Polymyositis and dermatomyositis overview

# Overview
In the late 19th century, polymyositis and dermatomyositis were described by different scientists. Polymyositis and dermatomyositis are subtypes of idiopathic inflammatory myopathy. Patients develop proximal muscle weakness manifested as difficulty to raise their arms above the shoulders or from chair, or climbing stairs and progress to distal muscle weakness. Patients with dermatomyositis suffer from skin lesions in which Gottron's papules and the heliotrope eruption considered as pathognomonic features. The exact pathogenesis of polymyositis and dermatomyositis is not fully understood. However, it is understood that polymyositis and dermatomyositis are the result of autoimmune attack but triggering factors are not well-known. Polymyositis is caused by inflammation and degeneration of the muscles. Dermatomyositis is caused by skin inflammation. In dermatomyositis, activation and deposition of complements may lead to lysis of endomysial capillaries and muscle ischemia. Genes including HLA DRB1*0301 and HLA DQA1*0501 alleles, and tumour necrosis factor 308A might be associated with development of polymyositis and dermatomyositis, especially in familial cases. Different conditions associated with polymyositis and dermatomyositis include respiratory and cardiac diseases, other connective tissue diseases, and malignancies. 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. Muscle biopsy is the gold standard test for the diagnosis of polymyositis and dermatomyositis. 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. 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. Effective measures for prevention of complications include regular follow ups, malignancy screening, pulmonary protection, lifestyle modification, and prevention of medication-induced complications.

# 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
The exact pathogenesis of polymyositis and dermatomyositis is not fully understood. However, it is understood that polymyositis and dermatomyositis are the result of autoimmune attack but triggering factors are not well-known. Polymyositis is caused by inflammation and degeneration of the muscles. In polymyositis, CD8-positive cytotoxic T cells invade muscle fibers that express MHC class I antigens which may leads to fiber necrosis via the perforin pathway. Hypoxia may reduce creatine phosphate and adenosine triphosphate (ATP) levels in muscle and lead to fatigue and muscle weakness. Dermatomyositis is caused by skin inflammation. In dermatomyositis, activation and deposition of complements may lead to lysis of endomysial capillaries and muscle ischemia. Genes including HLA DRB1*0301 and HLA DQA1*0501 alleles, and tumour necrosis factor 308A might be associated with development of polymyositis and dermatomyositis, especially in familial cases. Different conditions associated with polymyositis and dermatomyositis include respiratory and cardiac diseases, other [[Connective tissue diseaseconnective tissue diseases, and malignancies.The inflammatory cells are predominantly B-cells (with smaller numbers of CD4-positive T-cells) and are found around blood vessels, in the septa between muscle fascicles, and in fibroadipose tissue around muscle. The key pathological change of dermatomyositis is a vasculitis, which involves endomysial and perimysial capillaries and arterioles. This vasculitis begins with endothelial swelling and is followed by endothelial necrosis and capillary loss. Tubuloreticular cytoplasmic inclusions (TRIs) are often seen in endothelial cells. TRIs also occur in lupus and other collagen vascular diseases but are absent in polymyositis and inclusion body myositis. The vasculitis is thought to be caused by circulating anti-endothelial antibodies. Interaction of these antibodies with vascular antigensactivates complement, leading to formation of the membranolytic attack complex (MAC), which destroys endothelial cells.

# Causes
The cause of polymyositis and dermatomyositis has not been identified.

# Differentiating Polymyositis and Dermatomyositis from Other Diseases
Polymyositis and Dermatomyositis must be differentiated from other inflammatory myopathies which cause muscle weakness and systemic symptoms.

# 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. Pharmacologic medical therapies for polymyositis and dermatomyositis include corticosteroids and disease-modifying antirheumatic drugs (DMARDs).

# 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).

## Electrocardiogram
An ECG may be helpful in the diagnosis of cardiac complications of polymyositis and dermatomyositis. Findings on an ECG suggestive of cardiac involvement include conductionabnormalities, arrhythmias, left atrial abnormality, and ST-T changes.

## X-ray
X-ray may be helpful in the diagnosis of polymyositis, dermatomyositis, and their complications, which include dystrophic calcification in the soft tissue, interstitial lung diseaseand malignancy.

## Echocardiography and Ultrasound
Different ultrasound modalities might be used to diagnose polymyositis, which include doppler sonography, contrast-enhanced ultrasound, and sonoelastography. There are no echocardiography findings associated with polymyositis and dermatomyositis. However, an echocardiography may be helpful in the diagnosis of cardiac complications of polymyositis and dermatomyositis, which include left ventricular diastolic dysfunction, hyperdynamic heart, mitral valve prolapse, and endomyocardial fibrosis.

## CT scan
There are no CT scan findings associated with polymyositis and dermatomyositis. However, a CT scan may be helpful in the diagnosis of complications of polymyositis and dermatomyositis, which include interstitial lung disease or malignancy.

## MRI
MRI from muscles may be helpful in the diagnosis of polymyositis and dermatomyositis. Findings on MRI suggestive of polymyositis and dermatomyositis include areas of high signal intensity reflecting an active disease and muscle edema, fatty infiltration of muscles, and muscle calcification.

## Other Imaging Findings
There are no other imaging findings associated with polymyositis and dermatomyositis.

## Other Diagnostic Studies
Electromyogram may be helpful in the diagnosis of polymyositis and dermatomyositis. Findings suggestive of polymyositis and dermatomyositis include delay in electrical signals between the muscles and nerves when they are stimulated, polyphasic, short, small motor-unit potentials, and fibrillation, positive sharp waves, increased insertional irritability. The first sign of improvement in polymyositis and dermatomyositis is the disappearance of fibrillation potentials. Electromyogram may also be used to identifying active sites of myositis for biopsy, follow up, and the treatment efficacy. Skin biopsy may be used in dermatomyositis, which demonstrates poikiloderma, epidermal atrophy, and liquefaction degeneration of the basal cells. Histochemical staining might be helpful to differentiate polymyositis and dermatomyositis from lower-motor-neuron diseases.

# Treatment

## Medical Therapy
Pharmacologic medical therapies for polymyositis and dermatomyositis include corticosteroids and disease-modifying antirheumatic drugs (DMARDs). Patients with polymyositis and dermatomyositis might require long-term treatment. However, if there are no disease flares during the course of the taper, glucocorticoids could be discontinued at 9 to 12 months. DMARDs might be discontinued at 6 months. Patients with recurrent flare of polymyositis and dermatomyositis require higher dose of prednisone, adding or increasing dose of methotrexate or azathioprine. Rituximab, IVIg, and mycophenolate mofetil might be used in resistant diseases.

## Surgery
Surgical intervention is not recommended for the management of polymyositis and dermatomyositis.

## Primary Prevention
There are no established measures for the primary prevention of polymyositis and dermatomyositis.

## Secondary Prevention
Effective measures for the secondary prevention of polymyositis and dermatomyositis include regular follow ups, malignancy screening, pulmonary protection, lifestyle modification, and prevention of medication-induced complications.