Paramyotonia congenita is an autosomal dominant disease that presents with cold-induced myotonia due to a mutation in the gene SCN4A (a voltage gated Na channel) 

sx of paramyotonia congenita also worsens with activity which is considered a paradoxical response

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In myotonia congenita sx worsen after a period of rest and may improve with activity; associated with CLCN1 gene mutation