Aphasia and cognitive impairment decrease the reliability of rnTMS language mapping

Background Navigated transcranial magnetic stimulation (nTMS) is a non-invasive mapping tool to locate functional areas of the brain. While gaining importance in the preoperative planning process in motor eloquent regions, its usefulness for reliably identifying language areas is still being discussed. The aim of this study was to identify biometric factors which might influence and therefore bias the results of repetitive nTMS (rnTMS) over cortex areas relevant for language. Method We included data of 101 patients with language eloquent brain lesions who underwent preoperative rnTMS examination bihemispherically. Prior to rnTMS mapping, all patients performed two to three baseline runs of a picture-naming paradigm without stimulation, and only promptly and correctly named objects were retained for TMS mapping. Nine biometric factors (age, gender, baseline dataset, cognitive performance score, aphasia score, histology of lesion, affected hemisphere, location of lesion on the hemisphere, pain caused by examination) were included in the statistical analysis measuring their correlation with the incidence of errors during baseline naming as well as during rnTMS mapping. Results The incidence of baseline errors correlated with aphasia (p < 0.0001) and cognitive impairment (p < 0.0001). No significant correlation was observed between most biometric factors and errors during rnTMS mapping. Factors significantly affecting the incidence of errors during rnTMS mapping were again aphasia (p < 0.023) and cognitive impairment (p < 0.038). Patients affected by those factors showed a significantly higher baseline error rate, starting at 28% error rate. Conclusions Patients with pre-existing aphasia or severe cognitive impairment did still make significantly more mistakes during rnTMS mapping than non-aphasic patients despite baseline stratification, rendering the question of whether the procedure is reliable in those patient groups. Baseline testing revealed a cut-off point at 28% error rate. Interestingly, age or pain (caused by the examination) did not bias the results.