Traumatic Abducens Nerve Palsy Due to Avulsion in its Cisternal Segment: Illustrative Cases

Introduction

The abducens nerve is the longest and most complex cranial nerve in its path along the skull base, featuring at least 3 significant angulations that make it more vulnerable to injury in cases of trauma [1,2]. Traumatic abducens nerve palsy may result from direct compression of the nerve due to fractures near the petrous apex. However, some patients with abducens nerve palsy following trauma do not have fractures near the Dorello’s canal. Previous studies suggested that the mechanisms of such injury may involve nerve compression or tearing at the dura entry point and the Dorello’s canal, as well as damage in the cavernous sinus segment at its third angulation [3–5]. Research on abducens nerve injuries within the cisternal segment has been limited to recent imaging studies [6–13], with no surgical exploration or repair cases confirmed. In this report, we present 2 cases of abducens nerve avulsion in the cisternal segment, including 1 particularly interesting case in which the avulsion may also have been associated with a tortuous and enlarged vertebral artery. Our report is the first to provide intraoperative evidence of traumatic abducens nerve avulsion, which serves to enhance the understanding of the mechanisms of the disease.

Case Reports

CASE 1:

A 31-month-old boy, who had suffered from multiple traumatic injuries from a car accident 13 days before admission to our center, was transferred to our center for management of sequelae related to left abducens nerve palsy. Physical examination revealed lateral gaze palsy in the left eye, while no other neurological abnormalities were found. Preoperative CT showed fractures in the left occipital bone, left temporal bone, and left sphenoid bone, and the cisternal segment of the left abducens nerve close to the brainstem was unclear on magnetic resonance imaging (MRI) (Figure 1). Although there was no direct fracture near the Dorello’s canal, we hypothesized that the crush injury transmitted from the petrous bone resulted in an immediate displacement of the left petrous apex. This type of displacement can result in deformation of the Dorello’s canal and subsequent compression of the abducens nerve. Therefore, we intended to remove the petrous apex and open the Dorello’s canal to decompress the abducens nerve. Primary nerve anastomosis or nerve substitution was considered as an alternative intraoperative option.

We adopted the intradural subtemporal trans-Meckel’s-cave approach, planning to open the roof of Meckel’s cave, transpose the gasserian ganglia and trigeminal nerve root laterally, and then open the floor of Meckel’s cave to explore the petrous apex adjacent to Dorello’s canal. If fracture of the petrous apex was confirmed, we would remove any bony compression and decompress the abducens nerve. However, intraoperatively, when we opened Meckel’s cave and translocated the trigeminal nerve, it was clearly found that the cisternal segment of the abducens nerve was partially avulsed from the brainstem (Figure 2). Furthermore, no evidence of fracture of the petrous apex was found across the dura attached to the floor of Meckel’s cave. Therefore, we believe that the cause of the symptoms was nerve avulsion in the cisternal segment. Unfortunately, neuroanastomosis could not be performed because the avulsion point was too close to the brainstem. The child was very young, and the family was reluctant to proceed with nerve substitution, which could involve damaging other autogenous nerves. We recommended an ophthalmologic operation in the future.

CASE 2:

A 60-year-old woman presented with right abducent nerve palsy following a head injury 4 years previous to her visit to our center. Physical examination revealed lateral gaze palsy in the right eye, with no other neurological abnormalities. The patient reported multiple bone fractures at the time of the injury; however, a repeated CT at our center revealed no definite fractures at the region of Dorello’s canal and the petrous apex. Interestingly, magnetic resonance angiography (MRA) revealed a tortuous and enlarged vertebral artery, and the adjacent right abducens nerve was not clearly visible (Figure 3). We hypothesized 2 possible scenarios: in the first scenario, the right abducens nerve might be displaced and compressed by the contralateral tortuous vertebral artery; in the second scenario, the nerve may have been transected by the vertebral artery in the cisternal segment during the injury.

We opted for a retrosigmoid approach for surgical exploration. Intraoperatively, we found the right abducens nerve to be completely severed, with the proximal end absent and only about 1 cm of the distal end remaining before it entered the dura mater (Figure 4). We speculated that the avulsion at the cisternal segment could be associated with the tortuous and enlarged vertebral artery. We used fibrin glue to anastomose the distal end of the abducens nerve and the partial motor branch of the trigeminal nerve. However, after 7 months of follow-up, there was no significant recovery of nerve function, but no complications such as masseter atrophy or facial numbness were observed.

Discussion

The abducens nerve has a very complex course through the skull base, and can be divided into 3 intracranial segments: the cisternal segment, the petroclival segment (also known as the interdural segment), and the cavernous sinus segment. The cisternal segment extends from the root exit point at the pontomedullary sulcus to the point where it penetrates the dura mater. Upon penetrating the dura mater, the nerve enters a larger interdural venous space known as the petroclival venous confluence [14] or the sphenopetroclival venous gulf [15]. After passing through Dorello’s canal, it enters the cavernous sinus and continues through the superior orbital fissure, thereby marking the cavernous sinus segment. The abducens nerve has 3 angulations: the first occurs where the nerve penetrates the dura mater; the second, where it leaves the Dorello’s canal; and the third, where it reaches the intracavernous internal carotid artery. Previous microanatomical studies have largely focused on Dorello’s canal, which is bounded by Gruber’s ligament, the petrous apex, and the lateral border of the clivus. The abducens nerve is fixed by fibrous tissue in this canal. Dorello’s canal is surrounded by the periosteal dura, which together with the meningeal dura of the petroclival region forms an interdural space posterior to the cavernous sinus. It is continuous with the cavernous sinus, the basal sinus of the clivus, and the inferior petrosal sinus.

From previous clinical reports, traumatic abducens nerve injury can be unilateral or bilateral, and with or without the presence of fractures near the petrous apex [16]. Based on the presence or absence of petrous apex fractures, the mechanisms of traumatic abducens nerve injury can be categorized into 2 main types. The first type involves fractures close to the Dorello’s canal, which may result in compression or even transection of the abducens nerve by displaced bone fragments. The second type includes nerve injuries that occur without Dorello’s canal fractures. Schneider et al [17] thought that traumatic abducens nerve palsy resulted from compression by either the Gruber ligament or the petrous apex due to up-and-down movement of the brainstem. Subsequently, Takagi et al [3] and Arias [4] highlighted the role of the dural entry point in such injuries, noting that the damage primarily occurs at the dural entry point and the petrous apex, which is the base of Dorello’s canal. In 2004, Sam et al [5] investigated 10 specimens from autopsies of severe head trauma cases and found that the most common sites of abducens nerve injury were the dural entry point and the petrous apex. Additionally, the region where the sympathetic fibers anastomose with the abducens nerve lateral to the internal carotid artery in the cavernous sinus was also found to be prone to injury. The findings of Sam et al are consistent with the theories proposed by Takagi and Arias. Thus, it is now understood that, in the absence of petrous apex fractures, damage to the abducens nerve at the dural entry ring and within Dorello’s canal due to relative up-and-down movement of the brainstem is the primary mechanism leading to abducens nerve palsy following trauma.

However, with the development of MRI and the emergence of the fast imaging employing steady-state acquisition (FIESTA) sequence, people have further enhanced the understanding of abducent nerve injuries. Lopes et al [6] first reported the use of 1.5T MRI with the FIESTA sequence to observe discontinuity in the bilateral cisternal segments of the abducens nerve in 2011. The patient with neck hyperextension had symptoms of bilateral lateral gaze palsy and convergent strabismus, which remained unchanged in the 1-year follow-up period. In 2016, Azad et al [7] used the FIESTA sequence to confirm the avulsion of the right abducens nerve in the cisternal segment in a 26-year-old man with a temporal bone fracture who presented with diplopia and ipsilateral facial paralysis, whose facial nerve function totally recovered in long-term follow-up while abducens nerve function did not. In the same year, Yamasaki et al [8] also ascertained unilateral avulsion of the abducens nerve in a case of facial trauma with thin-slice MRI examination and FIESTA sequence. The patient’s symptoms showed no improvement after the injury. In 2017, Ravindran et al [9] presented 2 cases of bilateral avulsed abducens nerves in the prepontine cistern on oblique FIESTA without regional fractures, and they further emphasized the importance of high-resolution MRI in the diagnosis of abducens nerve injury. Subsequent studies have also documented cases of abducens nerve discontinuity in the cisternal segment using the FIESTA sequence, implying that avulsion in the cisternal segment is an under-recognized injury mechanism [10–13]

Based on our intraoperative findings in the 2 cases presented here, we propose that avulsion of the cisternal segment of the abducens nerve should be considered a significant cause of traumatic abducens nerve palsy. Trauma can lead to longitudinal relative displacement between brain tissue and the skull, potentially causing traction on cranial nerves which pass through various cranial base canals. When the brainstem is displaced inferiorly, the cisternal segment of the abducens nerve may be stretched, resulting in traction-induced damage at the dural entry ring. Additionally, excessive displacement may cause avulsion or even transection of the abducens nerve in the cisternal segment, as demonstrated in our cases. It is also worth noting that patients with significantly tortuous vertebral arteries may be more susceptible to injury, as these tortuous vessels may contribute to “laceration” of the abducens nerve under external force.

Reports of neurosurgical repair of the abducens nerve are quite rare. One is by Sekhar et al [18] in 1992, who discussed the reconstruction of cranial nerves III through VI following cavernous sinus surgery. In that study, 6 patients underwent abducens nerve repair with sural nerve grafting, with the graft bridging from the proximal cisternal segment to the orbital segment or directly to the lateral rectus muscle, as the distal abducens nerve was not accessible. In 1997, Sawamura et al [19] reported a case involving a petrous apex meningioma treated with a combined supra/infratentorial transpetrosal approach. During surgery, the abducens nerve was encased by the tumor at the dural entry point and was excised along with the tumor, followed by primary nerve anastomosis with 5 10-0 nylon sutures.

Our surgical procedures in the present 2 cases were mainly based on preoperative evaluation. In Case 1, although the site where the abducens nerve exits the brainstem was unclear on preoperative MRI, there was still anatomic continuity of the abducens nerve in imaging. Besides, the multiple cranial base fracture seen on CT, in the context of the previous literature, suggested that the injury might be related to the petrous apex or Dorello’s canal, so we adopted the intradural subtemporal trans-Meckel’s-cave approach and intended to preform decompression with direct visualization of the abducens nerve and its root entry/exit zone. In Case 2, the preoperative MRI approaches suggested that the vertebral artery occupied the anatomic position of the right abducens nerve, and the cisternal segment of the abducens nerve was unclear, so we used a retrosigmoid approach to explore this part. However, in our 2 cases, avulsion of the abducens nerve was confirmed in both cases intraoperatively, and we were unable to perform end-to-end anastomosis. We suggest that if both the proximal and distal segments of the abducens nerve are accessible and electrophysiological monitoring indicate that the lateral rectus muscle is not completely atrophied, direct nerve anastomosis or nerve grafting should be considered. Since the proximal segment of the abducens nerve was inaccessible but the distal segment was reachable, we tried the alternative approach of anastomosis of the distal end of the abducens nerve with partial motor branches of the trigeminal nerve. However, the efficacy of this method remains uncertain and patients may undertake the risk of weakness in chewing. We speculate that our patient’s poor recovery may be due to the long course of the injury and loss of function at the neuromuscular junction, or the inability of the central nervous system to remodel and replace the function of the abducens nerve. In cases where the lateral rectus muscle has undergone complete atrophy and cannot regain neural innervation, only ophthalmologic surgery can be considered [20].

Conclusions

Avulsion at the cisternal segment of the abducens nerve should be recognized as a significant cause of traumatic abducens nerve palsy. In patients with traumatic abducens nerve palsy when fractures near the petrous apex have been excluded, thin-slice FIESTA sequence MRI should be performed to precisely assess the continuity of the abducens nerve in its cisternal segment, especially in patients with tortuous vertebral arteries. The abducens nerve should be repaired whenever possible in cases with avulsion at its cisternal segment. If feasible, direct nerve repair or nerve grafting should be considered. When the proximal segment is inaccessible, nerve transfer may be considered. However, further research is needed to evaluate the efficacy and outcomes of these techniques.