Microsurgical anatomy of the abducens nerve

Functional anatomy of the orbit in strabismus surgery: Connective tissues, pulleys, and the modern surgical implications of the "arc of contact" paradigm

Oculomotricity is a multidimensional domain characterised by a delicate interplay of anatomical structures and physiological processes. This manuscript meticulously dissects the nuances of this interplay, bringing to the fore the integral role of the extraocular muscles (EOMs) and their intricate relationship with the myriad orbital connective tissues as it harmoniously orchestrates binocular movements, ensuring synchronised and fluid visual tracking. Historically, the peripheral oculomotor apparatus was conceptualised as a rudimentary system predominantly driven by neural directives. While widely accepted, this perspective offered a limited view of the complexities inherent in ocular movement mechanics. The twentieth century heralded a paradigm shift in this understanding. With advances in anatomical research and imaging techniques, a much clearer picture of the gross anatomy of the EOMs emerged. This clarity challenged traditional viewpoints, suggesting that the inherent biomechanical properties of the EOMs, coupled with their associated tissue pulleys, play a pivotal role in dictating eye movement dynamics. Central to this revised understanding is the "arc of contact" paradigm. This concept delves deep into the mechanics of eye rotation, elucidating the significance of the point of contact between the EOMs and the eyeball. The arc of contact is not just a static anatomical feature; its length and orientation play a crucial role in determining the effective torque generated by a muscle, thereby influencing the amplitude and direction of eye rotation. The dynamic nature of this arc, influenced by the position and tension of the muscle pulleys, offers a more comprehensive model for understanding ocular kinematics. Previously overlooked in traditional models, muscle pulleys have now emerged as central players in the biomechanics of eye movement. These anatomical structures, formed by dense connective tissues, guide the paths of the EOMs, ensuring that their pulling angles remain optimal across a range of gaze directions. The non-linear paths resulting from these pulleys provide a more dynamic and intricate understanding of eye movement, challenging two-dimensional, linear models of orbital anatomy. The implications of these revelations extend beyond mere theoretical knowledge. The insights garnered from this research promise transformative potential in the realm of strabismus surgery. Recognising the pivotal role of muscle pulleys and the "arc of contact" paradigm allows for more precise surgical interventions, ensuring better post-operative outcomes and minimising the risk of complications. Surgical procedures that previously relied on basic mechanical principles now stand to benefit from a more nuanced understanding of the underlying anatomical and physiological dynamics. In conclusion, this manuscript serves as a testament to the ever-evolving nature of scientific knowledge. Challenging established norms and introducing fresh perspectives pave the way for more effective and informed clinical interventions in strabismus surgery.

Petroclival Clinoidal Folds and Arachnoidal Membranes of the Anteromedial Incisural Space: Clinical Anatomy for Neuro Critical Care

A systematic and narrative literature review was performed, focusing attention on the anatomy of the area located at the junction of the sphenoid and the basal portion of the temporal bone (petrous bone, petrous apex, upper petro-clival region) encircled by the free edge of the tentorium, the insertion of the tentorium itself to the petrous apex and the anterior and posterior clinoid processes that give rise to three distinct dural folds or ligaments: the anterior petroclinoid ligament, the posterior petroclinoid ligament and the interclinoid ligament. These dural folds constitute the posterior portion of the roof of the cavernous sinus denominated "the oculomotor triangle". The main purpose of this review study was to describe this anatomical region, particularly in the light of the relationships between the anterior margin of the free edge of the tentorium and the above-mentioned components of the sphenoid and petrous bone.

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.

Diplopia Related to a Pediatric Cervical Rhabdoid Tumor in 18F-FDG PET

ABSTRACT

Increased muscle uptake is commonly seen with 18F-FDG PET/CT because of an important physiological muscle glucose metabolism. Muscle uptake can express a recent significant muscle activity. However, the absence of muscle uptake is almost never described or interpreted. We describe the case of an 8-year-old boy with extrarenal rhabdoid tumor in the right carotid space. An MRI and an 18F-FDG PET/CT were performed for the diagnostic workup. There was no uptake in the lateral rectus oculomotor muscle unlike all other oculomotor muscles. The ophthalmological examination found a diplopia confirmed by the Lancaster test.

Godtfredsen syndrome - recurrent clival chondrosarcoma with 6 years follow up: a case report and literature review

BACKGROUND

We report a rare case of Godtfredsen syndrome caused by clival chondrosarcoma and perform a review of literatures. This article also explains the clinico-anatomical correlation of this rare neurological syndrome.

CASE PRESENTATION

A 22-year-old gentleman presented with binocular diplopia. Clinical examination revealed an isolated right abducent nerve and right hypoglossal nerve palsy, with other cranial nerves intact. Neuroimaging revealed a right clival mass. Supraorbital craniotomy and tumour debulking were done in the same year. Histopathological examination showed low-grade chondrosarcoma. After 5-years of default, he came back with the tumour enlarged. He underwent a right orbitozygomatic craniotomy and tumour excision with 33 cycles of radiotherapy. Despite two surgeries and radiotherapy, the abducent nerve and hypoglossal nerve did not improve throughout 6 years of follow-up. Cranial nerve VI palsy is not always a false localizing sign, in Godtfredsen syndrome it serves as a localizing sign.

CONCLUSION

To the best of our knowledge, this is the first case report of Godtfredsen Syndrome secondary to clival chondrosarcoma. Cranial nerve VI and XII palsy with no involvement of other cranial nerves, most likely the pathology is located at the clivus.

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.

Clinical Course of Abducens Nerve Palsy in Patients with Ruptured Vertebral Artery Dissecting Aneurysms

Objective

There are few detailed reports on abducens nerve palsy due to a ruptured vertebral artery dissecting aneurysm (VADA). We investigated the clinical characteristics and long-term course of abducens nerve palsy in ruptured VADA patients treated by endovascular surgery.

Methods

Of the 51 cases of ruptured VADA treated by endovascular intervention from 2011 to 2019, 31 with a good/fair outcome, in which ocular motility disorder was able to be followed, were included and investigated.

Results

In all, 11 patients (35.5%) had abducens nerve palsy, and the World Federation of Neurological Surgeons (WFNS) grade and Hunt & Hess (H&H) grade at the time of arrival of patients with abducens nerve palsy were significantly higher than those of patients without abducens nerve palsy. Of the 10 patients who were able to be followed, abducens nerve palsy in 3 completely recovered in 7-180 days. Abducens nerve palsy improved in five patients and remained in two patients.

Conclusion

More severe neurological findings on admission reflect a higher rate of abducens nerve palsy. Diplopia induced by abducens nerve palsy is one of the most important sequelae of ruptured VADA, which impairs the daily activities of the patients. Some cases of abducens nerve palsy improve over a long period. Therefore, appropriate diagnosis and follow-up should be concerned.

Detailed imaging of abducens nerve anatomy using contrast-enhanced 3D-TOF MR angiography

PURPOSE

Cranial nerves (CNs), particularly CN IV and VI are difficult to visualize with conventional MRI techniques, particularly within the cavernous sinus region. The aim of this study was to evaluate the capacity of high-resolution contrast enhanced 3D time-of-flight (TOF) MR angiography using new generation 3 T imaging technology to provide detailed visualization of CN VI anatomy, particularly within the cavernous sinus and petroclival regions.

METHODS

Two neuroradiologists conducted bilateral evaluation of CN VI visibility in 23 patients for nerve segments located in the petroclival segment (dural cave and Dorello's canal), and three divisions of the cavernous sinus. All images were collected using contrast enhanced TOF MR angiography using a new generation 3 T machine.

RESULTS

Of the CN VI segments assessed, average visibility of CN VI was best achieved in Dorello's canal. Overall visibility of CN VI within the regions inspected was best achieved in the axial view, with the exception of the dural cave, which was best assessed using the coronal view. We also identified strong agreement in assessment of nerve visibility between the two reviewers. We also identified a putative CN6 duplication and a small schwannoma, highlighting the fidelity of our approach.

CONCLUSION

Contrast enhanced 3D TOF MR angiography can visualize CN VI anatomy, particularly within the petrocavernosal region and cavernous sinus with simultaneous visualization of arterial and venous structures. This cannot be easily achieved using traditional MRI techniques. This imaging technique might be used with new generation machines to evaluate CN VI anatomy and pathologies within the petrocavernosal region and cavernous sinus, especially relating to vascular pathologies.

Gross and Micro-Anatomical Study of the Cavernous Segment of the Abducens Nerve and Its Relationships to Internal Carotid Plexus: Application to Skull Base Surgery

The present study aims to provide detailed observations on the cavernous segment of the abducens nerve (AN), emphasizing anatomical variations and the relationships between the nerve and the internal carotid plexus. A total of 60 sides underwent gross-anatomical study. Five specimens of the AN were stained using Sihler’s method. An additional five specimens were subjected to histological examination. Four types of AN course were observed: a single nerve along its entire course, duplication of the nerve, division into separate rootlets at the point of contact with the cavernous part of the internal carotid artery (ICA), and early-branching before entering the orbit. Due to the relationships between the ICA and internal carotid plexus, the cavernous segment of the AN can be subdivided into a carotid portion located at the point of contact with the posterior vertical segment of the cavernous ICA and a prefissural portion. The carotid portion of the cavernous AN segment is a place of angulation, where the nerve always directly adheres to the ICA. The prefissural portion of the AN, in turn, is the primary site of fiber exchange between the internal carotid plexus and either the AN or the lateral wall of the cavernous sinus.

MRI of the intraorbital ocular motor nerves on three-dimensional double-echo steady state with water excitation sequence at 3.0 T

PURPOSE

To explore the capability of three-dimensional double-echo steady state with water excitation sequence (3D-DESS-WE) to determine the intraorbital ocular motor nerves (IOMN).

MATERIALS AND METHODS

A 3.0 T scanner was applied to investigate 30 healthy volunteers based on the 3D-DESS-WE sequence. Dunnett t test was conducted to evaluate the signal intensity (SI) of the left oculomotor nerve (CNIII), frontal white matter, cerebrospinal fluid (CSF), and lateral rectus (LR). The oculomotor nerve's detectability, trochlear nerve (CNIV), and abducens nerve (CNVI) were evaluated independently by two observers. The average assessment scores were determined, and interobserver variability for these nerves' detectability was determined using a weighted kappa analysis.

RESULTS

The SI of CNIII is similar to that of the frontal white matter (t = 2.26, P > 0.05), lower than the CSF, and higher than the LR (t = 3.81, - 3.45, P < 0.05). The average scores of the superior division of CNIII and the branch to medial rectus (MR), inferior rectus (IR), inferior oblique (IO), CNIV, and CNVI were 3.01, 3.07, 3.78, 2.98, 2.88, and 3.97, respectively. The interobserver variability was excellent (κ = 0.83-1.00).

CONCLUSION

The 3D-DESS-WE sequence shows an ability to detect the IOMN course in healthy volunteers effectively.

Putting the Pieces Back Together: Optimizing Function and Appearance after Orbital Surgery

Objectives  To describe medical and surgical options and techniques for functional and aesthetic abnormalities after orbital surgery and multidisciplinary approaches that include the orbit. Design  A review of current management options in outpatient clinics and ambulatory surgery centers with selected illustrative cases. The rationale for choosing specific medical and surgical interventions will be discussed with a focus on eyelid malposition and double vision. Setting  Outpatient clinics and ambulatory surgery centers. Participants  Patients with eyelid, orbital, eye muscle, and scalp contour abnormalities as a result of medical and surgical interventions for brain and/or orbital tumors. Main Outcome Measures  Descriptive outcomes. Results  A variety of medical and surgical options are available to optimize eyelid, orbit, extraocular muscle, and scalp structure and function.

Anatomical location of the abducens nerves (VI) in the ventral approach of clival tumors

The aim of this work was to determine reliable anatomical landmarks for locating and preserving the abducens nerves (6th cranial nerves) during trans-facial or trans-nasal endoscopic approaches of skull base tumors involving the clivus and the petrous apex. In order to describe this specific anatomy, we carefully dissected 10 cadaveric heads under optic magnification. Several measurements were taken between the two petro-sphénoidal foramina, from the bottom of the sella and the dorsum sellae. The close relationship between the nerves and the internal carotid artery were taken into account. We defined a trapezoid area that allowed drilling the clivus safely, preserving the 6th cranial nerve while being attentive to the internal carotid artery. The caudal part of this trapezium is, on average, 20 mm long at mi-distance between the two petro-sphenoidal foramina. The cranial part is at the sella level, a line between both paraclival internal carotid arteries. Oblique lateral edges between the cranial and caudal parts completed the trapezium.

Cavernous sinus and abducens nerve in human fetuses near term

A long tortuous course of the abducens nerve (ABN) crossing a highly curved siphon of the internal carotid artery is of interest to neurosurgeons for cavernous sinus surgery. Although a "straight" intracavernous carotid artery in fetuses can change into an adult-like siphon in infants, there is no information on when or how the unique course of ABN is established. Histological observations of 18 near-term fetuses (12 specimens of frontal sections and 6 specimens of sagittal sections) demonstrated the following: (I) the ABN consistently took a straight course crossing the lateral side of an almost straight intracavernous carotid artery; (II) the straight course was maintained when sympathetic nerves joined; (III) few parasellar veins of the developing cavernous sinus separated the ABN from the ophthalmic nerve; and (IV) immediately before the developing tendinous annulus for a common origin of extraocular recti, the ABN bent laterally to avoid a passage of the thick oculomotor nerve. Since the present observations strongly suggested morphologies at birth and in infants, major angulations of the ABN as well as the well-known course independent of the other nerves in the cavernous sinus seemed to be established during childhood. In the human body, the ABN might be a limited example showing a drastic postnatal change in course. Consequently, it might be important to know the unique course of ABN before performing endovascular interventions and skull base surgery for petroclival and cavernous sinus lesions without causing inadvertent neurovascular injuries to neonates or infants.

Acute Isolated Sixth Nerve Palsy Caused by Unruptured Intradural Saccular Aneurysm

BACKGROUND

Unruptured intracranial aneurysms that cause isolated sixth nerve palsies are usually located extradurally in the cavernous sinus or intradurally as part of fusiform enlargement of the parent artery. Intradural saccular aneurysms of the vertebrobasilar circulation are a rare but life-threatening cause of isolated sixth nerve palsy. We provide documentation of 2 such cases.

METHODS

Case records of the Neuro-ophthalmology Clinics at the University of Michigan.

RESULTS

A 56-year-old woman with an arteriosclerotic risk profile reported new diplopia and had an isolated left sixth nerve palsy caused by an unruptured saccular aneurysm of the anterior inferior cerebellar artery. It was successfully coiled, and the sixth nerve palsy markedly improved. A 62-year-old woman with an arteriosclerotic risk profile and new diplopia had an isolated right sixth nerve palsy caused by a large unruptured vertebral artery aneurysm. It was coiled and later stented and recoiled. These procedures isolated the aneurysm and relieved the sixth nerve palsy.

CONCLUSIONS

Isolated sixth nerve palsies in these 2 older adult patients with arteriosclerotic risk profiles were caused by unruptured intradural vertebrobasilar aneurysms. This report adds documentation to a rare phenomenon that must be taken into consideration in deciding whether brain imaging is needed in isolated sixth nerve palsy.

Variations of the 6th cranial nerve (nervus abducens) in the petroclival region: A microsurgical study

The aim of this study is to present the variations of nervus abducens in localization and number as it pierces the clival dura mater. The calvaria of 16 cadaveric heads were removed by making horizontal incisions from Glabella to Inion in both sides of the head. The dura mater was dissected. Cerebrum and cerebellum were taken out by obtuse dissection. Dissections of cavernous sinus were made under the stereomicroscope and the findings were photographed. Out of 16 specimens, one of them was excluded. Fifteen heads were bilaterally analyzed. Analysis of these nerves presented four different variations. Variation types a classified CN VI as a single trunk and entering a single dural pore with 77% occurrence. Variation type b classified CN VI with two branches running in the petroclival region and entering a single dural pore with 10% occurrence. Variation type c classified CN VI as 2 trunks and entering 2 separate but close dural pores with 10% occurrence. Variation type d classified CN VI with 2 distinct trunks and 2 branches entering 2 separate but close dural pores with 3% occurrence. CN VI plays a major role in the clinic of the eye. Due to its intracranial and extracranial course, injuries to the head and to the nerve may result in malfunctioning of the lateral muscles of the eye. Therefore, the variations of branching, relations and its course were analyzed.

Concurrent Atlantoaxial Septic Arthritis and Septic Thrombosis of the Ophthalmic Vein due to Staphylococcus aureus: A Case Report and Review of the Literature

Atlantoaxial joint septic arthritis and superior ophthalmic vein thrombosis are uncommon manifestations of Staphylococcus aureus infection. A 68-year-old man presented with acute-onset neck pain and diplopia. Imaging studies revealed atlantoaxial septic arthritis and right superior ophthalmic vein thrombosis. Blood cultures grew methicillin-susceptible S. aureus. We review the literature describing these 2 uncommon manifestations of a common pathogen.

Recent advances clarifying the etiologies of strabismus

BACKGROUND

Strabismus is commonly encountered in neuro-ophthalmology practice. Adult patients may present with symptoms including disabling diplopia and decreased quality of life. Although presentation to the neuro-ophthalmologist often prompts a thorough workup for a neurologic basis of ocular misalignment, advances in orbital imaging and understanding of orbital mechanics have revealed novel mechanical causes. A goal of this review is to clarify mechanical mechanisms of strabismus that were formerly assumed be neurologic in origin.

EVIDENCE ACQUISITION

The authors combine their own research and clinical experience with a literature review using PubMed.

RESULTS

Aberrant paths of the extraocular muscles can lead to strabismus. The extraocular muscles have connective tissue pulleys that control muscle paths and are, in turn, influenced by the extraocular muscle orbital layers. Orbital connective tissues, including the pulleys, constrain extraocular muscle paths. Abnormalities of these tissues may lead to strabismus that is not due to neurologic pathology. Some extraocular muscles are divided into independent neuromuscular compartments, so that partial motor nerve lesions may manifest as selective denervation of only 1 compartment, complicating the presentation of neuropathic strabismus.

CONCLUSIONS

Strabismus in adults due to nonneurologic causes can result from recently described abnormalities of the orbital connective tissue pulley system. Advances in understanding of compartmental extraocular muscle anatomy and innervation can explain cyclovertical strabismus in partial nerve palsies. Recognition of the underlying pathogenesis of the strabismus can lead to improved treatments.

Bilateral duplication of the abducens nerves: an incidental finding on magnetic resonance imaging

Although anomaly of the abducens nerve, including duplication, has been reported in anatomical papers, no radiological report exists regarding a duplicated abducens nerve observed on magnetic resonance (MR) imaging. We encountered a case of bilateral duplication of the abducens nerves, which was found incidentally on MR scans from an 11-year-old boy. He did not have any symptoms of eye movement related to abducens nerve abnormality; thus, the duplication was considered to be a normal variant in this patient. Radiologists should be aware that duplication of the abducens nerve may occur and can be diagnosed on MR, particularly when diagnosing symptomatic patients or as a preoperative assessment for microsurgery of the nerve.

An unusual variant of the abducens nerve duplication with two nerve trunks merging within the orbit: a case report with comments on developmental background

This study reports the first case of abducens nerve duplication along its entire intracranial course, ending within the orbit. A distinct abducens nerve duplication reaching the common tendinous ring (annulus of Zinn), as well as another split within the intraconal segment of the nerve have been revealed. Additionally, two groups (superior and inferior) of abducens nerve sub-branches to the lateral rectus muscle were visualised using Sihler's stain. The analysed anatomical variation has never been reported before and it seems to be in the middle of the spectrum between the cases of duplication occurring only within the intracranial segments of the abducens nerve found in the literature and those continuing throughout the whole course of the nerve. Abducens nerve duplication may be treated as a relic of early stages of ontogenesis. Such a variant might result from alternative developmental pathways in which axons of the abducens nerve, specific for a given segment of the lateral rectus muscle, run separately at some stage, instead of forming a single stem.

Compartmentalization of extraocular muscle function

Ocular motor diversity exceeds capabilities of only six extraocular muscles (EOMs), but this deficiency is overcome by the plethora of fibers within individual EOMs surpassing requirements of homogeneous actuators. This paper reviews emerging evidence that regions of individual EOMs can be differentially innervated to exert independent oculorotary torques, broadening the oculomotor repertoire, and potentially explaining diverse strabismus pathophysiology. Parallel structure characterizes EOM and tendon fibers, with little transverse coupling of experimentally imposed or actively generated tension. This arrangement enables arbitrary groupings of tendon and muscle fibers to act relatively independently. Coordinated force generation among EOM fibers occurs only upon potentially mutable coordination of innervational commands, whose central basis is suggested by preliminary findings of apparent compartmental segregation of abducens motor neuron pools. Humans, monkeys, and other mammals demonstrate separate, nonoverlapping intramuscular nerve arborizations in the superior vs inferior compartments of the medial rectus (MR) and lateral rectus (LR) EOMs that could apply force at the superior vs inferior portions of scleral insertions, and in the medial vs lateral compartments of the superior oblique that act at the equatorial vs posterior scleral insertions that might preferentially implement incycloduction vs infraduction. Magnetic resonance imaging of the MR during several physiological ocular motor behaviors indicates differential compartmental function. Differential compartmental pathology can influence clinical strabismus. Partial abducens palsy commonly affects the superior LR compartment more than the inferior, inducing vertical strabismus that might erroneously be attributed to cyclovertical EOM pathology. Surgery may selectively manipulate EOM compartments.