Abstracts

The development of magnetic and electrical high-frequency pulse train methods for motor testing make it possible to stimulate the motor cortex safely and use the evoked EMG to quantify responses. We describe a similar method for use at the bedside in intracranial implant patients requiring motor mapping as part of their presurgical workup. Pairs of adjacent subdural electrodes were systematically stimulated with brief trains of constant current pulses triggering single-trial EMG responses in a 100 msec observation window. Trains of 7 pulses with duration 300 usec/phase at 500 Hz and maximum 20 mA current were used, and repeated every 2 sec until threshold motor responses were elicited or the maximum current reached. A selection of 8 muscles on the contralateral face, arm and leg were used for the EMG. The same electrode pairs were stimulated using conventional 50 Hz pulse trains lasting 2-5 sec with duration 500 usec/phase and maximum 12.5 mA current, and repeated approximately every 30 sec until a positive motor response was elicited or the maximum current was reached without interference from induced afterdischarges or seizures. These stimulation parameters are very comparable in conforming to accepted safety standards for cortical stimulation in terms of charge density per phase. In the 2 patients analyzed for this preliminary study, triggered EMG responses in contralateral muscles were elicited from electrodes located over primary motor cortex using a little as 1 mA current, whereas no discernible EMG responses were elicted from electrodes located over other cortical areas. Although the patients often felt brief twitches in the triggered muscles, this was not uncomfortable, and no afterdischarges or seizures were observed. In one patient, the cortical motor map derived in this way showed very good concordance with the same map made via conventional 50 Hz stimulation and also was supported by anatomical renderings as well as SSEP mapping. In the second patient, the conventional mapping using 50 Hz stimulation could not be completed due to repeatedly induced seizures at very low stimulation current, while the pulse train method provided a well-delineated motor map that was well-supported by anatomical renderings and SSEP mapping. Stimulation of the supplementary motor area in this patient did not produce motor responses, which may indicate the method is specific to primary motor cortex. The described method appears to be a safe, efficient and quantitative approach for motor mapping in patients who are difficult to test due to a low seizure threshold or an inability to cooperate. It also has the advantage that it can be done (or repeated) in the OR to guide electrode implantation or resection. (Supported by Dartmouth-Hitchcock Medical Center)