Dermatomyositis and polymyositis present with these common features:
Dermatomyositis has the additional cutaneous manifestations:
Needle electromyography (EMG) of muscles shows myopathic changes.
Serum levels of muscle enzymes are elevated. The most sensitive enzyme is creatine kinase (CK).
Muscle tissue biopsy shows inflammatory changes in muscle tissue. This is the definitive test for establishing the diagnosis of myositis and for excluding other neuromuscular diseases .
Magnetic resonance imaging is not routinely used for the diagnosis of myositis, but it may guide the location of the muscle biopsy in certain clinical settings.
Blood tests for Myositis-specific autoantibodies (MSAs) and myositis associated autoantibodies (MAAs) can be used to predict the clinical pattern and prognosis of disease.
Cancer screening: In a patient of dermatomyositis screening for malignancy should be done at initial diagnostic evaluation and yearly thereafter.
The goal of the treatment in myositis is to improve muscle strength and the function in activities of daily living, and ameliorate the extramuscular manifestations like rash, dysphagia, dyspnea and fever.
Drugs used in the treatment of dermatomyositis and polymyositis include:
1. Glucocorticoids. The initial treatment of choice is oral prednisone1 mg/kg per day for 3- 4 weeks, followed by tapering over a period of 10 weeks to 1 mg/kg every other day. If treatment is effective, an objective increase in muscle strength is observed within 3 months. Steroid myopathy increasing the muscle weakness even further is a possible adverse effect.
2. Immunosuppressive therapy. Lack of response to prednisone or severe side-effects are indication. The drugs used are azathioprine, methotrexate, cyclophosphamide, chlorambucil and recently, mycophenolate mofetil.
3. Immunomodulation. Intravenous immunoglobulin (IVIg) has shown good results in a controlled trial of patients with refractory dermatomyositis.
Inclusion body myositis is difficult to treat, and is generally resistant to immunosuppressive therapies. Prednisone together with azathioprine or methotrexate is often tried in newly diagnosed patients .
The following are the causes of myositis:
Both of the above conditions are thought to arise from autoimmunity to environmental factors, possibly triggered by viral infections or certain drugs.
The prevalence of the myositis is estimated at 1 in 100,000. Polymyositis affects adults, peaking between 50 and 60 years, with females affected twice as much as males. Dermatomyositis affects both children and adults, with the female to male ratio being 2:1. Inclusion body myositis affects person above 50 years of age and the male to female ratio is 3:1. The mean age of onset in juvenile dermatomyositis is 7 years and the overall female to male ratio is 1.7 to 1 .
In up to 20% of patients with myositis, autoantibodies against nuclear antigens (antinuclear antibodies) and cytoplasmic antigens are found. In dermatomyositis, humoral immune mechanisms are implicated in causing microangiopathy and muscle ischemia. In polymyositis and inclusion body myositis, T cell–mediated cytotoxicity is the likely mechanism. The role of a degenerative process in inclusion body myositis is demonstrated by the presence of vacuoles in unaffected fibers together with β-amyloid deposits within the vacuolated muscle fibers and abnormal mitochondria with cytochrome oxidase-negative fibers .
While there are many causes and types of myositis, the most common underlying pathophysiology is autoimmune-mediated. Other causes include injury, infection and adverse effects of drugs. The condition presents with pain, swelling and weakness of the skeletal muscles. There are different kinds of inflammatory myopathies and the treatment and prognosis depend on the cause and type .
Definition: Myositis is a chronic inflammation of muscles leading to swelling and loss of muscles, and includes diseases like polymyositis, dermatomyositis, inclusion-body myositis, and juvenile forms of myositis.
Cause: Myositis is caused by autoimmune mechanism, which means that the body’s immune system attacks the body’s own normal and healthy tissue.
Symptoms: Muscles on both sides of your body are affected leading to progressive weakness, manifesting as difficulties in rising from a chair and climbing stairs. Other symptoms may include skin rash, difficulty in swallowing, speaking or breathing, fatigue and thickening and redness of the skin on the hands.
Diagnosis: After physically examining you, the doctor will order blood tests to assess muscle enzyme levels and autoantibodies. A needle electromyography will be done to assess the nature and severity of muscle damage. A muscle biopsy will be required which involves the examination of a piece of muscle removed through a nick in the skin. Sometimes, a magnetic resonance imaging scan may be required.
Treatment and follow -up: Treatment of myositis involves suppressing the autoimmune mechanism and the resulting damage to the muscle through corticosteroid drugs like prednisone and immunosuppressive therapy. Complete and early treatment of relapses is important to limit disability and avoid serious complications related to eating and breathing.