[Muscle weakness or rigidity due to hereditary ion channel diseases].

Three men, aged 36, 16 and 66 years, had suffered for several years from muscular weakness; after a low serum potassium level had been established, supplementary examination revealed hypokalaemic periodic paralysis. A woman aged 25 had suffered since youth from muscular stiffness on sudden movements; she suffered from hereditary myotonia of the recessive type (Becker's disease). Both rare skeletal muscle ion channel diseases are characterized by the fact that the variable clinical expression complicates making the diagnosis. Since the causal mutations are known, genetic analysis is an essential step in confirming the diagnosis. Additional EMG procedures may be of diagnostic value, even in cases that cannot be clarified genetically.