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).

Genetic myopathies

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:

• central core disease
• centronuclear (myotubular) myopathy
• myotonia congenita
• nemaline myopathy
• paramyotonia congenita
• periodic paralysis (hypokalemic and hyperkalemic forms)
• mitochondrial myopathies

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:

• Central core disease: mild weakness of voluntary muscles, especially in the hips and legs; hip displacement; delays in reaching developmental motor milestones; problems with running, jumping, and climbing stairs develop in childhood.
• Centronuclear myopathy: weakness of voluntary muscles, including those on the face, arms, legs, and trunk; drooping upper eyelids; facial weakness; foot drop; affected muscles almost always lack reflexes.
• Myotonia congenita: voluntary muscles of the arms, legs, and face stiff or slow to relax after contracting (myotonia); stiffness triggered by fatigue, stress, cold, or long rest periods, such as a night's sleep ; stiffness can be relieved by repeated movement of the affected muscles.
• Nemaline myopathy: moderate weakness of voluntary muscles in the arms, legs, and trunk; mild weakness of facial muscles; delays in reaching developmental motor milestones; decreased or absent reflexes in affected muscles; long, narrow face; high-arched palate; jaw projects beyond upper part of the face.
• Paramyotonia congenita: stiffness of voluntary muscles in the face, hands, and forearms; attacks spontaneous or triggered by cold temperatures; stiffness made worse by repeated movement; episodes of stiffness last longer than those seen in myotonia congenita.
• Periodic paralysis: attacks of temporary muscle weakness (muscles work normally between attacks); in the hypokalemic (low potassium) form, attacks triggered by vigorous exercise , heavy meals high in carbohydrates, insulin, stress, alcohol, infection, pregnancy; in the hyperkalemic (high potassium) form, attacks triggered by vigorous exercise, stress, pregnancy, missing a meal, steroid drugs, high potassium intake.
• Mitochondrial myopathies: symptoms vary quite widely with the form of the disease and may include progressive weakness of the eye muscles (ocular myopathy), weakness of the arms and legs, or multisystem problems primarily involving the brain and muscles.

Endocrine-related myopathies

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.

Inflammatory myopathies

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.

Muscular dystrophies

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.

When to call the doctor

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:

• periodic paralysis: medication and dietary changes
• hyperthyroid or hypothyroid myopathy: treatment of the underlying thyroid abnormality
• myositis ossificans: medication to help prevent abnormal bone formation, but there is no cure following onset
• central core disease: no treatment
• nemaline myopathy: no treatment
• centronuclear (myotubular) myopathy: no treatment
• paramyotonia congenita: treatment often unnecessary
• myotonia congenita: drug treatment (if necessary), but drugs do not affect the underlying disease, and attacks may still occur

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

The prognosis for patients with myopathy depends on the type and severity of the individual's disease. In most cases, the myopathy symptoms can be successfully treated, but in others, the disease can be fatal in childhood or adolescence.

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

BOOKS

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.

ORGANIZATIONS

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.

WEB SITES

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

Tish Davidson, A.M.

Carol A. Turkington