A BEND IN THE SIXTH CRANIAL NERVE-AND ITS PROBABLE SIGNIFICANCE

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.

Cranial nerve VI palsy after dural-arachnoid puncture

In this article, we provide a literature review of cranial nerve (CN) VI injury after dural-arachnoid puncture. CN VI injury is rare and ranges in severity from diplopia to complete lateral rectus palsy with deviated gaze. The proposed mechanism of injury is cerebrospinal fluid leakage causing intracranial hypotension and downward displacement of the brainstem. This results in traction on CN VI leading to stretch and neural demyelination. Symptoms may present 1 day to 3 weeks after dural-arachnoid puncture and typically are associated with a postdural puncture (spinal) headache. Resolution of symptoms may take weeks to months. Use of small-gauge, noncutting spinal needles may decrease the risk of intracranial hypotension and subsequent CN VI injury. When ocular symptoms are present, early administration of an epidural blood patch may decrease morbidity or prevent progression of ocular symptoms.

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.

Case report: Epidural blood patch in the treatment of abducens palsy after a dural puncture

PURPOSE

To describe a case of iatrogenically induced abducens nerve palsy following a diagnostic lumbar puncture, and to review the evidence for blood patching in the management of sixth cranial nerve palsy after dural puncture.

CLINICAL FEATURES

A 45-yr-old woman developed post-dural puncture headache with bilateral abducens palsy following a diagnostic lumbar puncture. Magnetic resonance imaging showed findings compatible with intracranial hypotension. An epidural blood patch was performed five days after the onset of diplopia and ten days following the dural puncture. After blood patching, the patient reported relief of the headache, but still complained of diplopia. The palsies recovered spontaneously 21 months after the dural puncture.

CONCLUSION

Experience from this case as well as other case report evidence suggest that an epidural blood patch performed more than 24 hr after the onset of a sixth cranial nerve palsy consistently fails to relieve diplopia. An epidural blood patch executed within 24 hr from the onset of diplopia could possibly lead to partial improvement and/or earlier resolution of symptoms.

Dorello's canal: a microanatomical study

The microsurgical anatomy of Dorello's canal has been studied in 20 specimens obtained from 10 cadaver heads fixed in formalin. The bow-shaped canal through which courses the abducens nerve before reaching the cavernous sinus is located inside a venous confluence which occupies the space between the dural leaves of the petroclival area. The petrosphenoidal ligament (Gruber's ligament), which forms the posteromedial wall of the canal, appears as a fibrous trabecula surrounded by venous blood. Canal measurements were performed and its anatomical relationship with the sixth cranial nerve is described. Angulations of variable degrees were observed in the course of the nerve inside and outside the canal. The influence of this relatively tortuous course of the abducens nerve upon its vulnerability in some pathological conditions is discussed.

Multiple cranial nerve palsies due to a hyperextension injury to the cervical spine. Case report

The case of a patient with multiple bilateral cranial nerve palsies and spinal cord sparing secondary to a stable hyperextension injury to C-1 is presented.

Abducens palsy after lumbar myelography with watersoluble contrast media

Five patients who developed abducens palsy after myelography with watersoluble contrast media are reported. These palsies can be compared to abducens palsies after spinal anesthesia and diagnostic lumbar puncture. They are most likely due to the lumbar puncture. The arguments for this explanation are discussed. The experience with these five patients suggests a greater incidence of postpuncture abducens palsy after myelography with watersoluble contrast media than after spinal anesthesia or lumbar puncture. If this first impression is verified, it could point to an additional toxic action.

The abducens nerve. Anatomical variations in its course

✓ The authors describe the origins and course of the sixth cranial nerve in 62 cadaver or autopsy cases and describe three patterns. In Pattern 1 the nerve originates and runs all its way as a single trunk. In Pattern 2 it originates as a single trunk, but splits into two branches in the subarachnoid space, while in Pattern 3 it originates as two separate trunks. In both Patterns 2 and 3 the trunks perforate the dura mater independently and enter the cavernous sinus by passing one above and the other below the petrosphenoidal ligament. In the sinus the two trunks fuse into a single trunk which then continues to the lateral rectus muscle. The practical neurological importance of these variations is discussed.

Bilateral traumatic abducens palsy. A mechanism of injury suggested by the study of associated cervical spine fractures

✓ Two patients with bilateral abducens palsy are presented. A study of the associated injury to the cervical spine suggests that a severe blow to the head in an upward and posterior direction may produce contusion or avulsion of the abducens nerve by the rigid petrosphenoidal ligament under which it passes.

Neurovascular relations and anomalies at the base of the brain

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