Lambert-Eaton筋無力症候群

Lambert-Eaton Myasthenic Syndrome (LEMS)

はじめに

　Lambert-Eaton筋無力症候群（Lambert-Eaton myasthenic syndrome：LEMS）は，悪性腫瘍に合併，あるいは腫瘍の発症に先行する傍腫瘍性症候群の1つである^1）。約50～60％の症例で肺小細胞癌（small-cell lung cancer；SCLC）が認められ，病態に深く関与している。残りの40％程度は腫瘍がない症例（non-tumor LEMS：NT-LEMS）である。いずれの場合でも電位依存性カルシウムチャネル（voltage-gated Ca^2＋ channel：VGCC）の機能を阻害する自己抗体が原因となり，臓器特異的自己免疫疾患とも考えられる^2）。神経筋接合部にあるVGCCは多様性を有しているが，特にP/Q型VGCCの抗体が重要である。

　LEMSの男女比は欧米，本邦とも2～4：1で男性に多く，50歳代にピークを認める。頻度は一般的には重症筋無力症（myasthenia gravis：MG）に対して100分の1程度であり，傍腫瘍性症候群と自己免疫疾患の両方の特徴を有している。疫学に関するデータとしては，オランダの調査で100万人あたり2～3人の有病率との報告がある^3）。

Abstract

　Lambert-Eaton myasthenic syndrome (LEMS) is a neuromuscular disorder in which autoantibodies inhibit the presynaptic release of acetylcholine.　Autoantibodies against P/Q-type voltage-gated calcium channels (VGCC) are detected in 85% of patients with LEMS.　In addition,autoantibodies to synaptotagmin,an M1-type muscarinic acetylcholine receptor and SOX1 are also found in the sera of patients with LEMS.　LEMS is closely associated with small cell lung cancer (SCLC) in 50-60% of patients.　Patients with SCLC who have anti-VGCC antibodies have been reported to have a favorable prognosis.　In contrast to paraneoplatic LEMS,other forms of LEMS may have an autoimmune aspect because of the established association between human leukocyte antigen and a family history of other autoimmune disorders in this condition.　The clinical features of LEMS include proximal weakness,areflexia,ptosis,cerebellar ataxia and autonomic dysfunction.　The findings of electrophysiological examination show that LEMS is characterized by compound muscle action potential potentials with a low amplitude and increment upon repetitive nerve stimulation at a high rate.　Tumor removal is the primary treatment of LEMS.　The efficacy of 3,4-diaminopyridine for the treatment of LEMS has also been established.　Patients with LEMS require the immunotherapies such as plasma exchange and the administration of high doses of immunoglobulin and prednisolone.