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Muscular dystrophy (MD) refers to a group of more than 30 genetic diseases that cause progressive weakness and degeneration of skeletal muscles. All forms of MD grow worse as muscles progressively degenerate and weaken. The majority of patients eventually lose the ability to walk.
Some types of MD also affect the heart, gastrointestinal system, endocrine glands, spine, eyes, brain, and other organs. Respiratory and cardiac diseases are common, and some patients may develop a swallowing disorder. MD is not contagious and cannot be brought on by injury or activity.
MD occurs worldwide, affecting all races. Its incidence varies, as some forms are more common than others. Its most common forms in children, Duchenne and Becker muscular dystrophy, alone affect approximately 1 in every 3,500 to 5,000 boys, or between 400 and 600 live male births each year in the United States. Some types of MD are more prevalent in certain countries and regions of the world. Most muscular dystrophies are familial, meaning there is some family history of the disease.
There are nine major groups of the muscular dystrophies. The disorders are classified by the extent and distribution of muscle weakness, age of onset, rate of progression, severity of symptoms, and family history (including any pattern of inheritance).
Four types of MD that begin in childhood:
Two types of youth/adolescent-onset MD:
Three types of MD that usually begin in adulthood.
Both the patient's medical history and a complete family history should be thoroughly reviewed to determine if the muscle disease is secondary to a disease affecting other tissues or organs or is an inherited condition. It is also important to rule out any muscle weakness resulting from prior surgery, exposure to toxins, or current medications that may affect the patient's functional status.
Various laboratory tests may be used to confirm the diagnosis of MD.
Blood and urine tests can detect defective genes and help identify specific neuromuscular disorders. The blood tests of children with Duchenne MD show an abnormally high level of creatine kinase, which is apparent from birth.
Electron microscopy can identify changes in subcellular components of muscle fibers. Electron microscopy can also identify changes that characterize cell death, mutations in muscle cell mitochondria, and an increase in connective tissue seen in muscle diseases such as MD. Changes in muscle fibers that are evident in a rare form of distal MD can be seen using an electron microscope.
Exercise tests can detect elevated rates of certain chemicals following exercise and are used to determine the nature of the MD or other muscle disorder.
Genetic testing looks for genes known to either cause or be associated with inherited muscle disease.
Genetic counseling can help parents who have a family history of MD determine if they are carrying one of the mutated genes that cause the disorder.
Magnetic resonance imaging (MRI) is used to examine muscle quality, any atrophy or abnormalities in size, and fatty replacement of muscle tissue, as well as to monitor disease progression.
Muscle biopsies are used to monitor the course of disease and treatment effectiveness.
Immunofluorescence testing can detect specific proteins such as dystrophin within muscle fibers. Following biopsy, fluorescent markers are used to stain the sample that has the protein of interest.
Neurophysiology studies can identify physical and/or chemical changes in the nervous system, such as Electromyography (EMG), which can record muscle fiber and motor unit activity.