A 56-year-old woman with Down syndrome presented with right-sided weakness and dysarthria, and was found on CT/CTA to have a left middle cerebral artery infarct secondary to moyamoya disease. Her left posterior inferior cerebellar artery (PICA) was fed both by the left vertebral artery, and the left ascending pharyngeal artery (APA), with a variant origin from the internal carotid artery (ICA), then passing through the jugular foramen (Figure 1). Her right PICA originated exclusively from her right occipital artery, also via the jugular foramen (Figure 2). The left vertebral artery originated directly from the aortic arch, whereas the right vertebral artery originated from the brachiocephalic trunk. In addition, she had a trifurcated anterior cerebral artery (ACA), and just prior to this trifurcation, her left ACA was partially supplied by the left ICA, via a superior hypophyseal artery. This case is noteworthy for several reasons. First, though it is exceedingly rare to have the PICA supplied by the jugular branch of the APA, this is the first reported case with an ICA origin of that APA.^{Reference Fujihara, Takahara, Katsuta, Takemoto, Higashi and Inoue1,Reference Uchino and Suzuki2} The fact that both PICAs in this patient originate from the anterior circulation should remind clinicians that in unexplained posterior circulation infarctions, vascular anatomy should be explored, as carotid-vertebrobasilar anastomoses such as these are rare, but possible. Lastly, the conjunction of moyamoya disease and anomalies of the vertebrobasilar system in a patient with Down syndrome raises interesting questions about the influence of trisomy 21 on the developing vasculature. Connections from the APA to the vertebrobasilar system are hypothesized to result from a lack of regression of an embryological anastomosis, in line with the more common persistent trigeminal and persistent hypoglossal arteries.^{Reference Fujihara, Takahara, Katsuta, Takemoto, Higashi and Inoue1} Patients with moyamoya disease have a significantly higher rate of persistent carotid-vertebrobasilar anastomoses than the general population,^{Reference Uchino, Saito, Takahashi, Kurita and Ishihara3} and are also 26 times more likely to have Down syndrome.^{Reference Kainth, Chaudhry, Kainth, Suri and Qureshi4} Correspondingly, patients with Down syndrome have significantly higher levels of moyamoya disease, and are more than 10 times as likely as the general population to have abnormalities of the Circle of Willis^{Reference Mito and Becker5} and vertebral arteries.^{Reference Rathore and Sreenivasan6} Several genes on chromosome 21 are known to affect angiogenesis, namely collagen XIII/endostatin (COL18A1), DYRK1A, and Down syndrome candidate region 1 (DSCR1), possibly through inhibition of VEGF activity.^{Reference Greene, Kim, Rogers, Fishman, Olsen and Mulliken7} Whether additional copies of these genes are responsible for the anomalous vascular development seen in Down syndrome, in turn predisposing to the development of moyamoya disease, could benefit from further exploration.

Figure 1: Left ICA to PICA via the APA.

Origin of the left APA from the left ICA (arrow) with a branch passing through the jugular foramen to supply the left PICA (arrowhead).

Figure 2: Right ECA to PICA via the occipital artery.

Origin of the right occipital artery from the right external carotid artery (arrow) with a branch passing through the jugular foramen to become the right PICA (arrowhead).