Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness. Muscular dystrophies are a group of inherited muscle disorders in which one or more genes Myotonia Congenita Myotonia congenita is an inherited disorder that causes muscle stiffness and muscle enlargement. It affects what most people think of as muscle, the type that can be contracted to move the various... read more needed for normal muscle structure and function are defective, leading to muscle weakness Weakness Weakness refers to loss of muscle strength. That is, people cannot move a muscle normally despite trying as hard as they can. However, the term is often misused. Many people with normal muscle... read more and muscle wasting (dystrophy) of varying severity.
Myotonic dystrophy is an autosomal dominant Dominant x-linked disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more disorder, which means only one affected parent is needed to pass the trait on to offspring. This dystrophy affects males and females and about 1 in 8,000 people.
Symptoms of Myotonic Dystrophy
Symptoms of myotonic dystrophy begin during adolescence or young adulthood.
The disorder causes myotonia. Myotonia is a delay in the ability to relax the muscles after contracting them. Other main symptoms are weakness and wasting of arm and leg muscles (especially in the hands) and muscles in the face. Drooping eyelids are also common. The heart muscle also becomes weak (cardiomyopathy Overview of Cardiomyopathy Cardiomyopathy refers to progressive impairment of the structure and function of the muscular walls of the heart chambers. There are three main types of cardiomyopathy: Dilated cardiomyopathy... read more 
) and the heart rhythm may become abnormal.
Symptoms of myotonic dystrophy begin during adolescence or young adulthood 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 Cataract A cataract is a clouding (opacity) of the lens of the eye that causes a progressive, painless loss of vision. Vision may be blurred, contrast may be lost, and halos may be visible around lights... read more 
, small testes (in men), premature balding in the front (in men), irregular heartbeats, diabetes Diabetes Mellitus (DM) Diabetes mellitus is a disorder in which the body does not produce enough or respond normally to insulin, causing blood sugar (glucose) levels to be abnormally high. Urination and thirst are... read more , and intellectual disability Intellectual Disability Intellectual disability is significantly below average intellectual functioning present from birth or early infancy, causing limitations in the ability to conduct normal activities of daily... read more . They usually die by about age 54.
Congenital myotonic dystrophy
Mothers with myotonic dystrophy may have children with a severe form of myotonia that appears during infancy. This disorder is called congenital myotonic dystrophy, which should not be confused with myotonia congenita Myotonia Congenita Myotonia congenita is an inherited disorder that causes muscle stiffness and muscle enlargement. It affects what most people think of as muscle, the type that can be contracted to move the various... read more , which is a separate disorder. Infants with congenital myotonic dystrophy have severely reduced muscle tone (hypotonia, or "floppiness"), feeding and breathing problems, bone deformities, facial weakness, and delays in development of thought processes and physical movement. Up to 40% of infants do not survive, usually because of difficulty breathing (respiratory failure Respiratory Failure Respiratory failure is a condition in which the level of oxygen in the blood becomes dangerously low or the level of carbon dioxide in the blood becomes dangerously high. Conditions that block... read more ) and perhaps cardiomyopathy. Up to 60% of survivors have intellectual disability.
Diagnosis of Myotonic Dystrophy
Genetic testing
The diagnosis of myotonic dystrophy is based on characteristic symptoms, the person's age when symptoms began, and family history.
Treatment of Myotonic Dystrophy
Drugs to relieve muscle stiffness
Treatment of myotonic dystrophy with mexiletine or other drugs (for example, lamotrigine, phenytoin, or carbamazepine) may relieve the stiffness, but these drugs do not relieve 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 (for footdrop) and other devices.
More Information
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Muscular Dystrophy Association: Information on research, treatment, technology, and support for people living with myotonic dystrophy
