Phase 2: Cortical and subcortical testing

In this video, we will present a few short clips showing the actual stimulation mapping during the operation. And I will explain to you the rules which we adhere to during the stimulation, and a surgical decision making used for maximal tumour removal while preserving quality of life. In the videos we will see part of the lateral surface area of the brain exposed, as cortical stimulation mapping is started. Central in the surgical field, a large vein can be seen, which marks the Sylvian fissure. In the image, the temporal lobe is above the fissure and the frontal lobe is below this fissure.

On the frontal lobe, a few marks can be seen. B, A, and H, marking the motor cortex. This area has been used to determine the threshold level of the stimulation current needed to evoke clear motor responses. Generally three to four Milliamps. This current level is then used to start the stimulation mapping of the brain area of interest. Notice that the stimulation, that is the bipolar electrode actually touching the brain, is limited to four second intervals, as was explained to you in the previous step. Notice also that one particular side has never tested two times in a row. The patient, in the meantime, is performing a naming test. [DUTCH] This is a glass. This is a mirror.

This is a tooth. This is a frog This is a swan. This is a heart. This is a chicken. In this first video, you’ve seen that a significant part of the temporal lobe was stimulated. The patient makes no clear mistakes in the picture naming test. The video illustrates an important aspect of stimulation mapping. The majority of stimulation areas are negative. That is, the patient makes no mistakes during stimulation of this particular area. One is inclined to think that the brain is full of functional areas, and this is undoubtedly true. However, when performing four-second tests, one test at a time, it’s actually quite difficult to find the areas of the brain that are essential for passing this particular test.

For the second and third video, it’s important to observe the patient’s response. Notice that, in the second video, a short hesitation can be heard, when the patient is naming the picture of a rabbit. His hesitation seems to be somehow released when a simulation probe is pulled away from the cerebral cortex. This is immediately followed by naming the picture of a soldier. Again with hesitation, albeit shorter than the previous one, and without concurrent stimulation. Also notice, in the third video, that a stimulation probe does not touch the cerebral cortex at all, at the moment of the picture naming. [DUTCH] This is refrigerator. This is - what do you call it - a rabbit. This is a soldier.

This is a table. This is a spider

Tom. This is a …? A key A haircut This is haircut. This is an eye. This is the cloud. This is a leg. This is a helmet.

MICHIEL WAGEMAKERS: These two videos illustrate that the patient responses to the test are not always unequivocal, and that the response to a test during stimulation of a particular area may vary in time. Therefore, specific brain areas that are negative during stimulation mapping, are tested at least three times before it is decided that this particular brain area may be removed. In this case, the stimulation site of the second video, in which hesitation was heard when naming the rabbit, may be considered a positive side. In the fourth video, you will see that this area has been marked as positive by letter E. During the fourth video, a phonological paraphasia can be heard, and you can hear the neuropsychologists reporting this.

Note during this response the patient closes his eyes. [ DUTCH] This is a comb. This is a plant. Mm-hm. This is a toothbrush. Very good. This is a helicopter. This is a squid. This is a toadstool.

This is a hatstand. This is a chameleol [SIC] Phonological parphasia

This is a mask. Although the stimulation side seems clearly positive, the closure of the eyes may be indicative of a larger part of the brain being affected by the stimulation. Although in this instance, no afterdischarges were reported by the neurophysiologist. It’s not entirely impossible that some epileptic activity is involved. The stimulation site may, therefore, inappropriately have been designated as positive. It’s important to keep in mind that the possibility of false positive tests during surgery. Subcortical resection is carried out under the same stimulation conditions using similar tests. Here you see the patient performing repetition taskwhile we stimulate his subcortical tracts.

The clips that were presented in this video showed that the test responses during stimulation can be hard to interpret. It’s mandatory that relevant areas are stimulated repeatedly, to get a more clear picture of the cerebral areas that are essential, in terms of language. And, thereby, get a clear picture of which part of the cerebral tissue can be removed to facilitate tumour resection.