Paramyotonia congenita

Paramyotonia congenita is a rare congenital autosomal dominant neuromuscular disorder characterized by paradoxical myotonia. The myotonia is called paradoxical because it becomes worse with exercise, whereas classical myotonia, as seen in myotonia congenita, is relieved by exercise. The condition can also be induced by cold temperatures, and although this is more typical of the periodic paralytic disorders, patients may also have potassium-provoked paralysis. It typically presents within the first decade of life and has full penetrance. Myotonia is most often seen in the face or upper extremities, and the lower extremities are generally less affected. Unlike some related disorders it does not normally cause muscle atrophy. It can also present as hyperkalemic periodic paralysis, and there is debate over whether the two disorders are actually distinct.