Though challenging, maximal resection of insular and thalamic gliomas enhances overall and progression-free survival. Both deep seated locations must be approached with careful attention both during surgical approach as well as throughout the resection. Here we review surgical anatomy, indications for surgery, and complication avoidance with particular attention on techniques to avoid violation of cortical and subcortical functional areas and vascular injury. Grading systems such as the Berger-Sanai classification system offer a framework through which to predict extent of resection and perioperative and morbidity. Surgical adjuvants such as cortical and subcortical direct stimulation mapping and fluorescence guided surgery can be used to minimize surgical morbidity while maximizing extent of resection.
Keywordsglioma, glioblastoma, insula, thalamus, oligodendroglioma, astrocytoma
Insular and thalamic intrinsic brain tumors pose perioperative risk due to their proximity to functional cortical and subcortical pathways and neurovascular structures.
Preservation of M2 insular and lenticulostriate arteries is critical to prevent postoperative stroke.
Cortical and subcortical mapping to identify language and motor sites, particularly the posterior limb of the internal capsule, limits the risk of postoperative deficits.
The role of surgery in the treatment of intrinsic brain tumors is to establish the correct histologic and molecular diagnosis, relieve mass effect, and provide maximal safe resection to improve both overall and progression-free survival. Nearly 50% of tumors are within difficult-to-access areas, with either presumed functional significance or close association with vascular structures. Surgical decisions therefore must balance reduction of tumor volume with avoidance of important neurovascular structures. Gliomas are the most common primary intrinsic brain tumor. The majority of gliomas are located in the cerebral hemispheres; however, 6.4% are located within the deep structures of the cerebrum, including primarily the insula and thalamus. Tumors within the insula and thalamus remain a challenge to manage given proximity to functionally significant areas and intimate relationship with vascular structures. Surgical techniques such as awake craniotomy with cortical and subcortical mapping permit maximal extent of resection while minimizing postoperative morbidity. Intraoperative violation of cortical and subcortical functional pathways may lead to immediate neurologic sequelae, negatively impacting quality of life and survival. Additionally, injuries to branches of the middle cerebral artery or lenticulostriate arteries are well reported and often have catastrophic consequences, which can range from (1) stroke to (2) hemorrhage, (3) vasospasm, or (4) thrombosis. This chapter will discuss surgical approaches for intrinsic brain tumors located within the thalamus and insula.
Indications for Surgery
Gliomas and brain metastasis are the most common deep-seated intrinsic brain tumors, with the most robust body of literature in support of maximal safe resection. There are four histologic grades for gliomas recognized by the World Health Organization (WHO). Grade I tumors have minimal proliferative potential and circumscribed growth. WHO grade II gliomas include diffuse astrocytoma, pleomorphic xanthoastrocytoma, and oligodendroglioma. These tumors have low mitotic activity; however, given their infiltrative nature, they have a tendency to recur, albeit most commonly near the site of initial presentation. WHO grade III gliomas, such as anaplastic astrocytoma, display nuclear anaplasia and increased cellularity. Glioblastomas are WHO grade IV gliomas and represent the most common primary brain tumor in adults. Numerous studies have examined the relationship between extent of resection and volume of residual tumor, overall survival, progression-free survival, and time to malignant transformation among patients with low- and high-grade gliomas. Although no class I data exist, the majority of published reports suggest that greater extent of resection improves overall survival and progression-free survival, and lengthens the time to malignant transformation.
Anatomic Insights and Surgical Approach
Intrinsic brain tumors within the insula or thalamus were previously considered inoperable due to the high risk of perioperative complications. However, a combination of improved microsurgical techniques, neuroanesthesia, and advanced structural and functional imaging permit greater access to many insular and thalamic tumors. Surgical approach and technical considerations for tumors within the insula and thalamus are discussed in this chapter.
Approaches to the Insula
Opercular landmarks at the cerebral surface may be beneficial to localize structures within the insula. The insula is a triangular-shaped structure within the sylvian fissure, which lies deep to the frontal, parietal, and temporal lobes. The optimal approach to insular lesions may require either splitting the sylvian fissure or resecting the overlying operculum; therefore it’s critical to know both sylvian fissure anatomy and cortical opercular landmarks. The sylvian fissure consists of a central stem in addition to horizontal, anterior ascending, and posterior rami. The longest portion of the sylvian fissure is the posterior ramus, which extends posterior and superior and terminates in the inferior parietal lobule. The anterior horizontal and anterior ascending rami are shorter and divide the inferior frontal gyrus into the pars opercularis, orbitalis, and triangularis. The insular surface faces laterally and is enclosed by the anterior and posterior limiting (also known as circular) sulci. The limiting sulcus has anterior, superior, and inferior parts.
The insular cortex is composed of an anterior limen insula, central sulcus, three anterior short gyri, and two posteriorly placed long gyri. The central sulcus of the insula is a continuation of the central sulcus of the cerebral hemispheres. Two anterior sulci separate the three short gyri, and a single sulcus separates the two long posterior gyri. The insular pole is located at the anteroinferior edge of the insula, where the short gyri converge to form a rounded area lateral to the limen. The insular apex is the highest and most prominent laterally projecting area on the insular convexity. Lying underneath the cortical surface of the pars opercularis is the superior portion of the anterior and middle short insular gyri. Posteriorly, the supramarginal gyrus overlies the superior limiting sulcus and the superior portion of the posterior long gyri. The limen insula overlies the uncinate fasciculus. Additionally, the anterior perforated substance lies medial to the limen. The middle cerebral artery bifurcates at the limen insula, forming M2 branches, which overlay the insular surface.
Insular tumors are among the most challenging neurosurgical lesions to manage. Tumor location and hemisphere of language dominance determine whether a transcortical or transsylvian approach should be considered. Patients are put in a semilateral position with the head parallel to the floor. For tumors located predominantly above or below the sylvian fissure, the vertex of the head is positioned 15 degrees toward the floor. The craniotomy is tailored based on tumor location and involvement of the overlying frontal or temporal operculum. Insular tumors may be approached based on their location within four zones. The sylvian line divides the insular cortex into dorsal and ventral parts. The foramen of Monro divides the insula into rostral and caudal portions, creating four zones. The transcortical exposure offers the maximal insular exposure with the widest surgical window and surgical freedom. The insular surface is exposed and surgical resection continues as the vessels of the sylvian fissure are skeletonized, which creates “surgical windows.” The resection is continued by working under the sylvian fissure along the uncinate fasciculus. Cortical and subcortical sensorimotor and language mapping may be utilized, particularly for posterior zone 2 and 3 tumors. The suprasylvian lenticulostriate arteries must be identified and preserved, and subcortical motor mapping of the corticospinal tract marks the medial border of the resection.
Approaches to the Thalamus
The choice of surgical approach for thalamic tumor resection balances tumor location, vascularity, and the location of the posterior limb of the internal capsule. Thalamic tumors are relatively rare, representing 2% of all brain tumors. Although lesions in this location have historically been surgically treated with stereotactic biopsy alone, advanced structural imaging and microsurgical techniques have allowed for surgical resection with acceptable perioperative morbidity. The surgical corridor of choice uses the shortest route to the tumor avoiding the internal capsule and normal thalamus. These decisions are made based on careful study of preoperative axial and coronal magnetic resonance imaging (MRI) with addition of diffusion tensor imaging (DTI) tractography. Operative approaches include (1) middle temporal gyrus approach, (2) occipital transtentorial approach, (3) middle frontal gyrus approach, (4) transcallosal approach, and (5) combined approaches.
Surgical resection is reserved for contrast-enhancing tumors with clear margins on preoperative imaging. Nonenhancing tumors with poorly defined margins should be treated with biopsy alone given their propensity to have functional tissue within the lesion. Surgical approach is based on tumor location (anterior or posterior within the thalamus) and proximity to the posterior limb of the internal capsule. The middle temporal gyrus approach is used predominantly for posterolateral tumors close to the temporal horn of the lateral ventricle. The occipital transtentorial approach offers maximal exposure for posterior medially placed thalamic tumors in close proximity to the third ventricle. The middle frontal gyrus approach is used for anterolateral thalamic tumors extending superiorly into the frontal lobe. The transcallosal approach is rarely used but is saved for anteromedial thalamic tumors. The majority of deep-seated intrinsic tumors cause anterolateral displacement of the posterior limb of the internal capsule, making the middle temporal gyrus corridor the common approach. After a temporal corticectomy, the tumor is approached through the lateral ventricle along the posterolateral margin of the tumor. Upon reaching the temporal horn of the lateral ventricle, the tumor is approached through the choroidal fissure. This approach ensures an entry corridor inferior to the insula and posterior to the internal capsule.
Surgery for intrinsic brain tumors within the insula and thalamus carries risk of developing postoperative medical and surgical complications. Neurologic complications may produce visual field, motor, sensory, cognitive, or language deficits. They result from violation of functional cortical and subcortical pathways, cerebral edema, hematoma, or vascular injury. In most series, the risk of a new neurologic deficit after craniotomy for resection of a thalamic or insular tumor ranges from 10% to 25%. These risks increase with older age, deep tumor location, tumor proximity to functional regions, and a low Karnofsky Performance Scale score at presentation. Neurologic complications can be minimized by individualizing the surgical approach based on anatomic and functional imaging, cortical mapping techniques, minimizing excessive brain retraction, meticulous hemostasis, and early identification of major vascular structures.
Additional complications are related to the surgical wound and surrounding brain parenchyma. These events include surgical wound infections, pneumocephalus, cerebrospinal fluid (CSF) leaks, hydrocephalus, seizure, brain abscess/cerebritis, meningitis, and pseudomeningocele. These complications occur in 1% to 5% of patients undergoing craniotomy for resection of an intrinsic brain tumor, and happen more readily in patients over the age of 65 years and in those undergoing reoperation. Posterior fossa location and reoperations are associated with a higher rate of pseudomeningocele, CSF fistula, and hydrocephalus. Postoperative wound infections occur in 1% to 2% of patients after supratentorial craniotomy, most commonly from Staphylococcus or Staphylococcus species. The risk of postoperative seizures after supratentorial craniotomy is 0.5% to 5%.