Cerebral Tumor Resection Notes   Back to home page

• Use of “functional neuronavigation”, despite an accuracy evaluated approximately between 1 and 4 mm, it is important to be careful with image-guided surgery for voluminous tumors. The caution is necessary because of the high risk of intraoperative brain shift, due to surgical retraction, mass effect, gravity, and the extent of the resection or cerebrospinal fluid leakage.

• As a consequence, despite the development of functional neuroimaging, but due to its current limitations, the additional use of invasive electrophysiological investigations has been advocated for surgery of tumors located near, or within, eloquent structures.

• more and more neurosurgeons advocate the additional use of intraoperative direct electrical stimulation (DES) under general or local, anesthesia during surgery of tumors in eloquent areas

• A bipolar electrode with tips, spaced 5 mm apart and delivering a biphasic current (pulse frequency 60 Hz, pulse duration 1 ms, and amplitude from 6 to 18 mA under general anesthesia or from 2 to 6 mA under local anesthesia), is applied to the cortex. DES allows the mapping of motor function (possibly under general anesthesia, by inducing unvoluntary motor response, if the stimulation is applied at the level of a motor site).

• DES allows somatosensory function by eliciting dysesthesia described by the patient himself intraoperatively, and also the mapping of cognitive functions. Examples of the functions that can be mapped include language (spontaneaous speech, oject naming, comprehension, reading, writing, bilinguism, etc. . . .), calculation, memory, or even visual–spatial processing, performed in these cases on awake patients by generating transient disturbances when the electrical stimulation is applied at the level of a functional “epicenter”

• Thus, DES is able to identify, in real time, the cortical sites essential for the function (i.e., to be imperatively preserved), following the dura-matter opening, but before the beginning of the resection, in order to select the best surgical approach and to define the cortical limits of the glioma removal

• Anatomical–functional organization of the supplementary motor area

  The supplementary motor area (SMA), namely the frontomesial area located in front of the primary motor area of the inferior limb, is a region involved in the planning of movement (Freund, 1996). Its resection induces the classical “SMA syndrome” (Laplane et al., 1977), which is characterized by a complete akinesia and even a mutism (in cases of lesions of the left dominant SMA), and which occurs 30 min following the end of the resection, as observed in awake patients (Duffau et al., 2001a). Around the tenth day following surgery, this syndrome suddenly and spontaneously resolves, though some rehabilitation is often needed during 1– 3 months to provide for a complete functional recovery.

  Interestingly, using IOM combined with fMRI, it was shown that the occurrence of this syndrome is not associated with the volume of the frontal resection, but it is directly related to the removal of the specific structure called the “SMA-proper” (Krainik et al., 2001, 2003). Moreover, the existence of a somatotopy within the SMA-proper was demonstrated

  — namely, from anterior to posterior: the representation of language (at least in the dominant hemisphere), the one of the face, then the superior limb, then the inferior limb (immediately in front of the paracentral lobule) (Fontaine et al., 2002). It is now possible to predict, before surgery, if a SMA syndrome will occur postoperatively, the severity and pattern of this transient postoperative deficit (e.g., only mutism, or mutism and akinesia of the superior limb, or akinesia of the entire hemibody).

   

  Anarthria: Loss of the motor ability that enables speech

   

• Tumors, especially gliomas, invade both subcortical and cortical structures, and moreover, show an infiltrative progression along the fibers

• Because of this, definitive deficits may occur because of surgical damage to pathways running in the white matter (Coenen et al., 2001; Lang et al., 2001). It is now necessary to incorporate methods of subcortical mapping in addition to the IOM techniques, allowing the study of the sole cortical functional organization. As a consequence, in order to avoid postoperative sequelae, the study of individual anatomical–functional connectivity underlying the networks between the eloquent areas is also mandatory

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  Subcortical motor pathways

  • For patients harboring a precentral tumor, after detection and preservation of the primary motor cortical areas using IOM, it is also possible to detect the corresponding descending motor pathways, using subcortical DES, and their somatotopy — that is the different fibers of the corona radiata, with the pyramidal bundles of the lower limb medially, and laterally, with the bundles of the upper limb and face (Duffau et al., 2003b). As at the cortical level, these subcortical motor fibers constituted the posterior and deep functional limits of the resection. The pyramidal pathways may also be identified at the level of the posterior limb of the internal capsule. In particular, they can be identified in cases of frontotemporal–insular glioma, in which the deep boundaries of the resection were givenwhen subcortical stimulations induced motor responses in the inferior part of the corona radiata up to the superior part of the mesencephalic peduncles (Duffau et al., 2000a, 2002a).

  Subcortical somatosensory pathways

  • Similarly, the thalamocortical somatosensory pathways and their somatotopy can be identified by subcortical DES, which induces dysesthesias in awake patients, in cases of retrocentral tumors (Duffau et al., 2003b).

   

  In summary, IOM allows the neurosurgeon to improve his knowledge of the anatomical–functional connectivity, thus integrating, more easily and more systematically, the concept of subcortical mapping in his surgical strategy: first, because the gliomas involve both cortical and subcortical structures, and second, because lessons from stoke studies have shown that the white matter lesions elicited more severe and more permanent neurological deficits than cortical damage.