Polymyositis and dermatomyositis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2] Ayesha A. Khan, MD[3]

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.

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.

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.

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.

The cause of polymyositis and dermatomyositis has not been identified.

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

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).

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.

There is insufficient evidence to recommend routine screening for polymyositis and dermatomyositis.

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.

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.

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 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.

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).

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

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.

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 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.

There are no other imaging findings associated with polymyositis and dermatomyositis.

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.

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.

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

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

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.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

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.

• In 1863 and 1887, Ernst Leberecht Wagner described polymyositis and dermatomyositis (also known as Unverricht-Wagner or Wagner-Unverricht syndrome) in two publications.^[1]
• In 1875, Potain was the first who published a case of dermatomyositis documented in Europe.^[2]
• In 1886, Urivericht was the first who described polymyositis.^[2]
• In 1916, Stertz was the first who described the association between dermatomyositis and malignancy.^[3]
• In 1975, Anthony Bohan and James B. Peter were the first physicians who classified polymyositis and dermatomyositis into 5 subtypes which include:^[4]
  • Primary idiopathic polymyositis (PM)
  • Primary idiopathic dermatomyositis (DM)
  • DM/PM associated with neoplasia
  • Childhood DM/PM associated with vasculitis
  • DM/PM with associated collagen-vascular disease
• By 1990, seven myositis-specific autoantibodies (MSA) were discovered and described. These myositis-specific autoantibodies (MSA) targeting different cytoplasmic ribonucleoproteins including:
  • Helicase protein (Mi2)
  • Signal recognition particle (SRP)
  • Anti-aminoacyl-tRNA synthetases such as:
    • Histidyl (Jo1)
    • Threonyl (PL-7)
    • Alanyl (PL-12)
    • Glycyl (EJ)
    • Isoleucyl (OJ)
• In 1991, Love et al. proposed another classification of idiopathic inflammatory myopathy based on myositis-specific autoantibodies (MSA).

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

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.

• Polymyositis and dermatomyositis is one of the subtypes of idiopathic inflammatory myopathy.^[1][2]
• The 2017 European League Against Rheumatism and American College of Rheumatology (EULAR/ACR) classified idiopathic inflammatory myopathy into 6 subtypes:^[3][4][5]
  • Polymyositis
  • Dermatomyositis
  • Inclusion body myositis
  • Immune-mediated necrotizing myopathies (IMNM)
  • Amyopathic dermatomyositis
  • Juvenile dermatomyositis

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

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 diseases, and malignancies. On microscopic histopathological analysis, partial invasion of lymphocytes within the muscle fibers and regenerating fibers with enlarged nuclei are characteristic findings of polymyositis.

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.^{[1][2][3][4][5]}

• 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 lead 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.
• Interleukin (IL) 21, tumor growth factor-b (TGF-b), and high-mobility group protein 1 (HMG-1) induce muscle fatigue by decreasing Ca release. 

• Dermatomyositis is caused by skin inflammation.^[6][7][8][9]
• In dermatomyositis, activation and deposition of complements may lead to lysis of endomysial capillaries and muscle ischemia.^[4]

• Genes might be associated with development of polymyositis and dermatomyositis, especially in familial cases, which include:^[10]
  • HLA DRB1*0301 alleles for polymyositis and inclusion-body myositis
  • HLA DQA1*0501 for juvenile dermatomyositis,
  • Tumour necrosis factor 308A polymorphism for photosensitivity in dermatomyositis

Different conditions associated with polymyositis and dermatomyositis include:

• Interstitial lung disease
• Conduction abnormalities and arrhythmia
• Myocardial infarction
• Other connective tissue diseases like
  • Scleroderma
  • SLE
  • Mixed connective tissue disease
  • Rheumatoid arthritis
  • Sjögren’s syndrome
• Malignancies are as follows:

Association	Polymyositis	Dermatomyositis
Malignancy	Lung Kidney Breast Bladder Endometrial Cervical Thyroid Lymphoma Myeloma Brain	5-7 fold more risks than the general population Lung Ovarian Breast Colorectal Cervical Bladder Nasopharyngeal Esophageal Pancreatic Kidney

• On microscopic histopathological analysis of polymyositis, partial invasion of lymphocytes within the muscle fibers and regenerating fibers with enlarged nuclei are characteristic findings.
• On microscopic histopathological analysis of dermatomyositis, perifascicular inflammation with perifascicular atrophy and loss of vessels around muscle fibers are characteristic findings.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

The cause of polymyositis and dermatomyositis has not been identified. To review risk factors for the development of polymyositis and dermatomyositis, click here.

The cause of polymyositis and dermatomyositis has not been identified. To review risk factors for the development of polymyositis and dermatomyositis, click here.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Template:Aditya, Anmol Pitliya, M.B.B.S. M.D.[2]

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

Organ system	Disease	Symptoms												History	Physical Examination	Diagnosis
		Age of onset	Muscle weakness	Fever	Myalgia	Contractures	Gait abnormality	Neuropathy	Atrophy	Stiffness	Myoglobinuria	Other features	Laboratory Findings			Creatine Kinase	Muscle Biopsy	Electromyogram
Inflammatory/ Rheumatologic	Dermatomyositis[1]	40s−50s Can affect children	Proximal	+	+	−	−	−	−	+	−	Rash Dyspnea Weight loss Cough	Viral infections Cancer	Heliotrope rash on face and hands Telangiectasia Erythema Mechanic’s hands Gottron’s sign ( violaceous scaly eruption )	↑↑ ESR ↑↑ CRP	↑↑	Perimysial mononuclear infiltrate	Myopathic
	Polymyositis[2]	> 18 years	Proximal	+	+	−	−	−	−	+	−	Similar to dermatomyositis without mucous and skin involvement	N/A	N/A	↑↑ ESR ↑↑ CRP	↑↑	Endomysial mononuclear infiltrate Patchy necrosis
	Inclusion body myositis[3]	50s	Proximal & distal	−	−	−	−	−	−	−	−	Dysphagia Asymmetric weakness	Retrovirus (most common)	N/A	Antibodies to cytoplasmic 5’−nucleotidase	↑↑	Inflammatory cells Invading muscle cells Vacuolar degeneration Inclusions or plaques	Neurogenic
	Fibromyalgia[4]	40−50s	Generalized	−	−	−	−	+	−	−	−	Anxiety or depression features Fatigue Sleep disturbance Numbness Muscle spasms	History of depression	Tenderness in the soft tissue anatomical location	Normal	Normal	Normal	Normal
	Polymyalgia Rheumatica[5]	50s	Diffuse	+	+	−	−	−	−	+	−	Weight loss	History of joints stiffness, worse in the morning	Restricted shoulder motion	↑↑ ESR ↑↑ CRP	Normal	Normal	Normal

Template:WH Template:WS

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief:

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.

• The incidence of polymyositis and dermatomyositis is approximately 2 per 100,000 individuals worldwide.^[1][2]
• In 2010, the incidence of polymyositis and dermatomyositis was estimated to be 1-1.3 cases per 100,000 individuals in Japan annually.^[3]
• In 1970, the age‐adjusted annual incidence of polymyositis was 5 cases per 100,000 individuals in USA hospital‐based retrospective study.^[4]

• The prevalence of polymyositis and dermatomyositis is approximately 5-22 per 100,000 individuals worldwide.^[5]
• In 2003, the prevalence of polymyositis and dermatomyositis was estimated to be 21.5 cases per 100,000 individuals in Quebec.^[6]
• In 2010, the prevalence of polymyositis and dermatomyositis was estimated to be 13.2 cases per 100,000 individuals in Japan.^[3]

•  The 5-year survival rate for polymyositis is 75% and for dermatomyositis is 63%.^[3]
• The median survival for polymyositis is 11.0 years and that for dermatomyositis is 12.3 years.^[3]

• Dermatomyositis has a bimodal pattern, commonly affects both children and adults over 50 years old.^[7][2]
• Polymyositis commonly affects adults after second decades of their lives and it is rare among children.^[7][8]

• There is no racial predilection to polymyositis and dermatomyositis.

• Females are more commonly affected by polymyositis and dermatomyositis than males. The female to male ratio is approximately 2 to 1.^[7]

• Prevalence of polymyositis and dermatomyositis are lower in young rural men and higher in older urban women.^[9]

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

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.

• 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.^{[1][2][3][4][5]}

• Common risk factors in the development of polymyositis and dermatomyositis include:^{[1][2][3][4][5]}
  • Viral infections such as:
    • Coxsackie B virus
    • Parvovirus
    • Enterovirus
    • Human T-cell lymphotropic virus (HTLV-1)
    • Human immunodeficiency virus (HIV)
    • Epstein-Barr virus (EBV)
    • Adenovirus
    • Influenza
  • Malignancy such as:
    • Ovarian
    • Lung
    • Gastric
    • Colorectal
    • Pancreatic
    • Non-Hodgkin’s lymphoma
  • Medications such as:
    • Statins
    • Immune checkpoint inhibitors such as:
      • Ipilimumab
      • Pembrolizumab
      • Interferon-alpha
      • IL-2 therapy 

• Less common risk factors in the development of polymyositis and dermatomyositis include:^[6]
  • Tobacco smoking (association with interstitial lung disease and rise in anti-Jo-1 antibody)
  • Maternal smoking during pregnancy (risk factor for a juvenile dermatomyositis)

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

There is insufficient evidence to recommend routine screening for polymyositis and dermatomyositis.

• There is insufficient evidence to recommend routine screening for polymyositis and dermatomyositis.

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Sadaf Sharfaei M.D.[2]

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.

• The symptoms of polymyositis and dermatomyositis usually develop very slowly and progressively from proximal muscle weakness to distal muscle weakness.^[1]
• Patients with polymyositis and dermatomyositis usually present with proximal muscle weakness manifested as difficulty to raise their arms above the shoulders or from chair, or climbing stairs. Patients might experience myalgia and muscle tenderness in the early stages of polymyositis.
• Polymyositis and dermatomyositis usually do not affect facial muscles.
• In dermatomyositis, myositis develops months to years after skin manifestations.

• Common complications of polymyositis and dermatomyositis include:^[2][3]

Complications	Polymyositis	Dermatomyositis
Malignancy	Lung Kidney Breast Bladder Endometrial Cervical Thyroid Lymphoma Myeloma Brain	5-7 fold more risks than the general population Lung Ovarian Breast Colorectal Cervical Bladder Nasopharyngeal Esophageal Pancreatic Kidney
Cardiac	Arrhythmia Conduction abnormalities Cardiac arrest Congestive heart failure (CHF) Myocarditis Pericarditis Angina Secondary fibrosis
Pulmonary	Hypoventilation and respiratory failure Aspiration pneumonia Interstitial lung disease
Gastrointestinal	Dysphagia Proximal esophageal skeletal muscle dysfunction
Medication related	Osteoporosis Myopathy
Infection	Pneumonia Infectious diarrhea

Factors associated with malignancy in patients with myositis
Increased risk of malignancy	Decreased risk of malignancy
Older age at diagnosis Male sex Smoking Skin disease Leucocytoclastic vasculitis Cutaneous necrosis Resistance to therapy Idiopathic pulmonary fibrosis   Muscle involvement Dysphagia Rapid onset of myositis Rare-infiltrative type muscle pathology on muscle biopsy Elevated inflammatory markers (ESR, CRP) Elevated CK Elevated tumor markers Positive anti-TIF-1γ Positive anti-NXP-2 Positive HLA-A28 Positive anti-HMGCR antibodies	Interstitial lung disease Arthritis/arthralgia Raynaud’s phenomenon Positive antinuclear antibodies (ANA) Positive anti-ENA antibodies Positive anti-Jo-1 antibody

• Prognosis of polymyositis and dermatomyositis varies depends on different studies.^[4]
• Prognosis is generally poor, and the overall mortality rate of patients with polymyositis and dermatomyositis is approximately 22%.^[5]
• Five-year survival rates in polymyositis and dermatomyositis have been estimated at more than 80%.
• 10-year survival rates in polymyositis and dermatomyositis have been estimated to be 62%.^[6]
• Polymyositis and dermatomyositis have a high morbidity rate in which patients would experience insignificant muscular disability after 3 years.
• Most common cause of death in patients with polymyositis and dermatomyositis include:^[6]
  • Cardiac complications (22%)
  • Pulmonary complications (22%)
  • Infections (15%)
  • Cancer (11%)
• The presence of following complications and symptoms are associated with a particularly poor prognosis among patients with polymyositis and dermatomyositis:^[7]
  • Extramuscular organ involvement like:
    • Cardiovascular involvement (one of the most common causes of death)
    • Pulmonary involvement (increased morbidity and mortality)
    • Gastrointestinal involvement
    • Cancer
  • Older age of diagnosis
  • Male gender
  • Non-Caucasian race
• The presence of following characteristics are associated with a better prognosis among patients with polymyositis and dermatomyositis:^[8]
  • Younger age of diagnosis
  • Shorter duration of clinical manifestations prior to therapy initiation