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Paramyotonia congenita, also called Eulenburg disease or paramyotonia congenita of Eulenberg, is a rare congenital neuromuscular disorder in which the muscles are slow to relax after contraction. In paramyotonia congenita, unlike in other neuromuscular disorders, the difficulty of relaxing the muscles, a symptom called myotonia, is paradoxical, meaning that it is worsened rather than improved by prolonged exercise. Eulenburg disease affects less than one in every 100,000 people.
Patients with Eulenberg disease and other myotonias may have difficulty, for instance, letting go of an object they have gripped or standing up from a squatting position. It may take significant effort or time to relax the muscles, and muscle stiffness or weakness can result even after the muscle is no longer contracted. Paramyotonia congenita does not typically cause muscle atrophy, unlike some related conditions.
Paramyotonia congenita is also characterized by the tendency of cold temperatures to induce myotonia. It most often affects the muscles of the face and upper extremities. Though the paradoxical myotonia associated with Eulenburg disease can lead to muscle weakness, persistent weakness is not a symptom of this particular condition. In addition to exercise and cold temperatures, some patients find that hunger or eating certain foods, particularly those rich in potassium, can trigger myotonia.
Paramyotonia congenita is caused by a mutation in the SCN4A gene, which encodes one of the sodium channel proteins responsible for muscular contraction. All people with the mutation will eventually develop the disorder. Most sufferers begin showing symptoms by the age of ten.
Many patients with paramyotonia congenita do not require any medical treatment. The condition can often be managed simply by avoiding whatever triggers myotonia and muscle weakness. Some patients may be prescribed medication such as mexiletine or acetazolamide to alleviate muscle stiffness.
Eulenburg disease has traditionally been considered a separate condition from other sodium channel myotonias, and from hyperkalemic periodic paralysis, a genetic disorder in which cold weather causes uncontrollable shaking followed by paralysis. However, it is now under debate whether these various genetic myotonias are distinct conditions or whether they belong to a spectrum of disorders.
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