Incisural Space Research Articles

The Use of the Suboccipital Transtentorial Approach to the Posterior Inferior Incisural Space.

Objective To describe our experience with the microsurgical technique of the suboccipital transtentorial (SOTT) approach in the removal of posterior fossa lesions located in the posterior incisural space. Method Between 2002 and 2020 we reviewed all patients who underwent microsurgical resection of lesions of the posterior incisural space at the Department of Neurosurgery, Essex Neuroscience Centre, London, England (eight patients, male to female 3:5, mean age: 51, range 35-69). We describe the preoperative symptoms, radiological findings, surgical techniques, histologyand postoperative outcomes in this cohort of patients. Results Eight patients with tumours located in the posterior incisural space underwent surgery during the study period including four meningiomas (50%), two haemangioblastomas (25%), one metastasis (13%) and one giant prolactinoma (13%). Gross or near total resection was achieved in six patients (75%): the giant prolactinoma could not be radically removed and one of the meningiomas required a small fragment to be left in place to protect the Vein of Galen.No patient developed a visual field deficit due to occipital lobe retraction.One patient developed a temporary trochlear nerve palsy (13%). Five patients had mild disability (Glasgow Outcome Scale (GOS) = 5), and four had moderate disability (GOS = 4). Conclusion In our series, the SOTT approach provided excellent access for all cases of tumours in the posterior incisural space. The tumour's size and relationship to the deep venous system contributed to the choice of approach and in one patient who had previously undergone surgery via the supracerebellar route, the SOTT approach enabled the avoidance of gliotic scar tissue. Success is dependent on careful case selection, though from our series of 8 patients, we conclude that this approach allows safe access to the posterior incisural space, with acceptable outcomes with regard to postoperative disability and cranial nerve palsy. As such, the approach should be in the armamentarium of any neurosurgeon who regularly deals with posterior fossa pathology.

Endoscopic Lateral and Superior Cerebellar Keyhole Approach to the Anterior and Middle Incisural Space and Meckel Cave: An Anatomic Study

We sought to assess the feasibility of endoscopic lateral and superior cerebellar keyhole approach for exposure of the anterior and middle incisural space and Meckel cave. The endoscopic lateral and superior cerebellar keyhole approach was performed in 6 cadaveric heads (12 sides) using 0- and 30-degree endoscopes, respectively. The anatomic structures for this approach to the anterior and middle incisural space and Meckel cave were observed. By grinding out the suprameatal tubercle and petrous apex and incising the tentorium, the anatomic structures in the anterior incisural space were visualized. The mean area exposed with a 0-degree endoscope in the anterior incisural space was 212.50 ± 6.04 mm2, significantly less than that exposed with a 30-degree endoscope (233.83 ± 8.72 mm2) (P < 0.05). The anatomic distance of the Meckel cave in the depth was the same between a 0-degree endoscope and a 30-degree endoscope; however, the distance in the width was 9.48 and 12.32 mm, respectively (P < 0.01). The area of petrous window grinded by a 30-degree endoscope was only increased by 5.83 mm2, compared with a 0-degree endoscope (P > 0.05). This approach provides access to the anterior and middle incisural space and Meckel cave, which is feasible to clearly expose the anatomic structures in those regions with minimal invasiveness. Additionally, better visualization and surgical space can be achieved under a 30-degree endoscope.

Cavernous Malformations: What They Have Taught Us.

Cavernous Malformations: What They Have Taught Us.

Petroclival Clinoidal Folds and Relationships with Arachnoidal Membranes of Medial Incisural Space: Old Neuroanatomical Terms for a New Neurosurgical Speech in Cadaver Labs with Limited Resources Era. Part III: Arachnoid Membranes, Cranial Nerves, and Surgical Implications.

BACKGROUND Anatomical dissections play an irreplaceable role in the training of new generations of effective neurosurgeons, especially when addressing skull base lesions is required.The Authors describe an inter-laboratory dissection study aimed at improving the knowledge of a complex region of the skull base. The anterior and middle incisural spaces are of remarkable anatomical and surgical interest due to complex relationships between bony, dural, arachnoidal, and neurovascular structures. The primary purposes of this study are to describe the anatomy of this region with particular emphasis on the relationships between the anterior margin of the free edge of the tentorium and the sphenoid and petrous bone; to identify surgical implications in many different types of neurosurgical procedures dealing with this challenging complex anatomic area.METHODS Thirteen anatomical specimens, including five injected specimens, were dissected in this study. In the formalin-fixed specimens, vessels were injected with colored silicone.RESULTS The anatomical study focused on the description of the relationships between bony dural, arachnoid, and neurovascular structures. Surgical implications are described accordingly.CONCLUSIONS Detailed anatomical knowledge of this region finds concrete applications in neurosurgical practice since the anterior and middle incisural spaces are often surgically exposed in neoplastic and vascular diseases. The high-definition pictures reported in this study could represent useful support to understand the anatomy of this complex region.Finally, our study could provide guidance to neurosurgical centers in which resources are limited that are either planning to establish their own cadaver dissection laboratory or failed to do so because of the supposed high-costs.

Subtemporal Approach and Its Infratentorial Extension: Review and a Comparative Analysis of Different Techniques.

Introduction Surgical resection of lesions occupying the incisural space is challenging. In a comparative fashion, we aimed to describe the anatomy and surgical approaches to the tentorial incisura and to the rostral brainstem via the intradural subtemporal approach and its infratentorial extensions. Methods Six fresh human head specimens (12 sides) were prepared for the microscopic dissection of the tentorial incisura using the intradural subtemporal approach and its infratentorial extensions. Endoscope was used to examine the anatomy of the region inadequately exposed with the microscope. Image-guided navigation was used to confirm bony structures visualized around the petrous apex. Results Standard subtemporal approach provides surgical access to the supratentorial brainstem above the pontomesencephalic sulcus and to the lateral surface of the cerebral peduncle. The linear or triangular tentorial divisions can provide access to the infratentorial space below the pontomesencephalic sulcus. The triangular tentorial flap in comparison with the linear incision obstructs the exposure of anterior incisural space and of the prepontine cistern. Visualization of the brainstem below the trigeminal nerve can be achieved by the anterior petrosectomy. Conclusion Infratentorial extension of the intradural subtemporal approach is technically demanding due to critical neurovascular structures and a relatively narrow corridor. In-depth anatomical knowledge is essential for the selection of the appropriate operative approach and safe surgical resections of lesions.

Two-stage endoscopic assisted approach for large pineal region and falcotentorial meningioma: first stage paramedian supracerebellar infratentorial approach, second stage interhemispheric occipital transtentorial approach: surgical cases and anatomical study.

Resection of complex falcotentorial meningiomas, growing along the pineal region (PR), and posterior incisural space (PIS) represents a neurosurgical challenge. Here, we present our strategy for effective resection of large falcotentorial meningiomas applying a paramedian supracerebellar infratentorial and interhemispheric occipital transtentorial approach in staged surgeries. We further systematically compared the effectiveness of midline (MSIA) and paramedian (PSIA) supracerebellar infratentorial, as well as interhemispheric occipital transtentorial approaches (IOTA) to operate along the PR and PIS in 8 cadaveric specimens. The staged PSIA and IOTA enabled successful resection of both falcotentorial meningiomas with an uneventful postoperative course. In our anatomo-morphometrical study, superficial vermian veins at an average depth of 11.38 ± 1.5mm and the superior vermian vein (SVV) at 54.13 ± 4.12mm limited the access to the PIS during MSIA. MSIA required sacrifice of these veins and retraction of the vermian culmen of 20.88 ± 2.03mm to obtain comparable operability indexes to PSIA and IOTA. Cerebellar and occipital lobe retraction averaged 14.31 ± 1.014mm and 14.81 ± 1.17mm during PSIA and IOTA respectively, which was significantly lower than during MSIA (p < 0.001). Only few minuscule veins were encountered along the access through PSIA and IOTA. The application of PSIA provided high operability scores around the pineal gland, ipsilateral colliculus and splenium, and acceptable scores on contralateral structures. The main advantage of IOTA was improving surgical maneuvers along the ipsilateral splenium. In summary, IOTA and PSIA may be advantageous in terms of brain retraction, vein sacrifice, and operability along the PR and PIS and can be effective for resection of complex falcotentorial meningiomas.

Extreme lateral supracerebellar infratentorial approach for tentorial arteriovenous fistula associated with a giant venous ectasia: how I do it

BackgroundSurgery of tentorial dural arteriovenous fistulas (DAVF) associated with large ectatic vein remains challenging due to the intimate neurovascular relationships in the incisural space. Interruption of the arterialized vein requires a good knowledge of the regional anatomy and a precise preoperative evaluation.MethodsWe describe the key steps extreme lateral supracerebellar infratentorial (ELSI) approach for tentorial DAVF with a video illustration. The surgical anatomy is described along with the advantages and limitations of this approach.ConclusionsIn cases of tentorial DAVF where the foot of the arterialized vein is located in the infratentorial compartment, ELSI offers good surgical exposure and outcomes.

Interhemispheric Precuneus Retrosplenial Transfalcine Approach for Falcotentorial Meningiomas: Anatomic Study and Clinical Series

Falcotentorial meningiomas are surgically challenging. Currently accepted approaches include occipital interhemispheric and supracerebellar infratentorial approaches, which have documented drawbacks. To propose an alternative approach to the posterior tentorial incisural space, the interhemispheric precuneus retrosplenial transfalcine approach (IPRTA). A total of 6 colored-silicone-injected adult cadaveric heads were dissected. We measured the interval between bridging veins and studied computed tomography venography scans of 20 subjects to confirm the feasibility of the IPRTA. Bony landmarks, the region's bridging veins, and the anterior and posterior borders of the approach were identified. The surgical corridor widths (veins' interval) and lengths were compared between alternative midline approaches. We also reviewed 4 consecutive clinical cases using this approach. The IPRTA provides the shortest distance to the anterior and posterior limits of the posterior tentorial incisura. Moreover, it is the only approach that provides direct visualization of the anterior and posterior limits of falcotentorial junction meningiomas. In all specimens and patients, a minimum 30-mm interval between the anterior and posterior parietal veins was found in at least one of the hemispheres. Tumor removal was successful in all 4 patients but resulted in mild paresthesia in the lower extremities of 2 patients and temporary foot drop in 1 patient. The IPRTA offers the shortest and most direct corridor for falcotentorial meningiomas and provides excellent visualization of most of the critical structures in the region. Detailed preoperative evaluation of the deep and superficial venous structures is recommended.

Indocyanine green illuminates the way to cut the tentorium in occipital transtentorial approach: technical note

Background and importance The occipital transtentorial approach is used to address lesions at the posterior incisural space or upper cerebellum. This approach is rarely used, making standardization of the surgical procedure challenging. Here we describe the effectiveness of indocyanine green (ICG) and dye markings before tentorial incision in charting a safe and bloodless surgical trajectory for improved manoeuvrability. Clinical presentation The first case was a 40-year-old man with a residual pineal mass after chemoradiation therapy for pathologically-proven germinoma. Surgical resection was performed via left occipital craniotomy. Incision of the left cerebellar tentorium by a radiofrequency knife was preceded by visualization of the straight sinus and venous lake, which were marked with dye, enabling safe entry into the quadrigeminal cistern. Finally, total-resection of the mature teratoma was achieved. The second case was a 50-year-old man with an enhancing mass at the cerebellar vermis and left hemisphere. Left occipital craniotomy was followed by ICG administration, illuminating the straight sinus and a complex structure of dural venous channels, which were marked with dye. This visualization maximized the tentorial incision by carefully avoiding venous structures and widely exposed the upper cerebellum. Subtotal-resection of the tumor was achieved, with a diagnosis of glioblastoma. Conclusion ICG administration and dye marking are feasible and useful methods for precise identification/visualization of venous structures. They enable maximization as well as safe and appropriate tentorial incision to provide a sufficient surgical corridor for the occipital transtentorial approach.

Pineal region tumours in the sitting position: how I do it

BackgroundPineal region tumours remain challenging neurosurgical pathologies.MethodsDetailed anatomical knowledge of the posterior incisural space and its variations is critical. An opaque arachnoidal membrane seals the internal cerebral and basal veins, leading to thalamic, basal ganglia, mesencephalic/pontine infarctions if injured. Medium-size tumours can be removed en-bloc with all traction/manipulation applied on the tumour side, virtually without contact of ependymal surfaces of the pulvinars or third ventricle. Sacrifice of the cerebello-mesencephalic fissure vein may be required.ConclusionsThe sitting position offers superior anatomical orientation and remains safe with experienced teams. Meticulous microsurgical techniques and detailed anatomical knowledge are likely to secure safe outcomes.

ANTERIOR INCISURAL SPACE (AIS): ITS TOPOGRAPHY AND SIZE IN THE CRANIAL CAVITY. POSITION OF THE BASILAR ARTERY APEX IN AIS

Objective : to explore microsurgical anatomy of the anterior incisural space (AIS), its topography and size in the skull, and to assess the topography of the basilar artery apex in AIS and find possible craniometric correlations. Materials and methods . In this anatomical study, we used 100 non-fixed heads of people who died from somatic pathology without diseases of the central nervous system. All heads were dissected using standard methods. Then we modelled the topography of the tentorial incisure (TI) in the skull by cutting off the hemispheres at the level of the brain stem in parallel to the free margin of TI. We measured length and width of TI, analyzed its topography in the cranial cavity, performed TI morphometry, and measured cranial indices and angular parameters of the skull base relief. Morphometric analysis of our data was conducted by calculating coefficients and indices. Data analysis was performed using the STATISTICA Statsoft. Microsurgical anatomy of the AIS and TI was studied using the operating microscope MBS-10 and digital video camera Sony HDR-CX560 E. The photos were taken with a 2–8-fold magnification. Results . Results. AIS length varied between 14 and 24 mm, whereas its width was between 24.0 and 36.0 mm. Almost half of all TI samples (42%) had AIS with a length of 17–19 mm. In all series of measurements, the length of the AIS was minimal – 14 mm (6%); 15–16 mm (19%), medium – 17–19 mm (42%), large – 20–22 mm (24%) and maximum – 23–24 mm – 9%. AIS were divided by TI types into four groups: A, B, C, and D. Minimum and small AIS were grouped into into ‘short-size AIS’ [14–16] mm, while large and maximum AIS were grouped into ‘long-size AIS’ [20–24] mm. The proportions of short AIS, long AIS, and medium AIS were 25%, 33%, and 42%, respectively. In group A, long-size AIS were found in 53.68% of samples; in garup B, long-size AIS were found in 3.7% of samples, in group C, there were only large AIS–33.3%; in group D, the maximum size of 24 mm was more frequent – 18.2 %, but the total number of samples with large AIS was two times lower–27.3% than in group A. Group A (sub- and brachycephaly) had the highest percentage of long AIS with a length of 20–24 mm (~54% of cases). The position of the BA in the AIS depends on the AIS length and clivus tilt. Conclusion. The size and topography of the AIS in the cranial cavity vary significantly; this must be taken into account when planning neurosurgical approaches. The concept of limited AIS boundaries, their structural flexibility in terms of surgical approach is the basis for choosing an optimal approach or its direction. The size of the AIS, its anatomical structure, and topography of the borders, access paths through AIS, possibility of manipulations with the content and boundaries of the AIS, the degrees of freedom or limits in acceptable surgical corridors, etc. should be taken into account when planning surgical intervention and choosing an optimal approach.

Unilateral Occipital Transtentorial Approach with Multimodal Assistance for Resection of Large Supracerebellar Hemangioblastomas: Preliminary Experience of 2 Cases

The surgical resection of large supracerebellar hemangioblastomas (SHBs) is exceptionally challenging due to their vascularity and deep anatomic location and is associated with a high risk of postoperative complications and mortality. Access to the posterior incisural space can be achieved by either an infratentorial supracerebellar approach or occipital transtentorial approach (OTA). However, the optimal surgical strategy has not yet been established. Here, we report 2 cases of large SHBs that were successfully and safely resected via a unilateral OTA with multimodal assistance. Two patients presented to our hospital with ataxia due to large, solid SHBs. After preoperative embolization, gross total resection of the SHBs was achieved via an OTA. Furthermore, endoscopic assistance was used to resect the remnant portion of the tumor in the second patient. Both patients experienced transient ataxia but were discharged from the hospital without serious complications. The combination of an OTA with preoperative embolization and endoscopic assistance may reduce the intraoperative risk and contribute to improved outcome in patients with such clinically challenging tumors.

Endoscopic transorbital transtentorial approach to middle incisural space: preclinical cadaveric study.

Endoscopic transorbital approach is a novel development of minimally invasive skull base surgery. Recently, anatomical studies have started to discuss the expanded utilization of endoscopic transorbital route for intracranial intradural lesions. The goal of this cadaveric study is to assess the feasibility of endoscopic transorbital transtentorial approach for exposure of middle incisural space. Anatomical dissections were performed in four human cadaveric heads (8 sides) using 0- and 30-degree endoscopes. A stepwise description of endoscopic transorbital transtentorial approach to middle incisural space and related anatomy was provided. Orbital manipulation following superior eyelid crease incision with lateral canthotomy and cantholysis established space for bone drilling. Extradural stage consisted of extensive drilling of orbital roof of frontal bone, lessor, and greater wings of sphenoid bone. Intradural stage was composed of dissection of sphenoidal compartment of Sylvian fissure, lateral mobilization of mesial temporal lobe, and penetration of tentorium. A cross-shaped incision of tentorium provided direct visualization of crural cistern with anterolateral aspect of cerebral peduncle and upper pons. Interpeduncular cistern, prepontine cistern, and anterior portions of ambient and cerebellopontine cisterns were exposed by 30-degree endoscope. The endoscopic transorbital transtentorial approach can be used as a minimally invasive surgery for exposure of middle incisural space. Extensive drilling of sphenoid wing and lateral mobilization of mesial temporal lobe are the main determinants of successful dissection. Further studies are needed to confirm the clinical feasibility of this novel approach.

Anatomical and MRI relations of the cerebral aqueduct to the adjacent parts of the brain and calvaria

Introducton/Objective. Insufficiency of relevant anatomic data and great neurological and neurosurgical significance were the reasons for this study with scientific and practical implications. The purpose was to determine, at the transverse in situ section of the head, the position and relations of the sylvian aqueduct of the mesencephalon by measuring its distances from particular brain and calvaria structures. Also, the aim was to determine the same distances according to axial sections by using MRI. Methods. The material consisted of twenty autopsy human heads. The section of the head was made at the level of the tentorial hiatus and the midbrain. After that, we measured the distances between the cerebral aqueduct and a) posterior border of the optic chiasm, b) upper border of the dorsum sellae, c) terminal bifurcation of the basilar artery, d) beginning of the straight sinus, e) internal occipital protuberance, f) tentorial edge (lateral from the aqueduct), and g) internal surface of the calvaria (lateral to the aqueduct). We determined the same distances by the MRI system. The measurements were made in 37 subjects. Results. The numerical data obtained by this study will be of benefit to neurosurgeons in choosing a surgical approach to the contents of the incisural space, and to neurologists for the exact localization of the lesion and interpretation of certain signs and symptoms. Conclusion. The results of a detailed examination of the sylvian aqueduct position and relations have shown that the use of MRI is the morphometric method of choice, because it is more precise for all the parameters monitored than in situ measurements.

Expanding the Horizon of the Suboccipital Retrosigmoid Approach to the Middle Incisural Space by Cutting the Tentorium Cerebelli: Anatomic Study and Illustration of 2 Cases

Complex skull base approaches are often used to treat lesions within the middle incisural space; yet the well-known retrosigmoid route may provide an effective avenue to this difficult-to-reach region. The purpose of this study was to quantify the exposure advantages on the middle incisural space provided by cutting of the tentorium cerebelli via a standard suboccipital retrosigmoid approach (i.e., via the cerebellopontine cistern route). Also, 2 illustrative cases are presented. A suboccipital retrosigmoid approach to the middle incisural space was performed bilaterally in 3 specimens in the semisitting position. A quantitative analysis of the brainstem surface exposed above the origin of the trigeminal nerve was performed before and after tentorial incision. Tentorial cutting significantly improved the exposure of the middle incisural space cisternal structures such as the oculomotor and trochlear nerves and the superior cerebellar and posterior cerebral arteries. The mean brainstem surface exposed more than doubled (2.13-fold increase) after tentorial incision with an average increase from 73.18 mm(2) to 155.76 mm(2). When the endoscope was used, it was possible to follow the entire course of the cisternal, infratentorial trochlear nerve segment during theopening of the tentorial free edge, facilitating its preservation. In the illustrative cases, 2 cerebellopontine angle meningiomas with supratentorial and infratentorial extension were removed without any postoperative complication. Tentorial incision may be useful to extend the rostral exposure of the middle incisural space via a simple retrosigmoid approach, avoiding the need for more complex skull base routes.

Endoscope-Assisted Microsurgical Approach to the Posterior and Posterolateral Incisural Space

Despite recent technological advances, direct approaches to the posterolateral incisural space remain surgical challenges. The choice of the operative route depends on the exact location and extent of the target lesion as well as individual preferences. The extreme lateral infratentorial supracerebellar approach to treat pathologies located in the ambient cistern and posterior incisural space is a technically feasible route in selected cases. In this cadaveric study, we demonstrate the benefits of endoscope-assisted microsurgical maneuvers using the extreme lateral supracerebellar infratentorial approach. An endoscope-assisted infratentorial supracerebellar approach was performed in six formalin-fixed cadaveric heads using standard microneurosurgical methods. Dissections were performed in a stepwise fashion, comparing the exposure afforded by the microsurgical route alone to the endoscope-assisted route, using 0- and 30-degree angled lenses. Relationships among the target and the surroundings neurovascular structures were described. Endoscope-assisted maneuvers for the extreme lateral supracerebellar approach provide an improved operative view and have the potential to reduce parenchymal trauma and neurovascular injuries. The endoscopic techniques bring the surgeon to the anatomy, enhancing illumination and surgical visualization. Direct visualization of the posterior and posterolateral incisural space avoids retraction of the occipital lobe and damage to the deep venous complex. The extreme lateral infratentorial supracerebellar corridor is effective for approaching the posterolateral mesencephalic junction and the posterior incisural space in selected cases. Endoscope-assisted microsurgery can improve visualization and minimize parenchymal retraction, which should enhance surgical control.

Basal vein in the posterior incisural space: An anatomical comparison based on autopsy, digital subtraction angiography and computed tomographic venography.

During surgical procedures, the basal vein in the posterior incisural space is susceptible to obstruction. In such circumstances, venous infarction can occur along with venous damage. The aim of this study was to correlate the microanatomy of the basal vein in the posterior incisural space with the digital subtraction angiography (DSA) and computed tomographic venography (CTV). Twenty cadavers and 42 patients were examined in this study. The head of each cadaver was injected with blue-colored gelatin via the internal jugular veins. Venograms for each patient were obtained from the venous phases of DSA or CTV. Compared to the cadavers, in the patients, DSA and CTV revealed 90% and 95% of the basal vein, respectively. According to difference of entrance, three types of basal veins were identified. No statistical difference of typing was found among the cadavers, DSA images and CTV images. On three sides of the cadavers and one side of the CTV images, the basal vein entered the straight sinus through the meningeal vein in the tentorium cerebelli. Preoperative DSA and CTV are useful in the design of individualized surgical approaches and the preservation of the basal vein in the posterior incisural space.

Anatomy of the pineal region applied to its surgical approach

IntroductionThe pineal region is situated in the posterior part of the incisural space. This region includes the pineal body inside the quadrigeminal arachnoidal cistern. This article reviews the anatomic features of this region, with particular emphasis on those aspects of importance for surgical access to the pineal region. Material & methodsFive cadaver heads fixed in 10% formalin and injected with colored latex were used for anatomic dissection (five other specimens were also prepared and dissected to illustrate the articles on surgical techniques and approaches presented elsewhere in this issue). ResultsThe pineal body is surrounded by several important structures such as: posterior part of the third ventricle, tectum, the complex of the great cerebral vein of Galen, pulvinar nuclei of the thalamus and splenium of corpus callosum. ConclusionThe surgical approach of the pineal body, whatever the route or the technique used (microsurgical, endoscopic or stereotactic), creates a great challenge for the neurosurgeons due to its location in the deep part of the brain and its close relationships with complex surrounded vascular structures.

Arachnoid membranes in the posterior half of the incisural space: an inverted Liliequist membrane-like arachnoid complex.

The aim of this study was to describe the similarity of configuration between the arachnoid complex in the posterior half of the incisural space and the Liliequist membrane. Microsurgical dissection and anatomical observation were performed in 20 formalin-fixed adult cadaver heads. The origin, distribution, and configuration of the arachnoid membranes and their relationships with the vascular structures in the posterior half of the incisural space were examined. The posterior perimesencephalic membrane and the cerebellar precentral membrane have a common origin at the tentorial edge and form an arachnoid complex strikingly resembling an inverted Liliequist membrane. Asymmetry between sides is not uncommon. If the cerebellar precentral membrane is hypoplastic on one side or both, the well-developed quadrigeminal membrane plays a prominent part in partitioning the subarachnoid space in the posterior half of the incisural space. The arachnoid complex in the posterior half of the incisural space can be regarded as an inverted Liliequist membrane. This concept can help neurosurgeons to gain better understanding of the surgical anatomy at the level of the tentorial incisura.

Acute Aneurismal Bilateral Subdural Haematoma without Subarachnoid Haemorrhage: A Case Report and Review of the Literature.

Spontaneous pure acute bilateral subdural haematoma (ASDH) without intraparenchymal or subarachnoid haemorrhage caused by a ruptured cerebral aneurysm is extremely rare. It can follow rupture of different aneurysms specially located in anterior incisural space; the most frequently encountered location is the PcoA aneurysms as demonstrated in the present case. We present a case report of a PcoA aneurysm presenting as pure bilateral ASDH. A high level of suspicion for bleeding of arterial origin should be maintained in all cases of acute subdural haematoma without history of trauma. The neurological status on admission dictates the appropriate timing and methodology of the neuroradiological investigations.