Muscular dystrophy (MD) refers to a group of genetic diseases characterized by progressive weakness and degeneration of the skeletal or voluntary muscles which control movement. The muscles of the heart and some other involuntary muscles are also affected in some forms of MD, and a few forms involve other organs as well. The major forms of MD include myotonic, Duchenne, Becker, limb-girdle, facioscapulohumeral, congenital, oculopharyngeal, distal and Emery-Dreifuss. Duchenne is the most common form of MD affecting children, and myotonic MD is the most common form affecting adults. MD can affect people of all ages. Although some forms first become apparent in infancy or childhood, others may not appear until middle age or later.
In most cases, the genetic abnormality for muscular dystrophy is inherited, so the condition tends to run in families. Some types of muscular dystrophy, including the most severe type, Duchenne, are X-linked, which means that the abnormality is carried on the X chromosome that the mother contributes. A girl receives two X chromosomes, one from each parent, whereas a boy receives a Y chromosome from the father and an X from the mother. As a result, it is almost always boys who develop symptoms; girls inherit a normal X chromosome that may cancel out the abnormal one. These girls are called carriers of the muscular dystrophy gene, but have no muscle weakness themselves. Girls who carry the abnormal gene have a 50% chance of passing the condition to their sons.
There is no specific treatment for any of the forms of MD. Physical therapy to prevent contractures, orthopedic appliances for support and corrective orthopedic surgery may be needed to improve the quality of life in some cases. The cardiac problems that occur with Emery-Dreifuss MD and myotonic MD may require a pacemaker. The myotonia (delayed relaxation of a muscle after a strong contraction) occurring in myotonic MD may be treated with medications such as phenytoin or quinine.
The prognosis of MD varies according to the type of MD and the progression of the disorder. Some cases may be mild and very slowly progressive, with normal lifespan, while other cases may have more marked progression of muscle weakness, functional disability and loss of ambulation. Life expectancy may depend on the degree of progression and late respiratory deficit. In Duchenne MD, death usually occurs in the late teens to early twenties.
Inclusion body myositis usually falls within a group of muscle disorders called inflammatory myopathies. There are two forms, sporadic inclusion body myopathy, which does not appear to be genetically inherited and hereditary inclusion body myopathy, of which there are two categories:
1. A recessively inherited form linked to chromosome 9 which means that to inherit this condition a faulty gene from each parent is needed to produce the symptoms. This form has been isolated in Jewish (of Middle Eastern descent), Japanese and Mexican families.
2. A dominantly inherited form (only need to inherit one faulty gene) linked to chromosome 17 but this form is very rare and currently has been isolated in one Swedish family.