Optic Neuropathy Due to Compression by an Ectatic Internal Carotid Artery Within the Orbital Apex

Neurovascular compression is a rare but potentially treatable cause of optic neuropathy. Although incidental contact of the cisternal optic nerve and internal carotid artery (ICA) is common, compressive optic neuropathy occurring within the orbital apex has not been comprehensively described. We report a case of intra-orbital and intracanalicular optic nerve compression due to an ectatic ICA in a patient with congenital absence of the contralateral ICA. This report describes the complementary roles of advanced neuroimaging and neuro-ophthalmologic examination in rendering the diagnosis.

Photo Essay Section Editors: Melissa W. Ko, MD Dean M. Cestari, MD Peter Quiros, MD Optic Neuropathy Due to Compression by an Ectatic Internal Carotid Artery Within the Orbital Apex Michael T. Caton Jr, MD, Amir A. Zamani, MD, Rose Du, MD, PhD, Sashank Prasad, MD Downloaded from http://journals.lww.com/jneuro-ophthalmology by BhDMf5ePHKav1zEoum1tQfN4a+kJLhEZgbsIHo4XMi0hCywCX1AWnYQp/IlQrHD3i3D0OdRyi7TvSFl4Cf3VC4/OAVpDDa8KKGKV0Ymy+78= on 05/04/2022 FIG. 1. Multimodal imaging and neuro-ophthalmologic characterization of optic neuropathy due to ICA compression: Time-of-flight MRA (A) shows complete absence of the left ICA (red arrowheads) with supply of the left ACA territory derived from anterior communicating artery complex. Note the left MCA is supplied entirely through the left posterior communicating artery. Gadolinium-enhanced T1 coronal fat-saturated MRI (B) shows decreased caliber of the right optic nerve and enhancement of the optic nerve sheath (white arrows). Asymmetrically increased fluid signal within the optic nerve sheath is seen on T2-weighted images (C). T2-weighted sequence shows the ectatic ICA exiting the orbit through the optic canal, displacing the optic nerve medially (D, white arrows). 3D cinematic volume rendered oblique sagittal image through the right orbit highlights the herniated ICA loop within the orbital apex with the ophthalmic artery origin shown by the white arrowhead (E). Coronal CTA reformation (F) shows the relationship of the optic canal (OC) and superior orbital fissure (SOF) with thinning and convex remodeling of the optic strut (red arrowheads) on the right due to the vascular loop. Funduscopic evaluation shows pallor of the optic disc in the right eye with the cup:disc ratio measuring 0.3, and normal appearance in the left eye. The visual field of the right eye was overall depressed with an indication of an inferior altitudinal defect. The visual field of the left eye was normal. OCT demonstrates severe thinning of the retinal ganglion cell layer complex in the right eye (red arrows) and normal thickness in the left eye. CTA, computed tomography angiogram; ICA, internal carotid artery;OCT, optical coherence tomography. Abstract: Neurovascular compression is a rare but potentially treatable cause of optic neuropathy. Although incidental contact of the cisternal optic nerve and internal carotid artery (ICA) is common, compressive optic neuropathy occurring within the orbital apex has not been comprehensively described. We report a case of intra-orbital and intracanalicular optic nerve compression due to an ectatic Departments of Radiology (MTC, AAZ) and Neurosurgery (RD), Brigham and Women’s Hospital, Boston, Massachusetts; and Department of Neurology (SP), Division of Neuro-Ophthalmology, Brigham and Women’s Hospital, Boston, Massachusetts. The authors report no conflicts of interest. Address correspondence to Michael T. Caton, MD, Department of Radiology and Biomedical Imaging, University of California, San Francisco, 505 Parnassus Avenue, L352, San Francisco, CA 94117; E-mail: [e-mail] Caton et al: J Neuro-Ophthalmol 2021; 41: e103-e104 ICA in a patient with congenital absence of the contralateral ICA. This report describes the complementary roles of advanced neuroimaging and neuro-ophthalmologic examination in rendering the diagnosis. Journal of Neuro-Ophthalmology 2021;41:e103–104 doi: 10.1097/WNO.0000000000000975 © 2020 by North American Neuro-Ophthalmology Society A 56-year-old man with diabetes mellitus and hypertension presented for evaluation of blurry vision in the right eye, which had progressed over 2 years. In addition, 3 weeks before presentation, the patient noted diplopia and right ptosis. Examination demonstrated 20/70 acuity, severe dyschromatopsia, relative afferent pupillary defect, and 50% e103 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited. Photo Essay reduction of adduction, elevation, and depression of the right eye. Forced duction testing was not performed. The pupils constricted from 6 to 4 mm with bright light but the reaction was more sluggish on the right. There was 4-mm ptosis of the right upper lid. Automated perimetry revealed an inferior visual field defect in the right eye. Funduscopic examination demonstrated right optic disc pallor without edema or cupping (Fig. 1 right eye). Optical coherence tomography revealed severe temporal thinning of the peripapillary retinal nerve fiber layer and severe thinning of the macular retinal ganglion cell layer in the right eye (Fig. 1 right eye). Visual function in the left eye was normal. Time-of-flight MRA showed complete absence of the left internal carotid artery (ICA) with filling of the left anterior cerebral artery through the anterior communicating artery and filling of the left middle cerebral artery through the left posterior communicating artery (Fig. 1A). MRI of the orbits confirmed mild proptosis of the right eye with peripheral gadolinium enhancement (Fig. 1B) and abnormal T2 hyperintensity of the right optic nerve sheath (Fig. 1C). There was a 9-mm inferiorly directed saccular aneurysm of the right posterior communicating artery–ICA junction. The right ICA had a markedly ectatic course with the anterior genu prolapsing into the orbital apex through the superior orbital fissure and exiting the orbital apex through the optic canal (Fig. 1D). Computed tomography angiogram (CTA) with 3D cinematic volume rendering provided excellent anatomic characterization of the ectatic ICA relative to the bony apex (Fig. 1E), and shows thinning of the optic strut and expansile remodeling of the optic canal from the vascular loop (Fig. 1E, F). The patient underwent successful clipping of the right posterior communicating artery aneurysm with no postoperative complications. The third nerve palsy subsequently resolved. Surgical decompression of the ectatic ICA by unroofing the orbit was discussed but has been deferred due to procedural risk. Symptomatic compression of the ON by the ICA is an uncommon cause of optic neuropathy. However, incidental, asymptomatic contact of the ICA and ON is frequently observed on high-resolution MRI (1). Therefore, deciphering whether visual symptoms are causally related to ICA mass effect can present a considerable diagnostic challenge. In addition to a comprehensive neuro-ophthalmologic examination, adjunct vascular imaging to characterize the anatomic relationship of vessel and nerve may be necessary. In documented cases, optic neuropathy due to ICA compression presents with typical symptoms including subacute, painless vision loss accompanied by impaired color vision and optic disc pallor (2). Cases of improved vision after surgical decompression are reported (3). Previous reports have described compression of the cisternal segment of the ON by dolichoectasia or aneurysms of the supraclinoid ICA (4). However, this report provides the first comprehensive clinical and imaging description of the intraorbital and intracanalicular segments of the ON. Jacobson described one patient presenting with subacute superior orbital fissure syndrome with bilateral elongated cavernous ICA but e104 comprehensive imaging documentation of the vessel course was not presented (2). Our case highlights the role of thincut CTA and 3D cinematic volume rendering in documenting the relationship of the vessel, clinoid process, and optic canal. The angiographic pattern of elongated anterior genu of the solitary ICA has not been previously described in cases of contralateral ICA agenesis (5,6). Congenitally absent ICA (aICA) is extremely rare with estimated prevalence of less than 0.01% (6). True agenesis, as opposed to hypoplasia of the ICA, is determined by the absence of the carotid canal in the petrous bone (5). The incidence of saccular aneurysms in aICA is much higher than the general population, reported at 67% in one case series; this predilection for aneurysm formation has been attributed to altered flow dynamics and large collateral demands of the solitary ICA (7). This increased demand in turn predisposes to dolichoectasia owing to compensatory remodeling from high wall shear stress. In summary, this report describes the first comprehensive radiologic–ophthalmologic correlation of symptomatic optic nerve compression by an ectatic ICA distal to the annulus of Zinn. Judicious use of advanced imaging techniques including MRI/MRA and CTA with 3D rendering serves as a complementary tool to the neuro-ophthalmologic examination in confirming the diagnosis. Rare patients with a solitary ICA may be predisposed to this phenomenon due to progressive dolichoectasia. STATEMENT OF AUTHORSHIP Category 1: a. Conception and design: M. T. Caton, A. A. Zamani, and S. Prasad, MD; b. Acquisition of data: R. Du and S. Prasad; c. Analysis and interpretation of data: M. T. Caton, A. A. Zamani, and S. Prasad, MD. Category 2: a. Drafting the manuscript: M. T. Caton and S. Prasad; b. Revising it for intellectual content: M. T. Caton, A. A. Zamani, R. Du, and S. Prasad. Category 3: a. Final approval of the completed manuscript: M. T. Caton, A. A. Zamani, R. Du, and S. Prasad. REFERENCES 1. 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Congenital absence of the internal carotid artery: case reports and review of the collateral circulation. Am J Neuroradiology. 2001;22:1953–1959. 7. Lee JH, Oh CW, Lee SH, Han DH. Aplasia of the internal carotid artery. Acta Neurochir (Wien). 2003;145:117–125. Caton et al: J Neuro-Ophthalmol 2021; 41: e103-e104 Copyright © North American Neuro-Ophthalmology Society. Unauthorized reproduction of this article is prohibited.