Congenital Absence of the Right A1 Segment and the Anterior Communicating Artery: A Previously Unreported Variant of the Circle of Willis

Congenital Absence of the Right A1 Segment and the Anterior Communicating Artery: A Previously Unreported Variant of the Circle of Willis | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Congenital Absence of the Right A1 Segment and the Anterior Communicating Artery: A Previously Unreported Variant of the Circle of Willis Isabel Echevarría Frutos, Gutenberg Carlos Navarro Zambrano, Amaury León Sosa, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8272687/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Background Variations of the Circle of Willis are common, but the combination of complete agenesis of a unilateral anterior cerebral artery A1 segment and absence of the anterior communicating artery (AComA) is exceptionally rare. Recognition of such patterns is essential for accurate radiologic interpretation and neurosurgical planning. Methods We describe a congenital CoW variant detected by brain MRI and TOF-MRA in an 18-year-old woman. Results Imaging demonstrated complete agenesis of the right A1 segment and total absence of the AComA. The left A1 was dominant and supplied both anterior hemispheric territories. No aneurysms, arteriovenous malformations, or stenoses were present. The patient remains neurologically asymptomatic at long-term follow-up. Conclusion This configuration complete unilateral A1 agenesis together with total absence of the AComA has not been previously described in contemporary angiographic literature. Awareness of this variant is important for preventing misinterpretation as arterial occlusion and for planning endovascular or surgical procedures. Circle of Willis anterior cerebral artery A1 agenesis anterior communicating artery anatomical variation MR angiography Figures Figure 1 Figure 2 Introduction The Circle of Willis (CoW) plays a key role in collateral cerebral perfusion. Although traditionally represented as a complete, symmetrical arterial polygon, anatomical studies consistently show that a classical configuration is present in only a minority of individuals 1 . Variations involving the anterior cerebral artery (ACA) A1 segment and the anterior communicating artery (AComA) are among the most frequent 2 . Most reported anomalies involve hypoplasia or aplasia of a single A1 segment, or morphological variants of the AComA such as duplication, fenestration, or hypoplasia 3 ,4 . However, the combined complete agenesis of one A1 segment with total absence of the AComA is extremely uncommon in modern angiographic literature. Only isolated case reports describe similar, though not identical, variants, often involving hypoplastic rather than absent segments 5 . Here, we present a well-documented case of congenital agenesis of the right A1 and complete absence of the AComA, confirmed by TOF-MRA, thereby expanding the known morphologic spectrum of anterior CoW anatomy. Case Presentation An 18-year-old woman underwent brain MRI and TOF-MRA after a single transient loss-of-consciousness episode. Neurological examination, EEG, and laboratory findings were unremarkable, and she had no vascular risk factors or relevant medical history. Imaging demonstrated complete congenital absence of the right ACA A1 segment and total absence of the anterior communicating artery, with the left A1 dominant and supplying both anterior hemispheric territories. No aneurysms, arteriovenous malformations, stenoses, or acute/chronic ischemic lesions were observed. The patient remains asymptomatic after nearly three years of follow-up (Fig. 1 , Fig. 2 ). An 18-year-old female patient without significant personal pathological history, except sulfa allergy and allergic rhinitis. Family history includes cerebral cavernoma and absence seizures in her father. The patient reported a single syncope episode occurring upon sudden standing after lying down, with brief loss of consciousness (approximately 1 minute), spontaneous recovery, and no subsequent neurological focality. She also reported prior exposure to solvents (thinner and paint) and moderate alcohol consumption. Initial evaluation by neurology and neurosurgery included electroencephalogram (normal during wakefulness and light sleep) and laboratory studies within normal limits. Consideration was given to performing contrast neuroimaging studies, which were authorized after allergy evaluation. Brain magnetic resonance imaging (MRI) and time-of-flight (TOF) MR angiography showed absence of the right anterior cerebral artery (ACA A1) and complete absence of the anterior communicating artery (ACoA) (Fig. 1 ). The left ACA was dominant, supplying both hemispheres. The rest of the Circle of Willis had a normal configuration without evidence of aneurysms, arteriovenous malformations, or other pathological findings (Fig. 2 ). The patient remained asymptomatic without new syncope episodes or neurological deficits. Cardiology evaluation diagnosed classical vasovagal syncope. Follow-up included isolated arterial hypertension and mild dyslipidemia, without pharmacological treatment requirement. Complementary care was provided by ENT and ophthalmology for unrelated findings. At 2 years and 11 months follow-up, the patient remains asymptomatic. Discussion The overall prevalence of Circle of Willis variants is high; complete circles are observed in fewer than 40% of individuals in most imaging series ( 1 , 2 ). Hypoplasia or aplasia of the A1 segment occurs in 1–13% depending on the criteria and imaging modality ( 3 ), and anterior communicating artery (AComA) anomalies—including duplication, fenestration, hypoplasia, or absence are also frequently reported 4 . However, complete agenesis of a single A1 segment combined with total absence of the AComA is rarely documented. Previously published cases typically involve hypoplastic or plexiform ACA segments rather than true agenesis, or they report atypical midline anterior channels such as azygos or infraoptic ACAs instead of a true absent AComA ( 5 – 7 ), making the present configuration distinctly uncommon. The absence of the AComA eliminates the main anterior collateral pathway, increasing dependence on the contralateral A1 segment for bilateral anterior cerebral perfusion. In cerebrovascular interventions such as carotid endarterectomy, stenting, aneurysm repair, or acute ischemic stroke management, the functional integrity of the A1–AComA network is critical for clinical outcomes ( 8 ). Yamashita et al. demonstrated that the absence or non-functioning of the AComA on selective MRA is associated with a higher risk of new ischemic lesions after carotid revascularization ( 9 ). Accurate recognition of congenital absence versus acute occlusion is therefore essential. From an embryological perspective, the ACA develops through the fusion and remodeling of longitudinal neural arteries interconnected by primitive olfactory and anterior carotid plexuses. Failure of formation or persistence of embryonic channels may lead to agenesis of the A1 segment or absence of the AComA. When compensatory hypertrophy of the contralateral A1 occurs, as in this case, patients may remain clinically asymptomatic ( 10 ). Conclusion We describe a rare congenital anatomical variation of the CoW characterized by complete agenesis of the right A1 segment and total absence of the AComA. Recognition of such variants prevents misdiagnosis of arterial occlusion and supports safer neurovascular and surgical planning. Declarations Ethics approval and consent to participate: Not applicable. Consent to Publish declaration Written informed consent was obtained from the patient’s legal guardian for the publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor of this journal. Competing interests: The authors declare that they have no competing interests. Conflict of interest: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Funding: No funding was received for this study. Author Contribution All authors contributed to data acquisition, manuscript drafting, and approval of the final version Acknowledgement We acknowledge Universidad Israel for their valuable contribution to this work. Availability of data and materials: Not applicable. References Alpers BJ, Berry RG (1963) Circle of Willis in cerebral vascular disorders. Arch Neurol Psychiatry 90:12–24 Krzyżewski RM et al (2014) Anatomical variations of the anterior communicating artery complex. Acta Neurochir (Wien) 156:561–569 Kapoor K, Singh B, Dewan LI (2008) Variations in the configuration of the circle of Willis. Anat Sci Int 83:96–106 Gunnal SA et al (2013) Anatomical variations of the anterior cerebral artery–anterior communicating artery complex. Clin Anat 26:778–791 Nayak B, Guru A, Shetty SD, Rao S (2013) Hypoplastic plexiform right anterior cerebral artery and absence of anterior communicating artery: a case report. Forensic Med Anat Res 1:47–49 Lakhotia M et al (2016) A case of anterior cerebral artery (A1) segment hypoplasia presenting with neurologic deficit. J Clin Imaging Sci 6:1–4 Shatri J et al (2019) Morphological variations of the anterior part of the circle of Willis. Folia Morphol (Warsz) 78:703–708 Liebeskind DS (2003) Collaterals in acute stroke. Stroke 34:2279–2284 Yamashita S et al (2022) Absence of the anterior communicating artery on selective MRA is associated with new ischemic lesions after carotid revascularization. AJNR Am J Neuroradiol 43:1124–1130 Padget DH (1948) The development of the cranial arteries in the human embryo. Contrib Embryol Carnegie Inst 32:205–262 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8272687","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":555867839,"identity":"43ec8f21-4890-4ce0-b641-7ef7fe1f7ece","order_by":0,"name":"Isabel Echevarría 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12:54:32","extension":"html","order_by":14,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":33839,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8272687/v1/9038d10d92838b2bcc0e7c65.html"},{"id":97705970,"identity":"48261813-26dd-4e6c-a130-22abb44a833c","added_by":"auto","created_at":"2025-12-08 12:54:25","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":159423,"visible":true,"origin":"","legend":"\u003cp\u003eBrain magnetic resonance imaging in anatomical sequences in the (a) coronal, (b) sagittal, and (c) axial planes, showing no structural abnormalities. The cerebral parenchyma, midline structures, and ventricular system display preserved morphology. These images correspond to the initial structural evaluation and complement the MR angiographic study used to identify the Circle of Willis variant.\u003c/p\u003e","description":"","filename":"FIGURE1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8272687/v1/00bc6b9fb5528129a829aaa9.jpeg"},{"id":97705928,"identity":"23ba5474-325e-4926-9c21-e36e9d547d71","added_by":"auto","created_at":"2025-12-08 12:54:19","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":319377,"visible":true,"origin":"","legend":"\u003cp\u003eTime-of-flight magnetic resonance angiography in different planes demonstrating congenital absence of the right A1 segment of the anterior cerebral artery and absence of the anterior communicating artery (arrows). The left A1 segment is dominant and supplies both anterior cerebral artery territories. No aneurysms or other vascular malformations are identified.\u003c/p\u003e","description":"","filename":"FIGURE2.jpeg.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8272687/v1/21326e58566b6c3c3ab4702d.jpg"},{"id":102182046,"identity":"a2677aff-5fc5-4a86-b02a-6c80b8fbf82b","added_by":"auto","created_at":"2026-02-09 07:27:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":910866,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8272687/v1/eea6d754-6c54-4a1e-9cfc-3094910122c7.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Congenital Absence of the Right A1 Segment and the Anterior Communicating Artery: A Previously Unreported Variant of the Circle of Willis","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Circle of Willis (CoW) plays a key role in collateral cerebral perfusion. Although traditionally represented as a complete, symmetrical arterial polygon, anatomical studies consistently show that a classical configuration is present in only a minority of individuals\u003csup\u003e1\u003c/sup\u003e. Variations involving the anterior cerebral artery (ACA) A1 segment and the anterior communicating artery (AComA) are among the most frequent\u003csup\u003e2\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eMost reported anomalies involve hypoplasia or aplasia of a single A1 segment, or morphological variants of the AComA such as duplication, fenestration, or hypoplasia\u003csup\u003e3 ,4\u003c/sup\u003e. However, the \u003cb\u003ecombined complete agenesis of one A1 segment with total absence of the AComA\u003c/b\u003e is extremely uncommon in modern angiographic literature. Only isolated case reports describe similar, though not identical, variants, often involving hypoplastic rather than absent segments\u003csup\u003e5\u003c/sup\u003e.\u003c/p\u003e\u003cp\u003eHere, we present a well-documented case of congenital agenesis of the right A1 and complete absence of the AComA, confirmed by TOF-MRA, thereby expanding the known morphologic spectrum of anterior CoW anatomy.\u003c/p\u003e"},{"header":"Case Presentation","content":"\u003cp\u003eAn 18-year-old woman underwent brain MRI and TOF-MRA after a single transient loss-of-consciousness episode. Neurological examination, EEG, and laboratory findings were unremarkable, and she had no vascular risk factors or relevant medical history. Imaging demonstrated complete congenital absence of the right ACA A1 segment and total absence of the anterior communicating artery, with the left A1 dominant and supplying both anterior hemispheric territories. No aneurysms, arteriovenous malformations, stenoses, or acute/chronic ischemic lesions were observed. The patient remains asymptomatic after nearly three years of follow-up (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eAn 18-year-old female patient without significant personal pathological history, except sulfa allergy and allergic rhinitis. Family history includes cerebral cavernoma and absence seizures in her father. The patient reported a single syncope episode occurring upon sudden standing after lying down, with brief loss of consciousness (approximately 1 minute), spontaneous recovery, and no subsequent neurological focality. She also reported prior exposure to solvents (thinner and paint) and moderate alcohol consumption. Initial evaluation by neurology and neurosurgery included electroencephalogram (normal during wakefulness and light sleep) and laboratory studies within normal limits. Consideration was given to performing contrast neuroimaging studies, which were authorized after allergy evaluation.\u003c/p\u003e\u003cp\u003eBrain magnetic resonance imaging (MRI) and time-of-flight (TOF) MR angiography showed absence of the right anterior cerebral artery (ACA A1) and complete absence of the anterior communicating artery (ACoA) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The left ACA was dominant, supplying both hemispheres. The rest of the Circle of Willis had a normal configuration without evidence of aneurysms, arteriovenous malformations, or other pathological findings (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The patient remained asymptomatic without new syncope episodes or neurological deficits. Cardiology evaluation diagnosed classical vasovagal syncope. Follow-up included isolated arterial hypertension and mild dyslipidemia, without pharmacological treatment requirement. Complementary care was provided by ENT and ophthalmology for unrelated findings. At 2 years and 11 months follow-up, the patient remains asymptomatic.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe overall prevalence of Circle of Willis variants is high; complete circles are observed in fewer than 40% of individuals in most imaging series (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e). Hypoplasia or aplasia of the A1 segment occurs in 1\u0026ndash;13% depending on the criteria and imaging modality (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e), and anterior communicating artery (AComA) anomalies\u0026mdash;including duplication, fenestration, hypoplasia, or absence are also frequently reported\u003csup\u003e4\u003c/sup\u003e. However, complete agenesis of a single A1 segment combined with total absence of the AComA is rarely documented. Previously published cases typically involve hypoplastic or plexiform ACA segments rather than true agenesis, or they report atypical midline anterior channels such as azygos or infraoptic ACAs instead of a true absent AComA (\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e), making the present configuration distinctly uncommon.\u003c/p\u003e\u003cp\u003eThe absence of the AComA eliminates the main anterior collateral pathway, increasing dependence on the contralateral A1 segment for bilateral anterior cerebral perfusion. In cerebrovascular interventions such as carotid endarterectomy, stenting, aneurysm repair, or acute ischemic stroke management, the functional integrity of the A1\u0026ndash;AComA network is critical for clinical outcomes (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e). Yamashita et al. demonstrated that the absence or non-functioning of the AComA on selective MRA is associated with a higher risk of new ischemic lesions after carotid revascularization (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e). Accurate recognition of congenital absence versus acute occlusion is therefore essential.\u003c/p\u003e\u003cp\u003eFrom an embryological perspective, the ACA develops through the fusion and remodeling of longitudinal neural arteries interconnected by primitive olfactory and anterior carotid plexuses. Failure of formation or persistence of embryonic channels may lead to agenesis of the A1 segment or absence of the AComA. When compensatory hypertrophy of the contralateral A1 occurs, as in this case, patients may remain clinically asymptomatic (\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe describe a rare congenital anatomical variation of the CoW characterized by complete agenesis of the right A1 segment and total absence of the AComA. Recognition of such variants prevents misdiagnosis of arterial occlusion and supports safer neurovascular and surgical planning.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate:\u003c/strong\u003e\u003cp\u003eNot applicable.\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eConsent to Publish declaration\u003c/strong\u003e\u003cp\u003eWritten informed consent was obtained from the patient\u0026rsquo;s legal guardian for the publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.\u003c/p\u003e\u003ch2\u003eCompeting interests:\u003c/h2\u003e\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\u003ch2\u003eConflict of interest:\u003c/h2\u003e\u003cp\u003eThe authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.\u003c/p\u003e\u003ch2\u003eFunding:\u003c/h2\u003e\u003cp\u003eNo funding was received for this study.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAll authors contributed to data acquisition, manuscript drafting, and approval of the final version\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe acknowledge Universidad Israel for their valuable contribution to this work.\u003c/p\u003e\u003ch2\u003eAvailability of data and materials:\u003c/h2\u003e\u003cp\u003eNot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAlpers BJ, Berry RG (1963) Circle of Willis in cerebral vascular disorders. Arch Neurol Psychiatry 90:12\u0026ndash;24\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKrzyżewski RM et al (2014) Anatomical variations of the anterior communicating artery complex. Acta Neurochir (Wien) 156:561\u0026ndash;569\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eKapoor K, Singh B, Dewan LI (2008) Variations in the configuration of the circle of Willis. Anat Sci Int 83:96\u0026ndash;106\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eGunnal SA et al (2013) Anatomical variations of the anterior cerebral artery\u0026ndash;anterior communicating artery complex. Clin Anat 26:778\u0026ndash;791\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eNayak B, Guru A, Shetty SD, Rao S (2013) Hypoplastic plexiform right anterior cerebral artery and absence of anterior communicating artery: a case report. Forensic Med Anat Res 1:47\u0026ndash;49\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLakhotia M et al (2016) A case of anterior cerebral artery (A1) segment hypoplasia presenting with neurologic deficit. J Clin Imaging Sci 6:1\u0026ndash;4\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eShatri J et al (2019) Morphological variations of the anterior part of the circle of Willis. Folia Morphol (Warsz) 78:703\u0026ndash;708\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eLiebeskind DS (2003) Collaterals in acute stroke. Stroke 34:2279\u0026ndash;2284\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eYamashita S et al (2022) Absence of the anterior communicating artery on selective MRA is associated with new ischemic lesions after carotid revascularization. AJNR Am J Neuroradiol 43:1124\u0026ndash;1130\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003ePadget DH (1948) The development of the cranial arteries in the human embryo. Contrib Embryol Carnegie Inst 32:205\u0026ndash;262\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Circle of Willis, anterior cerebral artery, A1 agenesis, anterior communicating artery, anatomical variation, MR angiography","lastPublishedDoi":"10.21203/rs.3.rs-8272687/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8272687/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground\u003c/h2\u003e\u003cp\u003eVariations of the Circle of Willis are common, but the combination of complete agenesis of a unilateral anterior cerebral artery A1 segment and absence of the anterior communicating artery (AComA) is exceptionally rare. Recognition of such patterns is essential for accurate radiologic interpretation and neurosurgical planning.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e\u003cp\u003eWe describe a congenital CoW variant detected by brain MRI and TOF-MRA in an 18-year-old woman.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e\u003cp\u003eImaging demonstrated complete agenesis of the right A1 segment and total absence of the AComA. The left A1 was dominant and supplied both anterior hemispheric territories. No aneurysms, arteriovenous malformations, or stenoses were present. The patient remains neurologically asymptomatic at long-term follow-up.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e\u003cp\u003eThis configuration complete unilateral A1 agenesis together with total absence of the AComA has not been previously described in contemporary angiographic literature. Awareness of this variant is important for preventing misinterpretation as arterial occlusion and for planning endovascular or surgical procedures.\u003c/p\u003e","manuscriptTitle":"Congenital Absence of the Right A1 Segment and the Anterior Communicating Artery: A Previously Unreported Variant of the Circle of Willis","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-08 12:53:48","doi":"10.21203/rs.3.rs-8272687/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"fdadedd5-c437-4f31-a936-2518a8f9d010","owner":[],"postedDate":"December 8th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-09T07:27:03+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-08 12:53:48","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8272687","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8272687","identity":"rs-8272687","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}