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).
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,
(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
Some myopathies are caused by inflammation. Inflammation is a protective
response of injured tissues characterized by redness, increased heat,
in the affected area. Examples of this type of myopathy include
, 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
. 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
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
. 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.
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
, 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
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.
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
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
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.
—A diagnostic test that records the electrical activity of the
muscles that control eye movement.
—An abnormally high level of potassium in the blood.
—A condition characterized by a deficiency of potassium in the
—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.
—Spherical or rod-shaped structures of the cell. Mitochondria
contain genetic material (DNA and RNA) and are responsible for
converting food to energy.
—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.
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
Individuals with known myopathies who wish to become parents may want to
seek genetic counseling before attempting to have children.
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:
National Organization for Rare Disorders Inc.
55 Kenosia Ave, PO Box 1968, Danbury, CT 06813–1968. Web site:
"NINDS Myopathy Information Page."
National Institute of
Neurological Disorders and Stroke
, January 6, 2005. Available online at
(accessed January 13, 2005).