Congenital myopathies are rare, inherited disorders of the muscles that cause reduced muscle tone and weakness. These disorders are present at birth or show up during infancy.
There are several types of congenital myopathies. The three most common types of congenital myopathy are
Symptoms and lifespan vary with the type and severity of the myopathy.
Central core myopathy and multiminicore myopathy (core myopathies) are the most common form of congenital myopathy.
Most newborns have severely reduced muscle tone (hypotonia, or "floppiness") and mild muscle weakness, but sometimes symptoms of core myopathy do not manifest until adulthood. Many people also have facial weakness. The weakness does not become worse, and life expectancy is normal, but some people are severely affected and require a wheelchair. People who have central core myopathy are at increased risk of a life-threatening disorder called malignant hyperthermia.
Centronuclear myopathies are inherited in various ways. The forms that are inherited via a gene carried on the X chromosome (see X-Linked Inheritance) are the most common and most severe, and most affected children do not survive beyond the first year of life. Symptoms of the autosomal dominant forms typically appear in adolescence or adulthood. People who have one of these forms have muscle pain caused by exercise, facial weakness, droopy eyelids, and weakness of the eye muscles.
Nemaline myopathy, one of the more common congenital myopathies, can be autosomal dominant or autosomal recessive.
Nemaline myopathy may be severe, moderate, or mild. People who have a severe form may have weakness of the muscles used for breathing and respiratory failure. People who have a moderate form may have weakness in muscles of the face, neck, trunk, and feet that becomes worse over time, but their life expectancy may be nearly normal. People who have a mild form have weakness that does not worsen, and their life expectancy is normal.
The diagnosis of congenital myopathy is based on characteristic symptoms of weakness that are caused by each specific type.
Doctors confirm the diagnosis by taking a sample of the weak muscle tissue for biopsy (removal of a piece of tissue for examination under a microscope).
Specific treatments are not available, but physical therapy may help maintain muscle function.