Syncope Associated with a Congenital Variant of the Circle of Willis: Absence of the Right Anterior Cerebral Artery and Anterior Communicating Artery

Background: Syncope is a transient loss of consciousness caused by global cerebral hypoperfusion. Although most commonly attributed to vasovagal or cardiac etiologies, anatomical variants of the Circle of Willis may alter cerebral hemodynamics and, in rare cases, contribute to syncope. Case presentation: We report the case of an 18-year-old female patient with no significant personal medical history and a family history of cerebral cavernoma. She experienced a single syncopal episode upon sudden standing, with brief loss of consciousness lasting approximately one minute and no subsequent neurological deficits. Laboratory tests and electroencephalogram were within normal limits. Brain magnetic resonance imaging (MRI) and time-of-flight magnetic resonance angiography (TOF-MRA) revealed absence of the right anterior cerebral artery (ACA) A1 segment and complete absence of the anterior communicating artery (ACoA). The left ACA was dominant, supplying both cerebral hemispheres. Cardiac causes were excluded, and a diagnosis of classic vasovagal syncope was established. The patient has remained asymptomatic after nearly three years of follow-up. Discussion: Congenital variants of the Circle of Willis are common anatomical findings but rarely linked to syncope. In this case, the bilateral absence of key vascular segments may have reduced cerebrovascular reserve during orthostatic stress, precipitating the syncopal event. This case underscores the importance of advanced neuroimaging techniques in the evaluation of atypical syncope. Conclusions: This report highlights the clinical significance of Circle of Willis anatomical variants as potential contributors to syncope when other causes have been excluded. A multidisciplinary approach and the use of appropriate imaging modalities are critical for accurate diagnosis.