Myopathies are diseases of skeletal muscle which are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weakor wasted. There are many different types of myopathies, some of which are inherited, some inflammatory, and some caused by endocrine problems. Myopathiesare rare and not usually fatal. Typically, effects are mild, largely causingmuscle weakness and movement problems, and many are transitory. Only rarelywill patients become dependent on a wheelchair. However, muscular dystrophy (which is technically a form of myopathy) is far more severe. Some types of this disease are fatal in early adulthood.
Myopathies are usually degenerative, but they are sometimes caused by drug side effects, chemical poisoning, or a chronic disorder of the immune system. Among their many functions, genes are responsible for overseeing the production of proteins important in maintaining healthy cells. Muscle cells produce thousands of proteins. With each of the inherited myopathies, a genetic defectis linked to a lack of, or problem with, one of the proteins needed for normal muscle cell function. Different kinds of myopathy caused by different defective genes.
Most, but not all, of these genetic myopathies are dominant, which means thata child needs to inherit only one copy of the defective gene from one parentin order to show symptoms of the disease. Male and female children are equally at risk.
The major symptoms associated with the genetic myopathies include: weakness of voluntary muscles (affected muscles almost always lack reflexes); delays inreaching developmental motor milestones; problems with running, jumping, andclimbing stairs that develop in childhood; muscle stiffness triggered by fatigue, stress, cold, or long rest periods, such as a night's sleep; attacks oftemporary muscle weakness (muscles work normally between attacks); or multisystem problems primarily involving the brain and muscles.
In some cases, myopathies can be caused by a malfunctioning endocrine gland (or glands), which produces either too much or too little of the chemical messengers called hormones. Hormones are carried by the blood and one of their many functions is to regulate muscle activity. Problems in producing hormones can lead to muscle weakness.
Other myopathies are inflammatory, leading to inflamed, weakened muscles. Inflammation is a protective response of injured tissues characterized by redness, increased heat, swelling, and/or pain in the affected area. Examples of this type include polymyositis, dermatomyositis, and myositis ossificans.
Dermatomyositis is a disease of the connective tissue that also involves weak, tender, inflamed muscles. In fact, muscle tissue loss may be so severe thatthe person may be unable to walk. Skin inflammation is also present. The cause is unknown, but viral infection and antibiotics are associated with the condition. In some cases, dermatomyositis is associated with rheumatologic disease or cancer.
Polymyositis involves inflammation of many muscles usually accompanied by deformity, swelling, sleeplessness, pain, sweating, and tension. It, too, may beassociated with cancer. Myositis ossificans is a rare inherited disease in which muscle tissue is replaced by bone, beginning in childhood.
While considered to be a separate group of diseases, the muscular dystrophiesalso technically involve muscle wasting and can be described as myopathies.These relatively rare diseases appear during childhood and adolescence, and are caused by muscle destruction or degeneration. They are genetic disorders caused by problems in the production of key proteins.
The forms of muscular dystrophy (MD) differ according to the way they are inherited, the age of onset, the muscles they affect, and how fast they progress. The most common type is Duchenne MD, affecting one or two in every 10,000 boys. Other types of MD include Becker's, myotonic dystrophy, limb-girdle MD,and facioscapulohumeral MD.
Early diagnosis of myopathy is important so that the best possible care can be provided as soon as possible. An experienced physician can diagnose a myopathy by evaluating a person's medical history and by performing a thorough physical exam. Diagnostic tests can help differentiate between the different types of myopathy, as well as between myopathy and other neuromuscular disorders. If the doctor suspects a genetic myopathy, a thorough family history will also be taken.
Treatment depends on the specific type of myopathy the person has and the prognosis for patients with myopathy depends on the type and severity. In most cases, the myopathy can be successfully treated and the patient returned to normal life. Muscular dystrophy, however, is a much more serious condition. Duchenne's MD is usually fatal by the late teens; Becker's MD is less serious and may not be fatal until the 50s.