Internal Carotid Artery Pseudoaneurysm Causing an Abducens Nerve Palsy: A Case Report

Sixth cranial neuropathies are the most common isolated ocular cranial nerve palsy. Despite advancements in MRI and MRA, the exact etiology remains unclear in 22%-30% of cases (1).

Clinical Correspondence Section Editors: Robert Avery, DO Karl C. Golnik, MD Caroline Froment, MD, PhD An-Guor Wang, MD Internal Carotid Artery Pseudoaneurysm Causing an Abducens Nerve Palsy: A Case Report Alexander S. Himstead, BS, Kevin M. Gustafson, MD, Anton N. Hasso, MD, Lilangi S. Ediriwickrema, MD REPORT OF A CASE S ixth cranial neuropathies are the most common isolated ocular cranial nerve palsy. Despite advancements in MRI and MRA, the exact etiology remains unclear in 22%–30% of cases (1). Neoplasm, trauma, and microvascular disease are among the more common etiologies, but sixth neuropathies can be caused by large vessel aneurysm, Wernicke encephalopathy, basilar meningitis, intracranial hypertension, giant cell arteritis, and intracranial or subarachnoid hemorrhage (1). Fibromuscular dysplasia (FMD) is an idiopathic, nonatherosclerotic, noninflammatory disease of the smooth muscle of small-sized and medium-sized arteries that primarily affects women (2). It causes sequential stenoses, aneurysms, and dissections throughout the vasculature, but most commonly in the renal, extracranial carotid, and vertebral arteries (2). An isolated abducens nerve palsy in the setting of FMD is rare. We present the case of a young woman with an isolated abducens nerve palsy caused by an internal carotid artery (ICA) dissection and pseudoaneurysm in the setting of previously undiagnosed FMD. A 45-year-old woman with a remote history of bilateral Laser-Assisted In Situ Keratomileusis and ruptured splenic artery aneurysm presented for a second opinion of persistent diplopia. The patient developed binocular, horizontal diplopia 14 months prior, 1 day after an episode of severe headache, nausea, and vomiting. She reports injuring her neck in a motor vehicle accident 20 years prior but denies any head or neck trauma since. She has no monocular blurry vision, loss of vision, pulsatile tinnitus, vertigo, weakness, numbness, or medication changes. MRI of the brain, MRA of the head and neck, and computed tomography angiography of the head were read by an outside radiologist as unremarkable. Her School of Medicine (ASH), University of California, Irvine, California; Department of Ophthalmology (KG, LSE), Gavin Herbert Eye Institute, University of California, Irvine, California; and Department of Radiological Sciences (ANH), University of California, Irvine, California. Supported by Research to Prevent Blindness. The authors report no conflicts of interest. Address correspondence to Alexander S. Himstead, BS, Department of Ophthalmology, Gavin Herbert Eye Institute, University of California, 850 Health Sciences Road, Irvine, CA 92697; E-mail: [e-mail] e50 diplopia improved significantly over the next 2 months and at the time of evaluation was only present with extreme left gaze and fatigue. On examination, there was esotropia (14 prism diopters [PD] on primary gaze and 18 PD on left gaze) and slight abduction deficit of the left eye. The remainder of extraocular motility was full. Slit-lamp and fundus examination were unremarkable without signs of optic nerve edema, pallor, or cupping. Automated visual field testing revealed no deficits. Laboratory results including acetylcholine receptor binding and blocking antibodies, angiotensin-converting enzyme, antineutrophil cytoplasmic antibody, muscle-specific kinase, and rapid plasma reagin were within normal limits. Given the negative laboratory workup, the prior imaging was reviewed. Findings included a pseudoaneurysm of the petrous left ICA and fusiform aneurysms consistent with FMD of the left cervical vertebral artery and bilateral cervical internal carotid arteries (Fig. 1). The patient was started on aspirin 325 mg daily and referred to neurovascular surgery. Serial imaging was obtained 9 months later and demonstrated persistent fusiform aneurysms of the bilateral internal carotid arteries with resolution of the previously demonstrated pseudoaneurysm. She was recommended to continue antiplatelet therapy and to undergo serial neuroimaging surveillance. The patient continues to have a mild sixth nerve palsy 26 months after onset of original symptoms. DISCUSSION Abducens nerve palsies are the most common isolated ocular motor cranial nerve palsy (1). Lesions to this nerve may occur along its entire course (Fig. 2), and vascular etiologies such as saccular aneurysm, carotid-cavernous fistula, or ICA dissection affect the nerve as it passes through the cavernous sinus (1). The mechanism of an abducens neuropathy in cavernous sinus depends on the etiology. An ICA saccular aneurysm or carotid-cavernous fistula may directly compress the nerve, leading to impaired function (3). Subintimal ICA dissections typically cause nerve ischemia through luminal narrowing, thromboembolism, or compression of the ostia of branch vessels (Fig. 2), whereas subadventitial dissections result in aneurysmal dilatations or Himstead et al: J Neuro-Ophthalmol 2023; 43: e50-e52 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence FIG. 1. MRA and 3-dimensional reconstructions. MRA demonstrating coronal (A) and axial (B) views of the pseudoaneurysm (indicated by arrows) of the petrous segment of the left ICA. MRA 3-dimensional reconstructions of the (C) ICA pseudoaneurysm (indicated by an arrow) and (D) fusiform dilatations (indicated by arrowheads) of bilateral cervical ICAs and beading pattern (indicated by an arrow) of the left vertebral artery. ICA, internal carotid artery. pseudoaneurysms if the adventita is breached (3). Subadventitial dissections may also disrupt sympathetic fibers traversing the carotid sheath, causing a concomitant Horner syndrome (3). Among other connective tissue diseases, FMD is associated with higher risk of spontaneous arterial dissections and aneurysms (2). A study by the US Registry for FMD found the prevalence of aneurysm and dissection in patients with FMD to be 21.7% and 25.7%, respectively (2). Dissections were most commonly found in extracranial carotid and vertebral arteries (63.7%), whereas a smaller proportion of aneurysms involved these arteries (31%) (2). Most patients with aneurysms and dissections presented with headaches (64.3% and 76.4%, respectively) and pulsatile tinnitus (32%), and less frequently patients presented with strokes Himstead et al: J Neuro-Ophthalmol 2023; 43: e50-e52 (10.8% and 20.1%), transient ischemic attacks (10.9% and 14.7%), amaurosis fugax (4.3% and 9.8%), or Horner syndrome (2,4,5). However, palsies of ocular motor nerves in FMD are rare overall. Treatment guidelines for cerebrovascular FMD are limited because there are no randomized trials comparing management strategies in these patients (4). In general, aspirin 75–100 mg daily is recommended for stroke prophylaxis in asymptomatic individuals (4,5). In patients with arterial dissection, antiplatelet or anticoagulant agents may be used (4). Percutaneous angioplasty is indicated for symptomatic cerebrovascular FMD, and endovascular stenting may be used in dissections with recurrent ischemia despite optimal medical therapy (4,5). In the US Registry for FMD, 17.3% of patients e51 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Clinical Correspondence FIG. 2. Course of sixth cranial nerve as it traverses from the abducens nucleus through the cavernous sinus adjacent to internal carotid artery before passing through the superior orbital fissure (SOF) to innervate the lateral rectus muscle. Internal carotid artery branching into the meningohypophyseal trunk (MHT) and inferolateral trunk (ILT), which supply the abducens nerve, is labeled. with dissection underwent angioplasty with or without stent placement (2). Pseudoaneurysms resulting from a dissection are typically monitored and resolve without intervention, whereas high-risk true aneurysms are managed with endovascular coiling, flow diverting stents, or surgical clipping (4,5). The choice of therapy for the abducens nerve palsy because of an ICA dissection or pseudoaneurysm depends on the mechanism by which the function of the abducens nerve is disrupted. If mass effect from a subadventitial dissection compresses the nerve directly, the deficit may persist despite medical management (5). Given the rarity of this entity, it is unknown whether continued observation will lead to resolution of the palsy or an irreversible impairment, and vascular bypass surgery may be warranted for persistent symptoms (5). In the case of subintimal dissection, narrowing of the ICA lumen or compression of branch vessel ostia results in a transient palsy that can improve significantly over the next several months without intervention (5). In the present case, previously undiagnosed FMD likely led to a subadventitial dissection and subsequent high petrosal ICA pseudoaneurysm, which compressed blood vessels supplying the abducens nerve, leading to a transient isolated palsy that significantly improved significantly over the ensuing 2 months. STATEMENT OF AUTHORSHIP Conception and design: L. S. Ediriwickrema, A. N. Hasso; Acquisition of data: A. S. Himstead; Analysis and interpretation of data: A. S. Himstead, L. S. Ediriwickrema, A. N. Hasso. Drafting the manuscript: e52 A. S. Himstead; Revising it for intellectual content: L. S. Ediriwickrema, K. Gustafson. Final approval of the completed manuscript: L. S. Ediriwickrema, L. S. Ediriwickrema, A. N. Hasso, K. Gustafson. ACKNOWLEDGMENTS The authors thank Sarah Djafri for assistance in creating Figure 1. REFERENCES 1. Elder C, Hainline C, Galetta SL, Balcer LJ, Rucker JC. Isolated abducens nerve palsy: update on evaluation and diagnosis. Curr Neurol Neurosci Rep. 2016;16:69. 2. Kadian-Dodov D, Gornik HL, Gu X, Froehlich J, Bacharach JM, Chi YW, Gray BH, Jaff MR, Kim ES, Mace P, Sharma A, KlineRogers E, White C, Olin JW. Dissection and aneurysm in patients with fibromuscular dysplasia: findings from the U.S. Registry for FMD. J Am Coll Cardiol. 2016;68:176–185. 3. English SW, Passe TJ, Lindell EP, Klaas JP. Multiple cranial neuropathies as a presentation of spontaneous internal carotid artery dissection: a case report and literature review. J Clin Neurosci. 2018;50:129–131. 4. 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