gms | German Medical Science

Objective: TMS is used for noninvasive, painless stimulation of the cerebral cortex. Presurgical mapping of the cortex, i.e. the handknob (HK), contributes to information on connectivity and helps to protect brain tissue during tumor surgery. The technique is used to stimulate the cortex in paresis of the hand or in aphasia by repetitive stimulation. The drawback of the method, i.e. hand holding of the magnetic coil (MC), may lead to stimulation of tissue distant to the focus. A neuronavigated robot holding the MC can be of advantage. We compared the targeting of the HK with the 8-shaped MC by either the conventional hand holding and visual control or by a robot recognizing its own position over the cortex.

Methods: A brain MRI (180 slices) presenting the cortex including the HK is connected to a PC. The 6-joint-robotic arm (antR), the MC, and the head of the person are identified by an infrared camera using ball tracks fixed to head and MC. Potential latency (LAT, ms) and amplitude (AMP, µV) of 3 hand muscles were recorded. 10 volunteers (4m/6f; 22-75 y) underwent the protocol: Stimulation of 13 regions of interest (ROI) of the HK with the hand-held MC and control in the MRI imaging followed by stimulation of the same ROI using the robotic arm.

Results: 13 ROIs of the HK of both hemispheres with evoked muscular response were identified. LAT of the 3 hand muscles activation did not differ between hand-held and robotic stimulation: Maximal LAT for both protocols was 23 ms; intraindividual and interindividual differences of the ROI responses were high. AMP of muscles often were significant higher using the robot than with the hand-held MC (Robot: mean 325µV-347µV, hand-held 298µV-315µV). ROI with the highest AMP (hot spot) varied from person to person, however, it was stable intraindividually in repeated measurements. Maximal AMP was above 2000 µV for the hot spots with both techniques. In a third protocol the volunteer moved the head by 3-5 cm in 4 directions which could be compensated by the robotic stimulation, giving the same LAT and AMP as in complete rest.

Conclusion: Mapping of the motor system using a high focus MC to measure potential responses of hand muscles with either a hand-hold coil or a robotic guided MC yields identical results for latency and higher values for the amplitude. Head movements were compensated by the robotic system. Robotic guided mapping of the motor system is a practical alternative to hand held stimulation.