There is no single disease called muscular dystrophy. The term designates a group of hereditary muscle-destroying disorders that vary in inheritance pattern, age of onset, initial muscles attacked and rate of progression.
Muscular dystrophy is a term used to describe a number of inherited disorders characterized by progressive weakness and wasting of the muscles.
The most common and severe type is Duchenne's muscular dystrophy (MD), in which a genetic defect leads to the formation of an abnormal type of muscle protein called dystrophin.
Other types include Becker's MD, limb-girdle MD, facioscapulohumeral MD, and myotonic dystrophy.
The types of MD are classified according to the location of the muscles involved, the age that symptoms appear, the rate that symptoms progress, and the manner in which the defective gene is passed on. For example, the genes for Duchenne's and Becker's MD are X-linked recessives - that is, they generally affect only boys but are carried and passed on by women.
Symptoms and prognosis vary, depending on the type of MD. The disease may affect some or all muscles, may develop during childhood or adulthood, may progress very gradually or rapidly, and may or may not be severely disabling.
Boys with Duchenne's MD are usually in a wheelchair by the age of 12 and rarely live past age 20; on the other hand, people with facioscapulohumeral MD often have a normal life span and usually remain able to walk, since the leg muscles are only mildly affected. Also, in some types of MD, the rate of progression and the extent of disability varies substantially from patient to patient.
Most forms of MD affect the heart muscle and this results in cardiomyopathy. All forms of MD result from some type of genetic defect. In the majority of cases, the defect is inherited and affects various relatives throughout a family. In a few cases, the disorder may result from a genetic mutation.
Muscular dystrophy is a genetically determined disease, although why the individuals who are genetically the same get MDs of different severity is not known.
Principal symptoms include:
- Progressive muscle wasting, weakness, and loss of function
- In Duchenne's MD, delayed development of basic muscle skills and coordination in children. Common signs include poor balance with frequent falls, walking difficulty with waddling gait and calf pain, and limited range of movement
- Joint contractures
- Cataracts, frontal baldness, drooping eyelids, gonadal atrophy, and mental impairment (with myotonic dystrophy)
The diagnosis of MD is based upon a combination of a characteristic clinical presentation and the results of muscle biopsy.
There is no specific treatment to cure or halt MD. Physical therapy, exercise, orthopedic appliances (such as braces and wheelchairs), or corrective orthopedic surgery may help to preserve muscle function and prevent joint contractures as much as possible and improve quality of life. Steroids have been used to slow disease progression, but do not effect the final outcome. Identification of the specific genes responsible for the various types of MD has led to extensive research on gene and molecular therapy, but all such treatments are still experimental.
Genetic counseling is recommended for families of affected individuals to ascertain the carrier status of other family members so that prenatal testing can be offered.