Polymyositis and dermatomyositis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]
Overview
Historical Perspective
In the late 19th century, polymyositis and dermatomyositis were described by different scientists. In 1916, Stertz was the first who described the association between dermatomyositis and malignancy. In 1975, Anthony Bohan and James B. Peter were the first physicians who classified polymyositis and dermatomyositis into 5 subtypes which were used for decades. By 1990, multiple myositis-specific autoantibodies (MSA) were discovered and described. These myositis-specific autoantibodies (MSA) targeting different cytoplasmic ribonucleoproteins and they are used by Love et al. to classify polymyositis and dermatomyositis.
Classification
Polymyositis and dermatomyositis is one of the subtypes of idiopathic inflammatory myopathy. The 2017 European League Against Rheumatism and American College of Rheumatology (EULAR/ACR) classified idiopathic inflammatory myopathy into 6 subtypes including polymyositis, dermatomyositis, inclusion body myositis, immune-mediated necrotizing myopathies (IMNM), amyopathic dermatomyositis, and juvenile dermatomyositis.
Pathophysiology
Causes
The cause of polymyositis and dermatomyositis has not been identified.
Differentiating Xyz from Other Diseases
Epidemiology and Demographics
The incidence of polymyositis and dermatomyositis is approximately 1-5 per 100,000 individuals worldwide. The prevalence of polymyositis and dermatomyositis is approximately 5-22 per 100,000 individuals worldwide. The 5-year survival rate for polymyositis is 75% and for dermatomyositis is 63%. The median survival for polymyositis is 11.0 years and that for dermatomyositis is 12.3 years. Dermatomyositis has a bimodal pattern, commonly affects both children and adults over 50 years old. Polymyositis commonly affects adults after second decades of their lives and it is rare among children. There is no racial predilection to polymyositis and dermatomyositis. The female to male ratio is approximately 2 to 1. Prevalence of polymyositis and dermatomyositis are lower in young rural men and higher in older urban women.
Risk Factors
Common risk factors in the development of polymyositis and dermatomyositis include environmental factors such as infection, malignancy, and drug toxicities from statins or immune checkpoint inhibitors.
Screening
There is insufficient evidence to recommend routine screening for polymyositis and dermatomyositis.
Natural History, Complications, and Prognosis
The symptoms of polymyositis and dermatomyositis usually develop very slowly and progressively from proximal muscle weakness manifested as difficulty to raise their arms above the shoulders or from chair, or climbing stairs to distal muscle weakness. In dermatomyositis, myositis develops months to years after skin manifestations. Common complications of polymyositis and dermatomyositis include malignancy, cardiac, pulmonary, gastrointestinal, infection, and medication related complications. Prognosis of polymyositis and dermatomyositis varies depends on different studies. Prognosis is generally poor, and the overall mortality rate of patients with polymyositis and dermatomyositis is approximately 22%. Most common cause of death in patients with polymyositis and dermatomyositis include cardiac and pulmonary complications, infections, and cancers. Extramuscular organ involvement, older age of diagnosis, male gender, and non-Caucasian race are associated with a particularly poor prognosis among patients with polymyositis and dermatomyositis. Younger age of diagnosis and quicker therapy initiation are associated with a better prognosis.
Diagnosis
Diagnostic Study of Choice
Muscle biopsy is the gold standard test for the diagnosis of polymyositis and dermatomyositis. Necrosis, increase in endomysial and perimysial connective tissue, atrophy and degeneration of both type I and II fibers, especially in a perifascicular distribution might be seen in muscle biopsy. The muscle biopsy should be performed when a patient presented with symptoms of muscle weakness and skin rash or elevated level of muscle enzymes.
History and Symptoms
Polymyositis and dermatomyositis is a multisystem disorder that involves many organs. The hallmark of polymyositis is symmetric muscle weakness. The hallmark of dermatomyositis is skin manifestation. Patients with polymyositis and dermatomyositis may have a positive history of gradual worsening of proximal muscle weakness, past medical history or family history of other autoimmune diseases. Common symptoms include constitutional symptoms, mild myalgias, skin eruptions, joint pain, and swelling. Less common symptoms of polymyositis and dermatomyositis include cough, dyspnea, aspiration, dysphagia, nasal regurgitation, swelling of periorbital area, and fingernails.
Physical Examination
Physical examination of patients with polymyositis and dermatomyositis is usually remarkable for muscle weakness, hyporeflexia, skin lesions, respiratory symptoms. The presence of Gottron's papules and the heliotrope eruption on physical examination is pathognomonic of dermatomyositis. Muscle atrophy in severe, long standing disease might occur.
Laboratory Findings
Elevated sarcoplasmic enzymes are consistent with the diagnosis of polymyositis and dermatomyositis which include creatine phosphokinase, aldolase, transaminases, lactic dehydrogenase, and myoglobin. High white blood cell counts, low lymphocytes, and low hematocrit levels might be detected on CBC. Low albumin levels, high ESR and high IgMand IgG levels could be seen in patients with polymyositis and dermatomyositis. 20 myositis-specific autoantibodies are identified in patients with polymyositis and dermatomyositis. Anti-Jo-1 antibody is the most frequent myositis-specific autoantibodies which causes antisynthetase syndrome. Anti Mi-2 might be seen frequently in patients with dermatomyositis. Anti-SRP antibody is associated with very poor prognosis. Some of these myositis-specific autoantibodies are associated with malignancy. Anti-HMGCR antibody might be elevated in patients with statin-associated necrotizing autoimmune myopathy (SANAM).