Microsurgical approaches to the perimesencephalic cisterns and related segments of the posterior cerebral artery: comparison using a novel application of image guidance

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 Labbé 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.     

Surgical indications and microsurgical anatomy of the transchoroidal fissure approach for lesions in and around the ambient cistern

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.     

The transsylvian corridor through minimally invasive transcranial approaches: a comparative anatomical study

Minimally invasive transcranial approaches (MITAs) continue to expand in popularity in neurosurgery. Only few MITAs allow sufficient sylvian exposure to enable wide use of the transsylvian corridor. In this study, we aim to compare the transsylvian corridor in two MITAs: the minipterional (MPTa) and the extended supraorbital eyebrow approaches (XSEa). Eight cadaver heads were used to quantify the surgical exposure and maneuverability along the sylvian fissure and the insular lobe provided by the MPTa and the XSEa. Surgical exposure was calculated by means of the exposed length of the sylvian fissure and by the area framed within three extreme points in the insular lobe. Maneuverability was assessed by means of the surgical freedom along the sylvian cistern. XSEa provides twice the frontal exposure and half of the temporal exposure in comparison to the MPTa (p?<?0.001 and p?=?0.02, respectively). No significant differences were found between the two craniotomies in the length of the exposure of the sylvian fissure, area of insular exposure, or surgical freedom. Both the MPTa and the XSEa afford sufficient grades of exposure along the sylvian fissure and the insular lobe, although the viewing angle is significantly different between the two approaches. Such properties allow either to be used for microsurgery deep within the sylvian cistern. The use of additional corridors, such as the subfrontal route (XSEa) and pretemporal route (MPTa), may influence selection of either the minipterional or the extended supraorbital approaches according to the origin of the surgical pathology addressed.

     

How I do it – selective amygdalohippocampectomy via subtemporal approach

Background

Surgery is superior over medicamentous treatment of pharmacoresistant mesial temporal lobe epilepsy caused by hippocampal sclerosis. The armamentarium of surgical procedures comprises standard temporal lobectomy and more selective procedures. Selective amygdalohippocampectomy can be performed via transcortical, transsylvian or subtemporal approach.

Method

Describe the selective amygdalohippocampectomy through the subtemporal approach

Conclusion

After the detailed preoperative epilepsy evaluation, surgery can be offered to pharmacoresistant epilepsy patient with hippocampal sclerosis. Selective amygdalohippocampectomy can be safely performed through the subtemporal approach. The good knowledge of the mesial temporal lobe anatomy is necessary when performing this procedure.

Key points

? Perform the subtemporal craniotomy with additional bone removal up to temporal petrous part to minimize retraction of the brain. ? Release the CSF from the subarachnoid sulcal space in order to relax the temporal lobe. Dissect the arachnoid around basal temporal veins and protect them with hemostatic material in order to avoid vein rupture. ? After gyrus fusiformis corticotomy, always follow the white matter in order to enter the temporal horn. ? Place the self-retraining retractor gently to secure an unobstructed view of the intraventricular mesial temporal lobe structures. ? Visualize the choroid plexus and the inferior choroidal point. They represent the two most important landmarks. ? While performing the anterior disconnection the goal is to reach the arachnoid of the interpeduncular and crural cistern medially and the tentorial edge laterally. ? Follow the tentorial edge and the arachnoid of the temporal base to securely perform the lateral disconnection. ? Perform the posterior disconnection at the level of the mesencephalon superior colliculi. ? During the medial disconnection the dissection of the arachnoid of the hippocampal sulcus must be done as close to the hippocampus as possible in order to avoid damage to the brain stem perforators or the loop of the anterior choroidal artery. ? Knowledge of mesial temporal lobe anatomy is crucial.     

A combined subtemporal and transventricular/transchoroidal fissure approach to medial temporal lesions

OBJECTIVE: To minimize therapeutic morbidity such as cognitive function disturbance and vascular injury to perforating arteries, preoperative functional mapping of the basal temporal lobe functions was performed and the medial temporal lesions were resected via a combined subtemporal and transventricular/transchoroidal fissure approach. METHODS: Twenty-one patients with medial temporal lesions including tumors, arteriovenous malformations, and medial temporal lobe epilepsy underwent operation. The neurovascular structures in the ambient cistern were first dissected free from the medial temporal lobe with a conventional subtemporal approach. Then, the temporal horn was opened through the basal surface of the temporal lobe. Finally, the ambient cistern was accessed from the temporal horn through the choroidal fissure. In five patients whose lesions were revealed to be located on the dominant side by preoperative intracarotid amytal administration test (Wada test), functional mapping of the basal temporal lobe language cortex was monitored for 1 week by use of a subdural electrocorticogram grid before the extirpation surgery. The entrance point from the temporal base to the temporal horn was determined by the result of the functional mapping. RESULTS: The lesions were resected safely and completely in all cases. Language and cognitive functions were preserved even in patients with the basal language area on the dominant side. CONCLUSION: Surgeons can confirm the important neurovascular structures from the subtemporal route and from the transtemporal horn route by a combined subtemporal and transventricular/transchoroidal fissure approach. This approach is especially effective for avoiding ischemic complications by allowing direct confirmation of the anterior choroidal and thalamoperforating arteries.     

Combined extradural subtemporal and anterior transpetrosal approach to tumors located in the interpeduncular fossa and the upper clivus

Background

Central skull base lesions in the interpeduncular fossa and the upper clival regions can be challenging to access because of their location anterior to the brainstem. We have modified the anterior transpetrosal approach by combination with the extradural subtemporal route to increase the surgical corridor.

Methods

Thirty-seven patients underwent surgical treatment via the anterior transpetrosal approach from 2002 to 2012. The combined surgical approach was primarily applied when the tumors arose from the upper clival portion and extended to the interpeduncular fossa. The combined approach was used in seven of these patients, comprising four patients with petroclival meningiomas, one patient with sphenoclival meningiomas, one patient with trigeminal schwannoma, and one patient with an epidermoid cyst extending from the interpeduncular fossa to the prepontine cistern.

Results

The combined approach permitted excellent visualization of the interpeduncular fossa in addition to the upper clivus and the lateral aspect of the brain stem. Mobilization of the temporal lobe by the entire epidural dissection of the lateral wall of the cavernous sinus facilitates access via the subtemporal route. The transient symptom of the temporal lobe in the dominant site may be the only drawback for this combined approach, although it may disappear immediately after the surgery.

Conclusion

The present approach combines Dolenc’s approach and Kawase’s approach, providing a wide exposure to lesions of the interpeduncular fossa and the clivus.     

The pretemporal approach to the interpeduncular and petroclival regions

Summary A pretemporal approach to the interpeduncular and petroclival regions is described.Through a frontotemporal craniotomy based very low in the middle fossa the temporal lobe is completely exposed. The Sylvian, carotid, chiasmatic, and lamina terminalis cisterns are widely opened. The arachnoid fibers between the uncus and the frontal lobe, as well as those binding the temporal lobe to the tentorial edge and to the oculomotor nerve are also separated. The bridging veins from the temporal pole to the spheno-parietal sinus are usually coagulated and sacrificed allowing for posterior displacement of the temporal lobe.The approach combines the advantages of both the classical pterional and subtemporal approaches providing unhindered exposure of the anterior portion of the tentorial incisura in dealing with vascular and tumoural lesions arising at the sellar, parasellar, and interpeduncular regions, and at the superior aspect of the petroclival region.     

Microsurgical anatomy of the choroidal fissure

The microsurgical anatomy of the choroidal fissure was examined in 25 cadaveric heads. The choroidal fissure, the site of attachment of the choroid plexus in the lateral ventricle, is located between the fornix and thalamus in the medial part of the lateral ventricle. The choroidal fissure is divided into three parts: (a) a body portion situated in the body of the lateral ventricle between the body of the fornix and the thalamus, (b) an atrial part located in the atrium of the lateral ventricle between the crus of the fornix and the pulvinar, and (c) a temporal part situated in the temporal horn between the fimbria of the fornix and the lower surface of the thalamus. The three parts of the fissure are the thinnest sites in the wall of the lateral ventricle bordering the basal cisterns and the roof of the third ventricle. Opening through the body portion of the choroidal fissure from the lateral ventricle exposes the velum interpositum and third ventricle. Opening through the temporal portion of the choroidal fissure from the temporal horn exposes the structures in the ambient and crural cisterns. Opening through the atrial portion of the fissure from the atrium exposes the quadrigeminal cistern, the pineal region, and the posterior portion of the ambient cistern. The neural, arterial, and venous relationships of each part of the fissure are reviewed. The operative approaches directed through each part of the fissure are also reviewed.     

Microsurgical anatomy of the interpeduncular cistern and related arachnoid membranes

OBJECT: The goal of this study was to investigate the microsurgical anatomy of the interpeduncular cistern and related arachnoid membranes. METHODS: The interpeduncular cistern and related arachnoid membranes were studied in eight Han Chinese adult human cadaveric brains with the aid of an operating microscope. The interpeduncular cistern is one area in the cranial cavity in which the arachnoid membranes and trabeculae are extremely luxuriant and complicated. The Liliequist membrane, the medial pontomesencephalic membrane, and the lateral pontomesencephalic membranes form the walls of the interpeduncular cisterns. The basilar artery (BA) bifurcation membrane, posterior perforated membrane, and arachnoid trabeculae fill the cistern. These arachnoid membranes and trabeculae adhere to the hypothalamus, brainstem, and oculomotor nerves, and bind the bifurcation of the BA, posterior cerebral arteries, superior cerebellar arteries, posterior communicating arteries, and their perforating branches. CONCLUSIONS: Arachnoid membranes and trabeculae complicate the exposure and dissection of lesions within the interpeduncular cistern. All arachnoid membranes and trabeculae should be dissected and incised sharply during surgical procedures. The BA bifurcation membrane and the posterior perforated membrane must be incised after opening the Liliequist membrane for sufficient exposure of deep structures within the interpeduncular cistern.     

Selection of surgical approach for quadrigeminal cistern arachnoid cyst

Preoperative magnetic resonance imaging of 4 cases of quadrigeminal cistern arachnoid cyst were retrospectively reviewed and patterns of extension to surrounding cisterns from the quadrigeminal cistern were examined. Relationship between patterns of extension to surrounding cisterns and selected surgical approach were evaluated. In 2 cases, the cyst extended anteriorly and compressed the quadrigeminal plate. These two cases had hydrocephalus due to aqueductal stenosis. The anteroirly extending cyst was treated with endoscopic ventriculocystocisternostomy via the lateral and third ventricles. In one case, the cyst extended superiorly to the velum interpositum cistern, and was treated with endoscopic ventriculocystocisternostomy via the lateral ventricle. In one case, the cyst extended laterally to the ambient cistern and compressed the posterior horn of the lateral ventricle. This case had loculated hydrocephalus of the inferior horn. The laterally extending cyst was treated with resection of the wall of the arachnoid cyst via an infratentorial supracerebellar approach assisted by endoscope. All cysts were successfully treated. Injury of the foramen of Monro occurred during a procedure using a flexible endoscope in a case with an anterior extending cyst. Exact analysis of the preoperative imaging and selection of appropriate surgical approach are key factors for successful treatment of a quadrigeminal cistern arachnoid cyst.     

White matter fiber dissection of the optic radiations of the temporal lobe and implications for surgical approaches to the temporal horn

OBJECT: The aim of this anatomical study was to define more fully the three-dimensional (3D) relationships between the optic radiations and the temporal horn and superficial anatomy of the temporal lobe by using the Klingler white matter fiber dissection technique. These findings were correlated with established surgical trajectories to the temporal horn. Such surgical trajectories have implications for amygdalohippocampectomy and other procedures that involve entering the temporal horn for the resection of tumors or vascular lesions. METHODS: Ten human cadaveric hemispheres were prepared with several cycles of freezing and thawing by using a modification of the method described by Klingler. Wooden spatulas were used to strip away the deeper layers of white matter progressively in a lateromedial direction, and various association, projection, and commissural fibers were demonstrated. As the dissection progressed, photographs of each progressive layer were obtained. Special attention was given to the optic radiation and to the sagittal stratum of which the optic radiation is a part. The trajectories of fibers in the optic radiation were specifically studied in relation to the lateral, medial, superior, and inferior walls of the temporal horn as well as to the superficial anatomy of the temporal lobe. In three of the hemispheres coronal sections were made so that the relationship between the optic radiation and the temporal horn could be studied more fully. In all 10 hemispheres that were dissected the following observations were made. 1) The optic radiation covered the entire lateral aspect of the temporal horn as it extends to the occipital horn. 2) The anterior tip of the temporal horn was covered by the anterior optic radiation along its lateral half. 3) The entire medial wall of the temporal horn was free from optic radiation fibers, except at the level at which these fibers arise from the lateral geniculate body to ascend over the roof of the temporal horn. 4) The superior wall of the temporal horn was covered by optic radiation fibers. 5) The entire inferior wall of the temporal horn was free from optic radiation fibers anterior to the level of the lateral geniculate body. CONCLUSIONS: Fiber dissections of the temporal lobe and horn demonstrated the complex 3D relationships between the optic radiations and the temporal horn and superficial anatomy of the temporal lobe. Based on the results of this study, the authors define two anatomical surgical trajectories to the temporal horn that would avoid the optic radiations. The first of these involves a transsylvian anterior medial approach and the second a pure inferior trajectory through a fusiform gyrus. Lateral approaches to the temporal horn through the superior and middle gyri, based on the authors' findings, would traverse the optic radiations.     

Transsylvian keyhole functional hemispherectomy

OBJECTIVE: To describe the technical steps, advantages, and limitations of a quicker, minimal-exposure, functional hemispherectomy procedure developed from a hemispherical deafferentation technique previously described. METHODS: The surgical approach using the transsylvian/transsulcal passage to the ventricular system, with the anatomic orientation points and key features for planning of the small trepanation, is described. Through a linear incision, a craniotomy (4 x 4 to 4 x 5 cm) is placed over the sylvian fissure. Transsylvian exposure of the circular sulcus allows transcortical exposure of the entire ventricular system, from the frontal horn to the temporal horn encircling the insular cortex. The frontobasal and mesial white matter is disconnected via the intraventricular approach, with a callosotomy. An amygdalohippocampectomy completes the dissection. The experience with 20 patients who were treated using the transsylvian keyhole hemispherectomy technique is summarized. RESULTS: The operation time was significantly shorter (mean, 3.6 h) than with the Rasmussen technique (mean, 6.3 h) and 25% shorter than with the transcortical perisylvian technique (mean, 4.9 h). The proportion of patients requiring blood replacements was lower (15 versus 58%), as was the mean amount of transfused blood. The mean follow-up period was 46 months; 88% of patients were in Engel Outcome Class I, 6% in Class III, and 6% in Class IV. CONCLUSION: The transsylvian keyhole procedure has been demonstrated to further reduce operation time and the need for blood replacement. It is most easily performed in cases with enlarged ventricles or perinatal ischemic cysts and is not recommended for hemimegalencephaly. The immediate seizure relief was satisfying. This minimal-exposure approach seems to be a satisfying alternative among possible functional hemispherectomy procedures.     

Clipping of an aneurysm of the posterior cerebral artery via the transcortical transchoroidal-fissure approach: a case report

A 65-year-old woman suddenly developed severe headache with nausea. Computed tomographic scans revealed a diffuse subarachnoid hemorrhage with thick hematoma of the left ambient cistern. Cerebral angiogram did not show any aneurysm. On the 7th day after admission, 3D-CT angiogram showed an aneurysm of the left posterior cerebral artery. On the 14th day, axial and coronal magnetic resonance images showed the aneurysm, surrounding structures and the choroidal fissure. On the 26th day after admission, successful neck clipping was performed through the temporal horn via the inferior temporal gyrus. The postoperative course was uneventful except for transient aphasia. This approach may be preferable in such cases, because it protects the brain from the detrimental effects of strong temporal retraction and provides a wider working space. In our case, thin slice MRI and MRA showing the aneurysm in the ambient cistern and the choroidal fissure were useful for deciding the appropriate approach.     

Microanatomy and surgical relevance of the olfactory cistern

All surgical approaches to the anterior skull base involve the olfactory cistern and have the risk of damaging the olfactory nerve. The purpose of this study was to describe the microanatomical features of the olfactory cistern and discuss its surgical relevance. In this study, the olfactory cisterns of 15 formalin-fixed adult cadaveric heads were dissected using a surgical microscope. The results showed that the olfactory cistern was situated in the superficial part of the olfactory sulcus, which separated the gyrus retus from the orbital gyrus. In coronal section, the cistern was triangular in shape; its anterior part enveloped the olfactory bulbs and was high and broad; its posterior part was medial-superior to internal carotid artery and was also much broader. There were one or several openings in the inferior wall of the posterior part in 53.4% of the cisterns. The olfactory cistern communicated with the surrounding subarachnoind cisterns through these openings. The middle part of the olfactory cistern gradually narrowed down posteriorly. Most cisterns were spacious with a few fibrous trabeculas and bands between the olfactory nerves and cistern walls. However 23% of the cisterns were narrow with the cistern walls tightly encasing the olfactory nerve. There were two or three of arterial loops in each olfactory sulcus, from which long, fine olfactory arteries originated. The olfactory arteries coursed along the olfactory nerve and gave off many terminal branches to provide the main blood supply to the olfactory nerve in most cisterns, but the blood supply was in segmental style in a few cisterns. Moreover, the veins of the cistern appeared to be more segmental than the olfactory arteries in most cisterns. These results suggested that most olfactory cisterns are spacious with relatively independent blood supply, and it is reasonable to separate the olfactory tract with its independent blood supply from the frontal lobe by 1-2 cm in the subfrontal approach, the pterional approach, or anterior interhemispheric approach. However, in the minority of cases, separation of the olfactory tract is not safe because of the anterior origin of the olfactory arteries or segmental blood supply. It is difficult to separate the olfactory nerve without any damage to the olfactory nerve, even with very skilled hands.     

Opening cranial cisterns by the anterior subtemporal keyhole approach to the superior petroclival region: anatomical study and comparative analysis

Background

Without drilling the bony structures, simple incision of the tentorium dorsal to the porus of trochlear nerve by the anterior subtemporal keyhole approach provides narrow exposure in the superior petroclival region. This study was designed to measure and compare the areas of exposure, linear distances, and angles of approach in the superior petroclival region before and after opening cranial cisterns.

Methods

The study was carried out on 20 sides of cadaver heads. With the use of a navigation system, the area of exposure in the dorsum sella and clivus, the ventrolateral area of exposure in brainstem around the BA, the linear exposure of the posterior petroclinoid fold and BA trunk, and the angles of approach to BA tip were evaluated. The length of oculomotor cistern and trochlear cistern, the distance of oculomotor porus between trochlear porus, the linear exposure of the trigeminal nerve, and the angle to the most inferior medial point in clivus were also measured.

Results

The area of exposure in the dorsum sella and clivus (157.6 ± 27.2mm²), the ventrolateral area of exposure in brainstem around BA (249.5 ± 29.7mm²), the linear exposure of the posterior petroclinoid fold (11.5 ± 0.6 mm) and BA trunk (10.3 ± 2.0 mm), and the angle of approach in the vertical axis to the BA tip (13.7° ± 1.7°) were significantly greater after opening cisterns. The angles of approach in the horizontal axis to the BA tip (24.5° ± 1.1° vs 24.7° ± 0.8°) were not statistically different. The length of oculomotor cistern and trochlear cistern, the distance of oculomotor porus and trochlear porus, the linear exposure of the trigeminal nerve, and the angle of approach to the most inferior medial point in clivus were 6.7 ± 1.3 mm, 10.7 ± 2.4 mm, 9.6 ± 2.3 mm, 15.8 ± 1.4 mm, and 13.0° ± 1.5°, respectively.

Conclusion

The areas of exposure, linear distances, and the angles of approach in the superior petroclival region can be increased after opening cranial cisterns by the anterior subtemporal keyhole approach.     

Zygomaticotemporal approach to the basis cranii and basilar artery

When using the zygomaticotemporal approach, one removes the whole of the zygomatic bone with its attachment to the masseter muscle, allowing a lower and more anterior approach to the interpeduncular cistern along the inferomedial surface of the temporal lobe. Minimal brain retraction is required to give an excellent view of the bifurcation of the basilar artery and of the suprasellar region.     

Orbital cortical approach to lesions around the frontal horn of the lateral ventricle: indication and surgical parameters

Background

An orbital cortical approach to lesions in the region of the frontal horn is described on the basis of surgical experience with five cases and dissections of three cadaveric brain specimens. The approach involves cortical incision over the orbital surface of the frontal brain and directing the surgical trajectory superiorly. The possible indications of the approach and the critical surgical parameters are described.

Method

To assess the landmarks that could be used to employ the approach, three formalin-fixed frozen cadaveric brains were appropriately dissected. A number of parameters were analysed to identify the safe entry points and the trajectory to approach the frontal horn. Five lesions located in the region of the frontal horn were operated upon by employing the discussed approach.

Results

The frontal horn is located at the depth of approximately 18 mm (range, 17–20 mm) from the orbital surface of the frontal brain. In a lateral perspective, the tip of the frontal horn is in line with the tip of the temporal pole. Wide opening of the Sylvian fissure, relaxation of the brain and lateral basal frontal exposure can be used effectively to obtain a suitable angulation for conduct of surgery. Avoidance of olfactory tracts and Heubner’s perforating artery at the site of medial orbital gyrus cortical incision and appropriately directing the corticectomy that avoids the association fibre tracts, caudate head and internal capsule can lead to a safe exposure of the frontal horn. The approach is suitable for lesions involving or in the vicinity of the inferior aspect of the frontal horn and in the region of the caudate head. Neuronavigation can be of assistance during surgery and avoid critical misdirection. All the five lesions were treated without consequence.

Conclusions

For selected indications, an inferior frontal or orbital cortical approach can be used effectively and safely to approach lesions in relation to the frontal horn. The approach needs to be precise to avoid injury to vital adjoining structures.     

Microsurgical anatomy of the posterior fossa cisterns

The microsurgical anatomy of the posterior fossa cisterns was examined in 15 cadavers using 3X to 40X magnification. Liliequist's membrane was found to split into two arachnoidal sheets as it spreads upward from the dorsum sellae: an upper sheet, called the diencephalic membrane, which attaches to the diencephalon at the posterior edge of the mamillary bodies, and a lower sheet, called the mesencephalic membrane, which attaches along the junction of the midbrain and pons. Several other arachnoidal membranes that separate the cisterns were identified. These include the anterior pontine membrane, which separates the prepontine and cerebellopontine cisterns; the lateral pontomesencephalic membrane, which separates the ambient and cerebellopontine cisterns; the medial pontomedullary membrane, which separates the premedullary and prepontine cisterns; and the lateral pontomedullary membrane, which separates the cerebellopontine and cerebellomedullary cisterns. The three cisterns in which the arachnoid trabeculae and membranes are the most dense and present the greatest obstacle at operation are the interpeduncular and quadrigeminal cisterns and the cisterna magna. Numerous arachnoid membranes were found to intersect the oculomotor nerves. The neural and vascular structures in each cistern are reviewed.     

Inferior temporal sulcus approach for amygdalohippocampectomy guided by a laser beam of stereotactic navigator

OBJECTIVE: To describe a surgical technique for a minimally invasive transcortical transventricular amygdalohippocampectomy via the inferior temporal sulcus (ITS) using a stereotactic navigator. METHODS: Seven patients with medically intractable mesial temporal lobe epilepsy underwent an amygdalohippocampectomy via the ITS. By use of a laser-guided navigation system, the epileptogenic foci of the mesial temporal lobe were resected through a small linear operative route that was made by a brain speculum inserted from the ITS to the anterolateral floor of the temporal horn in the lateral ventricle. RESULTS: All patients completed at least a 1-year follow-up (range, 14-45 mo) after surgery and had improved neuropsychological parameters as a result of the operation. All patients became seizure-free after surgery. A Humphrey visual field perimeter detected no hemianopsia. CONCLUSION: Combined with the stereotactic navigation system, the ITS approach provides the least invasive amygdalohippocampectomy that preserves optic radiation. This approach seems beneficial especially in patients in whom the epileptic lesions are limited to the anterior mesial temporal lobe.     

Temporal mediobasal tumors: a proposal for classification according to surgical anatomy

Objective  Development of a classification for temporal mediobasal tumors based on anatomical and neuroradiological aspects to help evaluate surgical accessibility and risk. Methods  Preoperative magnetic resonance imaging, surgical approaches and outcomes of 235 patients with a temporal mediobasal tumor were analyzed retrospectively. Surgical landmarks were defined in accordance with operative anatomy. Previous classifications of these tumors were reviewed and a new classification system was developed. Results  The new classification system recognises four types of temporal mediobasal tumor based on anatomical landmarks, location, and size. Type A comprises lesions confined to the uncus, hippocampus, parahippocampus, and/or amygdala. Type B comprises lesions in the area immediately lateral to the structures where type A tumors are located but sparing lateral gyri. Type C tumors are larger lesions, which occupy the area of type A and type B simultaneously. Type D tumors originate from the temporal mediobasal region and invade into the adjacent structures of the temporal stem, insular cortex, claustrum, putamen, or pallidum. The area occupied by a tumor in the axial plane was divided into anterior (a) and posterior (p) subregions. Progressive grading from A to D and from “a” to “p” was based on the view that larger and more posteriorly growing tumors were more difficult to remove. Lesions located in the anterior subregion (n = 173) were easier to remove by the transsylvian route (39%) or after partial anterior lobectomy (32%). For the posterior lesions (n = 62), a subtemporal approach was more appropriate (75%). Conclusions  Based on a series of 235 temporal mediobasal tumors, a classification system was designed to aid in decision making about operability, surgical risk, and approach.