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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.