抗VGKC抗体陽性辺縁系脳炎／Morvan症候群

Anti-VGKC Antibody-associated Limbic Encephalitis/Morvan Syndrome

はじめに

　抗K^＋チャネル抗体は，末梢・中枢神経系の電位依存性Kチャネル（voltage-gated potassium channel：VGKC）に対する抗体であり，再分極を阻害することで神経系に過剰興奮を引き起こす。抗VGKC抗体が関与する神経疾患としては，末梢神経疾患としてIsaacs症候群，cramp-fasciculation症候群，末梢および中枢性疾患としてMorvan症候群，中枢神経疾患として辺縁系脳炎，てんかんなどが知られている。ここでは前半で抗VGKC抗体陽性辺縁系脳炎について，後半でMorvan症候群について述べる。

Abstract

　Anti-voltage-gated potassium channel antibodies (anti-VGKC-Ab) cause hyperexcitability of the peripheral nerve and central nervous system.　Peripheral nerve hyperexcitability is the chief manifestation of Issacs syndrome and cramp-fasciculation syndrome.　Morvan syndrome is characterized by neuromyotonia with autonomic and CNS involvement.　Manifestations involving the CNS without peripheral involvement are characteristic of limbic encephalitis and epilepsy.　The clinical features of anti-VGKC-Ab-associated limbic encephalitis are subacute onset of episodic memory impairment,disorientation and agitation.　Hyponatremia is also noted in most patients.　Cortico-steroid therapy,plasma exchange and intravenous immunoglobulin are effective in treating to not only the clinical symptoms but also hyponatremia.　Unlike other anti-VGKC-Ab-associated neurological disorders,paraneoplastic cases are rare.　Thus,anti-VGKC-Ab-associated limbic encephalopathy is considered to be an autoimmune,non-paraneoplastic,potentially treatable encephalitis.　Morvan syndrome is characterized by widespread neurological symptoms involving the peripheral nervous system (neuromyotonia),autonomic system (hyperhidrosis,severe constipation,urinary incontinence,and cardiac arrhythmia) and the CNS (severe insomnia,hallucinations,impairment of short-term memory and epilepsy).　Many patients have an underlying tumor,for example thymoma,lung cancer,testicular cancer and lymphoma; this indicates the paraneoplastic nature of the disease.　Needle electro-myography reveals myokimic discharge.　In nerve conduction study,stimulus-induced repetitive descharges are frequently demonstrated in involved muscles.　Plasma exchange is an effective treatment approach,and tumor resection also improves symptoms.　Both VGKC-Ab-associated limbic encephalitis and Morvan syndrome can be successfully treated.　Therefore,when these diseases are suspected,it's important to measure the anti-VGKC-Ab level.