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.. Variants of the ACOMM are common variants in the CoW with a duplication prevalence BMS354825 of 18% and fenestrations in up to 21%.60 The ACOMM may not appear in angiography, but we cannot be sure about its real absence. Definitive absence of the ACOMM has been found in 5% of surgical dissections.61,62 Basilar artery One of the most common embryological variants of the BA is its fenestration, also known as segmental duplication (Figure 3B, C). In the literature, the prevalence of BA fenestrations ranges from 3.9% in autopsy reports to 1.1% in angiographic reports.34,37 The fenestration usually occurs more proximal than distal, closer to the vertebrobasilar junction.38 The morphological similarities between embryonic and postnatal specimens of BA fenestrations suggest that fenestrations are inborn variants and only rarely undergo further modifications in the adult.63 Vertebral artery In a large surgical series, variants involving the origin of the VA were not uncommon.64,65 The VA normally arises from the upper, posterior surface of subclavian artery 0.5 to 2.0 cm medial to thyrocervical trunk. Some of the variants of VA origin include a common trunk with the thyrocervical trunk (3.0%), an aortic origin of the left VA (2.5%, arising between the left GSK1349572 CCA and the left subclavian artery), a right VA arising at the site of bifurcation of the innominate artery into right subclavian and right CCA (1.1%.), and a right VA as a direct branch of the right CCA (0.3%). The segment between the VA origin and the site of entrance to the foramina of the transverse processes is called the V1 segment. In the majority of the cases, the V1 segment ends at the 6th vertebra (87.5%), but occasionally it ends at the 5th (6.6%), the 7th (5.4%) or less frequently the 4th vertebra (1.3%). Other VA variants such as dual (i.e., accessory) VA are rare (0.7%). Posterior cerebral artery A fetal origin of the PCA is the most frequent variant involving this artery. It has been reported in between 13%-15% of the population, and it is typically described in only one PCA, but it can also occur in both PCA in up to 0.5% of the population.39,44 More commonly (in up to 10% of the cases), the P1 segment is hypoplastic, which renders a fetal-like circulation to the occipital lobes, although it is not technically a fetal PCA given the physical link of the PCOMM with the BA.27,66 Duplications of the PCA SAR1B (in either the P1 or P2 segments) are extremely rare (1.0 %).61,67 Posterior communicating artery A hypoplastic PCOMM is overall the most common arterial variant reported in the brain.43,44 Duplication of the PCOMM artery is a rare event with a prevalence of 1.8% in surgical dissection series, and absence of the PCOMM artery is even rarer (