Overview of Myopathy
Myopathies are diseases that affect muscles connected to bones (called skeletal muscles), such as the biceps in the upper arm and the quadriceps in the thigh. Myopathies can be caused by inherited genetic defects (e.g., muscular dystrophies), or by endocrine, inflammatory (e.g., polymyositis), and metabolic disorders.
Nearly all types of myopathy produce weakening and atrophy of skeletal muscles, especially those muscles closest to the center of the body (called the proximal muscles), such as the thigh and shoulder muscles. Muscles further from the center of the body (called the distal muscles), such as those in the hands and feet, are generally affected less often.
Some myopathies, such as the muscular dystrophies, usually develop at an early age, and others develop later in life. Some conditions worsen over time and do not respond well to treatment and others are treatable and othe remain stable. When few treatments are available that address the root cause of the disease, the myopathy is labeled "nonspecific muscle myopathy."
Incidence and Prevalence of Myopathy
Worldwide incidence of inheritable myopathies is about 14 percent. Of all inheritable myopathyies, central core disease accounts for 16 percent of cases; nemaline rod myopathy accounts for 20 percent; centronuclear myopathy accounts for 14 percent; and multicore myopathy accounts for 10 percent.
Prevalence of muscular dystrophy is higher in males. In the United States, Duchenne MD 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 type of endocrine myopathy and endocrine disorders are more common in women than in men. Metabolic myopathies are rare, but diagnosis of these conditions is increasing in the United States.