The insular area is a predilection site for gliomas, with insular gliomas accounting for up to 25% of low grade gliomas and 10% of high-grade gliomas. Due to the complex vascular network and functional areas around the insula, the surgical resection of insular gliomas remains a huge challenge for neurosurgeons. Despite this, the standard management of insular gliomas continues to remain maximum resection followed by chemoradiation therapy.
LLSAs, which originates from segment of M1, courses under the anterior perforated substance and generally supply the internal capsule and basal ganglia. The vascular supply of the insula is mainly provided by the M2 branches, which overlie the insular surface. The course of M2 segment along the surface of insula constitutes a rich arterial network, which represents a substantial obstacle to access the insular region[1,7,25], especially, when the arterial network was encased and distorted and wrapped by the tumors. LLSAs may also be encased by the tumor and be a source of vascular supply to the tumor. All these vascular characteristics of insular gliomas make the maximal resection difficult and increase the incidence of ischemia.
Moreover, in contrast to other glioma locations, the insula is largely and frequently supplied by perforating arteries with no collateral flow. There is new evidence that motor deficits frequently arise from ischemia of the pyramidal tract due to compromise of the blood vessels supplying the corona radiata and the internal capsule.
As to the insular glioma resection, most of the procedure was done among the main branches of M2, which may cause ischemia to the surrounding structure. Various mechanisms have been proposed for the development of intraoperative brain ischemia, including direct vascular damage, vasospasm, and kinking of arteries by the retraction of the brain [2,5]. Therefore, the identification and protection of blood vessels is particularly important for insular gliomas resection.
In our experience, as to the insular gliomas, we use different strategies according to the location tumor based on the Berger-Sinai classification. For the anterior location, the protection the initial segment of LLSAs is especially important. If the initial segment of LLSAs was encased by the tumor, which make it difficult to totally resect the tumor and increase the risk of motor deficits. Given our experience, in such situation, we made the “Residual Triangle” at the initial segment of LLSAs to preserve and avoid LLSAs distortion resulting in ischemia. In this study, the multivariate analysis showed no significant association between the encased initial segment of LLSAs and Critical Ischemia. Meanwhile, the volume of “Residual Triangle” less than 2% of the tumor volume, has little influence on the EOR. This may confirm the effectiveness of this strategy for protection of proximal of LLSAs. As to posterior tumor location, the protection of distal of LLSAs is the key point. Due to the basal ganglia was adjacent to or even invaded by the insular gliomas when there was no clear border between the tumor and the basal ganglia, “Basal Ganglia Reconstruction” was made base on the texture of basal ganglia and distal branches of LLSAs, which not only avoided the direct injury of the posterior limb of internal capsule but also preserved the distal branches of LLSAs. Based on the identification of the texture of basal ganglia and distal branches of LLSAs to control the depth of resection, direct injury to the white matter motor fibers is unusual , and also the tumor maybe maximally and safely resected. Finally, the protection of main branches of M2 is also important, the “Sculpting Technique” was used to outline the frame of the main branches of M2. In this way, we could cut off the blood supply arteries of the tumor (insular arteries) and protect the arteries that pass through (mainly branches of M2). All these strategies of arteries protection may verify the importance and validity of the protection of supply arteries of critical structure as to the insular glioma resection.
Deep small infarcts are commonly found after glioma surgery, which is also true for insular glioma. To achieve maximal resection, surgeon pay much attention to the protection of LLSAs, mainly branches of M2 and the long insular artery, some small artery may be need to be sacrificed which may result in ischemia but not cause neurological deficits. The surgical manipulation among the main branches of M2 might aggravate such situation. The incidence of such small ischemia of insular glioma resection has not been fully revealed and need further investigation. As to insular glioma surgery in this study, ischemic lesions were found in 44 (58.7%) cases on DWI, while Critical Ischemia were in 11 patients. Though the ischemic incidence was higher than some earlier reports[2,5], the insular tumor location had been shown to be the strongest risk factor for the development of intraoperative ischemia, comprising almost all of their cases with ischemia. In other reports, the incidence was as high as 100%. In this report, multivariate analysis showed that the Critical Ischemia incidence was significantly association with clear flat inner boundary. Our explanation may be that the clear flat inner boundary between the tumor and basal ganglia means the basal ganglia haven’t invaded by the tumor yet, the tumor maybe totally resected and the risk of LLSAs damage is less. Obscure boundary means the risk of LLSAs damage increased greatly. Through the “Basal Ganglia Reconstruction” strategy to control the depth of resection, the risk of distal of LLSAs maybe decrease, especially to the patients with obscure boundary.
Intraoperative detection of impending stroke is important for operation because a prompt response may improve the declines to transient instead of permanent deficits. Although intraoperative MRI was efficient in estimating EOR, its role in detecting ischemic lesions is not recommended. Thus, IONM is one such technique, and multiple studies have verified that IONM parameters including MEP and SSEP are useful and reliable for predicting and preventing ischemic brain injury in neurosurgery. In general, MEP is thought to offer better diagnostic accuracy than SSEP. The monitoring of intraoperative MEPs reveals early ischemia and even prevents it from becoming permanent through the implementation of a therapeutic response, such as holding surgery, irrigating with warm saline, and releasing retraction on the brain parenchyma[5,17]. In this study, MEP 50% decline was found to be significantly associated with Critical Ischemia, postoperative paralysis and three months paralysis, but not with No Critical Ischemia. Thus, intraoperative continue IONM is necessary for insular gliomas.
The main limitations of our study are its retrospective nature and the limited number of enrolled patients. Moreover, our strategies for insular gliomas on survival need longer time and more cases to certify. Finally, although No Critical Ischemia has no effect on motor deficits, its influence on neurocognitive functions needs further study.