Influence of clinical and tumor-specific factors on the resting motor threshold in navigated transcranial magnetic stimulation.

Objective: Preoperative non-invasive mapping of motor function with navigated transcranial magnetic stimulation (nTMS) has become a widely used diagnostic procedure. Determination of the patient-individual resting motor threshold (rMT) is of great importance to achieve reliable results when conducting nTMS motor mapping. Factors which contribute to differences in rMT of brain tumor patients have not been fully investigated.

Methods: We included adult patients with all types of de novo and recurrent intracranial lesions, suspicious for intra-axial brain tumors. The outcome measure was the rMT of the upper extremity, defined as the stimulation intensity eliciting motor evoked potentials with amplitudes greater than 50µV in 50 % of applied stimulations.

Results: Eighty nTMS examinations in 75 patients (37.5 % female) aged 57.9 ± 14.9 years were evaluated. In non-parametric testing, rMT values were higher in patients with upper extremity paresis (p = 0.024) and lower in patients with high grade gliomas (HGG) (p = 0.001). rMT inversely correlated with patient age (r=-0.28, p = 0.013) and edema volume (r=-0.28, p = 0.012) In regression analysis, infiltration of the precentral gyrus (p<0.001) increased rMT values. Values of rMT were reduced in high grade gliomas (p<0.001), in patients taking Levetiracetam (p = 0.019) and if perilesional edema infiltrated motor eloquent brain (p<0.001). Subgroup analyses of glioma patients revealed similar results. Values of rMT did not differ between hand and forearm muscles.

Conclusion: Most factors confounding rMT in our study were specific to the lesion. These factors contributed to the variability in cortical excitability and must be considered in clinical work with nTMS to achieve reliable results with nTMS motor mapping.