Myopathy

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 muscle—muscles connected to bones, like the biceps in the upper arm and quadriceps in the thigh—are 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 metabolic disorders.

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 talking.

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 5–10 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 the 50s.