Myotonic Myopathies
Myotonic myopathies are inherited disorders in which the muscles are not able to relax normally after contraction; muscle weakness and spasms may also occur.
Myotonia congenita (Thomsen's disease) is a rare autosomal dominant disorder that affects males and females. Symptoms usually start in infancy. The hands, legs, and eyelids become very stiff because of an inability to relax the muscles. Muscle weakness, however, is usually minimal. The diagnosis is made from the child's characteristic appearance, inability to relax the grip of the hand rapidly after closing the hand, and prolonged contraction after the doctor taps a muscle. An electromyogram (a test in which electrical impulses from muscles are recorded (see Section 6, Chapter 77)) is needed to confirm the diagnosis. Myotonic congenita is treated with phenytoin, quinine, procainamide, or mexiletine to relieve muscle stiffness and cramping; however, each of these drugs has undesirable side effects. Regular exercise may be beneficial. People with myotonic congenita have a normal life expectancy.
Myotonic dystrophy (Steinert's disease) is an autosomal dominant disorder affecting males and females. The disorder produces weakness and stiff muscles, especially in the hands. Drooping eyelids are also common. Symptoms can appear at any age and can range from mild to severe. People with the most severe form of the disorder have extreme muscle weakness and many other symptoms, including cataracts, small testes (in men), premature balding in the front (in men), irregular heartbeats, diabetes, and mental retardation. They usually die by age 50. Treatment with quinine, phenytoin, procainamide, and other drugs has been used, but these drugs do not improve the weakness, which is the most bothersome symptom to the person. Also, each of these drugs has undesirable side effects. The only treatment for muscle weakness is supportive measures, such as ankle braces and other devices.
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