Isolated abducens nerve palsy after closed head injury in a child.

The authors present the rare case of a 5-year-old child with an isolated right abducens nerve palsy following a minor closed head injury. Occlusion of the left eye for 4 h daily was prescribed, and the child remained under close ophthalmological follow-up. Significant improvement was noticed in the following 6 months.

Introduction

Large series have indicated that 34–42% of abducens nerve palsy in pediatric patients can occur as a result of open or closed skull or cervical trauma, usually along with other cranial nerve injuries or neurological signs.[12] However, posttraumatic isolated abducens nerve palsy in the absence of skull fracture, hematoma or raised intracranial pressure is uncommon in children and only a handful such cases have been reported.[34] When treated conservatively, pediatric patients with posttraumatic isolated abducens nerve palsy have a very good prognosis and usually recover fully within 6 months. Occlusion therapy is necessary to alleviate diplopia and prevent amblyopia in children younger than 7 years.

Case Report

A 5-year-old boy with no previous ophthalmic or general medical history was referred to the University Eye Clinic of Ioannina, Greece due to acute onset of strabismus and diplopia. The patient had reportedly fallen off his bicycle 30 h ago. Clinical evaluation showed no signs of head injury, but the ophthalmic examination revealed a 30-prism diopters right esotropia, limitation of right abduction and compensatory right face turn [Figure 1a]. The boy was not particularly cooperative, and the uncorrected visual acuity was recorded as 8/10 and 5/10 in the right and left eye respectively. Pupillary reflexes were normal in both eyes. Anterior segment examination at the slit lamp and indirect fundoscopy were unremarkable in both eyes. A diagnosis of new-onset paralytic esotropia due to right abducens nerve palsy was established. Neurosurgical and pediatric consultation were unremarkable, and a computed tomography of the head was negative. Ophthalmic reviews over the next days showed that the strabismus angle was stable. In the meanwhile, brain and orbit magnetic resonance imaging of the head was performed but was unrevealing. Serological analysis for the herpes virus family was also normal. In the meanwhile, the patient progressively developed a tendency to fixate with the left eye. Thus, occlusion of the left eye for 4 h daily was prescribed, and close ophthalmic follow-up was scheduled. Over the next couple of months, the esotropia gradually decreased. At the 3 months visit, the boy's visual acuity was 10/10 in both eyes and ocular motility was normal, but intermittent esotropia for near was evident for the right eye. The stereoscopic vision was checked using the Titmus test (140 s of arc). Retinoscopy revealed a refractive error of +1,50 sph in both eyes. Six months after the accident, neither esophoria nor esotropia was noted [Figure 1b].

Figure 1

Picture of the patient at right gaze, primary position, and left gaze, respectively, at presentation (a), and after 6 months (b)

Discussion

Sixth nerve palsy occurs more commonly in children than in adults. Leading causes are neoplasms (range: 27–39%) such as meningioma, acoustic neurinoma, nasopharyngeal carcinoma, and head injuries (range: 34–42%).[12345] Less common causes are idiopathic, congenital, hydrocephalus, infections (herpes virus family, leptospirosis), otitis media, and others.[125]

The sixth nerve leaves the brainstem at the pontomedullary junction and follows an upward and outward path. It ascends vertically through the subarachnoid space, penetrates the dura mater, and courses vertically along the ridge of the petrous part of the temporal bone. About 3–5 mm above the petrous part of the sphenoid bone, the abducens nerve makes an angle of 120° and enters the cavernous sinus, where it runs alongside the internal carotid artery. Next, it enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle.

The long intracranial course of the abducens nerve makes it vulnerable to injuries, but it seems that several anatomic factors may contribute to this. Nerve injury occurs most often at the site of dural entry point and at the petrous apex. Although traumatic palsy of the sixth nerve is usually seen in combination with intracranial hemorrhage, skull fracture, facial fracture or elevated intracranial pressure, it may occur in the absence of such lesions. In such cases, it has been suggested that the most likely mechanism of injury is stretching of the nerve by acceleration in the mid-sagittal plane. The apex of the petrous part of the temporal bone acts as the fulcrum, so the nerve is compressed, contused, and stretched at this point.

The appropriate management of posttraumatic isolated abducens nerve palsy remains a matter of debate. It has been suggested that botulinum toxin should be used early to induce iatrogenic paralysis of the ipsilateral medial rectus, thus counterbalancing the original muscle paralysis and preventing or overcoming the contracture of the ipsilateral medial rectus.[67] On the other hand, a prospective multicenter study with patients with acute traumatic palsy has shown that the probability of spontaneous recovery is similar to that of recovery with botulinum toxin injection.[8] Spontaneous recovery from sixth nerve palsy usually occurs within 6 months, with improvement noted in most cases at 3 months.[349] Bilateral involvement and complete lateral rectus palsy may signify a worst prognosis.[910] In our case, given the fact that the patient could attend frequent ophthalmic follow-up visits, we opted for conservative management. In any event, to alleviate diplopia and prevent amblyopia in children younger than 7 years old, intermittent occlusion of the contralateral eye is important if the strabismus is not alternating. Prisms can also be used to maintain binocularity when the angle of deviation is <15-prism diopters.

Surgery may be considered if there is no recovery within 6 months to 1 year.[6] The type of surgery depends on the angle of deviation and the ability to abduct past the midline. In cases of total abducens palsy, a muscle transposition procedure is indicated. In cases with partial palsy, a resection/recession procedure should initially be tried. Nonetheless, success is variable and more than one operation, or the combination of botulinum toxin injections with surgery, may be needed.