Muscular dystrophy, «DIHS truh fee», is a name for certain types of serious muscle diseases. The disorders weaken skeletal muscles—that is, the muscles that hold the bones of the skeleton together and move the body. Therefore, muscular dystrophies seriously affect movement, posture, and breathing. Skeletal deformities are often associated with this group of diseases.
All types of muscular dystrophies are inherited. They are caused by a defect in one or more of the genes that control muscle structure and function. Some types are inherited as a dominant gene abnormality, while others are inherited as a recessive gene abnormality or an X-linked or sex-linked recessive gene abnormality. In an X-linked recessive gene abnormality, the gene is on the X chromosome, one of the pair of chromosomes that determine a person's sex.
In a dominant gene abnormality, a person who inherits the defective gene from either parent will develop the disease. In a recessive gene abnormality, a person must inherit the defective gene from both parents to develop the disease. In an X-linked recessive gene abnormality, women are not affected but may pass on the defective gene to their children. If they pass on the defective gene, generally only their sons will develop the disease because males have only one X chromosome. Females have two X chromosomes, one of which has a normal form of the same gene. The presence of the normal gene usually prevents the harmful gene on the other X chromosome from producing its effects.
Doctors have not yet found a cure for any of the muscular dystrophies. The four most common muscular dystrophies are Duchenne, facioscapulohumeral, limb-girdle, and myotonic.
Duchenne muscular dystrophy is the most common and most rapidly progressive of the childhood muscle diseases. In most cases, the first sign of the disease is difficulty in walking at an early age, from 2 to 6 years old. Later, the patients fall frequently and cannot run. Walking and standing become more difficult, and a wheelchair is needed by adolescence. The disease eventually affects most muscles of the body. Doctors use antibiotics and flu shots for lung complications and balanced nutrition for overall health in an effort to extend the life expectancy of the patients. In addition, stretching exercises and braces to reduce muscle shortening and joint deformities may also help to improve the patient's mobility. Duchenne dystrophy is inherited as an X-linked recessive gene abnormality. It usually affects only boys.
Scientists have identified the genes that cause Duchenne dystrophy. They also have discovered a protein called dystrophin whose absence in muscle tissues causes the disease. Researchers are attempting to improve muscle function in Duchenne patients with treatments that introduce normal genes or muscle cells into the muscles. These treatments may increase production of dystrophin.
Facioscapulohumeral and limb-girdle muscular dystrophies are slowly progressive and usually become evident in adolescence or early adult life. They affect both men and women. Most patients have a nearly normal life expectancy, though with increasing disability. Facioscapulohumeral dystrophy primarily affects the muscles of the face, shoulder, and upper arm. It is inherited as a dominant gene abnormality. Limb-girdle dystrophy primarily affects the muscles of the arms, shoulders, legs, and hips. It is inherited as a recessive gene abnormality.
Myotonic muscular dystrophy is the most common adult dystrophy. But symptoms may appear at any age. The signs of the disease include myotonia (stiffness) and slowly progressive muscle weakness in the arms, hands, and fingers, as well as in the feet and lower legs. Along with the onset of stiffness and weakness, patients develop disorders of nonskeletal muscles, including eye cataracts, diabetes, and weakening of the heart muscle. This disease can affect both men and women. It is inherited as a dominant gene abnormality.
The gene that causes myotonic dystrophy lies on one chromosome of a pair designated as chromosome 19. In 1992, scientists reported that with each generation of inheritance, the gene can grow bigger, causing the disease to become increasingly severe.
Other muscular dystrophies include Becker, congenital, and distal. Becker is similar to but less severe than Duchenne dystrophy. Congenital dystrophy involves early weakness at birth. Distal dystrophy initially involves the small muscles of the hands and feet. Other muscle diseases, such as Pompe disease (also called acid maltase deficiency or glycogen storage disease type II), cause progressive muscle weakness in infants and adults. These diseases are often caused by an inherited condition that interferes with the processing of carbohydrates in muscle tissue.
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