Bilateral internuclear ophthalmoplegia following minor head trauma

Introduction 

Internuclear Ophthalmoplegia (INO) is clinically characterized by total or partial failure to adduct one eye in lateral gaze and a monocular nystagmus of the abducting eye. It may be unilateral or bilateral. The differential diagnosis of INO can be approached by considering the age of the patient, the mode of onset of symptoms, and whether the findings are unilateral or bilateral. Bilateral INO among young patients is most commonly due to demyelination of the medial longitudinal fasciculus (MLF), often secondary to multiple sclerosis (1). Unilateral INO found in an elderly patient is most commonly caused by cerebrovascular disease (1). INO is rarely associated with minor head trauma. Most patients who suffer severe brainstem injuries are comatose and have additional clinical findings that preclude the detection of INO. We report on a patient with isolated bilateral INO occurring immediately after minor head trauma.

Case presentation 

A previously healthy 29-year-old man presented to our Emergency Department (ED) after sustaining head trauma from an altercation. The patient was intoxicated and had been wrestling with friends. During the struggle he was placed into a headlock, with his neck positioned in extreme flexion, initiating a syncopal episode. The patient was then released from the headlock and fell to the floor, striking his occiput. Witnesses reported a return of consciousness in less than 1 min. Emergency Medical Services (EMS) was called and the patient was placed in long board spinal immobilization and a cervical collar. On arrival in the ED, the patient complained of headache, nausea and double vision.

On examination, the patient was alert and oriented with normal vital signs. He was noted to have bilateral periorbital ecchymoses without retroauricular ecchymosis. No additional signs of head trauma or basilar skull facture were noted. The cervical spine was non-tender with no carotid bruits auscultated. Ocular examination revealed normal visual fields and pupils that were equal, round and reactive to light. On primary gaze he had bilateral exotropia (laterally divergent eyes). On attempted left lateral gaze, the right eye could not adduct past the midline, and there was horizontal nystagmus of the left eye. On attempted right lateral gaze, the left eye could not adduct past the midline, and there was horizontal nystagmus of the right eye (Figure 1). In primary gaze he had upbeating nystagmus. The remainder of the extraocular movements were intact. The funduscopic examination was normal. Aside from the ocular findings, the remainder of the neurological examination was non-focal. Gait was not tested initially.

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Figure 1. A) Patient’s eyes in right lateral gaze. B) Patient’s eyes in left lateral gaze.

Plain films of the cervical spine revealed no fracture, subluxation or soft tissue swelling. Computed tomography (CT) scan of the head without contrast demonstrated an increased density located at the base of the interpeduncular cistern and central midbrain without bony abnormalities, consistent with a localized, post-traumatic subarachnoid hemorrhage. Neurosurgical consultation was obtained and the patient was admitted to the neuroscience intensive care unit (ICU). Subsequent four vessel cerebral angiography was normal, without vasospasm. Magnetic resonance imaging performed within 24 h of injury exhibited a focal area of increased signal intensity ventral to the peri-aqueductal grey matter involving both medical longitudinal fasciculus (MLFs) on diffusion weighted imaging and long repetition-timed (RT) scans (Figure 2).

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Figure 2. MRI showing MLF infarct.

On follow-up examination the patient was not able to stand without support due to truncal ataxia. His diplopia, ataxia and nausea improved slowly over 2 weeks. He was discharged home and continues to improve on outpatient follow-up.

Discussion 

Our patient demonstrated the classic findings of a bilateral internuclear ophthalmoplegia. He had bilateral exotropia on primary gaze with bilateral paralysis of adduction on attempted lateral gaze. In addition, there was a prominent monocular horizontal nystagmus of the abducting eye.

Normal conjugate gaze is based on several distinct neuroanatomical pathways (Figure 3). The sixth nerve nucleus on one side of the brainstem receives supranuclear impulses to direct lateral saccades via the ipsilateral paramedian pontine reticular formation. It also receives pursuit and vestibular input for horizontal eye movements. The sixth nerve nucleus contains both motor neurons for the ipsilateral lateral rectus and interneurons that cross the midline of the brainstem and form the contralateral MLF. These excitatory interneurons travel through the MLF and synapse with neurons in the contralateral medial rectus subnucleus of the third nerve nuclear complex, resulting in contraction of the medial rectus. Unilateral lesions of the MLF allow for normal abduction while impairing ipsilateral adduction. Our patient had a small lesion affecting both MLFs, thus inducing a bilateral INO. A second pathognomonic sign of an INO is horizontal nystagmus of the abducting eye. The mechanism for this finding is not clearly understood and may be simply an adaptive response (2).

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Figure 3. Schematic diagram depicting the neuroanatomy of conjugate gaze. Left lateral gaze is demonstrated by the hashed lines. Cortical input reaches the sixth nerve nucleus via the paramedian pontine reticular formation (1,2). The sixth nerve nucleus contains motor neurons that innervate the ipsilateral lateral rectus (3a) and interneurons that cross the midline and form the contralateral MLF (3b). Interneurons from the MLF synapse with motor neurons in the contralateral oculomotor nucleus, which innervate the contralateral medial rectus (4). Adapted with permission from Kandel E, et al. Principals of Neural Science, 3rd edition. New York: Elsevier; 1991:722.

The mechanism by which localized injury to the MLF occurs is somewhat controversial. Shear forces directly injuring the nerve fibers was the prevailing injury theory until the mid-1970s 3, 4. More recently, the proposed mechanism involves shear forces damaging the vascular supply, leading to a localized infarction of the MLF. The deep nuclei of the brainstem are located in a “watershed” vascular distribution area. The vascular supply to the deep nuclei of the brainstem is provided by the paramedian perforating branches of the basilar artery. During trauma, shear forces stretch these vessels and ischemic or hemorrhagic infarction of the MLF may occur 5, 6, 7, 8, 9. In our case, with occipital trauma, the patient had a contrecoup, localized, prepontine, subarachnoid hemorrhage. Diffusion weighted images on MRI demonstrated ischemic infarction of both MLFs at the pontomesencephalic junction. Vasospasm was excluded in our patient as a possible pathogenic mechanism, and therefore contrecoup stretching of the perforating vessels was the most likely etiology of the ischemia.

The most common reason for INO in a young patient without head trauma is multiple sclerosis (MS). Bilateral INO was previously conscribed as “pathognomonic” of MS. In a case series, Smith and Cogan found MS as the etiology among 28 of 29 patients with bilateral INO. INO in elderly patients is usually unilateral and secondary to cerebrovascular disease (1). INO less commonly may be secondary to malignancy, with some cases of presumed post-traumatic gaze palsies actually secondary to an undiagnosed intracranial neoplasm (10). Cervical manipulation, post-irradiation demyelination, infection, Arnold-Chiari malformation, syphilis, pernicious anemia, Wernicke’s encephalopathy, maple syrup urine disease, abetalipoproteinemia, Fabry’s disease, hepatic encephalopathy, drugs, and progressive supranuclear palsy have all been reported as infrequent etiologies of INO. Pseudo INO may be seen in myasthenia gravis and Miller-Fischer’s syndrome (2).

INO is an uncommon finding in the ED. It represents a definite lesion in the brainstem with many possible etiologies. The evaluation of the patient with a new INO can be guided by the patient’s age and the presence or absence of trauma. With trauma, a non-contrast CT scan of the head and neurosurgical consultation should be obtained. In the elderly patient, when cerebrovascular disease is the most likely cause, a CT scan can be used to rule out hemorrhagic infarct and possibly permit the patient to be treated with thrombolysis. Younger patients without a history of trauma and no physical evidence of elevated intracranial pressure should undergo work-up for demyelinating disease.