Rare Case of Post-Traumatic Abducens-Oculomotor Nerve Synkinesis Exhibiting Unusual Ocular Motility

Introduction

Acquired synkinesis can occur after trauma and has frequently been described after oculomotor nerve injury. The abnormal movement appear within the distribution of one nerve or other cranial nerves may be involved. Abducens-oculomotor synkinesis is quite unusual. The present report is of case of unusual ocular motility due to abnormal connection between the sixth and third cranial nerves following severe head and orbit trauma. Very few similar cases have been published.

Case Report

A 21-year-old man, previously healthy with no history of preexisting ocular motility disorders, sustained severe left-sided head trauma in a motor vehicle accident (MVA). He was comatose for 6 weeks in the intensive care unit (ICU). He had left-sided frontal, lateral orbital wall, and zygomatic fractures with hemorrhagic contusion in the left frontal lobe. During his stay in the ICU, the family observed that the patient’s left eye movements were incomplete. An examination of the left eye in the ICU revealed a pale optic disc. He was scheduled to visit the ophthalmology clinic upon his discharge. On examination 6 months later, the best corrected visual acuity was 20\20 right eye and NLP left eye. His left pupil diameter was 6 mm and not reactive, and a left afferent pupillary defect was present. An ocular motility exam showed a small esotropia in the primary position. On duction testing, the left eye had markedly decreased adduction, elevation, and minimal depression, but on attempted abduction, the eye would adduct (Figure 1). Forced duction testing was negative, indicating that left oculomotor nerve function was subnormal. Right eye motility was full. The rest of his examination was normal apart from left optic atrophy (Figure 2). Brain MRI revealed no brain stem lesions. The patient provided informed consent for the publication of his case and accompanying photographs. The details of the patient’s file are available to interested researchers and clinicians upon request. The patient’s condition remained unchanged for 1 year.

Discussion

Various patterns of synkinesis have been observed following both congenital and acquired third nerve palsies [1–3]. The hypotheses that have been proposed to explain oculomotor synkinesis include misdirection of regenerating axons, ephatic transmission, central reorganization, and denervation supersensitivity [1,2,4].

Aberrant regeneration is a recognized neurophysiological phenomenon following peripheral third nerve palsy. It commonly occurs when the palsy is secondary to trauma, aneurysm, or tumor, and the nerve does not recover for several months [1].

Acquired abducens-oculomotor synkinesis is quite unusual, with few reported cases [1,4–6]. The paradoxic eye movement in the current case can be described by neuronal misdirection theories. Following trauma to the orbit, the oculomotor and abducens nerve functions were disrupted, with abnormal function, causing medial rectus muscle contracture when the lateral rectus muscle is stimulated. The theory behind this mechanism proposes that the regeneration of injured peripheral neurons leads to a bridging matrix of proliferating cells with sprouts of regenerated axons that innervate tissues inappropriately [2,4]. Experimental studies by Cajal and Whitman showed that during repair processes, newly formed axons can randomly grow into the wrong muscle or distal stump [4,7]. In contrast to Duane’s retraction syndrome [8], we could not see any globe retraction when the left eye attempted abduction, in addition to lack of ocular motility disorders, as evident by the history and normal results of a previous childhood examination.

Conclusions

The most likely pathophysiologic mechanism explaining the present case is the peripheral neuronal misdirection hypothesis after trauma. Careful and detailed examination of a patient presenting with unusual ocular motility after trauma is very important. The abnormal connection between the sixth and third cranial nerves is quite rare, but has been reported.