LegendsAxial DWI shows acute infarct in left MCA deep watershed territory. MRA and CTA (B, C) show left distal cervical, petrous, cavernous, and clinoid ICA occlusion and diffuse intracranial atheromatosis. There is a left carotid-basilar anastomosis, with complete absence of the cervical vertebral artery and a hypoplastic right vertebral artery. CTA (D, E) demonstrates the carotid-vertebral anastomosis coursing through the left hypoglossal canal (arrow in D, E).
Three primitive anastomoses occur between the carotid and vertebrobasilar systems: trigeminal artery, hypoglossal artery, and proatlantal intersegmental artery. Failure of obliteration of these results in a persistent embryonic artery. The “otic” artery has been reported as a fourth potential anastomosis, but its existence has been refuted.原始颈动脉和椎基底动脉的吻合包括三种：三叉动脉、舌下动脉、寰前节间动脉。这三种动脉发育过程中管腔闭合异常，即形成永存胚胎动脉。耳动脉曾被报道为第四个潜在的吻合，但这一理论已经被推翻。
Persistent hypoglossal artery (PPHA) is the second most common carotid-vertebrobasilar anastomosis, with an estimated prevalence of 0.02%–0.1%, with 65% of them being left-sided.永存舌下动脉（PPHA）是第二常见的颈动脉-椎基底动脉异常吻合，发病率约为0.02%-1%，其中约65%发生在左侧。
The PPHA originates from the cervical part of the internal carotid artery at the level of C-1 to C-3 vertebral bodies, penetrates the hypoglossal canal, and joins the ipsilateral verterbal artery. The remnant of the PPHA in the adult is the hypoglossal artery (branch of the neuromeningeal trunk of the ascending pharyngeal artery).PPHA常发生于颈内动脉约C1-C3椎体水平，通过舌下神经管吻合与通侧椎动脉。成年人PPHA的残余，即为舌下动脉（咽升动脉的脑膜支）。
Clinical PresentationPPHA is commonly an incidental finding, but it can present with XI and XII nerve compression. A higher incidence of intracranial aneurysms has been reported, especially in the basilar artery. In the setting of carotid bifurcation disease, posterior circulation embolic infarcts and vertebral arterial steal resulting in watershed infarcts may occur.
Key Diagnostic Feature 诊断要点
the artery leaves the ICA as an extra cranial branch;ICA走行异常，似乎沿颅外方向走行。
the artery passes through the hypoglossal canal;动脉穿过舌下神经管。
the ipsilateral VA is absent同侧椎动脉缺如。
DDx Compensatory hypoglossal artery hypertrophy due cervicial arterial occlusive disease.