ニューロミオトニア（Isaacs症候群）：カリウムチャネロパチー

Neuromyotonia（Isaacs'syndrome）as a potassium channelopathy

　Isaacs症候群は原因不明の末梢神経の興奮性異常に基づく特異な臨床症状を示す疾患として従来注目されていたが，本疾患では高頻度に自己抗体を合併し，血漿交換療法で症状が改善することから，自己免疫疾患の1つであることが明らかとなった。その後免疫学的手法およびパッチクランプを用いた電気生理学的手法により，末梢神経に存在する電位依存性カリウムチャネル（VGKC）に対する抗体が存在し，その機能を抑制することで末梢神経の興奮性亢進を来すことが明らかとなった。最近では，筋痙攣，筋硬直，pseudomyotoniaを主症状とする，より広範な末梢神経過剰興奮性の疾患として認識され，抗VGKC抗体との関連が指摘されてきつつある。さらに抗VGKC抗体はMorvan症候群や辺縁系脳炎などの中枢性疾患でもみられることが報告され，いわゆるカリウムチャネロパチーとしての概念が拡がりつつある。

　Isaacs'syndrome is a disorder presenting with signs and symptoms of peripheral nerve hyperexcitability of unknown origin. Since the existence of autoantibodies, such as anti-acetylcholine receptor antibodies, tumors such as thymoma and symptoms are improved by plasmapheresis, Isaacs'syndrome has been considered as one of the autoimmune disorders. Recent immunological and electrophysiological approaches have found the existence of anti-voltage-gated K channel（VGKCs）antibodies, which suppress K channels and induce the hyperexcitability of the peripheral nerve. Aside from Isaacs'syndrome, anti-VGKC antibodies have also been correlated with a wide spectrum of other disorders presenting with peripheral nerve hyperexcitability such as symptoms of muscle cramp, muscle stiffness and neuromyotonia. Furthermore, anti-VGKC antibodies have also been associated with some central nervous system disorders such as Morvan's syndrome and limbic encephalitis. From these developments, it appears that the concept of potassium channelopathies is currently expanding to involve not only the peripheral but the central nervous system as well.