and intraoperative histology, there can be no doubt that more low-grade gliomas are now suitable for an attempt at surgical cure than in the past. As discussed earlier, most of these tumors do not cross the pia arachnoid and hence are often well demarcated on the surface of the brain. The typical appearance of such a tumor is of an area of expanded, abnormally pale gyri, bounded by sulci. This superficial area can be identified in most cases by the experienced surgeon by eye alone, but others will find neuronavigation helpful. The area that is directly infiltrated is unlikely to be functional, but if eloquent speech or motor cortex is nearby, they should be identified by cortical mapping. The tumor can cause considerable distortion of the surrounding normal gyri, and the conventional anatomical and radiological landmarks for cortical localization no longer apply. In some cases it can be surprisingly difficult to know whether the tumor is in front of, or behind, the central sulcus. If tumors do not present on the cerebral surface, there can be considerable difficulties in finding them without navigation or ultrasound, but since they are more probably in the deep white matter, it is also less likely that they are suitable for an attempt at radical resection.
The reason why most low-grade gliomas are not curable by surgery is because they invade the deep white matter. It is impossible to establish any kind of surgical plane here. The marginal infiltrated areas of the brain adjacent to the central bulk of the tumor will look and feel no different from normal brain to the surgeon. Once one is operating in the deep white matter, there is also the risk of causing extensive neurological deficits from both undercutting the adjacent cerebral cortex and disrupting the association tracts. Neuronavigation will not help in the deep white matter as a result of both brain shift and distortion produced by surgery and the fact that MRI scanning often does not define the true boundaries of the tumor. The surgeon can only be guided by smear marginal biopsies (in some cases the author has sent more than 60 such biopsies during an operation) and by the patients themselves, who will need to be kept awake so that if any relatively deep resection is being carried out and a developing neurological deficit is identified early during resection, any further resection can then be abandoned.
Awake craniotomy allows the surgeon to operate with greater confidence close to eloquent areas of the brain, but "complete" removal of these tumors will often remain impossible. One difficulty with the awake-cran-iotomy technique is that patients can develop a degree of neurological deficit while the resection proceeds, which subsequently recovers. It can be a question of fine judgement as to when to abandon further resection as a deficit develops. It is only possible to carry out simple neurological testing during awake craniotomy - in particular of limb movements and speech, and sometimes of the visual fields. It is not possible to assess more subtle cognitive functions, but the patient's general alertness and responsiveness can serve as a guide to these to some extent. It is most important that the awake craniotomy is supervised by an anesthetist with experience in this area . In skilled anesthetic hands it is remarkable how relaxed, pain free, cooperative and alert patients can be, despite being subjected to a very extensive craniotomy.
Illustrative Example: a 26-year-old woman with a single generalized epileptic fit (Figs. 9.6, 9.7, 9.8)
The scan shows a large tumor arising in the region of the right primary sensory cortex. This was judged to be potentially resectable. Histology showed the tumor to be an oligodendroglioma, and she remains free from fits and without neurological deficit. Follow-up scanning shows persistent abnormality in the brain adjacent to the resection cavity. The abnormality has become a little smaller over time but it must remain likely that there is residual tumor here.
Comment: The very long natural history of oligodendrogliomas must be remembered when dealing with cases such as this. The author remains undecided as to what to recommend if further follow-up scans suggest tumor recurrence.
The interpretation of post-operative scans can be very difficult. Flair MRI sequences are much more sensitive to both post-operative changes and recurrent tumor than are other sequences. Areas of signal change in the brain adjacent to the resection cavity may represent post-operative inflammatory effects or tumor
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