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Myopathies

Definition

Myopathies are diseases of skeletal muscle that are not caused by nerve disorders. These diseases cause the skeletal or voluntary muscles to become weak or shrunken (atrophied).

Description

There are many different types of myopathies. Some are inherited, some inflammatory, and some caused by endocrine or metabolic problems. Myopathies usually are not fatal. Typically they cause muscle weakness and movement problems. The shoulders and thigh muscles are usually, but not always, affected earlier than the muscles of the hands and feet. Most myopathies are degenerative, meaning they become more pronounced over time. Some weaknesses are transitory. Only rarely do individuals become dependent on a wheelchair. However, muscular dystrophy (technically a myopathy) is far more severe. Some types of muscular dystrophy are fatal in early adulthood.

Causes and symptoms

There is great variety among myopathies, but what they all share are effects on the skeletal muscles. The main causes of myopathies are genetic, inflammatory (caused by infection), endocrine (hormonal), and metabolic (errors in how cells function). Often the cause of the myopathy is not known (idiopathic disease).

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 defect is linked to a lack of, or defect in, one of the proteins needed for normal muscle cell function.

There are several different kinds of myopathy caused by defective genes:

Most, but not all, of these genetic myopathies are inherited through an autosomal dominant pattern of inheritance. In this pattern of inheritance, one copy of each gene comes from each parent. Only one of these two copies needs to have the mutation (change) or defect in order for the child to have the disease. The parent with the defective gene has the disease, and each of this parent's children has a 50 percent chance of inheriting the disease. This percentage is not changed by results of other pregnancies. With this pattern of inheritance, male and female children are equally at risk of developing the disease.

However, for a child to have one type of myotonia congenita and some forms of nemaline myopathy, two defective genes must be inherited—one from each parents. This is called an autosomal recessive pattern of inheritance. Neither parent may have symptoms of the disease, but each carries a recessive defective gene for it. Each child of such parents has a 25 percent chance of inheriting both genes and showing signs of the disease, and a 50 percent chance of inheriting one defective gene from only one parent. If the child has inherited just one defective gene, he or she will be a carrier of the disease and can pass the gene on to his or her offspring, while showing no signs of the disease himself.

A few forms of centronuclear myopathy develop primarily in males. Females who inherit the defective gene are usually carriers without symptoms, like their mothers, but they can pass on the disease to their sons. Mitochondrial myopathies are inherited only through the mother, since sperm do not contain mitochondria.

The major symptoms associated with the genetic myopathies are:

In some cases, myopathies can be caused by a malfunctioning endocrine gland that produces either too much or too little of the chemical messengers called hormones. Hormones travel through the bloodstream. One of their many functions is to help regulate muscle activity. Problems in producing hormones can lead to muscle weakness.

Hyperthyroid myopathy and hypothyroid myopathy affect different muscles in different ways. Hyperthyroid myopathy occurs when the thyroid gland produces too much of the hormone thyroxine, leading to muscle weakness, some muscle wasting in hips and shoulders, and, sometimes, problems with eye muscles. The hypothyroid type of myopathy occurs when too little hormone is produced, leading to stiffness, cramps, and weakness of arm and leg muscles.

Some myopathies are caused by inflammation. 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 of myopathy include dermatomyositis , polymyositis, and myositis ossificans.

Dermatomyositis is a disease of the connective tissue that also involves weak, tender, inflamed muscles. Muscle tissue loss may be so severe that the individual may be unable to walk. Skin inflammation is also present. The cause of dermatomyositis is as of 2004 unknown, but viral infection and antibiotic use 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 be associated with cancer. Myositis ossificans is a rare inherited disease in which muscle tissue is replaced by bone, beginning in childhood.

While considered a separate group of diseases, the muscular dystrophies also involve muscle wasting and can be described as myopathies. Symptoms of muscular dystrophy (MD) diseases usually appear during childhood and adolescence . These are genetic disorders that result in defects in the production of specific proteins. The forms of muscular dystrophy differ according to the way they are inherited, the age at which symptoms begin, the muscles they affect, and how fast they progress.

Demographics

Myopathies are not common. About 14 percent of myopathies are inherited. Worldwide the rate of inflammatory myopathies is about five to ten individuals per 100,000. These myopathies are more often seen in women. MD is found in about 63 of every 1 million individuals, but the rates vary widely depending on the type of MD. The most common type is Duchenne MD, affecting one in every 3,300 boys. Other more common types of MD are Becker's, myotonic dystrophy , limb-girdle MD, and facioscapulohumeral MD. MD is more common in boys. The rate of metabolic and endocrine myopathies was, as of 2004, not known.

Parents should let the doctor know as soon as possible if there is a family history of muscle weakness or muscle wasting disease. Otherwise, they should contact their pediatrician if the child is showing any signs of delayed or abnormal growth or unexplained muscle weakness.

Diagnosis

Early diagnosis of myopathy is important in order to provide the best care possible. An experienced physician can diagnose a myopathy by evaluating a child's medical history and by performing a thorough physical examination. Diagnostic tests can help differentiate among the different types of myopathies, as well as between myopathy and other neuromuscular disorders. If the doctor suspects a genetic myopathy, a thorough family history will also be taken. Genetic tests are available for a few myopathies.

Diagnostic tests the doctor may order include: measurements of potassium, (K) creatine kinase,(CK) lactic dehydrogenase (LDH) and pyruvate kinase (PK) and certain antibodies in the blood; muscle tissue biopsy; and electromyogram (EMG).

Treatment

As of 2004, there was no cure for many myopathies. Treatment depends on the specific type of myopathy the person has and is aimed at controlling symptoms. Specific treatment approaches for specific forms of myopathies are as follows:

General treatments aim at supporting the individual's functioning and independence. Physical therapy can help preserve or increase strength and flexibility in muscles. Ankle and wrist braces can support weakened limbs. Occupational therapy is used to develop tools and techniques to compensate for loss of strength and dexterity. A speech-language pathologist can provide retraining for weakness in the muscles controlling speech and swallowing.

Prognosis

KEY TERMS

Electrooculography (EOG) —A diagnostic test that records the electrical activity of the muscles that control eye movement.

Hyperkalemia —An abnormally high level of potassium in the blood.

Hypokalemia —A condition characterized by a deficiency of potassium in the blood.

Inflammation —Pain, redness, swelling, and heat that develop in response to tissue irritation or injury. It usually is caused by the immune system's response to the body's contact with a foreign substance, such as an allergen or pathogen.

Mitochondria —Spherical or rod-shaped structures of the cell. Mitochondria contain genetic material (DNA and RNA) and are responsible for converting food to energy.

Voluntary muscles —Muscles that can be moved by conscious thought.

Muscular dystrophy is generally a more serious disease than many other types of myopathies. Duchenne's MD is usually fatal by the late teens; Becker's MD is less serious and may not be fatal until the 50s.

Prevention

As of 2004 there is no way to prevent the genetic mutations that cause myopathies, nor are there ways to prevent metabolic and endocrine failures that result in myopathies. Inflammatory myopathies often occur as a result of exposure to viruses or drugs, but it is almost impossible to predict their development.

Parental concerns

Individuals with known myopathies who wish to become parents may want to seek genetic counseling before attempting to have children.

See also Muscular dystrophy ; Myotonic dystrophy .

Resources

Barnes, P. R. J., et al. Myopathies in Clinical Practice. Oxford, UK: Isis Medical Media, 2003.

The Official Patient's Sourcebook on Mitochondrial Myopathies. San Diego, CA: Icon Group International, 2002.

Muscular Dystrophy Association. 3300 East Sunrise Dr., Tucson, AZ 85718. Web site: http://ww.mdausa.org.

National Organization for Rare Disorders Inc. 55 Kenosia Ave, PO Box 1968, Danbury, CT 06813–1968. Web site: http://www.rarediseases.org.

"NINDS Myopathy Information Page." National Institute of Neurological Disorders and Stroke , January 6, 2005. Available online at <www.ninds.nih.gov/health_and_medical/disorders/myopathy.htm> (accessed January 13, 2005).

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