Dorello's canal: a microanatomical study

Single-Stage Endoscopic Repair of Pediatric Basal Encephalocele: A Comprehensive Multimedia Case Report

BACKGROUND AND IMPORTANCE

Basal encephaloceles are the result of a concomitant cranial and dural defect that allows for inferior displacement of cerebral tissue, meninges, and cerebrospinal fluid into the paranasal sinuses and outside the cranial vault. This work illustrates a step-by-step surgical approach of a successful single-stage, endoscopic repair of a congenital basal encephalocele in a 10-year-old child, using a free mucosal middle turbinate graft that provided effective results without utilization of traditional open reconstructive techniques or vascularized flaps.

CLINICAL PRESENTATION

A previously healthy 10-year-old male with a history of unilateral clear rhinorrhea was admitted as an inpatient because of an acute episode of nausea, vomiting, and confusion, accompanied by fever, diplopia, and bilateral abducens nerve palsies. Preoperative imaging revealed a 2-cm right-sided intranasal mass accompanied by a subcentimeter skull base defect spanning the lateral lamella. After completing a course of intravenous antibiotic therapy for 1 week after a negative lumbar puncture to ensure clearance of intracranial infection, the decision was made to proceed with definitive endoscopic skull base repair to obviate recurrent bacterial meningitis episodes and potential neurological complications.

CONCLUSION

This study demonstrates technical feasibility of a single-stage endoscopic endonasal approach for pediatric basal encephalocele resection and repair which minimizes craniofacial morbidity associated with traditional open approaches and sinonasal morbidity associated with local pedicle-based flaps for small cranial base defects in this unique patient population.

Unveiling the vulnerability of the human abducens nerve: insights from comparative cranial base anatomy in mammals and primates

The topographic anatomy of the abducens nerve has been the subject of research for more than 150 years. Although its vulnerability was initially attributed to its length, this hypothesis has largely lost prominence. Instead, attention has shifted toward its intricate anatomical relations along the cranial base. Contrary to the extensive anatomical and neurosurgical literature on abducens nerve anatomy in humans, its complex anatomy in other species has received less emphasis. The main question addressed here is why the human abducens nerve is predisposed to injury. Specifically, we aim to perform a comparative analysis of the basicranial pathway of the abducens nerve in mammals and primates. Our hypothesis links its vulnerability to cranial base flexion, particularly around the sphenooccipital synchondrosis. We examined the abducens nerve pathway in various mammals, including primates, humans (N = 40; 60% males; 40% females), and human fetuses (N = 5; 60% males; 40% females). The findings are presented at both the macroscopic and histological levels. To associate our findings with basicranial flexion, we measured the cranial base angles in the species included in this study and compared them to data in the available literature. Our findings show that the primitive state of the abducens nerve pathway follows a nearly flat (unflexed) cranial base from the pontomedullary sulcus to the superior orbital fissure. Only the gulfar segment, where the nerve passes through Dorello's canal, demonstrates some degree of variation. We present evidence indicating that the derived state of the abducens pathway, which is most pronounced in humans from an early stage of development, is characterized by following the significantly more flexed basicranium. Overall, the present study elucidates the evolutionary basis for the vulnerability of the abducens nerve, especially within its gulfar and cavernous segments, which are situated at the main synchondroses between the anterior, middle, and posterior cranial fossae-a unique anatomical relation exclusive to the abducens nerve. The principal differences between the pathways of this nerve and those of other cranial nerves are discussed. The findings suggest that the highly flexed human cranial base plays a pivotal role in the intricate anatomical relations and resulting vulnerability of the abducens nerve.

Prevalence, Laterality, and Classification of Ossified Petroclival Ligaments: An Anatomical and Histological Study With Application to Skull Base Surgery

Background The petroclival ligament (PL) forms the roof of Dorello's canal (DC). In humans, partial and complete ossification of this ligament have been reported. When completely ossified, DC is transformed into a bony foramen for the abducens nerve and accompanying vascular structures. As this osteological finding might have an impact on skull base surgery, this anatomical study was performed. Methodology Using 100 adult human skulls, the presence of an ossified PL was noted and classified. The diameter of the resultant bony foramen and laterality were documented. Additionally, PL was evaluated histologically in 10 heads. Results Overall, 8% of the sides were found to have partial or complete ossification of the PL. Partial ossification (type I) was noted on 3% of the sides. Completely ossified PL was identified on 5% of the sides. Some ossified ligaments (2.5%) were seen as an ossified bridge (type II), and others (2.5%) were converted into small foramina (type III). Three skulls (3%) were found to have a completely ossified ligament bilaterally. The mean diameter of the underlying DC was 0.8 mm. Partially ossified ligaments were statistically more likely to be on the right sides, and the diameter of the underlying DC was statistically smaller in type III. Histologically, the PL was found to have bone within it on three skull sides. Conclusions An ossified ligament can be found on imaging of the skull base. Moreover, during surgical approaches to the petroclival region and, specifically, DC, skull base surgeons should be cognizant of this anatomical variation.

An atypical anatomical variation of the petrosphenoidal ligament in a newborn cadaver

PURPOSE

To report an atypical anatomical variation of the petrosphenoidal ligament in a newborn cadaver and to discuss its clinical significance.

METHODS

During a study of ten newborn cadavers, the skull base was dissected to reveal the anatomy of the petrosphenoidal ligament and its relationship with the abducens nerve. An atypical anatomical variation was observed, and this is described. The length of the right and left ligaments to the point where it splits into two arms, the joint length of the two ligaments at the junction of the clivus, the length of the ligament proceeding to the posterior clinoid process, and the abducens nerve's diameter as it passes below the ligament were measured on ImageJ software.

RESULTS

The petrosphenoidal ligaments were y-shaped, and the attachment of the proximal ligaments was bifid. In the midline above the clivus, some ligament bundles joined the contralateral petrosphenoidal ligament, whilst another group of bundles originated at the posterior clinoid process. At the entry to Dorello's canal, the abducens nerve had a diameter of 0.59 mm on the left and 0.65 mm on the right. The part of the ligaments converging on the clivus in the midline after dividing into two arms was 10.68 mm.

CONCLUSIONS

We think that this case report will provide useful information for surgical procedures to the petroclival region, transnasal surgical approaches, and surgical interventions involving the carotid artery.

Clinical analysis of cavernous sinus anatomy, pathologies, diagnostics, surgical management and complications - comprehensive review

For decades, the cavernous sinus (CS) has been the subject of debates and scientific studies aimed at elucidating its anatomical variability, and at choosing the best method for accessing it so that optimal diagnoses and related surgical treatments can be decided. The present review considers a series of issues related to the CS. The anatomy of the CS and its features is explored first, and the most important structures, spaces and morphological variations are considered. This is followed by CS pathology and selected diagnostic methods that have proved useful in therapy, and then the management of these pathologies is discussed. Examples of therapeutic steps that have proved helpful in specific cases are taken from the literature. Finally, the various surgical accesses and complications that can be encountered during invasive interventions in the CS area are discussed. The aim of this study is to summarize up-to-date anatomical and clinical knowledge about the CS, citing the most informative scientific papers and aggregating their results. Morphological variations of the CS are common but have not been well described in the literature.

High resolution 3D magnetic resonance imaging of Gruber's ligament: a pilot study

INTRODUCTION

Gruber's ligament (GL), a surgical landmark, extends from the lateral upper clivus to the petrous apex (PA), forming the superior boundary of Dorello's canal (DC). It overlies the interdural segment of the abducens nerve (CN VI). High-resolution 3D skull base MRI (SB-MRI) demonstrates anatomic details visible to the surgeon, but not well seen on traditional cross-sectional imaging. The aim of this study was to demonstrate visualization of the GL and its relationship to CN VI utilizing contrast enhanced high-resolution SB-MRI.

METHODS

Two neuroradiologists retrospectively reviewed in consensus the SB-MRIs of 27 skull base sides, among 14 patients. GL detection rate, confidence of detection, and GL length were recorded. When GL was successfully identified, the position of the interdural segment of CN VI within DC was recorded.

RESULTS

GL was readily identified in 16 skull base sides (59%), identified with some difficulty in 2 skull base sides (7%), and failed to be identified in 9 skull base sides (33%). The mean GL length was 7.1 mm (4.5-9.3 mm). Among the 18 cases where GL was successfully identified, CN VI was readily identified in all cases (100%), coursing the lateral third of DC in 72% of sides, and middle third in the remaining 28% of sides.

CONCLUSION

GL can be identified in approximately two-thirds of cases utilizing 3D high resolution SB-MRI. CN VI passes most commonly along the lateral third of DC. This is the first report demonstrating visualization of GL and its relation to CN VI, on imaging.

The effect of morphological variability of Dorello's canal on surgical procedures - a review

Dorello's canal is an arched structure of bone-fibrous character located in the petroclival venous confluence atop the petrosal bone in the petroclival region. It is bordered by the petrosphenoidal ligament, the petrous part of the temporal bone and the lateral border of the upper part of clivus. Its content in the vast majority of variants comprises the abducens nerve, the inferior petrosal sinus, the venous drainage and the dorsal meningeal artery or its medial branch. With the development of microsurgical techniques, this area has gained huge clinical importance, mainly concerning the order in which the above-mentioned elements (especially the position of the abducens nerve) are arranged in relation to each other. These structures appear in different variant forms and necessitate an individual clinical approach. The main purpose of this review is to present condensed information about possible intercorrelations among them and to indicate, on the basis of the available literature and research, possible surgical approaches and the need to consider the variability when treatments in this region are planned.

Factors Associated With Abducens Nerve Palsy in Patients Undergoing Surgery for Petroclival Meningiomas

BACKGROUND

During the surgical resection of petroclival meningiomas, preserving the cranial nerves is crucial. The abducens nerve is particularly vulnerable during surgery. However, the preoperative risk factors and postoperative prognosis of abducens nerve palsy (ANP) are poorly understood.

METHODS

We retrospectively analyzed 70 patients who underwent surgery for petroclival meningiomas between May 2010 and December 2019, divided into gross-total resection (GTR) and subtotal resection (STR) groups. The relationship of preoperative clinical factors with the incidence and recovery of postoperative ANP was analyzed.

RESULTS

Postoperative ANP was observed in 23 patients (32.9%). Multivariable logistic regression revealed that the tumor-to-cerebellar peduncle T2 imaging intensity index (TCTI) (P < 0.001) and internal auditory canal invasion (P = 0.033) contributed to postoperative ANP. GTR was achieved in 37 patients (52.9%), and 10 (27.0%) of them showed ANP. STR was achieved in 33 patients (47.1%), and 13 (39.4%) of them showed ANP. Recovery from ANP took a median of 6.6 months (range, 4.5-20.3 months). At 6 months after the operation, recovery of the abducens nerve function was observed in 16 patients (69.0%); of whom, 4 (40.0%) were in the GTR group and 12 (92.3%) were in the STR group (P = 0.025).

CONCLUSIONS

TCTI and internal auditory canal invasion were the risk factors for postoperative ANP. Although intentional STR did not prevent ANP immediately after the operation, recovery of the abducens nerve function after surgery was observed more frequently in the STR group than in the GTR group.

Review of the Petroclinoid Ligament

An ossified left petroclinoid ligament was found during routine dissection of the skull base in an adult male cadaver. The petroclinoid ligament is clinically and surgically important given its anatomical relationships to cranial nerves III, V, and VI, so its ossification is a risk factor for injuries due to trauma, increased intracranial pressure, and vascular and tumor compression resulting in abducens and oculomotor palsies. The causes of petroclinoid ligament ossification are yet to be completely elucidated although several reports have associated them with age-related and physiological processes. Assessing the integrity of the petroclinoid ligament is important during skull base surgical interventions to avoid postoperative complications. Therefore, this paper reviews the petroclinoid ligament and its variation, the ossified petroclinoid ligament.

Differences in surgical outcome between petroclival meningioma and anterior petrous meningioma

BACKGROUND

Petroclival meningiomas (PC MNGs) and anterior petrous meningiomas (AP MNGs) have similar locations. However, these are different tumors clearly divided by the trigeminal nerve. There has never been a study on the comparison of the surgical outcomes of these two meningiomas. In this study, we compared and analyzed the surgical outcome of PC MNGs and AP MNGs.

METHODS

The charts of 85 patients diagnosed with PC MNGs of AP MNGs who underwent surgical treatment were retrospectively reviewed. And we analyzed the characteristics of 49 PC MNGs (57.6%) and compared them with those of 36 AP MNGs.

RESULTS

Preoperative brainstem edema was observed in 11 patients (22.4%) of the PC MNG group and 1 patient (2.8%) of the AP MNG group (p = 0.024). Total tumor removal was achieved in 21 patients (58.3%) of the AP MNG group, but only 17 patients (34.7%) of the PC MNG group were able to completely (p = 0.047). In addition, sixth cranial nerve palsy occurred in 17 patients (34.7%) of the PC MNG group and 4 patients (11.1%) of the AP MNG group (p = 0.025).

CONCLUSIONS

In this study, we found that PC MNGs has a worse surgical outcome than AP MNGs, because PC MNGs were difficult to completely remove and were more likely to damage abducens nerve.

Microsurgical Anatomy of the Inferomedial Paraclival Triangle: Contents, Topographical Relationships and Anatomical Variations

The inferomedial triangle is one of the two surgical triangles in the paraclival subregion of the skull base. It is delineated by the posterior clinoid process, the dural entrance of the trochlear nerve and the dural entrance of the abducens nerve. The aim of the present article is to describe the anatomical variations within the inferomedial triangle. Measurements of the triangle’s borders and area were supplemented by detailed observations of the topographical anatomy and various arrangements of its contents. Nine adult cadaveric heads (18 sides) and 28 sagittal head sections were studied. The mean area of the inferomedial triangle was estimated to be 60.7 mm². The mean lengths of its medial, lateral and superior borders were 16.1 mm, 11.9 mm and 10.4 mm, respectively. The dorsal meningeal artery was identified within the inferomedial triangle in 37 out of 46 sides (80.4%). A well-developed petrosphenoidal ligament of Grüber was identified within the triangle on 36 sides (78.3%). Although some structures were variable, the constant contents of the inferomedial triangle were the posterior petroclinoid dural fold, the upper end of the petroclival suture, the gulfar segment of the abducens nerve and the posterior genu of the intracavernous internal carotid artery.

Central Skull Base Anatomy and Pathology: A Review

The central skull base is an anatomically complex region of the head and neck which hosts a variety of neoplastic, vascular, infectious, inflammatory, and developmental pathologies. Evaluation of its intricate anatomy requires dedicated and complementary imaging modalities of MRI and CT. This article will provide a brief review of the anatomy of the central skull base, followed by an overview of common pathologies encountered in this region and their characteristic radiological characteristics.

Arachnoid and dural reflections

The dura mater is the major gateway for accessing most extra-axial lesions and all intra-axial lesions of the central nervous system. It provides a protective barrier against external trauma, infections, and the spread of malignant cells. Knowledge of the anatomical details of dural reflections around various corners of the skull bases provides the neurosurgeon with confidence during transdural approaches. Such knowledge is indispensable for protection of neurovascular structures in the vicinity of these dural reflections. The same concept is applicable to arachnoid folds and reflections during intradural excursions to expose intra- and extra-axial lesions of the brain. Without a detailed understanding of arachnoid membranes and cisterns, the neurosurgeon cannot confidently navigate the deep corridors of the skull base while safely protecting neurovascular structures. This chapter covers the surgical anatomy of dural and arachnoid reflections applicable to microneurosurgical approaches to various regions of the skull base.

Effect of clival bone growth on the localization of the abducens nerve at the petroclival region: a postmortem anatomical study

PURPOSE

To investigate the effect of the clival bone pattern on the abducens nerve (AN) localization in the petroclival region between the Pediatric and Adult Groups.

METHODS

This study used 12 pediatric and 17 adult heads obtained from the autopsy. The length and width of the clivus and the length of the petrosphenoidal ligaments (PSLs) were measured. The ratio of the length and width of the clivus was accepted as the clival index (CI). The localization of the AN at the petroclival region below the PSL, classified as lateral and medial, were recorded.

RESULTS

The average length of the clivus was 26.92 ± 2.88 mm in the Pediatric Group, and 40.66 ± 4.17 mm in the Adult Group (p < 0.001). The average width of the clivus was 22.35 ± 2.88 mm in the Pediatric Group, and 29.96 ± 3.86 mm in the Adult Group (p < 0.001). The average value of the CI was 1.20 in the Pediatric Group and 1.36 in the Adult Group (p = 0.003). The length of the PSL was 7.0 ± 1.47 mm in the Pediatric Group and 11.05 ± 2.95 mm in the Adult Group (p < 0.001). The nerve was located below the medial side of the PSL in the Pediatric Group and below the lateral side in the Adult Group (p = 0.002).

CONCLUSIONS

The petrous apex localization of the AN in adults compared with pediatric subjects could be related to the increased growth in the length of the clivus than its width.

Enlargement of Dorello's Canal as a Novel Radiographic Marker of Idiopathic Intracranial Hypertension

Objectives  The objective of this study is to compare the visibility and size of Dorello's canal (DC) on magnetic resonance imaging between patients with idiopathic intracranial hypertension (IIH) and control patients, for its evaluation as a potential novel marker for chronic increased intracranial pressure (ICP). Design  Retrospective blinded case-control study. Setting  Tertiary care academic center. Participants  Fourteen patients with spontaneous cerebrospinal fluid (CSF) rhinorrhea and diagnosed IIH, as well as an equal number of age and gender-matched controls. Main Outcome Measures  Radiographic presence or absence of visible CSF sleeve within DC as well as CSF sleeve width when present. Results  Following review of 28 IIH canals and 28 control canals, IIH patients were significantly more likely to have a visible CSF sleeve within DC and to have a wider measured medial entrance to DC ( p  < 0.001). Conclusion  Identification of CSF evagination into DC may serve as a reliable marker for increased ICP in the IIH population. This finding should be considered in the future as paradigms for diagnosis of IIH continue to evolve.

The Petroclinoid Ligament: Its Morphometrics, Relationships, Variations, and Suggestion for New Terminology

The anatomy and definition of the petroclinoid ligament (PCL) and its relationship with the abducens nerve are variably described. The goal of this study was to clarify the anatomy of the PCL and better elucidate its relationship with the abducens nerve. Thirty-six sides from 18 fresh-frozen adult cadaveric heads were used in this study. Specimens were all Caucasian and derived from 10 males and 8 females. The mean age at death was 79 years. Dissection of the PCL and abducens nerve was performed using a surgical microscope. The anterior and posterior attachments of the PCL, and position of the abducens nerve were noted. Subsequently, the width, thickness, and length of the ligament, and diameter of the abducens nerve were measured. Thirty-one sides (86.1%) were found to have a PCL, on two sides (5.6%), the PCL was ossified, and on three sides (8.3%), the PCL was absent. The width, thickness, and length of the PCL ranged from 0.54 to 3.39, 0.07 to 0.49, and 3.27 to 17.85 mm, respectively. No PCL had an anterior attachment onto the posterior clinoid process but rather, the clivus. Therefore, based on our findings, the PCL would be better described as the petroclival ligament.

Surgical implementation and efficacy of endoscopic endonasal extradural posterior clinoidectomy

OBJECTIVE

The endoscopic endonasal approach (EEA) for skull base tumors has become an important topic in recent years, but its use, merits, and demerits are still being debated. Herein, the authors describe the nuances and efficacy of the endoscopic endonasal extradural posterior clinoidectomy for maximal tumor exposure.

METHODS

The surgical technique included extradural posterior clinoidectomy following lateral retraction of the paraclival internal carotid artery and extradural pituitary transposition. In cases with prominent posterior clinoid process, a midline sellar dura cut was added to facilitate extradural exposure. Forty-four consecutive patients, in whom this technique was performed between 2016 and 2018 at Osaka City University Hospital, were reviewed. The pathology included 19 craniopharyngiomas, 7 chordomas, 6 meningiomas, 6 pituitary adenomas, 4 chondrosarcomas, and 2 miscellaneous. Utilization and effectiveness of this approach were further demonstrated with neuroimaging.

RESULTS

Extradural posterior clinoidectomies were successfully applied in all patients without permanent neurovascular injury and with better maneuverability and greater resection rate of the tumors. Four patients experienced transient postoperative abducens nerve paresis, and 1 patient experienced transient postoperative oculomotor nerve paresis; however, the patients with deficits recovered within 3 months. On radiological examination, the surgical field was 2.2 times wider in cases with bilateral posterior clinoidectomy than in cases without posterior clinoidectomy.

CONCLUSIONS

The extended EEA with extradural posterior clinoidectomy creates an extra working space and allows adequate accessibility with safe surgical maneuverability to remove tumors that extend behind the posterior clinoid and dorsum sellae.

Novel Segmentation of Placed Coils in the Treatment of Cavernous Sinus Dural Arteriovenous Fistulas Provides a Reliable Predictor of the Long-Term Outcome in Abducens Nerve Palsy

OBJECTIVE

Abducens nerve palsy (ANP) after transvenous embolization (TVE) for cavernous sinus (CS) dural arteriovenous fistulas (DAVFs) sometimes persists. The aim of this study was to assess the correlation between the coil mass and the long-term outcome of ANP after TVE.

METHODS

Between January 2008 and July 2016, 33 patients with a CS DAVF underwent TVE at our institute. On the basis of the relationship to the internal carotid artery (ICA), we divided the lateral part of the CS into the following 3 portions: anterolateral, anterolateral to the anterior bend of the ICA; middle-lateral (ML), lateral to the horizontal segment of the ICA; and posterolateral, posterolateral to the posterior bend of the ICA.

RESULTS

ANP persisted in 4 patients. The number of coils (35.3 vs. 21.9 coils; P = 0.04), coil volume (198.4 vs. 103.6 mm³; P = 0.03), and coil volume in the ML (54.9 vs. 20.3 mm³; P = 0.01) were significantly greater in the ANP group than in the non-ANP group. In the logistic regression analysis, only the ML coil volume was significantly associated with the persistence of ANP (P = 0.04). Based on the receiver operating characteristic curve, the optimal cutoff value of the ML coil volume was 27.9 mm³ (sensitivity, 100%; specificity, 72.4%).

CONCLUSIONS

Overpacking in the ML of the CS should be avoided to prevent persistent ANP. The cutoff value of the ML coil volume may provide a good guide for the practical use of TVE for CS DAVFs.

Anatomic variation of the abducens nerve in a single cadaver dissection: the "petrobasilar canal"

Anatomic variations of the petrosphenoid ligament, Dorello's canal and the course of the abducens nerve have been extensively described over the past years. In the present report of a single cadaver dissection, we describe an unusual course of the abducens nerve at the level of the petrous bone. The right abducens nerve did not enter Dorello's canal, but ran below the petrous bone through a narrow canal in the petrobasilar suture, which we called the "petrobasilar canal". No anatomic variations of the left abducens nerve were noted.

Anatomic Eponyms in Neuroradiology: Head and Neck

In medicine, an eponym is a word-typically referring to an anatomic structure, disease, or syndrome-that is derived from a person's name. Medical eponyms are ubiquitous and numerous. They are also at times controversial. Eponyms reflect medicine's rich and colorful history and can be useful for concisely conveying complex concepts. Familiarity with eponyms facilitates correct usage and accurate communication. In this article, 22 eponyms used to describe anatomic structures of the head and neck are discussed. For each structure, the author first provides a biographical account of the individual for whom the structure is named. An anatomic description and brief discussion of the structure's clinical relevance follow.

Comparative analysis of the anterior transpetrosal approach with the endoscopic endonasal approach to the petroclival region

OBJECTIVE The endoscopic endonasal approach (EEA) offers direct access to midline skull base lesions, and the anterior transpetrosal approach (ATPA) stands out as a method for granting entry into the upper and middle clival areas. This study evaluated the feasibility of performing EEA for tumors located in the petroclival region in comparison with ATPA. METHODS On 8 embalmed cadaver heads, EEA to the petroclival region was performed utilizing a 4-mm endoscope with either 0° or 30° lenses, and an ATPA was performed under microscopic visualization. A comparison was executed based on measurements of 5 heads (10 sides). Case illustrations were utilized to demonstrate the advantages and disadvantages of EEA and ATPA when dealing with petroclival conditions. RESULTS Extradurally, EEA allows direct access to the medial petrous apex, which is limited by the petrous and paraclival internal carotid artery (ICA) segments laterally. The ATPA offers direct access to the petrous apex, which is blocked by the petrous ICA and abducens nerve inferiorly. Intradurally, the EEA allows a direct view of the areas medial to the cisternal segment of cranial nerve VI with limited lateral exposure. ATPA offers excellent access to the cistern between cranial nerves III and VIII. The quantitative analysis demonstrated that the EEA corridor could be expanded laterally with an angled drill up to 1.8 times wider than the bone window between both paraclival ICA segments. CONCLUSIONS The midline, horizontal line of the petrous ICA segment, paraclival ICA segment, and the abducens nerve are the main landmarks used to decide which approach to the petroclival region to select. The EEA is superior to the ATPA for accessing lesions medial or caudal to the abducens nerve, such as chordomas, chondrosarcomas, and midclival meningiomas. The ATPA is superior to lesions located posterior and/or lateral to the paraclival ICA segment and lesions with extension to the middle fossa and/or infratemporal fossa. The EEA and ATPA are complementary and can be used independently or in combination with each other in order to approach complex petroclival lesions.

Investigation of the calcification at the petroclival region through Multi-slice Computed Tomography of the skull base

OBJECTIVES

The aim of this paper was a retrospective investigation of calcification at the petroclival region using Multi-slice Computed Tomography (MSCT).

METHODS

One hundred thirty skull bases were reviewed. The images were acquired with a 64 slice CT (MSCT). At first images were taken at the axial plane; and then coronal and sagittal reconstructions of raw data were performed. Later investigations were carried out on these 3-dimensional images (3-D imaging). Petrosphenoidal ligament (PSL) (Gruber's ligament) and posterior petroclinoid ligament (PPCL) calcifications were evaluated as "none, partial or complete calcification" for the right and left sides.

RESULTS

In the right PSL, there were partial calcifications in 9.8% and complete calcifications in 2.3%. Calcification ratio was 9.8% partial and 2.9% complete in the left PSL. In the right side, there were 26.6% partial and 5.2% complete calcifications of PPCL. In the left side, there were 29.5% partial and 4.6% complete PPCL calcifications. PPCL calcification was detected more in males compared to females in the right and left sides. In older patients, left PSL; right and left PPCL calcification were detected more.

CONCLUSION

PPCL calcifications cannot be differentiated from PSL calcifications in MSCT slices. The distinction can be easily done in 3-D views. The presence of ossified ligaments may make surgeries in this region difficult, and special care has to be taken to avoid injuries to structures which pass under these ossified ligaments. Particularly in elderly patients, the appropriate surgical instrument for the PSL calcifications should be prepared preoperatively. If PSL is calcified, 6th cranial nerve palsy may not occur even though increased intracranial pressure syndrome is present. Whereas, in lateral trans-tentorial herniations, 3rd cranial nerve palsy occurs in earlier periods when PSL is calcified. Moreover, in subtemporal and transtentorial petrosal approaches, knowing the PSL calcification preoperatively is important to avoid damaging the 6th cranial nerve during surgery.

Bilaterally isolated abducens palsy after an aneursym rupture is related with intracranial hypertension

BACKGROUND

Bilateral and isolated abducens nerve palsy is a rare initial presentation after aneurysms rupture. Several possible mechanisms including intracranial hypertension have been purposed. To date, there have been no reports with objective measurements to demonstrate the relationship between intracranial pressure and isolated abducens palsy in the setting of acute subarachnoid hemorrhage due to aneurysm rupture.

FINDINGS

A 50 year-old female presented with severe headache and bilaterally isolated abducens nerve palsy. A series of image studies showed a ruptured aneurysm over right internal carotid artery and posterior communicating artery bifurcation with minimal subarachnoid hemorrhage. Surgery of aneurysm clipping was performed and intracranial pressure monitoring was applied. Postoperatively no new neurological deficit developed but persistent headache and increased intracranial pressure measured by a fiber-optic device had been observed. The intracranial hypertension then decreased gradually with rapid recovery from the bilateral abducens palsy 7 days after the surgery. The relationship between postoperative intracranial pressure, subarachnoid hematoma and isolated abducens palsy are illustrated.

CONCLUSIONS

The report demonstrated the clinical presentation of bilaterally isolated abducens palsy after an intracranial aneurysm rupture is related with the increased intracranial pressure level, rather than the hematoma compression to the nerve or vasospasm of pontine branches of basilar artery.

Gruber, Gradenigo, Dorello, and Vail: key personalities in the historical evolution and modern-day understanding of Dorello's canal

A century ago an ambitious young anatomist in Rome, Primo Dorello, who sought to understand the cause of abducent nerve palsy that often occurred in patients with severe middle ear infections, conducted intricate studies on the intracranial course of the nerve. In his findings, he identified that the abducent nerve passes through a narrow sinus near the apex of the petrous bone, which formed an osteofibrous canal. Dorello suggested that in this enclosed region the abducent nerve may be particularly vulnerable to compression due to the vascular edema accompanying the infection. Although his work was widely appreciated, it was not well received by all. Interestingly, Giuseppe Gradenigo, one of the most prominent Italian otologists of the early 20th century, who was known for his work on a triad of symptoms (Gradenigo's syndrome) that accompanies petrous apicitis, a result of severe middle ear infections, was obstinate in his criticism of Dorello's findings. Thus a scientific duel began, with a series of correspondence between these two academics-one who was relatively new to the otological community (Dorello) and one who was well reputed in that community (Gradenigo). The disagreement ultimately ebbed in 1909, when Dorello published a report in response to Gradenigo's criticisms and convinced Gradenigo to change his views. Today Dorello's canal is widely recognized as a key landmark in skull base surgery of the petroclival region and holds clinical significance due to its relation to the abducent nerve and surrounding vascular structures. Yet, although academics such as Dorello and Gradenigo are recognized for their work on the canal, it is important not to forget the others throughout history who have contributed to the modern-day understanding of this anatomical structure. In fact, although the level of anatomical detail found in Dorello's work was previously unmatched, the first description of the canal was made by the experienced Austrian anatomist Wenzel Leopold Gruber in 1859, almost 50 years prior to Dorello's landmark publication. Another critical figure in building the understanding of Dorello's canal was Harris Holmes Vail, a young otolaryngologist from Harvard Medical School, who in 1922 became the first person to describe Dorello's canal in the English language. Vail conducted his own detailed anatomical studies on cadavers, and his publication not only reaffirmed Dorello's findings but also immortalized the eponym used today-"Dorello's canal." In this article the authors review the life and contributions of Gruber, Dorello, Gradenigo, and Vail, four men who played a critical role in the discovery of Dorello's canal and paved the way toward the current understanding of the canal as a key clinical and surgical entity.

Pathological Location of Cranial Nerves in Petroclival Lesions: How to Avoid Their Injury during Anterior Petrosal Approach

Objectives Numerous surgical approaches have been developed to access the petroclival region. The Kawase approach, through the middle fossa, is a well-described option for addressing cranial base lesions of the petroclival region. Our aim was to gather data about the variation of cranial nerve locations in diverse petroclival pathologies and clarify the most common pathologic variations confirmed during the anterior petrosal approach. Method A retrospective analysis was made of both videos and operative and histologic records of 40 petroclival tumors from January 2009 to September 2013 in which the Kawase approach was used. The anatomical variations of cranial nerves IV-VI related to the tumor were divided into several location categories: superior lateral (SL), inferior lateral (IL), superior medial (SM), inferior medial (IM), and encased (E). These data were then analyzed taking into consideration pathologic subgroups of meningioma, epidermoid, and schwannoma. Results In 41% of meningiomas, the trigeminal nerve is encased by the tumor. In 38% of the meningiomas, the trigeminal nerve is in the SL part of the tumor, and it is in 20% of the IL portion of the tumor. In 38% of the meningiomas, the trochlear nerve is encased by the tumor. The abducens nerve is not always visible (35%). The pathologic nerve pattern differs from that of meningiomas for epidermoid and trigeminal schwannomas. Conclusion The pattern of cranial nerves IV-VI is linked to the type of petroclival tumor. In a meningioma, tumor origin (cavernous, upper clival, tentorial, and petrous apex) is the most important predictor of the location of cranial nerves IV-VI. Classification of four subtypes of petroclival meningiomas using magnetic resonance imaging is very useful to predict the location of deviated cranial nerves IV-VI intraoperatively.

Ossification of the petrosphenoidal ligament: multidetector computed tomography findings of an unusual variation with a potential role in abducens nerve palsy

PURPOSE

The petrosphenoidal ligament (PSL) forms the roof of Dorello's canal in which the abducens nerve courses. An ossified PSL may play a role in abducens nerve palsy although it is controversial. Therefore, we aimed to investigate the incidence and the imaging features of PSL ossification using multidetector computed tomography (MDCT).

MATERIALS AND METHODS

Consecutive head CT scans, performed between January 2014 and May 2014, were retrospectively evaluated by two radiologists to look for a partial or complete ossification at the expected site of the PSL using axial, reformatted coronal and sagittal images.

RESULTS

We found 46 patients (32 men, 14 women) with ossification of the PSL out of 523 head CT scans (46/523, 8.8%). A total of 61 PSL ossifications (31 unilateral; 15 bilateral) were found in 1,046 petroclival regions (61/1,046, 5.8%). Of these 61 ossifications, 34 (34/523, 6.5%) were on the right and 27 (27/523, 5.1%) were on the left; 38 (38/1,046, 3.6%) were classified as partial and 23 (23/1,046, 2.2%) were classified as complete.

CONCLUSION

Ossification of the PSL is not a rare finding on MDCT. This imaging technique can reliably demonstrate the imaging features of an ossified PSL.

Grüber's ligament as a useful landmark for the abducens nerve in the transnasal approach

OBJECT

Extended endoscopic transnasal surgeries for skull base lesions have recently been performed. Some expert surgeons have attempted to remove tumors such as chordomas, meningiomas, and pituitary adenomas in the clival region using the transnasal approach and have reported abducens nerve injury as a common complication. There have been many microsurgical anatomical studies of the abducens nerve, but none of these studies has described an anatomical landmark of the abducens nerve in the transnasal approach. In this study the authors used cadaver dissections to describe Grüber's ligament as the most reliable landmark of the abducens nerve in the transnasal transclival view.

METHODS

The petroclival segment of the abducens nerve was dissected in the interdural space-which is also called Dorello's canal, the petroclival venous gulf, or the sphenopetroclival venous confluence-using the transnasal approach in 20 specimens obtained from 10 adult cadaveric heads.

RESULTS

The petroclival segment of the abducens nerve clearly crossed and attached to Grüber's ligament in the interdural space, as noted in the transnasal view. The average length of the dural porus to the intersection on the abducens nerve was 5.2 ± 1.0 mm. The length of the posterior clinoid process (PCP) to the intersection on Grüber's ligament was 6.4 ± 2.6 mm. The average width of Grüber's ligament at the midsection was 1.6 ± 0.5 mm.

CONCLUSIONS

Grüber's ligament is considered a useful landmark, and it is visible in most adults. Thus, surgeons can find the abducens nerve safely by visualizing inferolaterally along Grüber's ligament from the PCP.

Skull base embryology: a multidisciplinary review

INTRODUCTION

The skull base represents a central and complex bone structure of the skull and forms the floor of the cranial cavity on which the brain lies. Anatomical knowledge of this particular region is important for understanding several pathologic conditions as well as for planning surgical procedures. Embryology of the cranial base is of great interest due to its pronounced impact on the development of adjacent regions including the brain, neck, and craniofacial skeleton.

MATERIALS AND METHODS

Information from human and comparative anatomy, anthropology, embryology, surgery, and computed modelling was integrated to provide a perspective to interpret skull base formation and variability within the cranial functional and structural system.

RESULTS AND CONCLUSIONS

The skull base undergoes an elaborate sequence of development stages and represents a key player in skull, face and brain development. Furthering our holistic understanding of the embryology of the skull base promises to expand our knowledge and enhance our ability to treat associated anomalies.

Delayed abducens nerve palsy after transvenous coil embolization for cavernous sinus dural arteriovenous fistulae

PURPOSE

Abducens nerve palsy is the most common complication after transvenous embolization (TVE) for cavernous sinus dural arteriovenous fistula. Abducens nerve palsy is reported to have a good prognosis after the symptoms have been alleviated. The purpose of this study was to identify cases of delayed abducens nerve palsy after successful TVE and discuss the physiological mechanisms responsible for this unusual complication.

METHODS

Between 1991 and 2012, TVE was performed for 73 patients. The patients were evaluated for clinical symptoms every 12 months during the follow-up period. Patients' data and information about abducens nerve palsy were obtained from clinical records retrospectively.

RESULTS

Abducens nerve palsy newly developed in 4 (5.5%) of 73 patients at 3-65 months after TVE. All four patients with delayed abducens nerve palsy were followed up for 8-84 months. However, delayed abducens nerve palsy persisted in all four patients. In these four patients, the shunt points were posterior cavernous sinus. The average coil length used for four patients was 206.5 ± 43.1 cm (n = 4), and the average coil length used for patients without delayed abducens nerve palsy was 112.8 ± 38.8 cm (n = 69).

CONCLUSION

The possibility of delayed abducens nerve palsy should be kept in mind, especially in the patients who were treated with transvenous coil packing in the posterior part of the cavernous sinus. Furthermore, our results suggest that long-term follow-up care is important for these patients, even after complete neurological and radiological recovery was attained.

[Anterior petrosal approach - analysis of the surgical technique in cadavers simulation]

The aim of the study was to present consecutive stages of the anterior petrosal approach (APA). Eight simulations of APA were performed on non-fixed human cadavers without any known pathologies of the head and neck. The consecutive stages of the procedure were documented with photographs and schemes. The starting point for APA is a temporal craniotomy and extradural exposition of the base of the middle cranial fossa. Mobilisation of the trigeminal nerve allows for removal of the apex of the petrous bone. Approach to the upper part of the clivus is achieved by elevation of the temporal lobe and section of the tentorium and superior petrosal sinus with surrounding dura. Anterior petrosal approach is a reproducible technique, which provides surgical penetration of the upper clivus and related regions. This approach is particularly useful in the treatment of tumors of the mentioned above anatomical areas.

Avoiding injury to the abducens nerve during expanded endonasal endoscopic surgery: anatomic and clinical case studies

BACKGROUND

Understanding the course of the most medially located parasellar cranial nerve, the abducens, becomes critical when performing an expanded endonasal approach.

OBJECTIVE

We report an anatomoclinical study of the abducens nerve and describe relevant surgical nuances to avoid its injury.

METHODS

Ten anatomic specimens were dissected using endoscopes attached to an high-definition camera. A series of anatomic measurements and relationships of the abducens nerve were noted. Illustrative clinical cases are described to translate those findings into practice.

RESULTS

Cisternal, interdural, gulfar, and cavernous segments of the abducens were identified intracranially. The mean distance from the vertebrobasilar junction (VBJ) to the pontomedullary sulcus (PMS) was 4 mm; horizontal distance between both abducens nerves at the PMS was 10 mm, and between both abducens at the interdural segment was 18.5 mm. The upper limit of the lacerum segment of the internal carotid artery was at the same level of the dural entry point of the sixth cranial nerve posteriorly. The sellar floor at the sphenoid sinus marks the level of the gulfar segment in the craniocaudal axis. At the superior orbital fissure, the abducens nerve and V2 were at an average vertical distance of 11.5 mm.

CONCLUSION

Anatomic landmarks to localize the abducens nerve intraoperatively, such as the VBJ for the transclival approach, the lacerum segment of the carotid, and the sellar floor for the medial petrous apex approach, and V2 for Meckel's cave approach, are reliable and complementary to the use of intraoperative electrophysiological monitoring.

The Dorello canal: historical development, controversies in microsurgical anatomy, and clinical implications

Interest in studying the anatomy of the abducent nerve arose from early clinical experience with abducent palsy seen in middle ear infection. Primo Dorello, an Italian anatomist working in Rome in the early 1900s, studied the anatomy of the petroclival region to formulate his own explanation of this pathological entity. His work led to his being credited with the discovery of the canal that bears his name, although this structure had been described 50 years previously by Wenzel Leopold Gruber. Renewed interest in the anatomy of this region arose due to advances in surgical approaches to tumors of the petroclival region and the need to explain the abducent palsies seen in trauma, intracranial hypotension, and aneurysms. The advent of the surgical microscope has allowed more detailed anatomical studies, and numerous articles have been published in the last 2 decades. The current article highlights the historical development of the study of the Dorello canal. A review of the anatomical studies of this structure is provided, followed by a brief overview of clinical considerations.

Giuseppe Gradenigo: Much more than a syndrome! Historical vignette

BACKGROUND

Giuseppe Gradenigo (1859-1926), a legendary figure of Otology, was born in Venice, Italy. He soon became a pupil to Adam Politzer and Samuel Leopold Schenk in Vienna, demonstrating genuine interest in the embryology, morphology, physiopathology, as well as the clinical manifestations of ear diseases. In this paper, the authors attempt to highlight the major landmarks during Gradenigo's career and outline his contributions to neurosciences, which have been viewed as looking forward to the 20(th) century rather than awkward missteps at the end of the 19(th).

METHODS

Several rare photographs along with many non-English, more than a century old articles have been meticulously selected to enrich this historical journey in time.

RESULTS

It was after Gradenigo that the well-known syndrome consisting of diplopia and facial pain due to a middle ear infection was named. However, Gradenigo was much more than a syndrome. Surprisingly, despite the fact that he is considered a pioneer of the Italian Otology of the late 19(th) and early 20(th) century, little is written of his life and his notable achievements in the English literature.

CONCLUSIONS

Even though his name lives on nowadays only in the eponym "Gradenigo's syndrome," his accomplishments are much wider and cast him among the emblematic figures of science. His inherent tendency for discovering the underlying mechanisms of diseases and his vision of guaranteeing quality of services, professional proficiency, respect, and dedication toward the patients is in fact what constitutes his true legacy to the next generations.

Bilateral abducens paralysis revealing bipartite spheno-occipital synchondrosis fracture in a child: case based update

BACKGROUND

After reviewing the literature about sella turcica and spheno-occipital synchondrosis fracture, we present the case of a 6-year-old girl who suffered complex sphenoid bone fracture produced by an indirect mechanism.

METHODS

The girl fell from her height after a blunt head injury due to a falling lamppost.

RESULTS

Computer tomography (CT) scan showed occipital, sellar and clival fractures with pneumatocephalus. The probable fracture mechanism is contre-coup injury linked to osseous and brain oscillations after head trauma.

CONCLUSION

Two complications, abducens nerve palsy and cerebrospinal fluid fistula, are observed and discussed.

The retrosigmoid approach to petroclival meningioma surgery

Petroclival meningiomas are technically challenging lesions. The authors retrospectively analyzed their experience between 2000 and 2010 in 82 patients with petroclival meningioma to evaluate changes in management strategy. A total of 42 patients (51%) were treated via the retrosigmoid approach. The patients received postoperative neurological and neuroradiological follow-up. The maximum diameter of the tumors ranged from 1.5 cm to 6.5 cm (mean, 3.8 cm). Gross total resection (Simpson Grade II) was achieved in 27 patients (64%), subtotal resection (Simpson Grade III) in 11 (26%), and partial removal (Simpson Grade IV) in four (9.5%). Ten patients (24%) had new neurological deficits or worsening of pre-existing deficits. One patient (2%) died because of brainstem dysfunction after surgery. The retrosigmoid approach is suitable for treatment of selected petroclival meningioma if the main part of the tumor is located in the posterior fossa in the cerebellopontine angle and the low clivus, and only a minor part of the tumor extends to the posterior wall of the cavernous sinus. This approach provides a low degree of surgical difficulty and a low complication rate.

Dural and arachnoid membraneous protection of the abducens nerve at the petroclival region

The goal of this study was to determine the membranous protection of the abducens nerve in the petroclival region. The petroclival portion of the abducens nerve was studied in ten dissections from five cadaveric head specimens. One of the heads was used for histological sections. Four heads were injected with colored latex for microsurgical dissections. The histological sections were prepared from petroclival dura mater, embedded in paraffin blocks, stained, sectioned in the axial, coronal, and sagittal planes, and evaluated by light microscopy. The abducens nerve was covered by a dural sleeve and arachnoidal membrane during its course within the petroclival area. Following the petrous apex, the abducens nerve was fixed by a sympathetic plexus and connective tissue extensions to the lateral wall of the cavernous segment of the internal carotid artery and to the medial wall of Meckel's cave. Fibrous trabeculations inside the venous space were attached to the dural sleeve. The lateral clival artery accompanied the dural sleeve of the abducens nerve and supplied the petroclival dura mater. The arterioles accompanying the abducens nerve through the subarachnoid space supplied the nerve within the dural sleeve. The arachnoid membrane covered the abducens nerve within the dural sleeve to the petrous apex, and arachnoid granulations found on the dural sleeve protruded into the venous space. The extension of the arachnoid membrane to the petrous apex and the presence of arachnoid granulations on the dural sleeve suggest that the subarachnoid space continues in the dural sleeve.

Microsurgical Study of the Dorello's Canal

The microsurgical anatomy of Dorello's canal was examined in 20 cadaver specimens. The study included dissections of this anatomic structure, under operative microscope and using microsurgical instruments. The detailed microanatomy of the canal was defined, as well as its relation to the sixth nerve, the clivus, the petrous apex, and the adjacent neurovascular elements.

Extradural total petrous apex resection with trigeminal translocation for improved exposure of the posterior cavernous sinus and petroclival region

We have analyzed a strategy for improved exposure of the posterior cavernous sinus and petroclival region through an extradural subtemporal approach to be utilized in the removal of neoplastic processes with involvement of the apical petrous bone and posterior cavernous sinus. This surgical approach includes the following elements for improved exposure of the posterior cavernous sinus through the middle fossa corridor: (1) maximal extradural exposure and mobilization of the trigeminal nerve complex, allowing its elevation and anterior displacement, (2) complete extradural removal of the anterior petrous pyramid from the porus acousticus to the petrous apex under direct vision, (3) total exposure of the abducens nerve from the posterior fossa to its point of cross over the intracavernous carotid artery, and (4) wide extradural exposure of the cavernous carotid artery in the foramen lacerum region. This strategy can be combined with other related approaches; specifically, frontotemporal or posterior transpetrosal exposures for extensive lesions.Microsurgical dissection and morphometric analysis were performed in 20 fixed cadaver specimens for the purposes of validating the method for clinical application and determining the key elements to maximization of exposure. The trigeminal complex could be anteromedially retracted 4.8 mm +/- 1.3 (range = 3 to 6 mm) without skeletonization of V(2) and V(3). Liberating these two divisions from their bony canals to their first peripheral branch (10.4 mm +/- 2.5 and 5.4 mm +/- 1.1, respectively) resulted in increased mobilization an average of 9.1 mm +/- 1.7 (7 to 14 mm). Further mobilization is achieved by dividing the attachment between the trigeminal connective tissue sheath and the fibrous carotid ring at the foramen lacerum. An average of 13.0 mm +/- 3.1 (7 to 20 mm) of the posterior intracavernous carotid artery was exposed. Detailed microanatomic observations and a comprehensive morphometric analysis of the relevant anatomic relationships were made.

A new look at an old canal

Attempts to explain abducens vulnerability have centered around the petroclival segment of its pathway in the skull base, in particular, its relations to the Dorello's canal and the petrosphenoidal ligament of Grüber. This study aims to contribute to the definition of the Dorello's canal and to the understanding of abducens vulnerability from an evolutionary perspective. The petroclival region and the Dorello's canal in particular were examined in a sample of 86 primate skulls. The sample contains 20 Pan troglodytes (common chimpanzee), 35 Gorilla gorilla, 20 Pongo pygmaeus (orangoutan), and 11 Hylobates moloch (gibbon) skulls. Distance between the petrous apex and tip of the posterior clinoid process was measured using a 10-mm scale that was inserted into the endocranium. Complete absence of the Grüber ligament and an uninterrupted osseous circumference of the Dorello's canal (foramen) was demonstrated in all nonhuman primate species. Whatever the reason for the morphological difference between the human and ape Dorello's canal, it is without a doubt suggestive of the source of vulnerability of the abducens nerve, as the ligament of Grüber, unique to the human configuration, renders the contents of the Dorello's canal susceptible to compression against the cranial base.