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Myopathy is a term used to describe muscle disease. The inflammatory myopathies are a group of diseases that involve chronic muscle inflammation, accompanied by muscle weakness. Another word for chronic inflammation of muscle tissue is myositis.
The three main types of chronic, or persistent, inflammatory myopathy are polymyositis, dermatomyositis, and inclusion body myositis.
These rare disorders may 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.
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 fibers, 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 difficulty swallowing or breathing. Some patients may have slight muscle pain or muscles that are tender to touch.
Polymyositis affects skeletal muscles (involved with making movement) on both sides of the body. It is rarely seen in persons under age 18; most cases are in patients between the ages of 31 and 60. In addition to symptoms listed above, progressive muscle weakness leads to difficulty swallowing, speaking, rising from a sitting position, climbing stairs, lifting objects, or reaching overhead. Patients with polymyositis may also experience arthritis, shortness of breath, and heart arrhythmias.
Dermatomyositis is characterized by a skin rash that precedes or accompanies progressive muscle weakness. The rash looks patchy, with bluish-purple or red discolorations, and characteristically develops on the eyelids and on muscles used to extend or straighten joints, including knuckles, elbows, heels, 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. Children and adults with dermatomyositis may develop calcium deposits, which appear as hard bumps under the skin or in the muscle (called calcinosis). Calcinosis most often occurs 1-3 years after disease onset but may occur many years later. These deposits are seen more often in childhood dermatomyositis than in dermatomyositis that begins in adults. Dermatomyositis may be associated with collagen-vascular or autoimmune diseases.
In some cases of polymyositis and dermatomyositis, distal muscles (away from the trunk of the body, such as those in the forearms and around the ankles and wrists) may be affected as the disease progresses. Polymyositis and dermatomyositis may be associated with collagen-vascular or autoimmune diseases. Polymyositis may also be associated with infectious disorders.
Inclusion body myositis (IBM) is characterized by progressive muscle weakness and wasting. IBM is similar to polymyositis but has its own distinctive features. The onset of muscle weakness is generally gradual (over months or years) and affects both proximal and distal muscles. Muscle weakness may affect only one side of the body. Small holes called vacuoles are seen in the cells of affected muscle fibers. Falling and tripping are usually the first noticeable symptoms of IBM. For some patients the disorder begins with weakness in the wrists and fingers that causes difficulty with pinching, buttoning, and gripping objects. There may be weakness of the wrist and finger muscles and atrophy (thinning or loss of muscle bulk) of the forearm muscles and quadricep muscles in the legs. Difficulty swallowing occurs in approximately half of IBM cases. 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). Children with juvenile myositis may also have difficulty swallowing and breathing, and the heart may be affected. Approximately 20 to 30 percent of children with juvenile dermatomyositis develop calcinosis. Juvenile patients may not show higher than normal levels of the muscle enzyme creatine kinase in their blood but have higher than normal levels of other muscle enzymes.
Diagnosis is based on the patient's medical history, results of a physical exam and tests of muscle strength, and blood samples that show elevated levels of various muscle enzymes and autoantibodies. Diagnostic tools include electromyography to record the electrical activity that controls muscles during contraction and at rest, and ultrasound and magnetic resonance imaging to reveal abnormal muscle and evaluate muscle disease. A muscle biopsy can be examined by microscopy for signs of chronic inflammation, muscle fiber death, vascular deformities, or the changes specific to the diagnosis of IBM. A skin biopsy can show changes in the skin layer in patients with dermatomyositis.
Muscle inflammation may be caused by an allergic reaction, exposure to a toxic substance or medicine, another disease such as cancer or rheumatoid 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 white blood cells (that normally fight disease) attack blood vessels, normal muscle fibers, and connective tissue in organs, bones, and joints.
The chronic inflammatory myopathies cannot be cured in most adult patients 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 agent, 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.
Polymyositis and dermatomyositis are first treated with high doses of prednisone or another corticosteroid drug. This is most often given as an oral medication but can be delivered intravenously. Immunosuppressant drugs, such as azathioprine and methotrexate, may reduce inflammation in patients who do not respond well to prednisone. Periodic treatment using intravenous immunoglobulin can further recovery in patients with dermatomyositis or polymyositis. Other immunosuppressive agents that may treat the inflammation associated with dermatomyositis and polymyositis include cyclosporine A, cyclophosphamide, and tacrolimus. Physical therapy is usually recommended to prevent muscle atrophy and to regain muscle strength and range of motion. Bed rest for an extended period of time should be avoided, as patients may develop muscle atrophy, decreased muscle function, and joint contractures. A low-sodium diet may help to counter edema and cardiovascular complications.
Many patients with dermatomyositis may need a topical ointment (such as topical corticosteroids or tacrolimus) or additional treatment for their skin disorder. A high-protection sunscreen and protective clothing should be worn by all patients, particularly those who are sensitive to light. Surgery may be required to remove calcium deposits that cause nerve pain and recurrent infections.
There is no standard course of treatment for IBM. The disease is generally unresponsive to corticosteroids and immunosuppressive drugs. Some evidence suggests that 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.