There are many different types of myopathies. All involve muscle dysfunction that can cause muscle weakness and deterioration. Myopathies can be inherited or acquired. They may appear at birth or later in life.

Myopathies are generally categorized into groups, according to certain characteristics. There may be other ways of grouping myopathies, but the four main groups are described here. 

One group of myopathies, muscular dystrophy (MD), includes a collection of disorders that cause muscle weakness and deterioration. Symptoms may also include impaired reflexes, movement and breathing. There are many types of MD that may differ in the typical age when symptoms begin, which muscle groups are affected, and how the disease progresses. MD occurs as the result of genetic mutations that block the body from producing proteins needed to build and maintain muscle. As muscle becomes damaged during normal use, it cannot be repaired through the typical body processes. The most common MD types are Duchenne muscular dystrophy and Becker muscular dystrophy. According to the Centers for Disease Control and Prevention, 400 to 600 boys are born with these types of MD each year.

Inflammatory myopathies are muscle diseases characterized by an intense immune system reaction in the muscle and surrounding tissue. They are considered autoimmune disorders. Because the immune system is occupied with attacking body cells, patients may be at greater risk of infection from bacteria, viruses and other organisms. This group of myopathies includes:

• Polymyositis. An uncommon disease that causes continued and progressive weakness of the body core, especially in the shoulders and hips. Muscle weakness tends to be symmetrical. It may also cause difficulty swallowing, joint or muscle tenderness and fatigue. The age of onset can vary greatly. Most cases begin between the ages of 40 and 60, although other age groups may be affected. Polymyositis rarely occurs in patients under 18.
  
  
• Dermatomyositis. Similar to polymyositis, except that a skin rash also develops, sometimes before muscle weakness is experienced. This rash may be patchy and blue/purple in color. It results from the immune system attacking blood vessels in the skin. The degradation of blood vessels in muscle leads to muscle weakness. In most cases, symptoms first occur during two different periods: between the ages of 5 and 15, and between 40 and 70. The childhood form may also include symptoms of calcium deposits under the skin, stomach ulcers and infections. Dermatomyositis is more common in women than in men.
  
  
• Inclusion body myositis (IBM). This type is similar to and may be confused with polymyositis. IBM involves the formation of empty spaces, known as vacuoles, in muscle tissue. IBM usually develops after age 50 and symptoms gradually become more severe over a period of months to years. Symptoms may include falling and tripping, weakness in the hands, shrinking of the forearms and difficulty swallowing. This type of inflammatory myopathy is more common in men than in women.

Congenital myopathies are a group of muscle diseases that are present at birth. They are often hereditary conditions, although they may also be due to nonhereditary factors, such as a spontaneous genetic defect. Unlike many other types of myopathy, the congenital types are usually not progressive. There are several forms, including:

• Central core disease. Muscle weakness occurs more frequently in the legs than arms and may affect a child’s motor development (e.g., delayed ability to walk, difficulty climbing stairs). This type of congenital myopathy is associated with malignant hyperthermia, a life-threatening condition that causes fever and muscle contractions when the patient is under general anesthesia.
  
  
• Nemaline myopathy. Disease characterized by mutations of filament proteins, which are vital components of muscle tissue involved in muscle contraction. Symptoms include generalized weakness (especially in the face, head and lungs), loss of muscle tone, skeletal abnormalities, delays in walking and respiratory difficulties. The most severe form of nemaline myopathy causes death in newborns with the disease. Other forms are less severe and may cause delays in typical motor development milestones in children. In rare cases, symptoms of increasing weakness in proximal skeletal muscles (those closest to the body’s core) may not occur until adulthood.
  
  
• Centronuclear myopathy (also known as myotubular myopathy). This type of congenital myopathy causes severe weakness (e.g., loss of muscle tone, paralysis) in newborns that is fatal in many cases. It is most likely to occur in males. Sometimes symptoms do not appear until late infancy. When this occurs, it can cause delays in a child’s motor development and may be associated with skeletal deformities (e.g., scoliosis). In some cases, symptoms may not appear until adolescence or young adulthood. Unlike many other congenital myopathies, centronuclear myopathy may be progressive with symptoms that become increasingly severe over time.  

Drug-induced myopathies are those caused by the use of medications or illicit drugs. In some cases, severe weakness may occur. Medications that may cause muscle damage include immunosuppressants (e.g., corticosteroids) and those used to treat high cholesterol, gout, HIV/AIDS and malaria. Other drugs that may cause myopathy include syrup of ipecac, alcohol, cocaine, amphetamines and heroin.

Mitochondrial myopathies are a group of neuromuscular diseases that may involve both nerve and muscle damage. Thus, although myopathy is in its name, disorder or dysfunction of the muscles alone may not be the primary problem in these diseases.