椎基底動脈之異常將導致後顱窩循環之對半分支，這類的患者並不多見。回溯2008 年至2010 年
間，由作者所安排之飛行時間效應血管磁振造影之96 名患者，發現後顱窩循環對半分支者有8 名(男5，女3)，發現率為8.3%(8 of 96)，平均年齡54.8 歲。臨床症狀主要以合併陣發性頭暈為主(75%，n=6)，其次為慢性緊縮型頭痛、典型預兆偏頭痛性頭痛、姿態性搏動性耳鳴與單側漿液性中耳炎。影像學發現以一側椎動脈終止於同側後下小腦動脈為主(87.5%，n=7)，大多數(62.5%，n=5)會合併基底動脈彎曲。後顱窩循環之對半分支目前尚無適切的治療方式，僅能針對臨床合併症狀給予保守治療，是否會增加後顱窩循環缺血性中風的風險？未來尚需更多的研究與討論。

The anomaly of vertebrobasilar artery can contribute to the dichotomy of posterior circulation, which is uncommon. From 2008 to 2010, 96 patients receiving time-of-flight magnetic resonance angiogram were enrolled in this study retrospectively. The incidence of the dichotomy of posterior circulation was 8.3%(8 of 96).These 8 patients (5 males and 3 females) averaged 54.8 years old. Their major symptom was episodic dizziness
(75%, 6 out of 8), followed by chronic tension-headache, typical migrainous headache with aura, positional pulsatile tinnitus and unilateral serous otitis media. The image study showed that 7 patients out of 8(87.5%) had one vertebral artery terminated in the ipsilateral posterior inferior cerebellar artery, and 5 out of 8(62.5%) were
accompanied by a tortuous basilar artery. There hasn’t yet been any good therapy for the dichotomy of posterior circulation. Symptom control is recommended. The risk of ischemic stroke of posterior circulation should be addressed.