Myopathy is a general term referring to any disease of muscles. Myopathies can be
acquired or inherited, and can occur at birth or later in life. The muscular dystrophies
are examples of myopathies. General symptoms of myopathy include weakness of limbs,
usually proximal (located close to the center of the body). Some individuals report that
their myopathy emerges during exercise. In some cases, the symptoms diminish as exercise
increases. Depending upon the type of myopathy, one muscle group may be more affected than
another. In some instances, individuals have myopathy but report no symptoms. In the
inherited myopathies, some family members may be unaffected, while other family members
may have a range of symptoms. Myopathy can result from endocrine disorders, metabolic
disorders, infection or inflammation of the muscle, certain drugs, and mutations in genes
Information about Myopathy
Diseases that affect skeletal musclemuscles connected to bones, like the biceps
in the upper arm and quadriceps in the thighare called "myopathies."
Myopathies can be caused by many types of conditions, including inherited genetic defects
(e.g., the muscular dystrophies), and endocrine, inflammatory (e.g., polymyositis), and
Nearly all of the myopathies produce weakening and atrophy of skeletal muscles,
especially those closest to the center of the body (called the proximal muscles), such as
the thigh and shoulder muscles. Muscles furthest from the center of the body (called the
distal muscles), such as those in the hands and feet, are generally less affected.
Some myopathies, like the muscular dystrophies, develop at a very early age; others
develop later in life. Some worsen over time and do not respond well to treatment; others
are treatable and remain stable. Many times a myopathy is simply labeled "nonspecific
muscle myopathy" because there are few treatments available that address the root
cause of disease.
Skeletal Muscle Myopathy
Every time the body moves, a skeletal muscle contracts. Skeletal muscles are attached
to parts of the skeleton and make possible voluntary movement like walking, reaching, and
Depending on where they are and how they function, skeletal muscles vary considerably
in size and shape, but they are all made up of bundles of fibers. Each fiber is made up of
a sophisticated system of sliding filaments that, when "told" what do by the
brain via the nervous system, cause contraction and movement. The nerves that command the
muscle are called motorneurons, and the place where a motorneuron meets the muscle is
called the neuromuscular junction.
Motorneurons communicate to muscles by secreting biochemical substances. The skeletal
muscles receive that biochemical energy and transform it into the mechanical energy that
causes muscles to contract and move the human body.
When muscles are affected by disease, many changes occur that may lead to weakness,
pain, and atrophy. The muscle fibers can be destroyed or can show significant atrophy
(shrinkage). In the inflammatory myopathies, white blood cells and other blood elements
may attack parts of the muscle and surrounding blood vessels. Scar tissue may take the
place of normal muscle. In some of the metabolic myopathies, abnormal amounts of
biochemical substances may accumulate in the muscles.
Incidence and Prevalence of Myopathy
Worldwide incidence of all inheritable myopathies is about 14%. Central core disease
accounts for 16% of cases; nemaline rod myopathy for 20%; centronuclear myopathy for 14%;
and multicore myopathy for 10%.
Prevalence of muscular dystrophy is higher in males. In the United States, Duchenne and
Becker MD occur in approximately 1 in 3300 boys. Overall incidence of muscular dystrophy
is about 63 per 1 million.
Worldwide incidence of inflammatory myopathies (e.g., dermatomyositis, polymyositis) is
about 510 per 100,000 people. These disorders are more common in women.
Incidence and prevalence of endocrine and metabolic myopathies are unknown.
Corticosteroid myopathy is the most common endocrine myopathy and endocrine disorders are
more common in women. Metabolic myopathies are rare but diagnosis of these conditions is
increasing in the United States.
Prognosis of Myopathy
The prognosis for patients with myopathy depends on the type and severity of the
individual disease. In most cases, the myopathy can be successfully treated and the
patient returned to normal life.
Muscular dystrophy, however, is generally 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