• Tomoya Taminato, Madoka Mori-Yoshimura, Jun Miki, Ryogen Sasaki, Noriko Sato, Yasushi Oya, Ichizo Nishino, and Yuji Takahashi.
• Department of Neurology, National Center Hospital, National Center of Neurology and Psychiatry, Tokyo, Japan.
• J Neuromuscul Dis. 2020 Jan 1; 7 (2): 193-201.

BackgroundParamyotonia congenita (PC; OMIM 168300) is a non-dystrophic myotonia caused by mutations in the SCN4A gene. Transient muscle stiffness, usually induced by exposure to cold and aggravated by exercise, is the predominant clinical symptom, and interictal persistent weakness is uncommon.Case ReportWe report a family with a history of PC accompanied by persistent hand muscle weakness with masticatory muscle involvement. Persistent weakness was exacerbated with age, and MR analysis showed marked atrophy of temporal, masseter, and finger flexor muscles with fatty replacement. The PC causative mutation T1313M in the SCN4A gene was prevalent in the family. Administration of acetazolamide chloride improved clinical symptoms and the results of cold and short exercise tests. Phenotypic variation within the family was remarkable, as the two younger affected patients did not present with persistent weakness or muscle atrophy.ConclusionsPC associated with the T1313M mutation is a possible cause of persistent distal hand weakness.

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