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.