Polymyositis and dermatomyositis are uncommon systemic rheumatic disorders characterized by inflammatory and degenerative changes in the muscles (polymyositis) or in the skin and muscles (dermatomyositis).

Polymyositis is a disease of muscle featuring inflammation of the muscle fibers. The cause of the disease is not known. It begins when white blood cells, the immune cells of inflammation, spontaneously invade muscles. The muscles affected are typically those closest to the trunk or torso. This results in weakness that can be severe. Polymyositis is a chronic illness featuring progressive muscle weakness with periods of increased symptoms, called flares or relapses, and minimal or no symptoms, known as remissions.

Polymyositis is slightly more common in females. It affects all age groups, although its onset is most common in middle childhood and in the 20s. Polymyositis occurs throughout the world. Polymyositis can be associated with a characteristic skin rash and is then referred to as “dermatomyositis.” Dermatomyositis in children is referred to as juvenile dermatomyositis. “Amyopathic dermatomyositis” is the term used to describe people who have skin changes compatible with dermatomyositis but do not have diseased muscle involvement.

Polymyositis can also affect other areas of the body and is, therefore, referred to as a systemic illness. Occasionally, it is associated with cancer or with other diseases of connective tissue (such as systemic lupus erythematosus, scleroderma, and rheumatoid arthritis). Depending on which other diseases it is associated with, it may be referred to as an “overlap syndrome” or “mixed connective tissue disease.”

Polymyositis is a immune mediated disorder of unknown aetiology. It is characterized by proximal muscular weakness and inflammatory and degenerative lesions of the striated muscles. When it is accompanied by skin lesions, it is called dermatomyositis.

It is not a very common disease and mainly involves adults in the age group of 30-55 years, females are affected twice as common as men. Children in the age group of 10-15 years may present with inflammatory myopathy, skin involvement and vasculitis.

Aetiology

A number of factors ranging from immunological, viral to environmental factors have been incriminated. Coxsackie viral infection in adults has been associated with dermatomyositis. Its association with other autoimmune disorders and presence of circulating antibodies (deposition of immunoglobulins on intramuscular blood vessels) suggest role of autoimmunity.

Cell-mediated immunity theory is supported by the presence of helper and cytotoxic T lymplocytes in the inflammatory infiltrate. These damage the muscle fibres by cell-mediated cytotoxicity or by lymphotoxis. There is higher incidence of dermatomyositis in elderly with malignancy.

Classification

Dermatomyositis – Polymyositis group has been broadly classified as follows:

1. Primary idiopathic polymyositis (Adult form)

2. Primary idiopathic dermatomyositis (Adult form)

3. Dermatomyositis or polymyositis associated with malignancy

4. Childhood dermatomyositis (or polymyositis) associated with vasculitis.

5. Polymyositis associated with collagen vascular disorders.

Pathology

The involved muscles to start with are slightly swollen and edematous. As the disease progresses the muscles become grey, atrophic and fibrous. Muscles show fragmentation, loss of cross striation cellular infiltration, necrosis, phagocytosis and regenerative activity.

Regenerative activity results in the appearance of sarcolemmal nuclei and nucleoli. Skin shows swelling with mononuclear infiltrates surrounding the vessels, atrophy of dermis followed by fibrosis. A large number of CD 8+ monoclear cells surround and invade non-necrotic muscle tissue.

Clinical features

The disease starts insidiously with symmetrical muscle weakness and wasting involving proximal muscles of shoulder and pelvic girdles. Muscle tenderness is present in acute form of the disease.

There is difficulty in arising from the squatting position, climbing or descending stairs. As the disease progresses, there is fibrosis and contractures. Reflexes are diminished.

Arthritis or arthralgia involving small joints of the hands may precede the onset of muscular weakness. Because of the involvement of esophageal and pharyngeal muscles dysphagia is found in 50% of the cases. Occasionally patients may exhibit Raynaud’s phenomenon.

Cases of dermatomyositis have a classic rash which may precede coexist or follow features of polymyositis. It is in the form of a liliac or heliotrope discoloration of the upper eyelids with periorbital oedema along with erythematous rash over the face, neck, upper trunk, knuckles, elbows, knees, medial malleoli and back. Sometimes cutaneous lesions may ulcerate.

Childhood dermatomyositis mainly involves female children in the age group of 10-15 years and has slow progression. Muscle wasting is associated with skin involvement and vasculitis. Subcutaneous calcification is frequently present in this group.

Because of vasculitis rarely infarctions may occur in kidney and gastrointestinal tract.

When elderly have picture of polymyositis or dermatomyositis there is commonly association of malaginancy of the stomach, lung or breast. Very often muscle weakness precedes features of polymyositis.

Investigations

1. Muscle enzymes: Levels of muscle enzymes such as C.K. (Creatine kinase) Aldolase, Serum amino transferase and lactic acid dehydrogenase (LDH) are elevated and are used to assess the actively of the disease.

2. Rheumatoid factor and antinuclear antibodies are positive in 30-40% of cases. Serum myoglobulin levels are raised and myoglobin can be found in urine when there is extensive muscle destruction.

3. ESR may be raised. Normocytic normochromic anaemia is present.

4. Electromyography shows increased insertional activity (muscle irritability) with action potentials (fibrillary) polyphasic low amplitude motor unit potentials and high frequency repetitive discharges.

5. Muscle biopsy. There is infiltration of inflammatory cells along with evidence of muscle degeneration, fragmentation, atrophy and regeneration of muscle fibers.

Diagnosis

It is based upon characteristic clinical picture (Proximal muscle weakness) elevated muscle enzymes, electromyographic abnormalities and muscle biopsy.

Treatment

Glucocorticoids in high doses are to be given in a severe case of polymyositisdermatomyositis. Dose of prednisolone is 40-60 mg/ day and is to be given for 2-3 months. Improvement generally begins by 1-3 weeks.

Once improvement has occurred the dose is gradually tapered off but it should not be stopped abruptly Chances of relapse are there and some patients may then require high doses.

When disease is severe and not responding to corticosteroids cytotoxic drugs are employed. Azathioprine (2.5 mg/kg body weight per day in divided doses) is the most commonly used and in combination with corticosteroids shows better response.

Other cytotoxic drugs being employed are cyclophosphamide and methotrexate. In addition to drug therapy patient should be advised bed rest in acute stage. As the acute condition subsides physiotherapy may be advised.

Prognosis

It shall depend on the severely of the disease process and response to treatment. Patients with polymyositis associated with malignancy carry poor prognosis.

Many patients of dermatomyositis make full recovery though some degree of weakness of muscles may persist.