特異な発話障害を認めた浸透圧性脱髄症候群の1例

A unique speech disorder in extra-pontine myelinolysis of osmotic demyelination syndrome

　浸透圧性脱髄症候群は低ナトリウム（以下，Na）血症の急速補正などによる血漿浸透圧の急激な変化で生じる中枢性脱髄症候群である．症例は70代の女性．新型コロナウイルス感染症（COVID-19）罹患に伴い意識障害が生じた．低Na血症の補正後に一過性に改善したが意識障害が再燃，発話困難と四肢不随意運動が生じた．MRIで大脳基底核と中心前回皮質に両側性の病巣を認めたが橋病変はなく，橋外髄鞘崩壊症と診断された．SPECTでは左側頭頭頂葉に血流低下を認めた．発声発語・言語症状として①口唇・舌の筋緊張が高く交互反復運動が拙劣，発話速度が遅く，絞扼努力声といった痙性構音障害，②プロソディ異常，構音器官の探索，音韻の歪み・付加・脱落，発話リズムの乱れといった発語失行，③喚語困難や錯書，複雑な文の理解低下などの失語症状を認めた．本例は，両側性の大脳基底核および中心前回病変と左半球の機能低下により，痙性構音障害，発語失行，失語など複数の要素が重なったために特異な発話症状を示したと考えた．

　Osmotic demyelination syndrome (ODS) is demyelination injury in the central nervous system, which is caused by an abrupt change of plasma osmolality, usually due to the rapid correction of hyponatremia (low sodium). The symptoms are variable, including quadriplegia, pseudobulbar palsy, impaired consciousness, convulsions, and locked-in syndrome. A right-handed woman in her 70s developed ODS by the rapid correction of hyponatremia after corona-virus infection (COVID-19), presenting with a unique speech disorder and systemic involuntary movements. Brain MRI documented symmetrical demyelinating lesions bilaterally in the basal ganglia and cortical precentral areas, indicating extra-pontine myelinolisis alone without central pontine myelinolysis. Brain SPECT image showed mild dysfunction in the left hemisphere. The speech characteristics of this case were variable combinations of spastic dysarthria with non-paralyzed strained-strangled voice, aphasia with anomia and paragraphia, apraxia of speech with abnormal prosody and slow speech rate due to poor articulation, buccofacial apraxia, and extra-pyramidal sign with freezing of speech output. Simultaneous and bilateral combined damage to the basal ganglia and cortical precentral areas, together with mild dysfunction in the left hemisphere, may be the neural mechanism for the development of this unique speech disorder.