myotonia congenita

[kən-jĕnĭ-tə]
n.
  1. A hereditary disease characterized by tonic spasm or temporary rigidity of certain muscles after an attempt has been made to move them.Thomsen's disease
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.