter-ictal spiking frequency or morphology. RR obtained after LF Hip-DBS was restricted to the stimulated temporal lobe and no contralateral activation was noted. HF stimulation yielded no visually recognizable EEG modification. Mean intensity for initial appearance of RR was 3V. In 5 of the 6 patients submitted to Hip-DBS, an increase in inter-ictal spiking was noted unilaterally immediately after electrode insertion. Intraoperative LF stimulation did not modify temporal lobe spiking; on the other hand, HF was effective in abolishing inter-ictal spiking in 4 of the 6 patients studied. There was no immediate morbidity or mortality in this series. SIGNIFICANCE: Macrostimulation might be used to confirm that the hardware was working properly. There was no typical RR derived from each studied thalamic nuclei after LF stimulation. On the other hand, absence of such RRs was highly suggestive of hardware malfunction or inadequate targeting. Thalamic-DBS (Th-DBS) RR was always bilateral after unilateral stimulation, although they somehow prevailed over the stimulated hemisphere. Contrary to Th-DBS, Hip-DBS gave rise to localized RR over the ipsolateral temporal neocortex, and absence of this response might very likely be related to inadequate targeting or hardware failure. Increased spiking was seen over temporal neocortex during hippocampal electrode insertion; this might point to the more epileptogenic hippocampal region in each individual patient. We did not notice any intraoperative response difference among patients with temporal lobe epilepsy with or without MTS. The relationship between these intraoperative findings and seizure outcome is not yet clear and should be further evaluated.

Speech preservation using a non-linear paradigm for determination of the extent of neocortical resection in patients with mesial temporal sclerosis submitted to cortico-amygdalo-hippocampectomy (CAH).

RATIONALE: The rationale for using a non-linear (proportional) paradigm for determining the extent of the neocortex to be removed in temporal lobe resection was based on anatomical and intra-operative cortical mapping findings. We present our results regarding speech preservation in patients submitted to CAH using the central artery as an anatomical landmark for determining the posterior border of neocortical resection. METHODS: Two hundred and fifty consecutive right-handed patients with left unilateral mesial sclerosis were studied. All patients were submitted to CAH under general anesthesia and without intraoperative electrocorticography. The posterior border of the lateral neocortical resection was defined by a line perpendicular to the temporal axis at the level of the central artery. RESULTS: Seven patients had transient (1-3 weeks; mean=9 days) receptive speech disturbance. There was no permanent speech deficit. Imaging documented edema or contusion at the posterior temporal cortical border in all patients who had transient speech deficits. The mean extent of cortical resection was 3.9 cm in adults and 3.1cm in kids. DISCUSSION: This is the first report in the literature discussing the use of a non-linear paradigm to determine the extent of lateral neocortical removal in this patient population. We found no permanent speech disturbances in this series. The non-linear approach used in this series proved to be safe and effective to avoid post-operative speech disorders. It was able to compensate for different brain and head sizes, and allowed smaller neocortical removal when compared to traditional linear approaches.

Revisiting hydrocephalus as a model to study brain resilience.

Hydrocephalus is an entity which embraces a variety of diseases whose final result is the enlarged size of cerebral ventricular system, partially or completely. The physiopathology of hydrocephalus lies in the dynamics of circulation of cerebrospinal fluid (CSF). The consequent CSF stasis in hydrocephalus interferes with cerebral and ventricular system development. Children and adults who sustain congenital or acquired brain injury typically experience a diffuse insult that impacts many areas of the brain. Development and recovery after such injuries reflects both restoration and reorganization of cognitive functions. Classic examples were already reported in literature. This suggests the presence of biological mechanisms associated with resilient adaptation of brain networks. We will settle a link between the notable modifications to neurophysiology secondary to hydrocephalus and the ability of neuronal tissue to reassume and reorganize its functions.