Anterior Transcallosal Approach Research Articles

Surgical approaches to the third ventricle of the brain in children

AIM: This study aimed to describe and analyze the advantages and disadvantages of various surgical approaches to neoplasms of the third ventricle of the brain in children.
 MATERIALS AND METHODS: This study analyzed surgical interventions to the third ventricle in 657 patients, performed at the Academician N.N. Burdenko of the Research Institute of Neurosurgery from 1998 to 2018. These included 375 patients with intra-extraventricular craniopharyngiomas and 282 patients with gliomas of the third ventricle and chiasm. The patients age ranged from 3 mon to 18 years old.
 RESULTS: The anterior transcallosal approach provides access to the anterior horn and bodies of the lateral ventricles, as well as the third ventricle. The transfornical approach provides more opportunities for access to both the anterior and posterior parts of the third ventricle; however, it has a high risk of trauma to the fornix. The subchoroidal approach provides a very good view of the posterior parts of the third ventricle, especially of the pineal region; however, it has even greater restrictions on viewing its anterior parts. When compared with the transcallosal approach, the transfrontal approach can be used more safely in the absence of hydrocephalus (if the tumor is located in the anterior horn). No specific complications were inherent in a particular approach (seizures were registered in 1%, transient hemiparesis was noted in 10%, and transient memory impairments were revealed in 5% of cases).
 CONCLUSION: The use of a transcallosal approach is safe even in infants. The transcortical approach is recommended mainly for large tumors of the lateral ventricles, and the transcallosal approach should be used for small tumors of the third ventricle. No specific complications were inherent in a particular approach, and the choice was determined by the assessment of the exact location of the tumor and calculation of the most relevant trajectory for its achievement as well as the aim (biopsy or radical removal). Analysis of magnetic resonance imaging and neuronavigation are significant in the selection of surgical approaches.

Dual approach for Difficult Intracranial Lesions and their Outcome in BSMMU: Experience of Two Cases

Some lesions in the brain are difficult to approach through a single common route / trajectory. For those, dual approach combined together may ensure easier safe removal with better outcome as we had lesions like left petroclival meningioma & large craniopharyngioma. Methods Two patients, one left petroclival meningioma and one suprasellar craniopharyngioma with intraventricular extension underwent surgery with dual approach at the Neurosurgery department, BSMMU. After detailed clinical and radiological evaluation, the left petroclival meningioma patient was planned for a combined pre and post sigmoid petrosectomy and retromastoid retrosigmoid route. These were accomplished in two stages, three months apart. The craniopharyngioma patient with intraventricular extension underwent surgery through pterional transsylvian and frontal anterior trans-callosal approach in a single sitting.
 Bang. J Neurosurgery 2020; 10(1): 111-118

Cognitive function assessment and comparison on lateral ventricular tumors resection by the frontal transcortical approach and anterior transcallosal approach respectively in children.

Currently, there are few studies on cognitive impairment caused by neurosurgery, and there have been no studies on cognitive impairment after resection of lateral ventricular tumors in children. Previously, our research team has reported that the frontal transcortical approach can impair cognitive function. In this study, we explored which functions would be damaged by the transcallosal approach and compared the cognitive function changes caused by the two surgical approaches, so as to provide a theoretical basis for the selection of pre-operative surgical approaches. The authors prospectively collected pediatric patients with lateral ventricular tumors who had undergone surgical resection through the frontal transcortical approach and anterior transcallosal approach in Beijing Tiantan Hospital from November 2012 to May 2017. The inclusion criteria according to the Children Wechsler Scale requirements and clinical performance were formulated. Wechsler Intelligence Scale for Children®-fourth edition: Chinese version (WISC-IV) was adopted for general intelligence and cognitive function assessment in the study. In addition, the resting-state functional magnetic resonance imaging (resting-state fMRI) and diffusion tensor imaging (DTI) were carried out to measure the level of co-activation and to explore the functional connectivity between the brain regions at the pre-operative period and 6-month follow-up in post-operation. A total of 30 patients were enrolled. Gross total resection was achieved in all patients, and no severe post-operative complications were observed. The frontal transcortical approach was applied in 19 patients, and the transcallosal approach was conducted for 11 patients. Compared with the pre-operative indices of WISC-IV, patients generally had a lower level of indices of the WISC-IV in post-operation. In patients accepting lateral ventricular tumors resection through the anterior transcallosal approach, the total IQ was declined to M = 84.82, SD = 8.072 from M = 93.27, SD = 6.635 within the 6-month convalescence. The data of working memory (t = - 2.990, p = 0.002) and total IQ (t = - 2.205, p = 0.028) pre- and post-operative showed statistical significance. But in the comparison of two surgical approaches, it was found that IQ had no statistical difference in WISC-IV tasks data. Previous studies suggest that the frontal transcortical approach impair perceptual reasoning, processing speed, and IQ, while this study indicates that the anterior transcallosal approach impairs patients' working memory and IQ. Both approaches make equal damage to IQ. Through comparing the two surgical approaches, it can be known that the anterior transcallosal approach cannot replace the frontal transcortical approach. The protection of cognitive function should be considered as one of the bases for neurosurgeons to select the operative approach before the operation. However, in an actual situation, the specific approach should be carefully selected by comprehensive consideration.

Human Midbrain Participates In Mismatch Detection Without Cognitive Activity Of The Cerebral Cortex: Case Report

Mismatch detection is the process of identification of rare stimuli from a sequence of stimuli. This important cognitive function is usually attributed to the cerebral cortex. To test the human midbrain involvement in sound mismatch detection, we recorded local field potentials in the states of deep anesthesia and clear consciousness from the drainage-electrode implanted in the cerebral aqueduct of an adult patient with an obstructive hydrocephaly who had undergone pineal region tumor removal through anterior interhemispheric transcallosal approach. We found a significant difference in the state of deep anesthesia at 256-364 ms after hearing a rarely presented sounds compared with frequently presented sounds and equally probable sounds. This difference was not found in the same experiment in the state of clear consciousness. The results suggest that human midbrain participates in mismatch detection and can do it even without cognitive activity of the cerebral cortex. Amplitude-frequency analysis of the midbrain records revealed that propofol affects the electrical activity of both human midbrain and cortex but the level of inhibition of the cortex is 6 times higher than the level of inhibition of the midbrain. We suppose that the human cortex is more susceptible to propofol than the human midbrain.

The Anterior Subcallosal Approach to Third Ventricular and Suprasellar Lesions: Anatomical Description and Technical Note

Surgical access to the third ventricle is challenging, given the depth of the operative field and close proximity of vital neural structures that must be traversed. For anterior third ventricular lesions, approach options include anterior transcallosal or transcortical, subfrontal, frontotemporal, or endonasal. The subcallosal approach, a translamina terminalis approach, is unique in that the surgical corridor is just below the corpus callosum, minimizes retraction and preserves corpus callosum integrity. Case examples are provided, and an anatomical study delineating the dimensions of the surgical corridor is performed. Two latex-injected cadaver heads were used to describe the subcallosal corridor. A magnetic resonance imaging scan was obtained and registered with neuronavigation for correlative anatomical illustration. Depth, dimensions, and cross-sectional area were measured for the subcommunicating and supracommunicating corridors. The surgical depth for anterior transcallosal, subcallosal, and subfrontal approaches was 7.5 cm, 7.7 cm, and 7.6 cm respectively. The average corridor dimensions for the subcallosal approach were 14.75 × 6.63 mm compared with 8.88 × 5.38 mm for the subcommunicating corridor. Cross-sectional area of the subcommunicating corridor was 30.62 mm(2) compared with 80.42 mm(2) for supracommunicating. This was easily enlarged to 156.62 mm(2) with gentle retraction. The anterior subcallosal approach is a safe approach for lesions of the third ventricle that avoids splitting the corpus callosum and resecting unnecessary brain and minimizes brain retraction. This corridor is superior to the traditional subfrontal approach in terms of working space and compares favorably to the anterior transcallosal approach without disrupting the corpus callosum.

A case of ganglioglioma extended to the lateral ventricle and associated with neurofibromatosis type 1

We encountered a rare case of intraventricular ganglioglioma associated with neurofibromatosis type 1. A 42-year-old woman presented with a feeling of heaviness of the head and dizziness. She was diagnosed with neurofibromatosis type 1 because she had multiple subcutaneous neurofibromas and café au lait spots. On admission, she deteriorated slightly(Japan Coma Scale 1)and suffered from cognitive dysfunction and right hemiparesis. A computed tomography(CT)scan showed that she had an obstructed hydrocephalus with a long and circular mass lesion, 2cm in diameter, in the anterior horn of the left lateral ventricle. The mass showed low signal intensity(SI)on the T1-weighted image(WI), heterogeneous high SI on the T2-WI, and dense enhancement on a Gd-DTPA contrast MRI, extending from the head of the left caudate nucleus to the lateral ventricle. The patient underwent an urgent operation via an anterior transcallosal approach because of an obstructed hydrocephalus. The tumor was removed in its entirety, including its origin at the caudate head. The pathological diagnosis was a ganglioglioma grade 1 according with the classification of the World Health Organization. Here we describe this case and discuss the rare association between gangliogliomas and neurofibromatosis type 1.

An Uncommon Cause of Headache

A 45 year old male presented with chronic headache for the last six months, with recent aggravation. The headache was holocranial in nature and not associated with vomiting, aura or any other neurological complaints. On examination, he had no neurological deficit. Contrast enhanced Magnetic Resonance Imaging (MRI), was performed which revealed features suggestive of intraventricular neurocytoma with intraventricular hemorrhage (Figs. 1-​-3).3). He underwent gross total excision of tumor after frontal craniotomy and through anterior transcallosal approach. Histopathology suggested a diagnosis of neurocytoma WHO grade II. Figure 1a Axial T1 weighted image of the brain revealed a mass lesion in right lateral ventricle with heterogeneous signal intensity and hyperintensity in the frontal horn of right lateral ventricle suggesting intraventricular hemorrhage. Figure 3 Axial T1 weighted contrast enhanced MR image of the brain revealed heterogeneous enhancement of the mass lesion.

Anterior callosal section is useful for the removal of large tumors invading the dorsal part of the anterior third ventricle: operative technique and results

Large tumors invading the dorsal part of the anterior third ventricle are difficult to manage. The anterior transcallosal approach is usually used to manage these tumors. In our clinic, anterior callosal section was combined with the anterior interhemispheric (AIH) translamina terminalis approach for these tumors with excellent results. The AIH approach is useful for removing tumors in and around the anterior part of the third ventricle. However, AIH alone is insufficient for large tumors invading the dorsal part of the anterior third ventricle. In such situations, simple anterior callosal section enables the neurosurgeon to extirpate the caudal part of the tumors deeply hidden from operative field, sparing the foramen of Monro, fornix, etc. We treated four large tumors (malignant teratoma, recurrent chordoid glioma, recurrent papillary tumor of pineal region occupying the third ventricle, and paraventricular meningioma) without major complications. The malignant teratoma case exhibited no recurrence with >10years follow-up. The chordoid glioma and papillary tumor of pineal region were totally removed. The meningioma was subtotally removed except only a small tumor around the bilateral anterior cerebral artery. This simple technique is a new way to manage difficult large lesions in and around the third ventricle.

Anterior interhemispheric transcallosal approach for removal of central neurocytoma in lateral ventricle

Objective To investigate the effect of anterior interhemispheric transcallosal approach for removal of central neurocytoma in lateral ventricle.Methods From Apt 1996 to Jun 2010,central neurocytomas in lateral ventricles were removed through anterior interhemispheric transcallosal approaches in 16 cases,and the clinical data were retrospective analyzed.Results The diameters of the tumors were between 2 centimetres and 4 centimetres in 7 cases,more than 4 centimetre in 9 cases.The bottom of the tumor was located in septum pellucidum in 9 cases,in lateral wall of the ventricle in 7 cases.7 cases had hydrocephalus.Total removal was achieved in 12 cases.4 patients of subtotal resection of tumors received radiotherapy after operation.There was no perioperative death.14 patients were followed - up from 1year to 7 years and there was no recurrence.Conclusions The anterior interhemispheric transcallosal approach is a valid choice for removal of central neurocytoma in lateral ventricle.Through this approach,protection of normal brain tissue by using of natural brain cavity and prevention of postoperative complications could be achieved.The relationship between the tumor and lateral wall or bottom of ventricle is the main factor that influences the total removal of tumor. Key words: Anterior interhemispheric transcallosal approach; Microsurgery; Central neurocytoma; Lateral ventricle

Intraventricular cystic meningioma

We report a case of a 45-year-old male patient with intraventricular cystic meningioma located in the left lateral ventricle. He presented with complaints of global headache, progressively increasing loss of memory, and frequent episodes of abnormal behavior, of 1 month duration. At the time of hospital admission, his general and neurological examination was normal. Neuroimaging studies showed a left lateral ventricular enhancing mass, composed of mixed solid and cystic areas. The tumor was completely excised via the anterior transcallosal approach. A histological examination revealed a meningothelial meningioma without any atypia. The aim of this report is to present the occurrence of an intraventricular cystic meningioma.

Subependymoma in the Lateral Ventricle Manifesting as Intraventricular Hemorrhage -Case Report-

A 32-year-old man presented with subependymoma in the lateral ventricle causing intraventricular hemorrhage and manifesting as severe headache and disturbance of consciousness. Computed tomography on admission showed a massive intraventricular hemorrhage and acute obstructive hydrocephalus. Cerebral angiography revealed no abnormal findings. Emergency external ventricular drainage was performed, and his neurological deficits gradually improved. Magnetic resonance imaging at 5 weeks after admission showed a tumor arising from the septum pellucidum or the floor of the right lateral ventricle, appearing as a mixed-intensity solid tumor, which was partially enhanced following gadolinium administration. The tumor had arisen from the septum pellucidum and was totally removed via an interhemispheric anterior transcallosal approach. Histological examination found typical subependymoma, with little vascularity. Intraventricular hemorrhage from cerebral neoplasms is usually due to highly vascular tumors. Since subependymomas are quite benign and show poor vascularity, intraventricular or subarachnoid hemorrhages are very rare, but do occasionally occur.

Clinical features and management of five patients with supratentorial subependymoma

Subependymoma is a rare low-grade glioma of the central nervous system that is often asymptomatic and discovered incidentally. If symptomatic, however, its clinical features vary widely. We report and discuss the management of five surgical patients (two males and three females, 27–65 years old). All tumors developed supratentorially around the foramen of Monro, and their diameters ranged from 18 mm to 90 mm. Of the three symptomatic patients, one presented with sudden loss of consciousness despite having only a small tumor, while in contrast, another patient showed only gradual memory disturbance despite having a very large tumor (90 mm diameter). Two patients were asymptomatic but had ventricular dilatation. Gross total resection was achieved via either a transcortical (three patients) or an anterior transcallosal approach (two patients). No recurrence was observed during follow-up. We concluded that as surgery is the only curative treatment for subependymoma, and even a small tumor can present with sudden deterioration, we recommend early and total resection.

Microanatomy of cerebral veins related to approaches through interhemisphere fissure and its clinical application

Objective To study means by which we can gain ideal operative exposure without hurting bridging veins in approach through interhemisphere fissure by microsurgical anatomy study and superficial location of superior cerebral veins. Methods The superior cerebral veins of twenty adult cadaveric heads fixed with formalin were anatomized and measured under operative microscope.Results (1)There was a segment of in posterior part of frontal region of superior sagittal sinus in which there was no bridging vein flows, which could be named as the first safe region;the occipital region of superior sagittal sinus had a segment of regiontoo, which could be named as the second safe region.The first safe was located 32.6 mm in front of coronal suture and 7.5 mm behind coronal suture,the second safe regionwas located between point 5.5 mm below lambdoid suture and external occipital protuberance.(2) Fully separation of bridging veins near interhemisphere fissure could broaden opening width of interhemisphere fissure.As data showed,it could broaden opening wi dth at 2～3 mm in frontal polar, 4～11 mm inthe first safe region,10～17 mm inthe second safe region. Conclusions (1)The external points opening cranial bone in anterior transcallosal approach should be set before 32.6 mm in front of coronal suture and 7.5 mm behind coronal suture.The external points opening crinial bone in Poppen approach should be set 5.5 nun below lambdoid suture and external occipital protuberance. (2) Fully separation of sticking segment and arachnoid segment of bridging veins in interhemisphere fissure could broaden opening width of interhemisphere fissure without raising tension of bridging veins, thus bridging veins could be protected. Key words: Interhemisphere fissure; Cerebral veins; Anatomy; Microsurgery

Hemangiopericytoma arising in the body of the lateral ventricle

A 65-year-old woman presented with a very rare hemangiopericytoma in the body of the lateral ventricle.Magnetic resonance imaging demonstrated a homogeneously enhancing mass lesion occupying the bilateral medial portions of the body of the lateral ventricle. Cerebral angiography disclosed a vascular-rich tumor, fed mainly by the left lateral posterior choroidal artery. After devascularization of the feeding vessel by endovascular coiling, the patient underwent complete surgical excision of the tumor via an anterior transcallosal approach, followed by radiation therapy, and has thus far been disease-free for 5 years. The present patient represents the first reported case of hemangiopericytoma arising in the body of the lateral ventricle. In this location of hemangiopericytoma, preoperative embolization could provide a promising option in terms of reducing the intraoperative blood loss and achieving total tumor extirpation with minimum damage to the surrounding structures. Since preoperative identification of hemangiopericytomas confers therapeutic advantages,it is important to be aware that they can occur at unusual ventricular sites.

Glioblastoma multiforme of the lateral ventricle: report of nine cases

Glioblastoma multiforme is the most common lethal primary central nervous system (CNS) tumour in adults and they are rarely seen as primary intraventricular tumours. We present nine cases with lateral ventricle glioblastoma multiforme treated in our department. Seven of the nine tumours were located at the body of the lateral ventricle, whereas the other tumours were arising from septum pellicidum. Five patients were operated through an anterior transcallosal approach and four patients were operated through frontal transcortical approach. Total tumour excision was achieved only in one patient. Median survival time was 18.8 months. Three patients were alive at the end of study, and received radiotherapy and chemotherapy. Our goal was the decompressive surgery for these tumours without harming eloquent structures. In our opinion, patient's life quality is critically important in the postoperative period for these mortal tumours with rapid progression.

Idiopathic distal lenticulostriate artery aneurysm in a child

The authors describe a rare case of idiopathic distal lenticulostriate artery (LSA) aneurysm in a 5-year-old boy who presented in the emergency department with a sudden onset of headache. Admission computed tomography scans revealed an intracerebral hemorrhage in the left caudate nucleus with intraventricular extension. Angiographic studies demonstrated a left medial LSA aneurysm. The patient underwent a left parasagittal frontal craniotomy, the lateral ventricle was accessed via the anterior transcallosal approach, and the aneurysm was removed after sectioning of the parent vessel. The child left the hospital after 5 days; at that time he was asymptomatic and without motor impairment. The optimum treatment of aneurysms involving small perforating arteries is controversial and depends mainly on the causative factors. The pathogenesis and treatment of these unusual aneurysms are discussed.

Anterior transcallosal approach to the colloid cysts of the third ventricle: case series and review of the literature.

To evaluate the results of the anterior transcallosal approach to the colloid cysts of the third ventricle. A retrospective analysis of the patients operated on between 1986 and 2003 was carried out. There were 19 patients (10 female, 9 male) with a median age of 43. The main presenting symptom was headache. One of the patients presented with acromegaly due to a pituitary tumor. The size of the cysts ranged from 15 to 43 mm. An anterior transcallosal approach was used in all patients. The cysts were excised totally in all cases. Postoperatively no recent memory loss has been detected in any patient. One patient with hemorrhagic papil stasis experienced temporary visual worsening. In one patient with hydrocephalus a ventriculoperitoneal shunt was needed. One patient with postoperative superior frontal gyrus venous infarction had a seizure. The follow-up period was from 1 month to 13 years (mean 5.2 years). To date, there has been no recurrence so far. The anterior transcallosal approach is a safe method for the treatment of third ventricular colloid cysts.

Commentary on the Article of M. N. Pamir, S. Peker, S. Özgen, T. Kılıç, U. Türe, M. M. Özek: Anterior Transcallosal Approach to the Colloid Cysts of the Third Ventricle: Case Series and Review of the Literature

The paper of Pamir et al. is an interesting contribution to the current debate about the optimal treatment of colloid cysts of the third ventricle. It deserves some comment though. The statement that colloid cysts should be completely excised is very welcome and cannot be repeated often enough. But then there is the issue of choice of the microsurgical approach. It is very rare to find a case in which there is not at least unilateral enlargement of the ventricles and therefore also thinning of the cortex. When comparing the transcortical and transcallosal approach, no proof is provided to date that the transcallosal approach is generally better in all cases. From the literature it may even be concluded that it is more dangerous and has more complications and that is easily understood when just looking at the beginning of the surgery where the interhemispheric cleft has to be opened in the presence of dilated ventricles and relatively raised intracranial pressure. When a small precoronary craniotomy is used and ultrasound guidance, a sulcus can be opened and from the bottom the ventricle punctured. Following that ventriculostomy path a white matter opening of 8 mm to 10 mm is sufficient to gain access and exposure of the foramen of Monro. With collapsing ventricles and relieved pressure this opening is mobile and can be moved as angulation requires. This allows for a constantly changing keyhole view which meets the foramen from a lateral perspective and allows for very good views inside the third ventricle. The beauty about colloid cyst surgery is the consistency of most cysts which can be collapsed and then detached from all their adherences in the choroids plexus. The transcallosal approach does not allow that flexibility and angulation because the corpus callosum is much more rigid than a thinned cortex and each change in the angulation of instruments or retractors will be accompanied by more injury fiber dissection. It is true, that the transcortical approach is used best only for cysts and that there may be large cysts with calcifications or extensions far back into the third ventricle where the shape of the cysts already leads to the suspicion that it will be adherent and cannot be mobilized. Then, as in tumors, a direct approach into the third ventricle may be needed and that may undisputedly be best achieved by transcallosal routes. But that is mainly for entities other than cysts and in that respect, one of the shortcomings of this article is a mix up of literature arguments for the virtues of the transcallosal approach in which many literature references are textbook articles and not original research series which refer to technique but not the issues at question. They mostly refer to tumors of the third ventricle and not colloidal cysts. It is problematic to compare apples to oranges. A tumor of the third ventricle should by all means be exposed by the transcallosal route which is optimal in that case. Of course is the transsection of the fornices associated with poor outcome but that must not occur in the present age when mobilizing a colloidal cyst.

Transcortical or transcallosal approach to ventricle-associated lesions: a clinical study on the prognostic role of surgical approach.

Most entities in and around the anterior two-thirds of the supratentorial ventricles can be reached via transcortical or transcallosal approach. This study examined the effect of surgical approach on the postoperative neurological outcome. Thirty-eight patients with intra- and periventricular supratentorial lesions were operated on by either frontal transcortical or anterior transcallosal approach. Postoperative diencephalic damage occurred in 22% of patients in the transcortical group and in 36% in the transcallosal group; transient mutism was virtually equivalent in the two groups. Postoperative epilepsy (26%) and subdural fluid collections (30%) occurred only in the transcortical group. The incidence of postoperative hemiparesis was higher in the transcallosal group. There was a high correlation between postoperative Glasgow Outcome Score of 5 and preoperative severity of neurological disease but no correlation between postoperative Glasgow Outcome Score of 5 and location of the lesion or between postoperative clinical course and surgical approach. Surgical outcome of ventricle-associated lesions depends mainly on the severity of preoperative symptoms and not on surgical approach. Additionally, the incidence of postoperative seizures and subdural fluid collections after transcortical surgery is high.

Interactive image-guided transcallosal microsurgery for anterior third ventricular cysts.

Although the treatment of colloid cysts remains controversial, high viscosity and small volume are poor prognostic indicators for successful burr hole aspiration and cortical incision via craniotomy may be associated with postoperative epilepsy. The anterior transcallosal approach provides a direct and adequate pathway to the lateral ventricles, where the foramen of Monro serves as a natural entrance into the anterior third ventricle, especially when the foramen is dilated by a lesion. When the midsuperior portion of the IIIrd ventricle cannot be reached, the interforniceal or the subchoroidal exposures have been advocated. Stereotactic techniques contribute to a minimal invasive approach and reduce morbidity. Nine patients harbouring anterior third ventricular cysts (seven colloids and two intrinsic craniopharyngiomas) underwent anterior transcallosal microsurgical excision assisted by an interactive infrared-based image guided system (EasyGuide, Neuro, Philips). There were 4 men and 5 women ranging in age from 15 to 42 years (mean 28.5). Transcallosal transforaminal (5 cases) or interforniceal (4 cases) approaches allowed total excision in eight patients and subtotal in one. Postoperative morbidity included a case of transient hemiparesis and a case of transient short-term memory disturbances; both resolved in the first months. Mortality was zero. Particular advantages of the method were accurate trajectory and position of callosotomy incision determination, visualisation and avoidance of superior saggital sinus, retraction of bridging veins and the often variable pericallosal arteries, spatial orientation within the ventricular system, and identification of the periventricular anatomical structures.