Dorello's Canal and Gruber's Ligament: Historical Perspective

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.

A Rare Case of Isolated Abducens Nerve Palsy With Contemporaneous Thalamic Infarct

We report a case of isolated left abducens nerve palsy accompanying a right thalamic infarct. The patient, a 43-year-old Malay male with newly diagnosed hypertension, diabetes mellitus, and dyslipidemia, initially reported binocular diplopia on left lateral gaze persisting for five weeks. Subsequently, he experienced acute left-sided body weakness and slurred speech for over one day. Clinical examination revealed restricted left eye lateral gaze (-3) with no relative afferent pupillary defect. Additionally, decreased power (4/5) was noted in the left upper and lower limbs. Brain magnetic resonance imaging (MRI) revealed restricted diffusion in the right thalamus extending to the right posterior internal capsule, left anterior cingulate gyrus, and left caudate nucleus. The patient was initiated on antiplatelet, antihypertensive, and oral hypoglycemic agents, resulting in symptom improvement. This rare neuroophthalmological finding has not been reported previously.

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.

Petrous Apicitis: A Systematic Review and Case Presentation

INTRODUCTION

Petrous apicitis (PA) is a rare but dangerous complication of acute otitis media. The objective of this study is to present a case of PA and systematically review the existing literature on PA to characterize clinical presentation, diagnosis, management, and outcomes in the antibiotic era.

METHODS

A comprehensive search from 1983 to June 1, 2020, of PubMed, MEDLINE, Cochrane Library, and EmBase databases was conducted. Studies with clinical data regarding patients with PA were included. Non-English literature or studies with insufficient individual patient data were excluded. Sixty-seven studies were included with a total of 134 patients.

RESULTS

A total of 67 articles were found to meet criteria for inclusion. The mean age of presentation was 33 years. Recent acute otitis media was reported in 78 patients (58.2%). Only 3 patients (2.2%) were immunocompromised, and 8 patients (6.0%) had a history of diabetes. Gradenigo's triad of abducens palsy, otorrhea, and retro-orbital or facial pain was reported in 28 patients (20.9%); however, these presenting symptoms were common individually (51.5%, 48.5%, and 64.2%, respectively). Hearing loss (35.8%), facial weakness (17.9%), and vertigo (7.5%) were also reported.The most frequently cultured pathogen was Pseudomonas (34.2%), followed by Streptococcus and Staphylococcus. All 134 patients underwent imaging, with computed tomography being the most frequently used modality (56.0%). Nearly all patients received antibiotic therapy (95.6%), with 91 (67.9%) undergoing surgery ranging from myringotomy (26.9%) to petrosectomy (25.4%). Five patients (5.7%) died because of complications related to PA. Mean follow-up was 11.0 months.

CONCLUSIONS

Petrous apicitis has a variable presentation with potential for severe morbidity. Mortality rates are low, and presentation with Gradenigo's triad is uncommon. Appropriate medical management with surgical drainage can avoid long-term sequelae.

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.

Management of non-vestibular schwannomas in adult patients: a systematic review and consensus statement on behalf of the EANS skull base section. Part I: oculomotor and other rare non-vestibular schwannomas (I, II, III, IV, VI)

BACKGROUND

Non-vestibular schwannomas are relatively rare, with trigeminal and jugular foramen schwannomas being the most common. This is a heterogeneous group which requires detailed investigation and careful consideration to management strategy. The optimal management for these tumours remains unclear, and there are several controversies. The aim of this paper is to provide insight into the main principles defining management and surgical strategy, in order to formulate a series of recommendations.

METHODS

A task force was created by the EANS skull base section along with its members and other renowned experts in the field to generate recommendations for the surgical management of these tumours on a European perspective. To achieve this, the task force performed an extensive systematic review in this field and had discussions within the group. This article is the first of a three-part series describing non-vestibular schwannomas (I, II, III, IV, VI).

RESULTS

A summary of literature evidence was proposed after discussion within the EANS skull base section. The constituted task force dealt with the practice patterns that exist with respect to pre-operative radiological investigations, ophthalmological assessments, optimal surgical and radiotherapy strategies and follow-up management.

CONCLUSION

This article represents the consensually derived opinion of the task force with respect to the treatment of non-vestibular schwannomas. For each of these tumours, the management of these patients is complex, and for those which are symptomatic tumours, the paradigm is shifting towards the compromise between function preservation and progression-free survival.

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.

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.

CyberKnife Radiotherapy for Skull Base Petroclival Metastases Including Dorello's Canal: Report of 10 Cases

Skull base petroclival metastases cause diplopia due to abducens nerve palsy. Diplopia is visually disabling, and skull base metastasis is extremely difficult to treat even with microscopic surgery. However, stereotactic radiotherapy with CyberKnife (Accuray Incorporated, Sunnyvale, California) has been very successful in 10 cases. As the abducens nerve runs through Dorello’s canal in the skull base, the radiation dose and fraction were adjusted to avoid damage to the nerve. Since these metastases are not located inside the brain but in the skull base, contrast magnetic resonance imaging (MRI) combined with fluorodeoxyglucose-positron emission tomography (FDG-PET) was essential to detect the cancers.

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.

Skull base ligamentous mineralisation: evaluation using computed tomography and a review of the clinical relevance

Objectives

To determine the frequency, morphologic and demographic characteristics, and clinical relevance of the mineralisation of six skull base ligaments (interclinoid, caroticoclinoid, petrosphenoid, posterior petroclinoid, pterygospinous, and pterygoalar).

Methods

This is a retrospective review of 240 CT scans of the paranasal sinuses (ages 6–80 years). A limited systematic review was performed primarily using Embase and Medline databases.

Results

Ligamentous mineralisation was well delineated on CT and occurred at ≥ 1 location in 58.3% of patients. There was a nonsignificant trend towards a greater incidence with advancing age. The interclinoid and posterior petroclinoid ligaments were most commonly mineralised (22.1% and 18.3%, respectively); the petrosphenoid and pterygoalar ligaments were least frequently mineralised (10.8% and 6.3%, respectively). The mean age of patients with posterior petroclinoid mineralisation was significantly greater than those with interclinoid and petrosphenoid mineralisation and was not seen in patients aged 6–20 years. The literature review highlighted the clinically relevant potential for mineralised ligaments to cause barriers to surgical access (e.g. to the foramen ovale), increase the risk of neurovascular injury during surgery at the skull base (e.g. during anterior clinoidectomy), and predispose to neural impingement.

Conclusions

Skull base ligamentous mineralisation is commonly encountered on CT imaging. Given the potentially significant clinical implications, an understanding of the morphological appearances is of importance to those planning interventions at the skull base. To the authors’ knowledge, this study is the first to comprehensively evaluate such a wide range of skull base ligaments using CT. For some ligaments, the incidence on CT has not been previously described.

Primary Sphenoidal Sinus Lymphoma with Initial Presentation as Unilateral Abducens Nerve Palsy Symptom

A 48-year-old man presented with 3 days of mild horizontal diplopia in the left direction, followed by the onset of headache 17 days later. A physical examination revealed isolated left abducens nerve palsy. Head computed tomography (CT) and magnetic resonance imaging (MRI) scans revealed soft-tissue density neoplasms that occupied the sphenoidal sinus and further invaded to destroy the clivus. Immunohistochemical staining of neoplasms was performed from biopsies samples. The pathological diagnosis was extranodal natural killer (NK)/T-cell lymphoma (ENKL), nasal type, associated with Epstein-Barr virus (EBV). The patient subsequently exhibited secondary symptoms (fever, night sweats), enlarged lymph nodes, renal metastases, and hemophagocytic syndrome, with clinical diagnosis stage IV of ENKL. The patient has a poor prognosis. This report is unique in two aspects: the unilateral abducens nerve palsy as the initial and isolated symptom of ENKL, and the primary sphenoidal sinus ENKL.

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.

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.

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.