P.12.11 Atypical paramyotonia congenita in a boy with a new mutation in the SCN4A gene

Abstract

Myotonia means reduced ability to relax the muscles after contraction and is caused by various ion channel defects. Mutations in the chloride channel gene CLCN1 are found in Thomsen and Becker congenital myotonia. Mutations in the sodium channel gene SCN4A may give rise to paramyotonia congenita, hyperkalemic periodic paralysis or potassium-sensitive myotonia. Our patient is a seven year old boy with healthy, non-consanguineous parents. His psychomotor milestones were normal. He was admitted at the age of six because of episodes when one or both of his eye lids “fell down”. These episodes had become more prominent during the years. The episodes could be provoked by clapping hands just in front of his eyes or when he was suddenly scared in other ways. It was then difficult for him to re-open the eyes for several seconds. A waxing and waning swelling might also be seen below his eyes, possibly reflecting ongoing myotonia in the orbicularis oculi muscles. He sometimes used his fingers to get the eyes opened more quickly. It was also reported that when he started to walk after having been seated for some time, he often transiently felt stiff in his legs. It appeared that sometimes even his chewing and articulation might be impaired because of stiffness. The problems did not seem to be temperature sensitive. Episodes of paralysis were not reported. His shoulder muscles and the masseter muscle looked hypertrophic. EMG was consistent with extensive myotonia. Gene test for myotonic dystrophy was negative, and no mutation was found in the CLCN1 gene. However, an unclassified heterozygous mutation was detected in the SCN4A gene. Neither of the parents carried this mutation which is estimated as probably pathogenic. Classification of his phenotype will be discussed.