Introduction The intracranial arterial vasculature has numerous anatomical variants, which vary from largely benign to having remarkable clinical significance. A lesser known variant can be described as a bulbous configuration of the basilar artery tip, also referred to as a ‘shield-like appearance’ or ‘junctional dilatation.’ However, this variant’s significance and standard nomenclature have not been described in the literature to as of yet.

Methods Retrospective query of radiology imaging reports was performed for vascular specific imaging of the head (CTA/MRA/cerebral angiography), which included descriptions related to the basilar artery anatomical variant from 2005–2019 at a single institution. Key-word search descriptions included ‘bulbous tip’, ‘junctional dilatation’, and ‘shield-like’ in addition to ‘basilar.’ Imaging studies were individually scrutinized for true presence of the basilar variant, and were assessed for any change in appearance over time. Primary data-points collected included length of time between imaging studies with stable imaging appearance, basilar artery aneurysm incidence, other intracranial aneurysm incidence, and frequency of subarachnoid hemorrhage associated with aneurysm rupture.

Results Initial query yielded 345 imaging reports which included the basilar artery search terms in question. Fifty-two patients were found to have multiple vascular exams of the head on different occasions, and 50 of these were confirmed to have the typical appearance of the basilar shield variant present on imaging. Risks factors of cerebral aneurysm development were variably present in this cohort, including hypertension, history of smoking, family history of cerebral aneurysms, connective tissue disease, and autosomal dominant polycystic kidney disease. On review of follow-up imaging studies, the basilar shield variant was found to be stable in appearance over time in all 50 patients, without interval expansion or basilar tip aneurysm development. Interval follow-up ranged from 39 to 4,664 days, with a mean follow-up length of 3.83 years, and total patient-years of follow-up of 192. More than half (54%) of this patient cohort demonstrated concurrent cerebral arterial aneurysms. Three patients were found to have history of subarachnoid hemorrhage related to cerebral aneurysm rupture, however none of these cases were related to the basilar artery. No adverse neurological events were found which could be attributed to the basilar shield variant.

Conclusion The bulbous configuration of the basilar artery tip dubbed the ‘basilar shield’ is a benign, normal variant of the posterior intracranial arterial circulation. Further studies may be necessary to determine if there is any association with cerebral aneurysms, but the variant itself is innocuous. It is important for both radiologists and clinicians to recognize its characteristic appearance and benign nature. True basilar tip aneurysms have a high morbidity and mortality in rupture, and confusion with the basilar shield may lead to unnecessary work-up, imaging, and/or treatment.

Disclosures K. McElroy: None. W. Brinjiki: None.