Morphometric Analysis of the Retrolabyrinthine Approach to the Posterior Fossa.

INTRODUCTION: The retrolabyrinthine approach provides shorter working distance and less cerebellar retraction compared with the retrosigmoid approach to the internal acoustic canal (IAC) and cerebellopontine angle cistern. However, exposure of the ventral surface of the brainstem and petroclival region may be restricted. Trautmann's triangle (TT), an area intimately related to this region, demonstrates significant anatomical variability, which may adversely affect the ease of the approach. The aim of this study is to evaluate anatomic parameters of the posterior fossa that may anticipate a challenging situation in approaching the IAC and the petroclival region through the retrolabyrinthine approach. METHODS: It was performed a radioanatomic analysis of 75 cerebral angiotomography exams to identify parameters that could potentially reduce areas of surgical exposure. RESULTS: Large variations were observed in the area of exposure of the TT (553%) and the height of the jugular bulb (234%). Shorter distances from the sigmoid sinus to the posterior semicircular canal and high-riding jugular bulb were associated with smaller areas of exposure. Dominant and laterally positioned sigmoid sinuses and less pneumatized mastoids were associated with potentially unfavorable conditions, including a narrower angle of attack to the IAC. Increased petrous slopes and petroclival angles were associated with smaller petroclival areas and shallower clival depths. CONCLUSIONS: This study of the posterior fossa reveals remarkable anatomic variation in the region. These findings should be taken into consideration during the preoperative planning of retrolabyrinthine approaches in order to offer safer and more effective surgical procedures.

Retromastoid Craniotomy: An Adaptable and Panoramic Approach to the Cerebellopontine Angle.

There are many approaches to the cerebellopontine angle, all of which involve different degrees of disruption of the normal anatomy. The retromastoid craniotomy has stood the test of time for the efficiency, panoramic exposure, and flexibility it provides while allowing excellent control over the vital cerebrovascular structures including the brainstem. This approach allows the surgeon not to get distracted by how to get there but instead to focus on what to do when he or she is there. Herein, I discuss the nuances of this simple approach with its "extended" variations.

A modified park bench position: the "Dormeuse" position.

BACKGROUND: Because of the restricted volume of the cisternal space, proper patient positioning on the operating table is of utmost importance during surgery by retrosigmoid approaches. Three positions are commonly used: supine, with the head rotated to the side contralateral to the lesion; the semi-sitting position; and the park bench position. Each position has advantages and disadvantages, and the surgeon should choose the one best suited to the individual patient and the pathology to be treated. METHODS: We describe a modified park bench position that we call the Dormeuse position. CONCLUSION: The Dormeuse position guarantees decrease in the posterior fossa pressure and allows optimal neural and vascular manipulation and control of any aspect of the cerebellopontine angle.

Anatomical characterization of the inferior petrosal sinus and adjacent cerebellopontine angle cistern for development of an endovascular transdural cerebrospinal fluid shunt.

BACKGROUND AND PURPOSE: We evaluated the inferior petrosal sinus (IPS) and adjacent cerebellopontine angle (CPA) cistern as a potential implantation site for a novel venous endovascular transdural CSF shunt concept to treat communicating hydrocephalus. We analyzed the dimensions of the IPS, CPA cistern, and distances to adjacent neurovascular structures. MATERIALS AND METHODS: Gadolinium enhanced T1 weighted brain MRI datasets of 36 randomly selected patients, aged 20-80 years, were analyzed with three-dimensional multiplanar reconstruction to measure IPS diameter and length, CPA cistern depth, and IPS proximity to the vertebrobasilar arteries and brainstem. Statistical analysis was used to assess gender, sidedness, and age dependence. RESULTS: Mean IPS diameter ranged from 2.27 mm to 3.31 mm at three axial levels, with >90% larger than 1.46 mm. CPA cistern adjacent to the IPS exhibited a mean depth of 3.86 mm to 7.39 mm between the dura and brainstem at corresponding axial levels. There was no side dependence except for a longer distance from the IPS to the basilar artery on the left compared with the right (9.72 vs 7.28, P<0.019). Linear regression analysis showed that the distance from the IPS to the brainstem was statistically significantly increased with age (P<0.0002) and was greater in men, with little side variation (P=0.524). CONCLUSION: Our results demonstrate sufficient CSF CPA cisternal space adjacent to the IPS and support the feasibility of an endovascular catheter delivered transdural implantable shunt. Such a device could serve to mimic the function of the arachnoid granulation by establishing a regulated path for CSF flow from the intracranial subarachnoid space to the venous system and provide a treatment for communicating hydrocephalus.

Endoscopically Assisted Targeted Keyhole Retrosigmoid Approaches for Microvascular Decompression: Quantitative Anatomic Study.

OBJECTIVE: We describe and quantitatively assess minimally invasive keyhole retrosigmoid approaches targeted to the upper, middle, and lower cranial nerve (CN) complexes of the cerebellopontine angle (CPA). METHODS: Anatomic dissections were performed on 10 sides of 5 fixed, silicone-injected cadaver heads. Surgical views through various trajectories were assessed in endoscopic videos and 3-dimensional (3D) interactive virtual reality microscope views. Surgical freedom and angles of attack to the proximal and distal areas of CN complexes of the CPA were compared among upper and lower keyholes and conventional retrosigmoid craniotomy using neuronavigation. RESULTS: Compared with keyholes, the conventional approach had superior surgical freedom to most areas except for the distal CN V, the root of CN VII, and the root of CN IX, where differences were not significant. The conventional retrosigmoid approach provided a larger horizontal angle of attack than either the upper or lower keyholes for all selected areas; however, the vertical angles of attack were not different. Splitting the petrosal fissure resulted in a significant increase in the vertical angle of attack to the root zones of CNs V and VII but not to the distal areas of these nerves or CN IX. Illustrative cases of endoscope-assisted keyhole retrosigmoid approaches for the treatment of trigeminal neuralgia, hemifacial spasm, and glossopharyngeal neuralgia are presented. CONCLUSIONS: Targeted keyhole retrosigmoid approaches require detailed understanding of the 3D anatomy of the CPA to create appropriate locations of corridors, including skin incisions and keyholes. Endoscope assistance complements the standard microsurgical technique by maximizing the visualization and identification of the delicate neurovascular structures.

Mastoid foramen, mastoid emissary vein and clinical implications in neurosurgery.

BACKGROUND: Mastoid emissary vein is especially important from the neurosurgical point of view, because it is located in variable number in the area of the occipitomastoid suture and it can become a source of significant bleeding in surgical approaches through the mastoid process, especially in retrosigmoid craniotomy, which is used for approaches to pathologies localized in the cerebellopontine angle. Ideal imaging method for diagnosis of these neglected structures when planning a surgical approach is high-resolution computed tomography. The aim of this work was to provide detailed information about this issue. METHODS: We studied a group of 295 skulls obtained from collections of five anatomy departments and the National Museum. Both quantitative and qualitative parameters of the mastoid foramen were evaluated depending on side of appearance and gender. Individual distances of the mastoid foramen from clearly defined surface landmarks (asterion, apex of mastoid process, foramen magnum) and other anatomical structures closely related to this issue (width of groove for sigmoid sinus, diameters of internal and external openings of mastoid foramen) were statistically processed. RESULTS: The most frequently represented type of the mastoid foramen is type II by Louis (41.2%). The differences between right and left sides were not statistically significant. In men there was a higher number of openings on the right side and in qualitative parameters the type III and IV predominated, whereas in women the types I and II were more frequent. In men, greater distances from the mastoid foramen were observed when evaluating qualitative parameters for defined surface landmarks. Mean size of the external opening diameter was 1.3 mm; however, several openings measured up to 7 mm. CONCLUSIONS: Despite excellent knowledge of anatomy, however, good pre-operative examination using imaging methods and mastering of microsurgical techniques create the base for successful treatment of pathological structures in these anatomically complex areas.

Angular and metric analysis of the neural structures in the cerebellopontine angle.

INTRODUCTION: The cerebellopontine angle (CPA) is a subarachnoid space in the lateral aspect of the posterior fossa. In this study, we propose a complementary analysis of the CPA from the cerebellopontine fissure. METHODS: We studied 50 hemi-cerebelli in the laboratory of neuroanatomy and included a description of the CPA anatomy from the cerebellopontine fissure and its relationship with the flocculus and the 5th, 6th, 7th, and 8th cranial nerves (CN) origins. RESULTS: The average distance from the 5th CN to the mid-line (ML) was 19.2 mm, 6th CN to ML was 4.4 mm, 7-8 complex to ML was 15.8 mm, flocculus to ML was 20.5 mm, and flocculus to 5th CN was 11.5 mm, additionally, and the diameter of the flocculus was 9.0 mm. The angle between the vertex in the flocculus and the V CN and the medullary-pontine line was 64.8 degrees. DISCUSSION: The most common access to the CPA is through the retrosigmoid-suboccipital region and this approach can be done with the help of an endoscope. The anatomy of origins of neural structures tends to be preserved in cases of CPA lesions. CONCLUSION: Knowledge of the average distances between the neural structures in the cerebellar-pontine fissure and the angular relationships between these structures facilitates the use of surgical approaches such as microsurgery and endoscopy.

Endoscopically Assisted Drilling, Exposure of the Fundus through a Presigmoid Retrolabyrinthine Approach: A Cadaveric Feasibility Study.

The presigmoid retrolabyrinthine approach to the cerebellopontine angle is traditionally described to not provide access to the internal auditory canal (IAC). We aimed to evaluate the extent of the IAC that could be exposed with endoscopically assisted drilling and to measure the percentage of the IAC that could be visualized with the microscope and various endoscopes after drilling had been completed. Presigmoid retrolabyrinthine approaches were performed bilaterally on 4 fresh cadaveric heads. We performed endoscopically assisted drilling to expose the fundus of the IAC, which resulted in exposure of the entire IAC in 8 of 8 temporal bone specimens. The microscope afforded a mean view of 83% (n = 8) of the IAC. The 0°, 30°, 45°, and 70° endoscope each afforded a view of 100% of the IAC in 8 of 8 temporal bone specimens. In conclusion, endoscopic drilling of the IAC of can provide an extradural means of exposing the entire length of the IAC while preserving the labyrinth.

Endoscopic transorbital route to the petrous apex: a feasibility anatomic study.

BACKGROUND: While the subtemporal approach represents the surgical module milestone designed to reach the petrous apex, a novel ventral route, which is the superior eyelid endoscopic transorbital approach, has been proposed to access the skull base. Accordingly, we aimed to evaluate the feasibility of this route to the petrous apex, providing a qualitative and quantitative analysis of this relatively novel pathway. METHODS: Five human cadaveric heads were dissected at the Laboratory of Surgical NeuroAnatomy of the University of Barcelona. After proper dissection planning, anterior petrosectomy via the endoscopic transorbital route was performed. Specific quantitative analysis, as well as dedicated three-dimensional reconstruction, was done. RESULTS: Using the endoscopic transorbital approach, it was possible to reach the petrous apex with an average volume bone removal of 1.33 ± 0.21 cm3. Three main intradural spaces were exposed: cerebellopontine angle, middle tentorial incisura, and ventral brainstem. The first one was bounded by the origin of the trigeminal nerve medially and the facial and vestibulocochlear nerves laterally, the second extended from the origin of the oculomotor nerve to the entrance of the trochlear nerve into the tentorium free edge while the ventral brainstem area was hardly accessible through the straight, ventral endoscopic transorbital trajectory. CONCLUSION: This is the first qualitative and quantitative anatomic study concerning details of the lateral aspect of the incisura and ventrolateral posterior fossa reached via the transorbital window. This manuscript is intended as a feasibility anatomic study, and further clinical contributions are mandatory to confirm the effectiveness of this approach, defining its possible role in the neurosurgical armamentarium.

Morphometry and microsurgical anatomy of Bochdalek's flower basket and the related structures of the cerebellopontine angle.

BACKGROUND: Bochdalek's flower basket (Bfb) is the distal part of the horizontal segment of the fourth ventricle's choroid plexus protruding through the lateral aperture (foramen of Luschka). The microsurgical anatomy of the cerebellopontine angle, fourth ventricle and its inner choroid plexus is well described in the literature, but only one radiological study has investigated the Bfb so far. The goal of the present study was to give an extensive morphometric analysis of the Bfb for the first time and discuss the surgically relevant anatomical aspects. METHOD: Forty-two formalin-fixed human brains (84 cerebellopontine angles) were involved in this study. Photomicrographs with scale bars were taken in every step of dissection to perform further measurements with Fiji software. The lengths and widths of the Bfb, rhomboid lip and lateral aperture of the fourth ventricle as well as the related neurovascular and arachnoid structures were measured. The areas of two sides were compared with paired t-tests using R software. Significance level was set at p < 0.05. RESULTS: Protruding choroid plexus was present in 77 cases (91.66%). In 6 cases (7.14%), the Bfb was totally covered by the rhomboid lip, and in one case (1.19%), it was absent. The mean width of the Bfb was 6.618 mm (2-14 mm), the mean height 5.658 mm (1.5-14 mm) and mean area 25.80 mm2 (3.07-109.83 mm2). There was no statistically significant difference between the two sides (p = 0.1744). The Bfb was in contact with 20 AICAs (23.80%), 6 PICAs (7.14%) and 39 vestibulocochlear nerves (46.42%). Arachnoid trabecules, connecting the lower cranial nerves to the Bfb or rhomboid lip, were found in 57 cases (67.85%). CONCLUSIONS: The Bfb is an important landmark during various surgical procedures. Detailed morphology, dimensions and relations to the surrounding neurovascular structures are described in this study. These data are essential for surgeons operating in this region.

The Retrolabyrinthine Presigmoid Approach to the Anterior Cerebellopontine Region: Expanding the Limits of Trautmann Triangle.

OBJECTIVE: The anatomic area exposed through exposure of Trautmann triangle may not be sufficient. We studied the additional exposure provided by skeletonizing the sigmoid sinus (SS). METHODS: In 5 human cadaveric heads, thin-cut computed tomography images of 2 sides of the temporal bone were obtained for a total of 10 sides. The estimated surgical working angle was calculated based on the relationship of the SS to the posterior semicircular canal (PSC), superior petrosal sinus, and jugular bulb on imaging. Mastoidectomy was performed, and heads with and without SS skeletonization were remeasured. RESULTS: Working angle calculated on computed tomography was 56° ± 11.3. Skeletonization of the SS increased the distance between the PSC and SS by 5 mm (P = 0.01) and between the lateral semicircular canal and SS by 4 mm (P = 0.01). Skeletonization and retraction of the SS significantly increased the distance between the PSC and lateral semicircular canal to the SS. On images obtained after mastoidectomy, skeletonizing the SS helped improve anterior visibility on most samples that had an SS that was lateral to the PSC on axial imaging. In samples in which the SS was medial to the PSC or had only minimal lateral displacement, skeletonizing the SS did not markedly improve visibility of the retrolabyrinthine space. CONCLUSIONS: Working area and visibility improved as the PSC and SS approached the same plane on axial imaging. Preoperative evaluation of the laterality of the SS to the PSC may assist a surgeon in determining the need for skeletonizing the SS and avoiding possible vascular injuries.

[Applied anatomy of the cerebelloponotine angle through retrosigmoid approach].

Objective:To privde anatomical basis for the surgical operation of endoscope-assisted retrosigmoid approach. Method:In 20 adult head cadavers fixed with formalin (40 sides), the surgery via retrosigmoid approach was simulated. The anatomic features of the cerebellopontine angle were examined and measured. Result:The distance from the midpoint of posterior border of sigmoid sinus to acoustic nerve near internal acoustic pore, trigeminal nerve near Meckel's cave, glossopharyngeal nerve near jugular foramen and the posterior margin of internal acoustic meatus were (32.8±1.7)mm, (46.3±1.8)mm, (29.4±3.0)mm, (31.2±2.0)mm. The tracks of degree of angle of median sagittal plane were (32.6±7.7)°, (31.4±4.1)°, (40.2±6.9)°, (32.7±4.9)°. Conclusion:The measurement provides some useful references for defining the position in operating.

Neural connections between the nervus intermedius and the facial and vestibulocochlear nerves in the cerebellopontine angle: an anatomic study.

PURPOSE: Unexpected clinical outcomes following transection of single nerves of the internal acoustic meatus have been reported. Therefore, this study aimed to investigate interneural connections between the nervus intermedius and the adjacent nerves in the cerebellopontine angle. METHODS: On 100 cadaveric sides, dissections were made of the facial/vestibulocochlear complex in the cerebellopontine angle with special attention to the nervus intermedius and potential connections between this nerve and the adjacent facial or vestibulocochlear nerves. RESULTS: A nervus intermedius was identified on all but ten sides. Histologically confirmed neural connections were found between the nervus intermedius and either the facial or vestibulocochlear nerves on 34 % of sides. The mean diameter of these small interconnecting nerves was 0.1 mm. The fiber orientation of these nerves was usually oblique (anteromedial or posterolateral) in nature, but 13 connections traveled anteroposteriorly. Connecting fibers were single on 81 % of sides, doubled on 16 %, and tripled on 3 %, six sides had connections both with the facial nerve anteriorly and the vestibular nerves posteriorly. On 6.5 % of sides, a connection was between the nervus intermedius and cochlear nerve. For vestibular nerve connections with the nervus intermedius, 76 % were with the superior vestibular nerve and 24 % with the inferior vestibular nerve. CONCLUSIONS: Knowledge of the possible neural interconnections found between the nervus intermedius and surrounding nerves may prove useful to surgeons who operate in these regions so that inadvertent traction or transection is avoided. Additionally, unanticipated clinical presentations and exams following surgery may be due to such neural interconnections.

Microvascular anatomy of the cerebellar parafloccular perforating space.

OBJECTIVE: The cerebellopontine angle is a common site for tumor growth and vascular pathologies requiring surgical manipulations that jeopardize cranial nerve integrity and cerebellar and brainstem perfusion. To date, a detailed study of vessels perforating the cisternal surface of the middle cerebellar peduncle-namely, the paraflocculus or parafloccular perforating space-has yet to be published. In this report, the perforating vessels of the anterior inferior cerebellar artery (AICA) in the parafloccular space, or on the cisternal surface of the middle cerebellar peduncle, are described to elucidate their relevance pertaining to microsurgery and the different pathologies that occur at the cerebellopontine angle. METHODS: Fourteen cadaveric cerebellopontine cisterns (CPCs) were studied. Anatomical dissections and analysis of the perforating arteries of the AICA and posterior inferior cerebellar artery at the parafloccular space were recorded using direct visualization by surgical microscope, optical histology, and scanning electron microscope. A comprehensive review of the English-language and Spanish-language literature was also performed, and findings related to anatomy, histology, physiology, neurology, neuroradiology, microsurgery, and endovascular surgery pertaining to the cerebellar flocculus or parafloccular spaces are summarized. RESULTS: A total of 298 perforating arteries were found in the dissected specimens, with a minimum of 15 to a maximum of 26 vessels per parafloccular perforating space. The average outer diameter of the cisternal portion of the perforating arteries was 0.11 ± 0.042 mm (mean ± SD) and the average length was 2.84 ± 1.2 mm. Detailed schematics and the surgical anatomy of the perforating vessels at the CPC and their clinical relevance are reported. CONCLUSIONS: The parafloccular space is a key entry point for many perforating vessels toward the middle cerebellar peduncle and lateral brainstem, and it must be respected and protected during surgical approaches to the cerebellopontine angle.

Microsurgical Anatomy of the Labyrinthine Artery and Clinical Relevance.

AIM: To describe the origin, the course, and relationships of the labyrinthine artery (LA). MATERIAL AND METHODS: Thanks to a colored silicone mix preparation, ten cranial bases were examined using x3 to x40 magnification under surgical microscope. RESULTS: The LA often arose from the meatal loop of the anterior inferior cerebellar artery (AICA) (90%), or basilar artery (10%). The loop was extra-meatal of the internal auditory meatus (IAM) in 30%, at the opening of the internal auditory meatus in 20%, or intra-meatal in 35%. The AICA coursed in closed relationship to the VII and VIII cranial nerves. It coursed between VII and VIII cranial nerve roots in 85%, or passed over the ventral side of both VII and VIII cranial nerve. The average diameter of the LA was 0.2 +/- 0.05 mm. LA was single trunk in 60%, and bi-arterial in 40%. CONCLUSION: The implication of these anatomic findings for cerebello-pontine angle tumors surgery and neurovascular pathology such as infarction, aneurysm of the LA or the AICA are reviewed and discussed.

Via-cerebellar-fissures approach for microvascular decompression of trigeminal nerve.

Although the infratentorial superior-lateral cerebellar approach has been traditionally chosen for exposure of the V cranial nerve root in the process of microvascular decompression for treatment of trigeminal neuralgia, those petrosal veins often block this surgical corridor. To detour these petrosal veins, we require a new approach. We provide a via-cerebellar-fissures approach to expose well the trigeminal nerve. With microscopy, cerebrospinal fluid was drained sufficiently to relax the cerebellum. Caudally to petrosal veins, the dissection was started from the cerebellar fissures. With the arachnoid membranes around the petrosal fissure and superior cerebellopontine fissures being opened thoroughly, the root entry zone of V nerve was visualized directly. This new approach was used in 106 patients. Among them, the block veins were encountered in 17 (16.0%). Among the 17 vein-blocked cases, 1 or 2 branches of the veins were finally cut in 2 (1.9%). The postoperative relief rate was 95.3% without complications. This via-cerebellar-fissures approach may access the root entry zone of the V cranial nerve without killing those petrosal veins, which is worth to be recommended and popularized.