Abstracts

RATIONALE: Intraoperative electrophysiologic verification of stimulating electrode placement in the brain plays an important role in the implantation of anterior thalamic nucleus (ANT) stimulators. A knowledge of the morphology, distribution and proper parameters for evoking the ANT "recruiting response" is required for successful device placement. METHODS: ANT electrode placement in two patients was identified by intraoperative detection of the recruiting response on scalp EEG. Stimulation was performed through a range of voltages and pulse widths in order to establish appropriate parameters for evoking this response. RESULTS: Using a pulse width of 90 microseconds and a stimulation frequency of 5-10 Hz, the recruiting response was evoked at 3.5 Volts in patient #1 and 4.0 Volts in patient #2. Bipolar stimulation through contacts 0 and 2 was used, with the entire 4 contacts (numbered 0 through 3)spanning the ANT from its inferior border to near the lateral ventricle. Amplitude of the scalp response correlated with amplitude of ANT stimulation. In patient #1, gradually increasing stimulation voltage from 3.5 V to 5.0 V resulted in increasing sharpness of the evoked response then a spike and slow wave morphology, which did not propagate or have any clinical correlate. In patient #2 response threshold decreased with increasing pulse width to a minimum of 3.0 Volts at a pulse width of 180 microseconds. Further increases in pulse width did not lower threshold. In all cases there was a 1-2 second delay from initiation of stimulation to appearance of a recruiting response on scalp EEG. In addition to electrophysiology, electrode placement was confirmed by clinical response and postoperative MRI imaging. CONCLUSIONS: With proper placement of stimulating electrodes in the ANT, a robust recruiting response was detectable at a threshold of between 3.0-4.0 Volts, utilizing a pulse width of 90 microseconds and 5-10 Hz stimulation. Increasing voltage and pulse width provoked a more robust response. Higher stimulation voltage provoked subclinical bifrontal spike and slow wave activity in one patient.