Neurosurgery

blockade of cortical function rather than on eliciting function [29]. Most patients, even children as young as 10 years old, have little difficulty with the procedure, especially when propofol anesthesia is used during placement of the field block, cranial opening, the majority of the resection, and during closure. It is important to bear in mind that cortical stimulation mapping of language cortex is used to identify essential language cortex. This is distinctly different from involved language cortex, which is identified by functional imaging techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Furthermore, the specific language task performed by the patient may lead to identification of different language sites.

It is first necessary to determine the afterdischarge threshold so that depolarization is not propagated to nearby cortex, which may elicit local seizure phenomena or give false-negative or false-positive results. Therefore, electrocor-ticography (ECoG) must be performed during stimulation (Fig. 10.7). Using a U-shaped electrode holder, which is attached to the skull at the edge of the craniotomy (Grass model CE1), carbon-tip electrodes are placed over the exposed cortical surface and spaced approximately 10 mm apart. Bipolar stimulation, as described above, is then used, starting with a current of 2 mA. The current is gradually increased (0.5-1.0 mA increments) with successive stimulations until the after-discharge threshold is determined. The current used for language mapping is then set to 0.5-1.0mA below the after-discharge threshold.

Prior to mapping, 15-20 peri-Sylvian sites are selected and marked with small (5 x 5 mm)

Fig. 10.7. a Intraoperative electrocorticogram showing three afterdischarges following cortical stimulation (in lead E17). The montage is referential to three neck electrodes (balanced neck reference, to minimize ECG interference). bA higher current than in a has produced a train of afterdischarges. cThe cortical electrode arrangement is shown with numbered tickets on the left frontal lobe. dTicket "29" shows the location of motor speech; the suture is on the central sulcus. The frontal lobe tumor resection can be seen.

Fig. 10.7. a Intraoperative electrocorticogram showing three afterdischarges following cortical stimulation (in lead E17). The montage is referential to three neck electrodes (balanced neck reference, to minimize ECG interference). bA higher current than in a has produced a train of afterdischarges. cThe cortical electrode arrangement is shown with numbered tickets on the left frontal lobe. dTicket "29" shows the location of motor speech; the suture is on the central sulcus. The frontal lobe tumor resection can be seen.

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