Isolated Abducens Nerve Palsy: A Case Report

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1 [Case Report] 47 Isolated Abducens Nerve Palsy: A Case Report I-Chiang Shy 1, Ching-Yun Han 2, Ping-Hsueh Lee 1, Chi-Wen Juan 1 and Chung-Shing Wu 1 Abducens nerve palsy is the most commonly encountered ocular motor nerve palsy. Because abducens nerve runs a long course from the brainstem to the lateral rectus muscle, abducens nerve palsy can result from numerous etiologies. The most common cause of non-traumatic isolated abducens nerve palsy is microvascular disease, which results from ischemia of the nerve trunk. Isolated ANP due to focal brainstem infarction is very rare because many neural structures are located near this nerve. We describe a 66-yearold man who suffered from acute diplopia. On examination, he demonstrated signs of right abducens nerve palsy. Magnetic resonance imaging showed acute infarction in right anteriolateral pontomedullary junction. His diplopia resolved in one month. (Taiwan J Fam Med 2016; 26: 47-52) DOI: / Key words: abducens nerve palsy, brainstem, cranial nerve, infarction INTRODUCTION Abducens nerve palsy (ANP) is the most commonly encountered ocular motor nerve palsy [1]. However, isolated ANP due to focal brainstem infarction is very rare because of close spatial relationship with other pontine structures. The purpose of the article is to present a case of acute isolated ANP due to anteriolateral pontomedullary infarction, which was confirmed by brain magnetic resonance imaging (MRI). A brief literature review of ANP was also carried out. CASE REPORT A 66-year-old man with a history of minor cerebral infarction (right corona radiate), diabetes, and hypertension was admitted to our ward for rehabilitation. On admission, he had left hemiparesis and left 1 Department of Family Medicine, Kuang Tien General Hosptial; 2 Department of Family Medicine, Lin Shin Hospital, Taichung, Taiwan. Received: September 14, 2015; Revised: October 12, 2015; Accepted: October 15, Correspondence: Dr. Chung-Shing Wu, Department of Family Medicine, Kuang Tien General Hospital, No.117, Shatian Road, Shalu District, Taichung, Taiwan.

2 Taiwan J Fam Med 48 Isolated abducens nerve palsy 2016 Vol.26 No.1 facial palsy. During the hospitalization, his diabetes was controlled well with glimepride and metformin, and his fasting blood sugar levels were 100 to 200 mg/dl. He also took bisoprolol and valsartan for hypertension control. We prescribed clopidogrel for secondary prevention of ischemic stroke. Six weeks later, he presented with acute onset of diplopia when looking toward the right. He denied head trauma, headache, or other new neurologic symptoms. On physical examination, his vital signs were as follows: blood pressure of 132/77 mmhg, pulse rate of 80/min, respiratory rate of 16/ min, and body temperature of 36.5ºC. The patient s consciousness level was fully clear. His pupils were the same size and reactive to light. There was no visual field deficit or ptosis. Evaluation of his extraocular movements (EOMs) revealed right lateral gaze palsy. (Figure 1A,B) The rest of EOMs were normal. Although he had left hemiparesis and left facial palsy associated with prior ischemic stroke, there was no remarkable change in his muscle power and facial expression. No other abnormality was found on the remainder of his neurological and ophthalmological examinations. A workup was ordered, including MRI of the brain and orbits, computed tomography (CT) scans of the paranasal sinus, nasopharyngoscope, and thyroid function test. Diffusion-weighted brain MRI disclosed increased signal intensity in right pontomedullary junction, about 7mm in size, compatible with acute infarction. (Figure 2) No evidence of tumor, dissection, aneurysm or vascular malformation was found. There was no sphenoid sinus lesion in sinus CT. Flexible nasopharyngoscopy showed no space occupying lesion. His thyroid function tests were within normal limits. The neurologist and ophthalmologist recommended close observation, supportive therapy (ie, eye patching) and control of underlying cardiovascular disease risk factors. Follow-up 1 month later revealed complete resolution of the abducens nerve palsy. (Figure 1C) A B C Figure 1. A,B: Lateral gaze palsy of right eye C: Full recovery of gaze palsy. Figure 2. Diffusion-weighted MRI showing the acute infarction (arrow).

3 Shy IC, Han CY, Lee PH, et al 49 DISCUSSION The abducens nucleus is located in the pons, on the floor of the fourth ventricle. Motor axons (abducens nerve fascicle) leaving the nucleus run ventrally and caudally through the pons and exit the brainstem at the pontomedullary junction. The abducens nerve climbs up the clivus, passes through the cavernous sinus, and then enters the orbit to innervate the lateral rectus muscle of the eye [2]. ANP is more common than palsies of the other cranial nerves that control exatraocular movements, accounting for more than 50% of cases [1]. This condition results in inability to direct the eye laterally. The incidence of ANP is 11.3/10000, with a peak in the seventh decades of life [3]. Because of the long course of the nerve, ANP can result from numerous etiologies. (Table 1) Although etiology distributions were various in previous studies, the most commonly reported etiology was vascular disease [1-5]. This variety may be resulted from ambiguous inclusion criteria for vascular disease [4,5]. Differential diagnosis between a vascular and an undetermined etiology may be difficult and is usually based on the presence of vascular risk factors [1]. According to a Korean study, the most common cause was vascular disease, accounting for about 30% of cases, followed by undetermined (26%) and head trauma (21%) [1]. In this study, vascular causes were defined as having at least one vascular risk factor (diabetes, hypertension, ischemic heart disease, or peripheral vascular disease). Despite the introduction of useful diagnostic tools such as brain MRI, the exact etiology cannot be Table 1. Etiology of Abducens Nerve Palsy and Diagnostic Workup Underlying Cause Test Microvascular disease (diabetes Blood lab mellitus/hypertension) Trauma and intracranial CT of head hemorrhage Acute infarction MRI of brain and orbits Intracranial mass lesion and MRI of brain and orbits neoplasm Vasculopathic infarction CT angiography Multiple sclerosis MRI of brain Inflammatory process or viral Blood lab and lumbar puncture with CSF study infection Myasthenia gravis* Acetylcholine receptor antibody test and electromyography Thyroid eye disease* Thyroid function test *mimicker of abducens nerve palsy from reference 1,4,7, Eye 2008; 22: 691-6, J Oral Maxillofac Surg 2008; 66: , J Ophthlmic Vis Res 2013; 8:

4 Taiwan J Fam Med 50 Isolated abducens nerve palsy 2016 Vol.26 No.1 determined in many cases [1,4,5]. Fortunately, most patients recover spontaneously within weeks to several months, especially those with unknown or vascular etiology [4-6]. Diplopia from ANP should be differentiated from diplopia caused by isolated lateral rectus weakness, such as thyroid dysfunction, myasthenia gravis and orbital fracture [7]. The workup of a patient with ANP depends on the likely underlying cause and on the presence of other neurologic findings. (Table 1) ANP is considered isolated if it is present in the absence of orbital disease, severe headache, and other neurologic deficits. Isolated ANP can be divided into traumatic and nontraumatic [2]. Nontraumatic isolated ANP is a diagnostic challenge in daily routine and needs thorough investigation. The most common cause of nontraumatic isolated ANP in old patients who have vascular risk factors is microvascular disease, which results from ischemia of the nerve trunk as it is in isolated pupil-sparing third nerve palsy and nontraumatic fourth nerve palsy. A multicenter study evaluated 62 patients aged 50 years or older presenting with nontraumatic isolated ANP. Fifty patients (80.6%) were found to have presumed microvascular ischemia [5]. These presumed microvascular ANPs have an excellent prognosis, often fully recovering in 3 to 6 months [4,6]. With the development of MRI, less benign and potentially treatable causes have been documented, including tumor, aneurysm, infection, multiple sclerosis and brainstem infarction, though these etiologies are much rarer. Nevertheless, in older adults, tumor, infection, infarction, and increased intracranial pressure should be kept in mind. In younger adult population, multiple sclerosis, collagen vascular diseases and tumors are the other considerations [4]. Overall diagnostic yield of early brain MRI is low in patients with nontraumatic isolated ANP who have vascular risk factors alone [5]. However, a normal MRI finding can help to allay patient s anxiety and fears of brain tumor or other serious diseases, which in turn may have significant social and psychological benefits. The decision to perform an early MRI for nontraumatic isolated ANP depends on expertise of the clinician and physician-patient discussion [5]. If there is no resolution of the palsy, brain images should be obtained as soon as possible [4,5]. Our patient had a history of ischemic stroke, which is a strong risk factor for recurrent stroke. Prior carotid phonoangiograph also disclosed distal stenosis of bilateral vertebral artery without adequate vertebral flow. Recurrent ischemic stroke in brainstem was highly suspected. Therefore we arranged an early brain MRI to confirm our diagnosis. ANPs resulting from brainstem infarction are usually associated with other neurologic findings, such as ipsilateral Horner syndrome, ipsilateral internuclear ophthalmoplegia or contralateral hemiparesis. Isolated ANP from a pontine infarction is very rare because of close spatial relationship with other pontine structures [2,4,8,9]. Most previously reported infarctions related to isolated ANP were

5 Shy IC, Han CY, Lee PH, et al 51 confined to the pontine tegmentum, presumably involving the abducens nucleus. Only very few reported cases illustrated an anterolateral pontine syndrome, presumably affecting the intrapontine abducens nerve fascicle [10]. Our patient had isolated ANP and his brain MRI showed hyperintensity in the right anteriolateral pontomedullary area. Acute infarction involving abducens nerve fascicle without affecting any other surrounding structure such as corticospinal tract or medial lemniscus was highly suspected. Anteriolateral pontine infarction may cause Raymond syndrome (ANP and contralateral hemiparesis). Our patient has a history of minor cerebral infarction with left hemiparesis. Although there was no remarkable change in his limb muscle power, this syndrome cannot be ruled out. After confirming the diagnosis, we encouraged the patient to use an eye patch, which can help mask the diplopia by occluding the effected eye. Neostigmine eye drops were also prescribed to ease his eye discomfort. We evaluated his diplopia and checked his EOMs every day. During this period, his diabetes and hypertension were both controlled well. We found his diplopia resolved completely in one month and his EOMs became normal. Nontraumatic isolated ANP usually results from microvascular ischemia of abducens nerve trunk. Because of long course of this cranial nerve, many etiologies should also be considered, especially for younger patients and those without cardiovascular risk factors, although these etiologies are much rarer. REFERENCES 1. Park UC, Kim SJ, Hwang JM, Yu YS: Clinical features and natural history of acquired third, fourth and sixth cranial nerve palsy. Eye 2008; 22: Brinar VV, Habek M, Ozretic D, Djakovic Visnja, Matijevic V: Isolated nontraumatic abducens nerve palsy. Acta Neurol Belg 2007; 107: Patel SV, Mutyala S, Leske DA, et al: Incidence, associations, and evaluation of sixth nerve palsy using a population-based method. Ophthalmology 2004; 111: Hsu CS, Closmann JJ, Baus MR: Idiopathic unilateral cranial nerve VI palsy: A case reports and review of the literature. J Oral Maxillofac Surg 2008; 66: Tamhankar MA, Biousse V, Ying GS, et al: Isolated third, fourth, and sixth cranial nerve palsies from presumed microvascular versus other causes 2013; Ophthalmology 2013; 120: Tiffin PA, MacEwen CJ, Craig EA, et al: Acquired palsy of the oculomotor, trochlear and abducens nerve. Eye 1996; 1: Azarmina M, Azarmina H: The six syndromes of the sixth cranial nerve. J Ophthlmic Vis Res 2013; 8: Thomke F, Gutmann L, Stoeter P, Hopf HC: Cerebrovascular brainstem diseases with isolated cranial nerve palsies. Cerebrovasc Dis 2002; 13: Atilla H, Isikay CT, Kansu T: Isolated sixth nerve palsy from pontine infarction. Acta Neurol Belg 2000; 100: Pail JW, Kang SY, Sohn YH: Isolated abducens nerve palsy due to anterolateral pontine infarction. Eur Neurol 2004; 52:

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