Myopathy is a term used to describe any form of muscle disease. The inflammatory myopathies are a group of diseases that involve chronic muscle inflammation (myositis), accompanied by muscle weakness.
There are three main types of chronic or persistent inflammatory myopathy; polymyositis, dermatomyositis, and inclusion body myositis.
Muscle inflammation may be caused by an allergic reaction, exposure to a toxic substance, another disease such as cancer or rheumatic conditions, or a virus or other infectious agent. The chronic inflammatory myopathies are idiopathic, meaning they have no known cause. They are thought to be autoimmune disorders, in which the body’s own white blood cells (that normally fight disease) attack blood vessels, normal muscle fibres, and connective tissue in organs, bones, and joints.
The inflammatory myopathies are a group of rare disorders that affect both adults and children, although dermatomyositis is the most common chronic form in children. Polymyositis and dermatomyositis are more common in women than in men. A rare childhood onset form of polymyositis and dermatomyositis can occur in children between the ages of 2 and 15 years. Inclusion body myositis usually affects individuals over age 50.
General symptoms of chronic inflammatory myopathy include slow but progressive muscle weakness that starts in the proximal muscles—those muscles closest to the trunk of the body. Inflammation damages the muscle fibres, causing weakness, and may affect the arteries and blood vessels that run through the muscle. Other symptoms include fatigue after walking or standing, tripping or falling, and difficulties in swallowing or breathing. Some individuals may have slight muscle pain or muscles that are tender to touch.
Polymyositis affects skeletal muscles on both sides of the body. It is rarely seen in persons under age 18 and is most common the ages of 31 and 60. People with polymyositis may also experience arthritis, shortness of breath, and heart arrhythmias.
Dermatomyositis is characterised by a skin rash that precedes or accompanies progressive muscle weakness. The rash is patchy, with purple or red discolorations, and characteristically develops on the eyelids and on muscles used to extend or straighten joints, including knuckles, elbows, knees, and toes. Red rashes may also occur on the face, neck, shoulders, upper chest, back, and other locations, and there may be swelling in the affected areas. The rash sometimes occurs without obvious muscle involvement. Adults with dermatomyositis may experience weight loss or a low-grade fever, have inflamed lungs, and be sensitive to light. Adult dermatomyositis, unlike polymyositis, may accompany tumors of the breast, lung, female genitalia, or bowel.
Inclusion body myositis (IBM) is characterized by progressive muscle weakness and wasting. The onset of muscle weakness is generally gradual – over months or years – and affects both proximal and distal muscles, often only one side of the body. Small holes called vacuoles are sometimes seen in the cells of affected muscle fibers. Falling and tripping are usually the first noticeable symptoms of IBM. Symptoms of the disease usually begin after the age of 50, although the disease can occur earlier. Unlike polymyositis and dermatomyositis, IBM occurs more frequently in men than in women.
Juvenile myositis has some similarities to adult dermatomyositis and polymyositis. It typically affects children ages 2 to 15 years, with symptoms that include proximal muscle weakness and inflammation, edema (an abnormal collection of fluids within body tissues that causes swelling), muscle pain, fatigue, skin rashes, abdominal pain, fever, and contractures (chronic shortening of muscles or tendons around joints, caused by inflammation in the muscle tendons, which prevents the joints from moving freely).
There is no cure for the chronic inflammatory myopathies but many of the symptoms can be treated. Options include medication, physical therapy, exercise, heat therapy (including microwave and ultrasound), orthotics and assistive devices, and rest. Inflammatory myopathies that are caused by medicines, a virus or other infectious agents, or exposure to a toxic substance usually abate when the harmful substance is removed or the infection is treated. If left untreated, inflammatory myopathy can cause permanent disability.
There is no standard course of treatment for IBM. The disease is generally unresponsive to corticosteroids and immunosuppressive drugs. Some evidence suggests that immunosuppressive medications or intravenous immunoglobulin may have a slight, but short-lasting, beneficial effect in a small number of cases. Physical therapy may be helpful in maintaining mobility. Other therapy is symptomatic and supportive.