Extraocular muscle hypertrophy in myotonia congenita.

Myotonia congenita (MC) is a rare disorder of skeletal muscle caused by mutations in the CLCN1 gene,(1,2) which encodes the chloride ion channel found in the t-tubule of skeletal muscle. MC is characterized by impaired relaxation of voluntary muscle after sudden contraction that diminishes with muscle activity, known as the "warm-up effect." Individuals with MC can develop muscular hypertrophy despite little physical activity. Esotropia and reduced saccadic velocities have been reported in the dominant form of MC. We report two cases in which orbital magnetic resonance imaging (MRI) imaging showed extraocular muscle hypertrophy.