Anatomy of the interhemispheric fissure may widely influence feasibility and challenges in planning a transcallosal approach. The presence, number, and size of bridging veins should be carefully evaluated preoperatively. Even small bridging veins running from the frontal cerebral lobe to the superior sagittal sinus should be preserved to avoid postoperative edema and infarction leading to disastrous clinical outcome. After wide dissection of the interhemispheric fissure, both pericallosal and calloso-marginal arteries should be identified as anatomical landmarks before performing the callosotomy. The corpus callosum appears as a pale white-colored and hypovascular structure, usually well distinct from the cortex of the cingulate gyrus. A small incision along the sagittal plane, usually 15–20 mm, on the corpus callosum revealed the lateral ventricle chamber. At this point the thalamostriate and septal vein come into view. This step is mandatory to identify the foramen of Monro and, even more important, to be oriented in the discrimination of the left lateral ventricle from the right one. Posteriorly, thalamostriate and septal veins join each other to form the internal cerebral vein. On the lateral ventricle floor, the choroidal plexus hides the choroidal fissure, where the internal cerebral vein runs. Opening of the choroidal fissure may increase the third ventricle exposure allowing access of the middle and posterior portion of the third ventricle. The third ventricle is a deep-seated, narrow, vertically oriented, median cavity. Its roof goes from the suprapineal recess posteriorly to the foramen of Monro anteriorly, and it is formed by two thin membranous layers of tela choroidea, a layer of blood vessels between these sheets, and a neural layer formed by the fornix. The body of the fornix forms the anterior part of the roof, and the crura and hippocampal commissure form the posterior one. The velum interpositum is the space that contains the vascular layer, composed of the two medial posterior choroidal arteries and their branches and internal cerebral veins and their tributaries. The upper layer of the tela choroidea is attached to the lower surface of the fornix and the lower wall contacts the stria medullaris thalami and extends along the superomedial border of the thalamus, from the foramen of Monro to the habenular commissure; the posterior part of the lower wall is attached to the superior surface of the pineal gland. The paired strands of choroid plexus are attached to the lower layer of tela choroidea.

The floor extends from the optic chiasm to the opening of the aqueduct of Sylvius posteriorly; between those structures, the infundibulum of the hypothalamus, the tuber cinereum, the mammillary bodies, the posterior perforated substance (located in a space limited anteriorly and laterally by the optic chiasm and tracts, and posteriorly by the cerebral peduncles), and the part of the tegmentum of the midbrain located above the medial aspect of the cerebral peduncles are comprised. The posterior part of the floor extends posterior and superior to the medial part of the cerebral peduncles and superior to the tegmentum of the midbrain.

The boundaries of the anterior wall are formed by the optic chiasm, the optic recess, the lamina terminalis (a thin sheet of gray matter and pia matter that connect the chiasm with the rostrum of corpus callosum), anterior commissure (a 1.5–6 mm anterior-posterior diameter bundle of fibers that crosses the midline in front of the columns of the fornix), foramina of Monro, and the columns of the fornix. The lamina terminalis fills the interval between the anterior commissure and the optic chiasm [8]. The lamina attaches to the midportion of the superior surface of the chiasm, leaving a small cleft between the upper half of the chiasm and the lamina, called the optic recess.

The posterior wall, that extends from the suprapineal recess to the aqueduct of Sylvius, contains the habenular commissure, the pineal body and its recess, and the posterior commissure. The pineal gland projects posteriorly into the quadrigeminal cisterns and is concealed by the splenium of the corpus callosum above, the thalamus laterally, and the quadrigeminal plate and the vermis of the cerebellum inferiorly.

The patient is placed in supine position with the head in a neutral position, slightly flexed, and fixed in a three-pin Mayfield-Kees headholder. Some authors propose a lateral position with the operative side down to let the frontal lobe fall by gravity from the midline, avoiding unnecessary retraction during the interhemispheric fissure dissection. In our opinion the neutral position allows a better orientation in the midline and an improved cerebral venous outflow, while a strict microsurgical technique using sharp dissection and early CSF release minimize retraction damage to the frontal lobe. The approach is usually performed on the right side. An inverse U-shaped laterally based skin incision is usually performed 2/3 anteriorly and 1/3 posteriorly to the coronal suture. The skin incision can be placed more anteriorly to obtain a better exposure of the posterior portion of the third ventricle avoiding unnecessary manipulation of the prerolandic frontal lobe (Fig. 13.2). The use of neuronavigation to identify bridging veins may help in tailoring the position of skin incision and bone flap. The bone flap should be designed to expose the superior sagittal sinus in the midline. Usually a partial exposition of the sinus is enough to allow a direct access into the interhemispheric fissure, thus avoiding frontal lobe retraction. Before opening the dura, bridging vein position should be verified by means of neuronavigation. ICG video angiography may help in this task [9]. Dural opening is performed according to the bridging vein anatomy and with the base on the sagittal sinus. All the bridging veins should be carefully preserved. In some instances the bridging veins enter into a dural duplication before reaching the SSS. Careful incision of these dural duplications improves vein mobilization and access into the interhemispheric fissure. At this point sharp dissection is mandatory to widely open the fissure. Using cottonoids at the extreme anterior and posterior portion of the dissected fissure may help in maintaining it open avoiding the use of retractors. In case of tensive hydrocephalus, interhemispheric dissection may be very difficult; in such condition positioning of a ventricular catheter through the exposed frontal cortex may help in obtaining enough brain relaxation, facilitating arachnoidal dissection. Corpus callosum is a whitish, relatively avascular structure usually clearly different from the cortex of the cingulate gyrus. The correct identification of the pericallosal and calloso-marginal arteries is mandatory adjunctive landmarks. At this point, with the aid of the bipolar tips, a sagittal 15–20 mm wide incision is performed in the anterior portion of the corpus callosum. A transverse (coronal) callosal incision has been proposed by some authors especially in case of azygos anterior cerebral artery, with regard to neural fibers sparing [10]. In our experience, according to most other authors, the sagittal incision allows a wide exposure without permanent neuropsychological impairment. This deficit is most consequence of bilateral fornices manipulation more than callosal incision. The thickness of the corpus callosum is widely variable according to the preexisting hydrocephalus. Entering the lateral ventricle cavity allows further CSF outflow and brain relaxation. At this point a retractor may help in maintaining the fissure and callosal split opened to improve access to the deeper operative field. As soon as the lateral ventricle cavity is entered, orientation is granted by identification of foramen of Monro and thalamostriate vein. In the right lateral ventricle, the thalamostriate vein runs from the foramen to the right, while in the left lateral ventricle, it runs to the left (Fig. 13.3). If no vein is visualized, then a cavum septum pellucidum has been entered.