gms | German Medical Science

Objective: Maximizing the extent of resection whilst preserving important brain functions is a major goal in the surgical treatment of brain tumours. For primary motor functions, the presurgical diagnostic techniques such as functional MRI (fMRI) and navigated transcranial magnetic stimulation (TMS) have improved a lot over the last decades and are extremely useful for preoperative risk evaluation and the planning of the surgical approach. By contrast, presurgical mapping techniques for language-relevant areas are regarded poorly reliable (fMRI) and / or rather unspecific (navigated repetitive TMS; nrTMS).

Method: We investigated the retest reliability of nrTMS for speech mapping in 10 right-handed healthy volunteers in 3 consecutive sessions, spaced by 2-5 (short-term) and 21-40 (long-term) days. 10 Hz nrTMS (Nexstim eXimia 4.2) was applied over the dominant hemisphere after determination of the individual inhibition threshold over the primary motor representation of the face / tongue. The bursts were triggered to picture presentation (naming task) without delay (picture-to-trigger interval = 0). Errors were categorised as follows: arrest, delay, anomia, dysarthria, semantic and phonematic paraphasia.

Results: A good feasibility in terms of the evocation of errors was observed. Per session, speech delays occured most frequently (9.2 ± 2.1), followed by speech dysarthria (5.1 ± 2.4) and speech arrests (3.8 ± 1.8) whilst paraphasias were rather rare. There was a high variability in the distribution and the frequency of speech errors between the subjects. The reliability of the error frequency was rather good regarding long-term comparisons (Cohen's weighted Kappa κ = 0.41 [session 1-3] / κ = 0.42 [session 2-3]) and still fair for the short-term (κ = 0.29 [session 1-2]). Analyses of the spatial reliability of error representation are ongoing.

Conclusions: Presurgical speech mapping by nrTMS is a promising tool to improve the risk evaluation and the planning of the surgical approach. In this preclinical study a rather satisfactory retest reliability of nrTMS speech mapping at 10 Hz could be shown. However, since the method is still young, optimization of the stimulation protocol and further intraoperative validation in clinical trials are required.