Channelopathies myotonias

Data in part from Ackerman NJ, Clapham DE Ion channels— basic science and clinical disease. N Engl J Med 1997,-336 1575. Other channelopathies include the long QT syndrome (MIM 192500) pseudoaldosteronism (Liddle syndrome, MIM 177200) persistent hyperinsulinemic hypoglycemia of infancy (MIM 601820) hereditary X-linked recessive type II nephrolithiasis of infancy (Dent syndrome, MIM 300009) and generalized myotonia, recessive (Becker disease, MIM 255700). The term "myotonia" signifies any condition in which muscles do not relax after contraction. 

Clinically, skeletal muscle Na+ channelopathies appear as recurring episodes of muscle stiffness or weakness triggered by typical circumstances such as cold, exercise, oral K+ load, or drugs. Muscle stiffness, termed myotonia, ameliorates by exercise and can be associated with transient weakness during quick movements that lasts only for seconds. It is the clinical phenotype brought about by uncontrolled repetitive firing of action potentials that lead to involuntary muscle contraction. On the other hand, the weakness is characterized by lack of action potentials or inexcitability. 

Most channelopathies have some features in common. There are paroxysmal attacks of myotonia or paralysis, migraine, or ataxia precipitated by physiological stressors. They are often suppressed by membrane-stabilizing agents... 

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