DES vs nTMS mapping

Apart from intraoperative mapping, you now have seen how we can achieve a language map even preoperatively with navigated Transcranial Magnetic Stimulation. One can wonder why we put the patient through an awake brain surgery, if nTMS mapping has the same effect and result. The answer to this is: because it does not have the same result and reliability (yet). Here is what we mean by that:

When the areas around the tumour are mapped for positive and negative language involvement, a map can be created. It shows the patient’s MRI. Areas on the cortex that have shown to elicit errors after several item testings are marked as “positive areas” for language involvement. Areas not eliciting any errors are marked as “negative areas” for language involvement. Such a map can now help in different ways:

• The surgical planning can be enhanced.

• The time of the surgery can be reduced due to the preexisting knowledge.

• The size of the craniotomy can be reduced.

However, the current state-of-the-art is that the surgical team does not solely rely on a nTMS map to decide what to resect. Reason for that is that currently the nTMS maps are not 100% accurate and can not for the full 100% predict what will happen during the intra-operative mapping. This paper discusses the exact values and tells you how the nTMS map does not 100% overlap with the intra-operative finding.

The main reason for the lack of perfect overlap is that nTMS is overcalling positive language sites: it seems to disrupt areas functionality, that turns out to not host language-relevant areas in the OR. This means, the surgeon will interrogate an area that supposedly is positive to then find out he or she cannot find any error elicitation intra-operatively. Reasons for this overcalling of positive sites are still unknown, because the underlying mechanisms of nTMS, that has to travel through the scalp and skull, is less understood than the effect of Direct Electrical Stimulation (DES), directly on the cortex.

To conclude, this means two things: research needs to investigate the effects of nTMS and how to bring them as close as possible to the effects of DCS. And secondly, this entails that for now, the surgical teams do consult nTMS maps for surgical planning, but do not replace the intra-operative mapping with them yet.