What is it?

Myositis is defined as the inflammation of muscles (“myo” means muscle and “itis” means inflammation). The inflammatory myopathies are a group of disorders characterized by inflammation and weakness mainly of the muscles closest to the trunk of the body (proximal muscles). These disorders include polymyositis, dermatomyositis and inclusion body myositis. Myositis may be associated with inflammation in other organs, including the joints, heart, lungs, intestines and skin. In dermatomyositis, a rash develops in addition to the muscle inflammation.

Muscle inflammation also can occur in people with other rheumatic diseases, such as rheumatoid arthritis, lupus and scleroderma.

Many people with myositis respond well to treatment. Some people have a single attack and are able to stop taking medications after about a year. Others have recurrent attacks of the disease that do respond to medication. Although less common, some people experience chronically active myositis and must receive continuous treatment.

What causes it?

The inflammatory myopathies are believed to be autoimmune diseases, meaning that the body’s immune system goes awry and attacks its own tissues. It is thought that these diseases are triggered by environmental factors in genetically susceptible individuals. The specific causes or triggering events of the inflammatory myopathies are unknown, but viruses have been implicated.

In all people with myositis, lymphocytes (white blood cells) attack muscle. The body generates antibodies that attack healthy tissue in different parts of the body. These autoantibodies and some of the lymphocytes turn against the body’s own tissues and damage the muscles and skin.

What are the effects?

The clinical features of polymyositis in the adult are representative of the inflammatory myopathies.

Polymyositis
Polymyositis usually begins in a person’s 20s to 40s. It is rare in a person younger than 18 years. For most people, polymyositis develops slowly over a period of months or even years. The disease often comes and goes for no apparent reason. Characteristics of polymyositis include:

• Flares: Flares usually are recognized by increasing symptoms of muscle weakness, fatigue, skin changes or arthritis.

• Proximal muscle weakness: Symmetric weakness of the large muscles closest to the trunk of the body is the main symptom of polymyositis. Initially, hips, thighs and shoulders are commonly affected. Shoulder and pelvic girdle muscles are most severely affected. The weakness may make it difficult to lift heavy objects, climb stairs, or lift your arm to comb hair or put on a coat. The weakness can progress to the point that it is difficult to rise from a chair, get out of the tub or roll out of bed. If your neck muscles are involved, which is the case for about half of patients, it may be difficult to lift your head when lying down.

• Throat muscle weakness: Less commonly, throat muscles may be affected, making it hard to swallow and possibly affecting your voice. Swallowing difficulty can result in aspiration pneumonia.

• Lung problems: Polymyositis can cause weakness of the muscles required for breathing. It may also cause fibrosis (build up of excessive scars tissue) of the lungs, which results in coughing and shortness of breath.

• Heart problems: In rare cases, myocarditis (inflammation of the muscular walls of the heart) and congestive heart failure (heart disease accompanied by breathlessness and excessive retention of sodium and water) occur in polymyositis.

• Muscle atrophy: After having the disease for a long time, you may lose muscle mass.

• Systemic symptoms: You may experience fever, weight loss, general malaise and Raynaud’s phenomenon (an extreme sensitivity and discoloration to cold, especially in your fingers).

• Muscle pain: In some patients, the muscles ache and are tender to the touch, but these symptoms usually go away with treatment.
• Joint pain: Pain in the joints commonly occurs during periods when the disease is active, but the joints are not usually warm or swollen.

Dermatomyositis

• Dermatomyositis is an inflammatory muscle disease, like polymyositis; however, it has a somewhat severe onset and affects both children and adults. Clinical features of dermatomyositis include all those of polymyositis, plus a variety of skin manifestations, such as:

• Rashes: Dermatomyositis is characterized by a rash accompanying or, more often, preceding muscle weakness by a year or more. A patchy, red-to-purple rash may be found on the back of the shoulders and neck (shawl sign), front of the neck and upper chest (V-sign), face and forehead.

• Gottron’s papules: Lacy, pink or violet areas (raised or scaly) found symmetrically on the knuckles, elbows, knees and ankles are characteristic or diagnostic for dermatomyositis.

• Eyelids: purplish discoloration of the eyelids with swelling around the eyes

• Dystrophic cuticles: changes around the bed of the nails especially in the hands

• Calcium deposits: Calcification is hardening of skin and muscles as a result of calcium salt deposits. Calcification doesn’t often occur in adults, but children with dermatomyositis may develop calcium deposits years after the disease starts. Deposits usually occur in the shoulder, pelvis, hip, calf and thigh and may severely limit motion.

Juvenile dermatomyositis
Juvenile dermatomyositis differs from the adult form because of the coexistence of vasculitis (inflammation of blood vessels), calcium deposits and defective metabolism of fat. In juvenile dermatomyositis, the skin lesions and weakness almost always occur at the same time, but the severity and progression of each symptom varies from patient to patient.

Inclusion body myositis
Inclusion body myositis mainly affects older individuals. The symptoms begin and progress slowly. Symptoms often are present for five to six years before diagnosis. Swallowing difficulties are noted in more than 20 percent of patients. As muscle weakness becomes severe, it is accompanied by muscle wasting and diminished deep-tendon reflexes. Unlike polymyositis and dermatomyositis, the muscle weakness is often not as symmetric, and may be prominent in the smaller musches of the forearms and calfs. In some patients, the disease continues a slow, steady progression; in others it plateaus, leaving the individual with fixed weakness and atrophy of the involved muscles.