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Are the Variants of the Circle of Willis Determined by Genetic or Environmental Factors? Results of a Twin Study and Review of the Literature

  • Bianka Forgo (a1), Adam Domonkos Tarnoki (a1) (a2), David Laszlo Tarnoki (a1) (a2), Daniel Tamas Kovacs (a1), Laszlo Szalontai (a1), Aliz Persely (a1), Anita Hernyes (a1), Marcell Szily (a1), Levente Littvay (a3), Emanuela Medda (a4), Adam Szabo (a5), Lajos Rudolf Kozak (a5), Gabor Rudas (a5), Attila Sas (a6), Marianna Sepsi (a7), Laszlo Kostyal (a7) and Csaba Olah (a8) (a9)...


Background: Anatomic variants of the circle of Willis (CW) are commonly observed in healthy subjects. Genetic and environmental factors influencing these variants remain unclear. Our aim was to assess the genetic and environmental background affecting variant CW phenotypes. Methods: A total of 122 adult healthy twins from the Hungarian Twin Registry (39 monozygotic (MZ) and 22 dizygotic (DZ) pairs, average age 49.7 ± 13.4 years) underwent Time-of-Flight magnetic resonance angiography and transcranial Doppler sonography. We investigated the anterior and posterior CW according to morphological categories. Prevalence and concordance rates of CW variants were calculated. MZ twins discordant for CW variants were analyzed for cardiovascular risk factors and altered blood flow. Results: Complete CW (45.0%) and bilaterally absent posterior communicating artery (PCoA) (22.5%) were the most prevalent variants in the anterior and posterior CW, respectively. There was no significant difference regarding the prevalence of variants across zygosity except for bilaterally hypoplastic PCoA (p = .02). DZ concordance was higher compared to MZ twins regarding morphological categories of the CW. Cardiovascular risk factors were not significantly associated with variant CW in MZ twins discordant to CW morphology. Flow parameters did not differ significantly among MZ twins discordant to CW variants. Conclusion: CW variants may not be determined by substantial genetic effects and are not influenced by altered blood flow in healthy individuals. Further investigations are needed to identify potential environmental factors affecting these variants.


Corresponding author

address for correspondence: Bianka Forgo, Department of Radiology, Semmelweis University, 78/a Üllői street, Budapest H-1082, Hungary. E-mail:


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This paper was presented at the Twins 2017 Congress, Madrid, Spain, on November 17, 2017, which is the Joint 4th World Congress on Twin Pregnancy and 16th Congress of the International Society of Twin Studies (ISTS).



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