Homozygosity for dominant mutations increases severity of muscle channelopathies.

Muscle channelopathies caused by mutations in the SCN4A gene that encodes the muscle sodium channel are transmitted by autosomal-dominant inheritance. We report herein the first cases of homozygous patients for sodium channel mutations responsible for paramyotonia congenita (I1393T) or hypokalemic periodic paralysis (R1132Q). A parallel was drawn between this unprecedented situation and that of myotonia congenita by including patients homozygous or heterozygous for the CLCN1 I556N channel mutation, which is known for incomplete dominance and penetrance.

Glucocorticoids may trigger attacks in several types of periodic paralysis.

Hypokalemic periodic paralysis is a rare disorder characterized by episodic attacks of muscle flaccidity associated with low serum potassium levels. We report twelve patients with normokalemic and hypokalemic periodic paralysis due to various mutations who developed hypokalemic paralytic episodes following a single dose or short-term administration of glucocorticoids. We hypothesize that glucocorticoids cause hypokalemia due to their stimulation of the Na(+)-K(+) ATPase mediated by insulin and amylin and due to their side effect of insulin resistance resulting in hyperglycemia.

Cold extends electromyography distinction between ion channel mutations causing myotonia.

Objective: Myotonias are inherited disorders of the skeletal muscle excitability. Nondystrophic forms are caused by mutations in genes coding for the muscle chloride or sodium channel. Myotonia is either relieved or worsened by repeated exercise and can merge into flaccid weakness during exposure to cold, according to causal mutations.