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Purpose: Leakage of a small amount of intravenously administered gadolinium-based contrast agents (GBCAs) into the cerebrospinal fluid (CSF) space has been reported, even in healthy subjects without blood–brain barrier disruption. Several candidates including the choroid plexus and cortical veins have been proposed as the source of the leakage. The purpose of this study was to evaluate the distribution of intravenously administered GBCA leakage into the CSF by comparing the contrast enhancement of the cerebral cisterns to the lateral ventricles (LVs). Methods: In 26 patients with a suspicion of endolymphatic hydrops (21–80 years old), a three-dimensional real inversion recovery (3D-real IR) image was obtained at pre-, and at 5 min, and 4 h post-intravenous administration of a single dose of GBCA (IV-SD-GBCA). In the 3D-real IR image, the signal intensities (SIs) in the anterior horn of the LV (LVante), the trigone of the LV (LVtri), the Sylvian fissure (SyF), the ambient cistern (Amb), the prepontine cistern (PPC), the cerebellopontine angle cistern (CPA), and the vitreous (Vit) were measured. The differences in the SI at pre-, and at 5 min and 4 h post-IV-SD-GBCA were evaluated for each region. The change in the SI pre- to post-IV-SD-GBCA (SIchange) were calculated for each region. The differences in the SIchange in each region were evaluated at 5 min and 4 h post-IV-SD-GBCA. A Steel-Dwass’s test was applied to correct for multiple comparisons. Results: The SIs of all regions at 4 h post-IV-SD-GBCA were significantly higher compared with pre-IV-SD-GBCA (P < 0.05). The SIchange in the SyF, Amb, PPC, and the CPA were significantly higher compared with those of the LVante, LVtri, and the Vit at 4 h post-IV-SD-GBCA (P < 0.05). Conclusion: The contrast enhancement in the cerebral cisterns was greater than that in the LVs.

Objective Localization of the temporal horn of the lateral ventricle (TH) may be required during temporal lobe and ambient cistern surgery. Most available anatomic landmarks for TH localization are based on adjacent cortical landmarks that are inherently variable or subtle. This study aimed to localize the anterior tip of the TH relative to adjacent bony landmarks. Methods The TH was exposed on 21 sides of 11 cadaveric heads via removal of the middle temporal gyrus. Two lines were defined: (1) a perpendicular line to the zygomatic arch projected from the anterior concavity of the posterior zygomatic root (line A), and (2) a parallel line passing through the anterosuperior corner of the external auditory canal (line B). Sagittal distances from lines A and B to a parallel line passing through the anterior recess of the TH (line H) were measured. Results Mean (standard deviation) distances from lines A and B to line H were 13.3 (2.5) mm and 11.9 (2.2) mm, respectively. Line H was at 53% (8%) of the line A–line B interval measured from line A. The best way to search for the TH was to start approximately 15 mm posterior to line A and progress posteriorly such that a more posteriorly located TH tip would not be missed. Conclusions The zygomatic-meatal landmark is a reliable tool to localize TH during various approaches. It is independent from the approach trajectory. This landmark may be used as an ancillary tool in conjunction with other cortical landmarks and image guidance.

The subtemporal approach provides a narrow operative corridor to the crus cerebrum and adjacent structures of the crural, interpeduncular, and ambient cistern. Addition of a zygomatic osteotomy widens this narrow corridor and spares retraction of the temporal lobe. We investigate and compare the morphometric parameters of the subtemporal approach with versus without zygomatic osteotomy. On each side of four cadaveric heads, a temporal craniotomy was performed to gain access to the crus cerebrum and adjacent subarachnoid cisterns using a subtemporal approach. Operative corridor width and corridor working angle were measured with and without brain retraction on each specimen side. Next, a zygomatic osteotomy was performed followed by full downward reflection of the temporalis muscle and further drilling of the squamous part of the temporal bone. Lastly, operative corridor width and corridor working angle were measured again for comparison. The subtemporal operating corridor was (mean/SD): 5.8/2.6 mm without retraction, 11.4/4.3 mm with retraction, and 13.5/6.5° working angle. After addition of a zygomatic osteotomy, the operative corridor was 8/9.2/4.3 mm without retraction, 14.7/4.5 mm with retraction, 31.8/3.1° working angle. Zygomatic osteotomy significantly increased the operative corridor working angle of the subtemporal approach. Furthermore, we demonstrate a direct approach into the interpeduncular fossa. Clin. Anat. 32:710-714, 2019. © 2019 Wiley Periodicals, Inc.

BACKGROUND: Approaching ambient cistern lesions is still a challenge because of deep location and related white matter tracts (WMTs) and neural structures. OBJECTIVE: To investigate the white matter anatomy in the course of 3 types of transtemporal-transchoroidal fissure approaches (TTcFA) to ambient cistern by using fiber dissection technique with translumination and magnetic resonance imaging fiber tractography. METHODS: Eight formalin-fixed cerebral hemispheres were dissected on surgical corridor from the temporal cortex to the ambient cistern by using Klingler's method. The trans-middle temporal gyrus, trans-inferior temporal sulcus (TITS), and trans-inferior temporal gyrus (TITG) approaches were evaluated. WMTs that were identified during dissection were then reconstructed on the Human Connectome Project 1021 individual template for validation. RESULTS: The trans-middle gyrus approach interrupted the U fibers, arcuate fasciculus (AF), the ventral segment of inferior frontoocipital fasciculus (IFOF), the temporal extensions of the anterior commissure (AC) posterior crura, the tapetum (Tp) fibers, and the anterior loop of the optic radiation (OR). The TITS approach interrupted U fibers, inferior longitudinal fasciculus (ILF), IFOF, and OR. The TITG approach interrupted the U fibers, ILF, and OR. The middle longitudinal fasciculus, ILF, and uncinate fasciculus (UF) were not interrupted in the trans-middle gyrus approach and the AF, UF, AC, and Tp fibers were not interrupted in the TITS/gyrus approaches. CONCLUSION: Surgical planning of the ambient cistern lesions requires detailed knowledge about WMTs. Fiber dissection and tractography techniques improve the orientation during surgery and may help decrease surgical complications. Copyright © 2020 by the Congress of Neurological Surgeons.

Cranial nerve palsies are relatively common after trauma, but trochlear nerve palsy is relatively uncommon. Although traumatic trochlear nerve palsy is easy to diagnose clinically because of extraocular movement disturbances, radiologic evaluations of this condition are difficult to perform because of the nerve's small size. Here, we report the case of a patient with delayed traumatic trochlear nerve palsy associated with a traumatic subarachnoid hemorrhage (SAH) and the related radiological findings, as obtained with high-resolution three-dimensional (3D) magnetic resonance imaging (MRI). A 63-year-old woman was brought to the emergency room after a minor head trauma. Neurologic examinations did not reveal any focal neurologic deficits. Brain computed tomography showed a traumatic SAH at the left ambient cistern. The patient complained of vertical diplopia at 3 days post-trauma. Ophthalmologic evaluations revealed trochlear nerve palsy on the left side. High-resolution 3D MRI, performed 20 days post-trauma, revealed continuity of the trochlear nerve and its abutted course by the posterior cerebral artery branch at the brain stem. Chemical irritation due to the SAH and the abutting nerve course were considered causative factors. The trochlear nerve palsy completely resolved during follow-up. This case shows the usefulness of high-resolution 3D MRI for evaluating trochlear nerve palsy.

Arteriovenous malformations (AVMs) of the ambient cistern are an extremely rare and complex subgroup of vascular malformation, representing a clinical challenge due to the deep-seated, highly eloquent anatomic location and the debilitating, life-threatening consequences related to hemorrhagic presentation and surgical morbidity. Ultimately, a tailored treatment, based on the presenting symptoms, AVM angioarchitecture, and annual risk of hemorrhage should be discussed among a multidisciplinary team to find the best individualized strategy balancing between the pros and cons of each approach. In Video 1, we present the case of a 60-year-old man with a hemorrhaged AVM of the right ambient cistern, present the pros and cons of each possible treatment strategy, and illustrate the successful resection of this lesion through a subtemporal-transtentorial microsurgical approach.

Purpose: In textbooks, the perivascular space (PVS) is described as non-enhancing after the intravenous administration of gadolinium-based contrast agent (IV-GBCA). We noticed that the PVS sometimes has high signal intensity (SI) on heavily T2-weighted 3D-FLAIR (hT2-FL) images obtained 4 h after IV-GBCA. The purpose of this study was to retrospectively evaluate the contrast enhancement of the PVS. Materials and Methods: In 8 healthy subjects and 19 patients with suspected endolymphatic hydrops, magnetic resonance cisternography (MRC) and hT2-FL images were obtained before and 4 h after a single dose of IV-GBCA. No subjects had renal insufficiency. On axial MRC at the level of the anterior commissure (AC)-posterior commissure (PC) line, 1 cm circular regions of interest (ROIs) were drawn centering on the PVS in the bilateral basal ganglia and thalami. Three-millimeter diameter ROIs were set in the cerebrospinal fluid (CSF) of the bilateral ambient cistern. The ROIs on MRC were copied onto the hT2-FL images and the SI was measured. The SI ratio (SIR) was defined as SIRPVS = SI of PVS/SI of the thalami, and SIRCSF = SI of CSF/SI of the thalami. The average of the bilateral values was used for the calculation. The SIRCSF, SIRPVS, and SI of the thalami were compared between before and 4 h after IV-GBCA. Results: The SIR was increased significantly from 1.02 ± 0.37 to 2.65 ± 0.82 in the CSF (P < 0.01) and from 1.20 ± 0.35 to 2.13 ± 1.23 in the PVS at 4 h after IV-GBCA (P < 0.01). The SI of the thalami showed no significant difference. Conclusion: The enhancement of the PVS at 4 h after IV-GBCA was confirmed even in subjects without renal insufficiency. It is possible that the GBCA in the blood vessels might have permeated into the cerebrospinal fluid (CSF) space and the PVS. This might be a first step in the imaging evaluation of the glymphatic system (waste clearance system) of the brain.

Brainstem arteriovenous malformations (AVMs) are rare lesions (2%-6% of all intracranial AVMs) that are surgically challenging because of the high eloquence of the brainstem, including dense fiber tracts, cranial nerves, and multiple vital cerebrovascular structures. All these lesions possess eloquence and deep venous drainage, making them innately Spetlzer-Martin grade III or above. This patient had a large midbrain AVM with a complex clinical course beginning with ventriculoperitoneal shunting due to mass effect; the patient experienced 4 hemorrhages, underwent radiation treatment, and finally underwent surgical resection. The surgical approach involved a large torcular craniotomy exposing all posterior sinuses at the confluence. The vein of Galen was exposed and carried deeply to permit ambient cistern opening and relaxation of the cerebellum. Because of earlier radiation therapy, the vessels had undergone hyalinosis, which resulted in difficult manipulation of the nidus but did permit excellent coagulation of the vessels. The complete nidus was removed, as confirmed on postoperative angiography. The patient gave informed consent for surgery and video recording. Institutional review board approval was deemed unnecessary. Used with permission from Barrow Neurological Institute, Phoenix, Arizona.

Resumen Objetivo: Describir paso a paso el abordaje a la cisterna ambiens por la vía suboccipital retrosigmoidea supracerebelosa infratentorial (SRSI). Descripción: El abordaje SRSI se realiza de la misma manera que el abordaje suboccipital retrosigmoideo (SR), utilizado habitualmente para acceder a la patología del ángulo pontocerebeloso, con las siguientes modificaciones: (1) utilizamos siempre la posición semisentado, (2) la craneotomia-craniectomia debe exponer el seno transverso y extenderse 5 cm medialmente hacia el inion, (3) al realizar la apertura dural es necesario rebatir la duramadre junto con el seno transverso hacia cefálico con puntos de tracción, (4) bajo magnificación con microscopio quirúrgico se debe realizar la apertura de la cisterna cerebelobulbar para drenar líquido cefalorraquídeo, (5) en el plano supracerebeloso es fundamental cortar las bridas aracnoidales y de ser necesario debemos coagular y cortar las venas puente, todas estas maniobras sumadas al efecto de la gravedad brindan mayor apertura del corredor supracerebeloso. Conclusión: El abordaje a la cisterna ambiens por la vía SRSI es una opción segura para el acceso de patologías tumorales que se alojan en esta zona con un componente predominantemente infratentorial.

Key points for the approach include the use of a supracerebellar infratentorial craniotomy, dynamic and gravity retraction, lighted bipolar forceps and suction, and stereotactic navigation. This patient has a large cavernous malformation extending from the midbrain to the thalamus in craniocaudal extent. The cavernous malformation extends to the midbrain surface along the interface with the ambient cistern, making it appropriate for the supracerebellar infratentorial approach. By cutting the tentorium access to the superior extension becomes feasible. The cavernous malformation is excised in a piecemeal manner utilizing a CO2 laser. Because removal of the large mass allowed the partial collapse of the cavity, a small segment of the cavernous malformation obscured by the collapse is retained cranially along the foramen of Monro. This situation required a return to the operating room for complete excision. The patient tolerated both procedures well and remained at her neurological baseline postoperatively. The patient gave informed consent for surgery and video recording. Institutional review board approval was deemed unnecessary. The surgical video has been used with permission from the Barrow Neurological Institute.

Purpose Since the first report on the deposition of gadolinium in the brain parenchyma after repeated intravenous administrations of gadolinium-based contrast agent GBCA (IV-GBCA), the mechanisms of penetration and retention are still remaining a hot topic of discussion and a target of investigation. We routinely obtain endolymphatic hydrops (EH) images at 4 h after IV administration of a single dose (SD) of GBCA (IV-SD-GBCA) using heavily T2-weighted three-dimensional fluid-attenuated inversion recovery imaging (hT2W-3D-FLAIR). Occasionally, we have encountered cases, which indicate high-signal intensity (SI) in the cerebrospinal fluid (CSF) surrounding the vein of Labbe. The purpose of the present study was to investigate the degree of contrast enhancement of the CSF surrounding the vein of Labbe on hT2W-3D-FLAIR after IV-SD-GBCA in comparison with other CSF spaces. Materials and methods In 25 patients with a suspicion of EH, a magnetic resonance cisternography (MRC) and an hT2W-3D-FLAIR were obtained at 4 h after IV-SD-GBCA. The perivascular space (PVS) in the basal ganglia, CSF spaces in the ambient cistern (CSF-Amb), the CSF surrounding the superficial middle cerebral vein (CSF-SMCV), and the CSF surrounding the vein of Labbe (CSF-VL) were segmented on MRC. The PVS and CSF regions were co-registered onto the hT2W-3D-FLAIR and the SI of the PVS and CSF spaces were measured. The SI ratio (SIR) of the post-contrast hT2W-3D-FLAIR to the pre-contrast hT2W-3D-FLAIR was measured. Significant differences were evaluated using Steel-Dwass's test for multiple comparisons. Results The SIR of the CSF-VL was significantly higher than that of the PVS (P = 0.008), the CSF-Amb (P = 0.021), and the CSF-SMCV (P = 0.023). Conclusion The strong contrast enhancement of CSF space around the vein of Labbe was confirmed on hT2W-3D-FLAIR at 4 h after IV-GBCA compared to the PVS and the other CSF spaces.

Background Posterior cerebral artery (PCA) aneurysms are relatively rare, and neck clipping is often difficult due to their fusiform shape. We report a case of a thrombosed aneurysm of the distal PCA for which curative trapping and parent artery reconstruction by in situ bypass were performed through an occipital transtentorial approach (OTA). Case Description A 67-year-old woman had been suffering from numbness in the right face and limbs for 4 months. Radiologic imaging demonstrated a thrombosed aneurysm on a distal portion of the left PCA. Curative trapping of the aneurysm and in-situ bypass between the distal PCA and superior cerebellar artery were performed through the OTA. Before surgery, we had evaluated access to the PCA and feasibility of the bypass in a cadaveric simulation. The PCA was well exposed in the posterior half of the ambient cistern, and the proximity of the distal PCA to the superior cerebellar artery through a tentorial incision was confirmed. Conclusions This OTA could represent a useful option for definitive treatment of distal PCA aneurysms.

The ambient cistern is an arachnoid complex that extends from the crural cistern to lateral border of cerebral colliculi. The subtemporal approach has been recognized as the best access to reach pathologies in the ambient cistern, however many disadvantages exist. The present work aims to analyze quantitatively the area of exposure provided by the subtemporal access. The objective is to evaluate if there are advantages of using the neuroendoscope in conventional subtemporal access when compared to the subtemporal access with resection of the parahippocampal gyrus. A subtemporal approach was performed in six brain hemispheres. Qualitative and quantitative analyses were made. The linear exposition of the vascular structures and the surgical exposure area were evaluated. The linear exposure to the posterior cerebral artery was 5.95 for subtemporal access (ST) and 13.6 for subtemporal access with resection of the parahippocampal gyrus (STh) (p = 0.019). The total exposure area was 104.8 mm2 for ST and 210.5 for STh (p = 0.0001). Regarding endoscope assistance the medial area, ST was 81.0 mm2, and STend was 176.2 mm2 (p = 0.038). For the total area of exposure, we obtained a value of 210.5 mm2 for ST and a value of 391.3 mm2 for STend (p = 0.041). In conventional subtemporal access, the use of the neuroendoscopes avoids the need for resection of the parahippocampal gyrus for better visualization of the ambient cistern structures.

Background Meckel's cave involvement in tumors pose a challenge due to their surrounding neurovascular structure and deep location. Case Review A 24-year-old male presented with progressive headaches and right sided trigeminal neuralgia with a large epidermoid. The tumor extended from the ambient cistern to the cerebellomedullary cistern and involved Meckel's cave (Fig. 1). Technical Note/Video Description A retrosigmoid craniectomy was performed. Cranial nerves 3, 4, 6, 7, and 10, and auditory brainstem responses were monitored. Once the craniectomy was completed the dura was opened and cerebrospinal fluid (CSF) was released from the cisterna magna to allow for the tumor resection to be done without the use of any retractors (Fig. 2). Care was taken to ensure that cranial nerves in the posterior fossa were detethered to prevent any traction injury. Using ring curettes the pearly white epidermoid tumor was able to be debulked. After all the possible tumor was resected with the microscope, the 30-degree endoscope was used to identify the porus trigeminus. Malleable ring curettes and a malleable suction were used to remove the soft tumor from this location. The patient transiently had loss of hearing but this returned within 2 weeks after surgery. Conclusions The retrosigmoid approach is familiar to all neurosurgeons and with the adjunct of an angled endoscope, the posterior Meckel's cave can be easily reached. This is particularly useful for tumors with soft consistency. The assistance of the endoscope allows Meckel's cave visualization without additional drilling while still allowing safe resection of tumor from around the trigeminal nerve.The link to the video can be found at: https://youtu.be/01aqOyUmSW0.

Tentorial dural arteriovenous fistulas (DAVFs) are uncommon, complex fistulas located between the leaves of the tentorium cerebelli with a specific anatomic and clinical presentation characterized by high hemorrhagic risk. They have an extensive arterial supply and complex venous drainages, making them difficult to treat. There is recent literature favoring treatment through an endovascular transarterial route. The authors present an uncommon tentorial/ambient cistern region DAVF with feeders arising from the external and internal carotid arteries. The patient underwent a combined transarterial and transvenous approach with successful obliteration of the DAVF. The authors discuss the management challenges faced in this case.The video can be found here: https://youtu.be/VXDD8zUvsSQ.

Objective and Importance: The authors report the first known case of a giant multiloculated grape-like enterogenous cyst extending from the upper cervical canal to the ambient cistern. Clinical Presentation/Methods: We report the case of a 40-year-old male who had a prior transmastoid craniotomy at an outside facility 14 months prior with an indeterminate diagnosis, who presented to the University of New Mexico with recurrent headaches and nausea. Scans demonstrated a giant multiloculated cystic lesion in the right cerebellopontine angle that extended superiorly and inferiorly with brainstem compression and hydrocephalus. Intervention/Results: We took the patient to the operating room for a retrosigmoid suboccipital craniectomy for tumor resection. Post-operatively, the patient improved but required ventriculoperitoneal shunting for continued communicating hydrocephalus. Conclusion: This is the first known case of a giant multiloculated grape-like enterogenous cyst extending simultaneously from the upper cervical canal to the ambient cistern. Enterogenous cysts should be considered on the differential diagnosis of giant grape-like lesions extending from the cervical canal to the prepontine cistern.

Approximately 30% of epilepsy patients are refractory to antiepileptic drugs. In these cases, surgery is the only alternative to eliminate/control seizures. However, a significant minority of patients continues to exhibit post-operative seizures, even in those cases in which the suspected source of seizures has been correctly localized and resected. The protocol presented here combines a clinical procedure routinely employed during the pre-operative evaluation of temporal lobe epilepsy (TLE) patients with a novel technique for network analysis. The method allows for the evaluation of the temporal evolution of mesial network parameters. The bilateral insertion of foramen ovale electrodes (FOE) into the ambient cistern simultaneously records electrocortical activity at several mesial areas in the temporal lobe. Furthermore, network methodology applied to the recorded time series tracks the temporal evolution of the mesial networks both interictally and during the seizures. In this way, the presented protocol offers a unique way to visualize and quantify measures that considers the relationships between several mesial areas instead of a single area. Fil: Sanz-García, Ancor. Hospital de la Princesa; España Fil: Vega Zelaya, Lorena. Hospital de la Princesa; España Fil: Pastor, Jesús. Hospital de la Princesa; España Fil: Torres, Cristina V.. Hospital de la Princesa; España Fil: Sola, Rafael G.. Hospital de la Princesa; España Fil: Ortega, Guillermo José. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Hospital de la Princesa; España

The middle tentorial incisural space, located lateral to the midbrain and medial to the temporal lobe, contains the ambient cistern through which courses the third, fourth, and fifth cranial nerves, posterior cerebral artery (PCA), superior cerebellar artery, and the choroidal arteries. Arteriovenous malformations (AVMs) in this compartment are supplied by the thalamogeniculate and posterior temporal branches of the PCA, and drain into tributaries of the basal vein of Rosenthal. We present a case of an AVM in this middle tentorial incisural space that persisted after embolization and radiosurgery, and was microsurgically resected through a subtemporal approach. This case demonstrates the anatomy of the middle incisural space and technical aspects in microsurgical resection of these rare AVMs.The video can be found here: https://youtu.be/V-dIWh8ys3E.

Although venous hemangiomas are one of the most common soft-tissue tumors, venous hemangiomas in the central nervous system are extremely rare. We present an unusual case of venous hemangioma originating from the interdural space of the tentorium. A 32-year-old woman was incidentally found to have extra-axial mass occupying the left ambient cistern. This tumor was observed for the first 4 years as it was completely asymptomatic. Surgical resection was later recommended when the tumor grew. The mass originated from between the two layers of the anteromedial tentorial incisura. There were no findings indicative of previous hemorrhage inside the mass. The matrix of the mass was firm and vascular, resembling a fibrous meningioma. Gross total resection was achieved without any neurological deficit. Pathological examination revealed a dense fibrous connective tissue with a proliferation of vessels marked by thickened walls. A spindle cell proliferation in the vessel walls did not stain with the antibody to S-100 protein. Movat stain demonstrated the venous character of the vessels. These results were histologically compatible with a venous hemangioma. Albeit extremely rare, a venous hemangioma, a distinct clinical and pathological entity from a venous angioma, can present an intracranial mass lesion.

Neurenteric cysts are rare congenital lesions of endodermal origin occurring in the spinal canal and infrequently in the posterior cranial fossa. The authors report the case of a 3-year-old child who presented with a recurrent third cranial nerve palsy. Magnetic resonance imaging showed a large cystic mass lesion in the ambient cistern on the right side, with compression of the anterolateral aspect of the brainstem. The patient underwent a craniotomy, complete excision, and a primary third cranial nerve repair. While there have been 3 reported cases of neurenteric cysts arising from the oculomotor nerve, this is the first documented case with a primary nerve repair.

Object A precise understanding of the ambient cistern and its associated arachnoid membranes is helpful for accessing perimesencephalic lesions. However, few studies of the arachnoid membranes related to the ambient cistern have been published, and, additionally, some aspects of the ambient cistern also require further examination. The goal of this study was to reinvestigate and expound on the anatomical features of the cistern and membranes. Methods The ambient cisterns and its associated arachnoid membranes were examined in 20 adult cadaveric brains using an operative microscope. Results The perimesencephalic membrane is a set of inner arachnoid membranes surrounding the midbrain at the level of the tentorial incisura. It arises from the outer arachnoidal membranes covering the tentorial edge and the dorsum sellae and can be subdivided into anterior and posterior portions. The anterior membrane is actually the mesencephalic leaf of Liliequist membrane, which is divided into medial and lateral parts by the oculomotor nerve. The posterior membrane can be divided into horizontal and ascending parts. The ambient cistern is located above the perimesencephalic membrane and contains the anterior choroidal arteries, the posterior cerebral arteries, the basal vein, and sometimes the segments of the superior cerebellar arteries. It communicates with the carotid cistern, the interpeduncular cistern, the oculomotor cistern, the cerebellopontine and cerebellomesencephalic cistern, and the quadrigeminal cistern. Conclusions This study updates some information about the ambient cistern and its arachnoid membranes. The perimesencephalic membrane was identified and described in detail. The ambient cistern was verified to be a supratentorial cistern above the perimesencephalic membrane. The borders and contents of this cistern, as well as its relationship with adjacent cisterns, were also redefined.

A 58-year-old woman presented with a rare tentorial schwannoma manifesting as a chief complaint of headache. Magnetic resonance imaging showed an infratentorial extra-axial mass lesion with both cystic and solid components and dural tail sign. The tumor was easily separated from the surrounding structures, and the origin was apparently the tentorial branch of the trigeminal nerve. Tentorial schwannoma should be considered in the differential diagnosis of mixed cystic and solid mass lesions associated with the tentorium around the ambient cistern.

Objectives The current video presents the nuances of the endoscopic endonasal transoculomotor triangle approach for the resection of a pituitary adenoma with extension into the ambient cistern. Design The video analyzes the presentation, preoperative workup and imaging, surgical steps and technical nuances of the surgery, the clinical outcome, and follow-up imaging. Setting The patient was treated by a skull base team consisting of a neurosurgeon and an ENT surgeon at a teaching academic institution. Participants The case refers to a 62-year-old female who presented with vision loss and headaches, and was found to have a pituitary adenoma with extension into the ambient cistern. Main Outcome Measures The main outcome measures consist of the reversal of the patient symptoms (headaches), the recurrence-free survival based on imaging, as well as the absence of any complications. Results The patient's headaches improved. There was no evidence of recurrence. Conclusions The endoscopic endonasal transoculomotor triangle approach is safe and effective for addressing pituitary tumors which extend into the ambient cistern.The link to the video can be found at: https://youtu.be/EBLwEWhohxY.

Simultaneous occurrence of dual intracranial pathology is very rare. Most of such reported cases occur in the setting of a genetic or hereditary predisposing condition like neurofibromatosis. Though some cases of coexistence of multiple intracranial pathologies have been reported, occurrence of an arachnoid and epidermoid cyst in the same patient has never been reported. A 16-year-old boy was referred to us with a diagnosis of left temporal arachnoid cyst. Careful review of the MRI showed the presence of 2 different signal intensities with restricted diffusion in the cyst in the ambient cistern and normal diffusion in the cyst anterior to the temporal tip suggesting the presence of an arachnoid cyst adjacent to an epidermoid cyst which was managed using endoscope-assisted microsurgical transcranial surgery. The possibility of such an association could be explained based on the congenital origin of both lesions. Diffusion-weighted imaging can be invaluable in unraveling such a relationship.

A 74-year-old woman presented with malignant progression of remnant epidermoid cyst manifesting as sudden onset of right ptosis and double vision. She had right oculomotor nerve paresis. She had a history of surgery for right cerebellopontine angle epidermoid cyst 20 years previously. T(1)-weighted magnetic resonance (MR) imaging demonstrated a hypointense mass lesion in the right cerebellopontine angle and basal cistern, and an isointense mass in the right paraclinoid region which was strongly enhanced. Diffusion-weighted MR imaging showed hyperintense areas in the right cerebellopontine angle, ambient cistern, and basal cistern, and the paraclinoid mass as hypointense. Surgery was performed using Dolenc's approach. Histological examination revealed that the paraclinoid tumor adjacent to the epidermoid tumor remnant was malignant transformation of epidermoid cyst into squamous cell carcinoma. She was treated with 46 Gy linac radiotherapy. She has been without tumor recurrence for 17 months. Malignant change of epidermoid cyst is extremely rare, but rapid progress of the symptoms suggests malignant transformation. MR imaging with gadolinium is useful for diagnosis.

Trochlear nerve schwannomas are extremely rare. We present a surgically excised case of trochlear nerve schwannoma followed by Gamma Knife (ELEKTA, Stockholm, Sweden) after histologic confirmation. A 52-year-old man presented with diplopia, gait disturbance, and sensory disturbance. Magnetic resonance imaging demonstrated a large cystic mass at ambient cistern on the right side. Subtotal excision followed by Gamma Knife surgery was undertaken. His neurologic symptoms disappeared with radiologic reduction in size at 15-month follow-up. Because Gamma Knife can manage trochlear nerve schwannomas well, surgical excision should be reserved for large ones and limited to subcapsular or subtotal removal with the expectation of possible recovery of neurologic dysfunction.

We used microscopy to conduct qualitative and quantitative analysis of 4 surgical approaches commonly used in the surgery of the ambient cistern: infratentorial supracerebellar (SC), occipital interhemispheric, subtemporal (ST), and transchoroidal (TC). In addition, we performed a parahippocampal gyrus resection in the ST context.Each approach was performed in 3 cadaveric heads (6 sides). After the microscopic anatomic dissection, the parahippocampal gyrus was resected through an ST approach. The qualitative analysis was based on anatomic observation and the quantitative analysis was based on the linear exposure of vascular structures and the area of exposure of the ambient cistern region.The ST approach provided good exposure of the inferior portion of the cistern and of the proximal segments of the posterior cerebral artery. After the resection of the parahippocampal gyrus, the area of exposure improved in all components, especially the superior area. A TC approach provided the best exposure of the superior area compared with the other approaches. The posterolateral approaches (SC/occipital interhemispheric) to the ambient cistern region provided similar exposure of anatomic structures. There was a significant difference (P0.05) in linear exposure of the posterior cerebral artery when comparing the ST/TC and ST/SC approaches.This study has demonstrated that surgical approaches expose dissimilarly the different regions of the ambient cistern and an approach should be selected based on the specific need of anatomic exposure.

We used microscopy with endoscopic assistance to conduct an objective analysis of 4 surgical approaches commonly used in the surgery of the ambient cistern: infratentorial supracerebellar (SC), occipital interhemispheric (OI), subtemporal (ST), and transchoroideal (TC). In addition, we performed a parahippocampalis gyrus resection in the ST context.Each approach (SC, OI, ST, TC) was performed on 3 cadaveric heads (6 sides). After the microscopic anatomic dissection, the 30-degree endoscope was used to explore the exposure. The parahippocampalis gyrus was resected through an ST approach and the exposure was evaluated. The quantitative analysis was based on linear exposure of the vascular structures (linear exposure), such as the posterior choroidal artery (PChA), the P2 and P3 segments of the posterior cerebral artery (PCA) with their branches, the basal vein of Rosenthal, and the area of exposure of the ambient cistern region (area of exposure) limited by points on its superior, mesial, and anterior walls. In all cases, a P value of less than 0.05 was considered significant.There was a significant difference (P0.05) in linear exposure of the PCA and medial PChA between microsurgery and endoscopic assistance using the ST approach. This approach also improved the medial, superior, and total exposure of the ambient cistern region.This study demonstrates that endoscope assistance improved exposure of the ambient cistern region when using the ST approach. Endoscopic assistance provided similar surgical exposure compared with ST associated with parahippocampalis resection.

Idiopathic herniation of uncus and parahippocampal gyrus into the ambient cistern is a very rare entity, which could be mistaken for other pathology such as tumor. To the best of our knowledge, two prior cases of this kind of herniation have been described. One of these cases was with associated symptomatology and other abnormalities, and the other was characterized as idiopathic. In this case report, we report a case of accidental finding of a herniation of uncus and parahippocampal gyrus into the ambient cistern, without any other accompanying abnormalities, well depicted by magnetic resonance imaging without further necessity for surgical brain intervention.

Knowledge of detailed anatomy of the skull base has allowed considerable progress and advances in surgical techniques. Knowledge of dural, bony, and neurovascular spatial relationships of the trochlear nerve has played a critical role in preventing complications during complex microsurgical and endoscope-assisted transcranial and endoscopic endonasal approaches to the supracerebellar region and middle cranial fossae, and to cavernous sinus, parasellar, and orbital regions. The trochlear nerve is long and thin, and the diplopia caused by its lesion due to imprudent surgical maneuvers can have tremendous effects on the patient. The anatomy of the trochlear nerve has been assessed and examined along its entire course from the midbrain to the orbit, in order to perform a classification, anatomically and clinically oriented, of its segments. The nerve has been divided into five segments according to the areas where it is located: cisternal, tentorial, cavernous, fissural, and orbital.

The P2 bypass is one of the most difficult procedures in cerebrovascular surgery. Creating an anastomosis between the arterial graft and the P2 segment of the posterior cerebral artery is challenging because the P2 segment is located deep and high within the ambient cistern. We describe the application of a skull base technique, mastoidectomy and partial rhomboid drilling of the middle fossa, in order to achieve the anastomosis deep in the ambient cistern without temporal lobe injury. Mastoidectomy with hearing preservation creates a presigmoid space, which enables a presigmoid transtentorial approach to the ambient cistern. After complete skeletonization of the semicircular canals, rhomboid drilling of the middle fossa can be performed, confirming the landmarks of the rhomboid, arcuate eminence, greater superficial petrosal nerve, posterior margin of the third branch of the trigeminal nerve, and petrous margin. Partial rhomboid drilling, in which the posterior half of the rhomboid is drilled away, helps to reduce the requirement for temporal retraction during P2 bypass.

A 55-year-old woman presented with a sudden onset of severe headache. Computed tomography (CT) revealed SAH (grade I according to the Hunt and Hess classification), predominantly in the right ambient cistern. digital subtraction angiography (dsA) demonstrated a posterior communicating artery (pCoA) aneurysm (3.2 × 5.1 mm) (Fig. 1c). This patient had suffered sAH because of rupture of a right pCoA 12 years back (Fig. 1a) and had undergone endovascular treatment. Tungsten mechanical detachable coils (MDS; Balt Extrusion, Montmorency, France) were inserted into the aneurysm, and the occlusion was considered to be greater than 90 % (Fig. 1b). she was discharged and returned to normal life without further angiography. After comparing the dsA images of this presentation and the ones 12 years before, the two aneurysms were the same, but the tungsten coils had disappeared. Cranial CT and plain radiographs did not find high radiopacity in other places. On admission, endovascular embolisation was selected. While the patient was under a general anaesthetic, platinum coils (GDC, Boston Scientific, Fremont, CA, USA) were inserted, including two 4 mm × 10 cm coils, a 4 mm × 6 cm coil and a 2 mm × 3 cm coil. The coils were inserted until aneurysmal circulatory exclusion was achieved (Fig. 1d). Whole blood, serum and urine tungsten levels were assayed. The values were 0.00025, 0.00024 and 0.00018 μg/ ml, respectively. The normal serum value is 0.0002 μg/ml. Liver and renal functions were tested, and they were normal. X-ray scan showed no lung fibrosis or dermatitis. Subsequently, she was discharged with no neurological deficit.

Objective To describe the exposure obtained through six approaches to the perimesencephalic cisterns with an emphasis on exposure of the posterior cerebral artery and its branches. Methods Dissections in 12 hemispheres exposed the crural, ambient, and quadrigeminal cisterns and related segments of the posterior cerebral artery. A Stealth Image Guidance workstation (Medtronic Surgical Navigation Technologies, Louisville, CO) was used to compare the approaches. Results The transsylvian approach exposed the interpeduncular and crural cisterns. The subtemporal approach exposed the interpeduncular and crural cisterns as well as the lower half of the ambient cistern. Temporal lobe retraction and the position of the vein of Labbe limited exposure of the quadrigeminal cistern. Occipital transtentorial and infratentorial supracerebellar approaches exposed the quadrigeminal and lower two-thirds of the ambient cistern. Transchoroidal approaches exposed the posterior third of the crural cistern, the upper two-thirds of the ambient cistern, and the proximal quadrigeminal cistern. Transchoroidal approaches exposed the posterior portion of the P2 segment (P2p) in 9 of 10 hemispheres and were the only approaches that exposed the lateral posterior choroidal arteries and the plexal segment of the anterior choroidal artery. Occipital transtentorial and infratentorial supracerebellar approaches provided access to the P3 segment in all cases and exposed the P2p segment in 4 of 10 hemispheres. The subtemporal approach provided access to the cisternal and crural segments of the anterior choroidal and medial posterior choroidal arteries and exposed the P2p segment in 3 of 10 hemispheres. Conclusion Surgical approaches to lesions of the perimesencephalic cisterns must be tailored to the site of the pathological findings. The most challenging area to expose is the upper half of the ambient cistern, particularly the P2p segment of the posterior cerebral artery.

Hydatid cysts of the posterior fossa are extremely rare. Intracranial hydatid cysts are more common in children and occur more frequently in the supratentorial space. A 7-year-old boy was admitted to the emergency department because of intense headache, nausea, vomiting, and progressive drowsiness that developed within the period of a week. On radiological examination a round, 2.5 × 2.5-cm cystic lesion appeared in the ambient cistern and caused hydrocephalus as a result of extrinsic aqueductal stenosis. The cyst was successfully removed using the puncture, aspiration, irrigation, and resection technique via an infratentorial-supracerebellar approach with the patient in the sitting position. The authors here described an unusual case of a hydatid cyst in the left ambient cistern with hydrocephalus due to extrinsic aqueductal stenosis, which seems to be the first such case in the literature. Hydatid cyst may be considered in the differential diagnosis of arachnoid cysts of the quadrigeminal cistern to determine which surgical procedure to perform and to avoid unexpected complications.

A 21-year-old man developed an ipsilateral fourth nerve palsy, contralateral hemianopia, and contralateral hemisensory deficit as manifestations of a proximal right posterior cerebral artery aneurysm. This unusual constellation of signs reflects the involvement of the structures that run in the ambient cistern. The fourth nerve palsy and homonymous hemianopia are attributed to compression by the aneurysm. The hemisensory loss is ascribed to compromise of thalamoperforate arteries emanating from a thrombosed portion of the aneurysm.

OBJECTIVE: Opening the temporal part of the choroidal fissure (CF) makes it possible to expose the crural cistern, the ambient cistern, and the medial temporal lobe. We examined the microsurgical anatomy and the surgical indications for use of the trans-CF approach. METHODS: The microsurgical anatomy encountered in the trans-CF approach for lesions in and around the ambient cistern was studied in three cadavers. On the basis of these cadaveric studies, the trans-CF approach was used during surgery in three live patients with such lesions. RESULTS: The angiographic “plexal point,” which indicates the entrance of the anterior choroidal artery as it enters the temporal horn of the lateral ventricle, was thought to be a key anatomic landmark of the trans-CF approach. A cortical incision for entry into the temporal horn should be made in the inferior temporal gyrus to minimize the potential damage to the optic radiations and to the speech centers. After the CF is opened posteriorly to the plexal point between the tenia fimbria and the choroid plexus, the posterior cerebral artery (PCA) in the ambient cistern can be observed with minimal caudal retraction of the hippocampus. In this study, surgical procedures using the trans-CF approach were successfully performed on patients with high-positioned P2 aneurysms whose PCA ran close to the plexal point or higher, whose medial temporal arteriovenous malformations were fed mainly by the PCA, and whose tentorial hiatus meningiomas protruded into the temporal horn through the CF, with no resulting postoperative visual or memory disturbances. CONCLUSION: The trans-CF approach is especially useful in surgery for lesions in and around the ambient cistern.

OBJECTIVE: The P2 segment of the posterior cerebral artery has remained a challenging region to expose surgically. We establish a surgical strategy for P2-segment aneurysms. METHODS: Each segment of the posterior cerebral artery was classified according to Zeal and Rhoton's classification. Fourteen of 18 P2-segment aneurysms were surgically treated. The patient's outcome, the aneurysm location, and the surgical procedures were evaluated. A cadaveric study was performed to clarify the surgical view obtained via three different surgical approaches. RESULTS: Nine aneurysms were clipped, two were trapped, one was coated, and one was excised with parent artery reconstruction. Cerebral revascularization techniques were used for three patients. The pterional approach exposed the anterior half of the P2a segment. The subtemporal approach revealed the P2a segment, but its exposure was restricted by its localization in the ambient cistern. Via the occipital interhemispheric transtentorial approach, the P2p segment was visible and could be manipulated. When the posterior half of P2a segment was high on a coronal view of magnetic resonance imaging, it was extremely hard to access via any approach. CONCLUSION: A surgical strategy for the P2 aneurysm can be planned with accurate prediction of the aneurysm location. When the localization of an aneurysm on the posterior half of P2a segment is high, a resection of brain tissue may be required.

Object The basal vein of Rosenthal (BV) courses from the premesencephalic cistern, through the ambient cistern, and terminates in the quadrigeminal cistern. The aim of this study was to describe and quantitate the surgical anatomy of this structure and specifically to provide landmarks for identifying this vessel along its course. These data may be of use, for example, to surgeons using subtemporal operative approaches through regions where this vessel is concealed. Methods The authors examined 15 latex-injected adult cadaveric brains (30 sides) to delineate the morphological characteristics of the BV. Dissections of the BV were then performed and measurements were made between this structure and the tentorial incisura at the anterior, middle, and posterior borders of the lateral midbrain. All specimens were found to have a left and right BV with varying morphological characteristics. The mean distance between the BV and posterior cerebral artery at the midpoint of the lateral midbrain was 16 mm. The BV was always found superomedial to the posterior cerebral artery along the lateral aspect of the midbrain, and the BV ranged in diameter from 1 to 5 mm. The BV drained into the vein of Galen in all but two specimens. The mean distances from the tentorial edge to the BV at the anterior, middle, and posterior borders of the lateral midbrain were 11, 13, and 4 mm, respectively. No statistically significant differences were found when comparing left and right sides or male and female specimens. Conclusions The authors hope that these data will help the neurosurgeon operating near the BV to avoid injury to this important structure.

A 38-year-old male presented with a posterior fossa ependymoma with unusual extension from the cerebellopontine angle to the pineal region. Magnetic resonance imaging clearly demonstrated that these two components were continuous through the right ambient cistern. Computed tomography using a bone algorithm revealed enlargement of the right internal auditory canal. This case suggests that ependymoma can extend anywhere within the subarachnoid space along the path of least resistance.

An 18-year-old man presented with a 1-month history of global headache with lassitude and dyspnoea on exertion. Clinical examination was unremarkable except for pallor and tachycardia. An unenhanced CT of the cranium showed marked hypodensity in the region of the superior sagittal and transverse venous sinuses (figure 1A). The ambient cistern appeared to be effaced on CT. In view of the headache and hypoattenuating venous sinuses on CT, the possibility of thrombosis of …

A 66-year-old man presented with clival chordoma associated with subarachnoid hemorrhage. Computed tomography showed subarachnoid hemorrhage in the right ambient cistern and a well-enhanced tumor in the petroclival region. Surgical exploration performed on the day of admission showed a clot in the tumor. The tumor was totally removed. Histological examination showed hemorrhage between the tumor and the dura. The diagnosis was clival chordoma. Subarachnoid hemorrhage in chordoma seems to occur by spreading of intratumoral hemorrhage into the subarachnoid space due to dural invasion.

A 61-year-old woman presented with aneurysmal subarachnoid hemorrhage (SAH) associated with multiple remote intracerebral hemorrhages (ICHs). She had undergone microsurgical neck clipping for ruptured right middle cerebral artery aneurysm and ventriculo-peritoneal shunting 16 years previously. Computed tomography revealed SAH predominantly in the basal cistern and ambient cistern, in association with multiple ICHs in the bilateral tegmentum of the brain stem and right caudate nucleus. Digital subtraction angiography revealed an aneurysm at the P(1) segment of the left posterior cerebral artery. The ruptured aneurysm was microsurgically clipped via a left subtemporal approach without complications. Simultaneous occurrence of aneurysmal SAH and multiple remote ICHs is rare. The spatial pattern of the ICHs in the present case is apparently unique.

Introduction The resection of petroclival meningiomas presents great neurosurgical challenges. Although multiple surgical approaches have been developed, the retrosigmoid route tends to be used to address tumors that are predominantly located in the posterior fossa. Our modification of the lateral suboccipital retrosigmoid approach with the placement of a tentorial incision yields good visualization of the supratentorial part of the tumor around the midbrain. Methods We treated four patients, one with primary and three with recurrent petroclival meningioma, by our modified approach. After lateral suboccipital craniotomy, the infratentorial part of the tumor was removed after detaching it from the tentorial surface. The cerebellar tentorium was then carefully incised from the supracerebellar angle, taking care not to damage the superior cerebellar artery and trochlear nerve. Results The operative field surrounding the midbrain was widened by this procedure, and safe dissection of the tumor from the brainstem and other neurovascular structures was performed with direct observation of the interface. Conclusions Our approach is a useful modification of the retrosigmoid approach to petroclival meningiomas. It facilitates the safe resection of the supratentorial part of the tumor in the ambient cistern behind the tentorium.

The anatomy of the temporal lobe is reviewed as it relates to surgical resections for epilepsy. The major parenchymal structures are defined, and some mention of their functions is made. The arterial supply and venous drainage for the major regions are described. Pertinent anatomic relationships with surrounding structures in the cavernous sinus, ambient cistern, and sylvian fissure are identified.

Objective Lateral supracerebellar-infratentorial approaches are established for lesions in ambient cistern and posterolateral midbrain, but published surgical experiences do not describe results with this approach in the sitting position. Gravity retraction of the cerebellum opens this surgical corridor and dramatically alters exposure, creating 2 variations of the lateral supracerebellar-infratentorial approach: the supracerebellar-supratrochlear approach and the infratentorial-infratrochlear approach. Methods We reviewed our experience treating cavernous malformations and arteriovenous malformations (AVMs) of the posteroinferior thalamus and posterolateral midbrain by use of supracerebellar-supratrochlear and infratentorial-infratrochlear approaches. Microsurgical technique, clinical data, radiographic features, and neurological outcomes were evaluated. Results During an 11-year surgical experience with 341 cavernous malformation patients and 402 AVM patients, 8 patients were identified, 6 with cavernous malformations and 2 with AVMs. Infratentorial-infratrochlear approaches were used in 4 patients (50%), including 3 with inferolateral midbrain cavernous malformations. Supracerebellar-supratrochlear approaches were used in 4 patients (50%), including 2 with posterior thalamic lesions surfacing on pulvinar. Resections were radiographically complete in all cases. There were no new, permanent neurological deficits, nor were there any medical or surgical complications. There has been no evidence of rebleeding or recurrence. Conclusions Gravity retraction of the cerebellum transforms the lateral supracerebellar-infratentorial approach, enhancing exposure and approach trajectories that can be achieved with patients in prone or lateral positions. The increased upward viewing angle of the supracerebellar-supratrochlear approach accesses the posteroinferior thalamus. The increased downward-viewing angle of the infratentorial-infratrochlear approach accesses cerebellomesencephalic fissure and posterolateral midbrain. These approaches open wide corridors for safe surgical resection of symptomatic cavernous malformations and AVMs.

The aim of this study is to describe microsurgical anatomy and to quantitatively analyze exposure using the posterior subtemporal keyhole (PSK) approach combined with the transchoroidal keyhole (TCK) approach to the ambient cistern.We determined the proper location of craniotomy of such combined keyhole approach on 16 sides of cadaver heads. The PSK approach and the TCK approach were performed in the same minicraniotomy to observe microanatomic features and to quantitatively measure exposure limits of the ambient cistern and related structures using image-guided system.Some superficial and bone landmarks could be used to find the proper location of such combined minicraniotomy. In the PSK approach, the exposure distances of the trigeminal nerve and the anterior portion of the P2 segment (P2a) were 10.02 ± 0.76 mm and 16.32 ± 2.02 mm, respectively. The superior, inferior, anterior, and posterior exposure limit of brainstem from the intersection point of the lateral mesencephalic sulcus and pontomesencephalic sulcus was 7.5 ± 0.19 mm, 11.04 ± 0.27 mm, 15.72 ± 0.52 mm, and 10.16 ± 0.38 mm, respectively. In the TCK approach, the vertical distances between the taenia fimbriae and the lateral geniculate body without and with mild caudal retraction of the hippocampus were 5.28 ± 0.46 mm and 11.18 ± 0.57 mm, respectively. The linear exposure distances of the posterior portion of the P2 segment (P2p) or P3 segment were 12.14 ± 1.88 mm. Except of one case, the P2p could be exposed using the TCK approach. The midpoint of the medial edge of the parahippocampal gyrus on the coronal magnetic resonance images provides landmark to choose the appropriate approach.The PSK combined with the TCK approach can simultaneously expose the lower and upper ambient cistern in a proper minicraniotomy.