A case of paramyotonia congenita without periodic paralysis: electrophysiological and molecular genetic studies.

Paramyotonia congenita (PC), first described in 1886 by Eulenberg, is characterized by cold and exercise-induced muscle stiffness and intermittent flaccid paresis not necessarily related to cold or myotonia. Several authors segregated a pure form of PC, which has no periodic paralysis, even after cold exposure. The existence of this phenotype has been debated in the literature. We describe electrophysiological and molecular genetic features of a patient with PC who had no history of periodic paralysis. Immersion in cold water or potassium load could not induce clinical paralysis. However, repetitive nerve stimulation and exercise test demonstrated a drop in compound muscle action potential amplitude. Genetic analysis revealed the substitution of valine for glycine on the human skeletal muscle sodium channel (SCN4A) gene. The G1306V mutation is rare in the classic form of PC, and moreover might be the first in pure paramyotonia.