Paramyotonia Congenita (PC) is a type of familial periodic paralysis due to a mutation in the skeletal muscle sodium channel.
Chromosome location: 17q22
Gene affected: SCN4A
Most cases are related to a substitution at Arg-1448 in the transmembrane segment S4 of repeat IV.
Clinical features:
(1) The onset is during infancy.
(2) Other members of the family are affected in an autosomal dominant pattern.
(3) The patient experiences frequent episodes that last less than 24 hours.
(4) During an episode the patient experiences myotonia (stiffness) and muscle weakness. The most commonly affected areas are the face, neck and long muscles of the hand.
(5) Exposure to cold and/or exercise can trigger an episode.
(6) The serum potassium is normal. Administration of potassium has no effect.
Findings that are not features of PC: muscle hypertrophy, muscle atrophy, muscle pain
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