Neurological Surgery 脳神経外科39巻6号

2011年06月発行

連載 先天奇形シリーズ

（7）キアリ奇形

著者： 長嶋達也¹ 山元一樹¹ 河村淳史¹ 長嶋宏明¹

所属機関： ¹兵庫県立こども病院脳神経外科

ページ範囲：P.617 - P.628

Ⅰ．概説

　Chiari（1891）は⁹⁾，剖検例の検討から小脳扁桃下垂の程度の異なる一連の後脳奇形として4つのタイプを報告した．現在では，キアリ奇形は小脳扁桃や脳幹の一部が大孔を越えて脊椎管内に下垂する形態的特徴をもつ疾患の総称であり⁴⁵⁾，臨床的にはⅠ型は脊髄髄膜瘤を合併しないもの，Ⅱ型は合併するものと考えるのが理解しやすい．

　キアリⅠ型奇形は小脳扁桃の大後頭孔への下垂であり，高率に脊髄空洞症を伴う（Fig. 1A）．後頭骨の形成不全による後頭蓋窩狭小化の関与が指摘されている³⁶⁾．後頭蓋窩が狭窄しているものの小脳扁桃自体は下垂していない例にも脊髄空洞症を合併することがあり，大孔減圧術で空洞が縮小することからキアリ0型奇形と呼んだり（Fig. 1B），キアリⅠ型奇形と同様の所見に延髄と第四脳室の下垂を伴い，キアリⅡ型奇形に似たものをキアリ1.5型奇形と呼ぶ教科書もある⁴⁷⁾．いずれも大孔減圧術で改善することからキアリⅠ型奇形と共通した病態と考えられる．

　キアリⅡ型奇形は小脳虫部，第四脳室，延髄が大孔へ下垂したものである（Fig. 4）．大部分は小児であり脊髄髄膜瘤と水頭症を伴うことが重要な特徴である．McLoneは脳形成過程に生じた神経管閉鎖不全部から髄液が羊水中に流出するために，脳室が発生途上で正常に拡張できないことが，多様な脳・頭蓋の形成不全の基になると一元的に説明している（unified theory）²⁸⁾．Tulipanらは，胎内における脊髄髄膜瘤修復術により，既に存在した小脳扁桃下垂が改善し正常の後脳形態に回復し得ることを示した⁴⁹⁾．胎内における脊髄髄膜瘤部からの髄液流出がもたらす頭蓋内と脊椎管内の圧較差が小脳と脳幹を下垂させる機序であることを示唆している．

　キアリⅢ型奇形は小脳が大後頭孔より下垂してoccipito-cervical encephalocele内へ脱出したものであり，キアリⅣ型奇形は小脳の形成不全であるがいずれも極めて稀であり臨床的意義は乏しい．Barkovichは，キアリⅢ型奇形は上位頚椎の脊髄髄膜瘤であるとしている⁵⁾．

　後天性キアリⅠ型奇形（aquired Chiari malformation）と呼ばれる病態として，頭蓋縫合早期癒合症による頭蓋容積の狭小化や後頭蓋窩腫瘍，両側慢性硬膜下血腫などの頭蓋内占拠性病変，腰椎─腹腔髄液短絡術や低髄液圧症候群などに伴う小脳扁桃下垂がある（Fig. 1D）⁴⁷⁾．

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