Neurological Surgery 脳神経外科38巻9号

2010年09月発行

総説

脳動脈瘤発生・増大のメカニズム

著者： 横井俊浩¹ 野崎和彦¹

所属機関： ¹滋賀医科大学医学部脳神経外科

ページ範囲：P.787 - P.793

Ⅰ．は じ め に

　破裂脳動脈瘤は年間で人口10万人あたり10人に発症し，このうち26～42％は死亡する，予後不良な疾患の1つである．そのため，近年MR angiography（MRA），CT angiography（CTA）などの普及に伴い，未破裂脳動脈瘤の発見と破裂前の予防的治療が行われている．一方，未破裂脳動脈瘤の年間破裂率は1～2％程度と報告されているが，International Study of Unruptured Intracranial Aneurysm（ISUIA）が1998年および2003年に報告した年間破裂率は極めて低く，その後の脳動脈瘤治療に大きな影響を与えた⁴²⁾．この報告では，10mm以下の脳動脈瘤をもつ者のうち破裂脳動脈瘤の既往のない例の年間破裂率は0.05％であり，対象患者や研究方法などにより破裂率にはばらつきが生じることがわかる．また，anterior circulationでくも膜下出血の既往がない7mm以下の脳動脈瘤の年間破裂率は0％であるのに対して，実際の臨床では7mm以下の破裂脳動脈瘤にしばしば遭遇するという矛盾は，こうした未破裂脳動脈瘤と破裂脳動脈瘤の病理所見の違いに起因するものと思われる．すなわち偶然発見される未破裂脳動脈瘤の多くは，炎症や退行変化の少ない安定した脳動脈壁をもつタイプで破裂の可能性は極めて低いが，一部に脳動脈壁の炎症や退行変化が著しいものが存在し，それは短期間に増大し，比較的小さいサイズで破裂することが推察される．

　破裂危険因子としては，脳動脈瘤の大きさ，高血圧，喫煙，多発例や家族歴などが挙げられており，脳動脈瘤の発生には環境因子だけでなく，遺伝的な要素も関与していることが示唆されている．近年では遺伝子の解析やそれに伴う病理学的な検討，コンピュータシミュレーションによる血流の解析などにより，脳動脈瘤の発生の原因や増大，破裂のメカニズムの解析が試みられている．現時点では，脳動脈瘤破裂の危険性を予測する因子として，前述した大きさ，形状や脳動脈瘤の家族歴などが指標となっているが，将来的に脳動脈瘤壁の安定性を評価する方法が開発されれば，より正確な情報を患者に提供することが可能になると期待される．本稿では，脳動脈瘤発生・増大のメカニズムにつき，いくつかの文献を踏まえて考察する．

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