gms | German Medical Science

Objective: Recent work has demonstrated that the resting motor threshold (RMT) obtained by navigated transcranial magnetic stimulation (nTMS) is correlated with the probability of postoperative motor deficits in brain tumor surgery. Here, we use more sophisticated methods to assess cortical excitability, namely the TMS-based Recruitment Curve (RC) and the Cortical Silent Period (CSP), to analogously determine the probability of postoperative motor deficits.

Method: nTMS was used to map the cortical motor representation area of the FDI muscle bilaterally in 38 patients with malignant gliomas in motor eloquent areas prior to surgery. In addition to RMT determination, the RC was measured by applying 80 stimuli of randomized intensity between 80% and 150% RMT over the FDI hot spot bilaterally. The resulting MEP-amplitudes were plotted against the respective stimulation intensities and the slope of the resulting interpolated linear graph was calculated. CSP was determined by applying 10 stimuli of RMT intensity during tonic muscle contraction and measuring the duration of the plateau phase from EMG recording.

Results: The mean RC value of the slopes on the tumor hemisphere (215.50 µV/%) was significantly higher compared to the healthy hemisphere (110.91 µV/%) and the average CSP duration on the tumor hemisphere (124.66 ms) was considerably shorter compared to the healthy hemisphere (142.15 ms). These differences between tumor and healthy hemisphere were not correlated to the preoperative motor status and signify reduced cortical GABAergic inhibition resulting in increased recruitment of motor units on the tumor hemisphere. These significant changes were not detected by RMT measurements (mean values tumor: 34.71%, healthy: 34.13%). Postoperatively, patients with a new paresis at the three months follow-up also demonstrated a steeper RC slope on the tumor hemisphere (237.79 µV/%) in contrast to patients with no new deficits (156.40 µV/%), confirming the proposed pathomechanism.

Conclusions: The presented results demonstrate that the RC and the CSP are sensitive markers to detect subclinical impairment of the motor system that is not detectable by RMT measurements. If the presented preliminary results are confirmed on larger samples, RC slope and CSP durations might serve as valuable markers to predict imminent motor deficits in the future.