Accessory anterior cerebral arteryに発生した前大脳動脈瘤の2症例--文献的考察と診断における留意点

Two Cases of Anterior Cerebral Artery Aneurysm Associated with Accessory Anterior Cerebral Artery: Review of the Literature and Points of Diagnosis

Ⅰ．はじめに

　前交通動脈および前大脳動脈（anterior cerebral artery：ACA）は比較的奇形が発生しやすい血管系といわれている．そのなかでもaccessory anterior cerebral artery（AccACA）は前交通動脈から直接分枝し，大脳半球内側面を支配するACAの血管破格であり，通常の2本のACAに加え前交通動脈から大きな3本目の枝があるようにみえることからthird ACA，triple ACA，median ACAなどと呼ばれることもある^1,6,12)．AccACAに発生した動脈瘤の報告は稀とされ，以前筆者らは1例の報告をしたが⁸⁾，新たに2例を経験した．自験例を含めこれまでに報告された全11例のAccACAに発生した動脈瘤についての特徴および診断における留意点について述べる．

　The accessory anterior cerebral artery（AccACA）is an anomalous vessel arising from the anterior communicating artery. Although AccACA is not particularly rare, aneurysms arising from the AccACA is extremely rare. Here, we report two cases of unruptured AccACA aneurysms. Patient 1 was a 58-year-old woman with an unruptured distal AccACA aneurysm. Magnetic resonance imaging and three-dimensional computed tomography angiography（3D-CTA）demonstrated a left middle cerebral artery aneurysm that was subsequently clipped successfully by direct surgery. No aneurysm was detected in the distal anterior cerebral artery（ACA）due to the narrow imaging range at that time. Postoperatively, an aneurysm of the distal ACA was incidentally identified on 3D-CTA. This AccACA aneurysm was also clipped by direct surgery about 5 months later, and the patient was discharged without any neurological deficits. Patient 2 was a 46-year-old woman with an aneurysm at the proximal portion of the AccACA. Since the aneurysm was small and patient was asymptomatic, the observation-approach was selected. In introducing these cases, we discuss AccACA aneurysms and the process of diagnosis. Aneurysm can arise over the entire length of the ACA, from the anterior communicating artery to the peripheral portion, particularly the supracallosal portion, so observation and imaging of the peripheral region is important in cases where an AccACA is present.