Unilateral abducens nerve avulsion injury following trauma

Abstract

Dear Sir, While abducens nerve palsy is commonly seen in clinical practice, abducens nerve palsy secondary to an avulsion injury is uncommon. In a retrospective study documenting the causes of abducens nerve palsy, only up to 3.1% were attributed to trauma; the most common cause in adults was related to vascular ischaemia.[1] Unilateral abducens nerve palsy is found in only 1%–2.7% of all head traumas.[2] Here, we present a case of unilateral abducens nerve injury following trauma. A 60-year-old woman with no significant medical history was involved in a road traffic accident. Initial computed tomography (CT) of the brain revealed bilateral acute subarachnoid haemorrhage, as well as an acute subdural haemorrhage along the posterior interhemispheric falx. Multiple facial bone fractures were identified [Figure 1], including fractures of both orbital floors and lateral walls, lamina papyracea, maxilla and left zygomatic process. No fracture was identified along the course of the left abducens nerve. Of note, the left petrous apex in the region of Dorello’s canal, cavernous sinus and lateral rectus muscle appeared unremarkable.Figure 1: Axial CT images in the bone windows. (a) Acute fractures of both lateral orbital walls, lamina papyracea and left zygomatic process (circles). (b) There is also an acute fracture of the left lateral sphenoid wall (circle). Of note, the left petrous apex appears unremarkable, with no acute fracture.The patient subsequently underwent open reduction internal fixation of both zygomaticomaxillary complex fractures with left orbital floor reconstruction 26 days after the accident. Intraoperatively, forced duction test did not reveal any restriction on eye movement. Postoperatively, it was noted that the patient possessed signs of left abducens nerve palsy, with persistent medial deviation and failure of abduction of the left eye. These were likely not detected during the initial few weeks in view of extensive periorbital soft tissue swelling from the facial and orbital fractures, which resolved after surgical fixation. Contrast-enhanced CT of the brain did not demonstrate any new finding to explain the left abducens nerve palsy. Magnetic resonance imaging of the brain and orbits, including a three-dimensional constructive interference in steady-state (3D CISS) sequence of the cranial nerves was performed 35 days after the accident. This showed a discontinuous left abducens nerve in the prepontine cistern, with subtle linear enhancement corresponding to its root exit zone [Figure 2]. Findings pointed towards a traumatic unilateral left abducens nerve avulsion injury resulting from the road traffic accident. An iatrogenic aetiology of the left abducens nerve avulsion was deemed less likely, as the surgery was mainly focused on facial and orbital fracture repairs, with no surgical intervention performed along the course of the left abducens nerve. The patient was subsequently referred to ophthalmology and managed conservatively.Figure 2: (a) Axial MR image with constructive interference in steady state (CISS) sequence shows discontinuity of the left abducens nerve in the prepontine cistern (circle). (b) Sagittal oblique reconstruction of the MR image with CISS sequence shows left abducens nerve avulsion (circle). (c) Axial pre-contrast and (d) post-contrast T1-W MR images show a linear enhancement corresponding to the left abducens nerve root exit zone (circle).The abducens nerve is an important nerve involved in the extraocular movements of the globe. Its long course, in addition to the nerve being fixed in Dorello’s canal, makes it vulnerable to trauma. A common site of injury is at the petrous apex as the nerve enters Dorello’s canal.[3] The usual mechanism of injury is contusion/stretching along its course during vertical displacement of the brain, which is evident during head trauma.[3] With the advent of MR imaging, visualisation of the abducens nerve is possible. The most useful sequence for imaging includes the steady-state Gradient Echo technique, 3D CISS or fast imaging employing steady-state acquisition. These are heavily T2-weighted 3D volumetric sequences capable of acquiring very thin slices and allowing reconstruction in all three planes.[4] In this way, it is possible to analyse the morphological features of structures next to the cerebrospinal fluid-containing spaces, including the cranial nerves. In conclusion, we encountered an uncommon case of unilateral abducens nerve avulsion injury following trauma, of which there are fewer than five documented cases reported in the English literature.[5-7] Magnetic resonance imaging using CISS sequences plays an important role in diagnosis. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.