eMedicine Specialties > Rheumatology > Systemic Rheumatic Disease

Polymyositis

Author: Ramesh Pappu, MD, Associate Professor of Medicine, Drexel University College of Medicine; Consulting Staff, Department of Internal Medicine, Division of Rheumatology, Hahnemann University Hospital
Coauthor(s): Mythili Seetharaman, MD, Consulting Staff, Einstein Arthritis Center, Albert Einstein Medical Center, St Christopher's Hospital for Children
Contributor Information and Disclosures

Updated: Nov 6, 2008

Introduction

Background

Polymyositis (PM) is an idiopathic inflammatory myopathy that causes symmetric proximal muscle weakness, elevated skeletal muscle enzyme levels, and characteristic electromyography (EMG) and muscle biopsy findings. Clinically similar to polymyositis, dermatomyositis (DM) is an idiopathic inflammatory myopathy associated with characteristic dermatologic manifestations. Inclusion body myositis (IBM) is a slowly progressive idiopathic inflammatory myopathy with characteristic pathological findings generally found in older males. Bohan and Peter classify the idiopathic inflammatory myopathies as follows:¹

• I - Primary idiopathic polymyositis
• II - Primary idiopathic dermatomyositis
• III - Polymyositis or dermatomyositis associated with malignancy
• IV - Childhood polymyositis or dermatomyositis
• V - Polymyositis or dermatomyositis associated with another connective-tissue disease
• VI - Inclusion body myositis
• VII - Miscellaneous (eg, eosinophilic myositis, myositis ossificans, focal myositis, giant cell myositis)

Pathophysiology

Although the agent that initially causes polymyositis remains unknown, possibilities include virus-mediated muscle injury or microvascular insult leading to release of muscle autoantigens. These autoantigens are then presented to T lymphocytes by macrophages in the muscle. Activated T lymphocytes proliferate and release cytokines such as interferon gamma (IFN-gamma) and interleukin 2 (IL-2). IFN-gamma promotes further macrophage activation and release of mediators of inflammation such as IL-1 and tumor necrosis factor-alpha (TNF-alpha).

Additionally, these cytokines induce aberrant expression of major histocompatibility complex (MHC) class I and II molecules and adhesion molecules on muscle cells. Muscle fibers are destroyed when CD8⁺ T lymphocytes (cytotoxic) encounter antigens in conjunction with MHC class I molecules on muscle cells. Macrophages further the process of destruction both directly and by secreting cytokines.

Frequency

United States

Idiopathic inflammatory myopathies are relatively rare diseases, with an incidence that ranges from 0.5-8.4 cases per million population.

International

Polymyositis is less common among Japanese persons.

Mortality/Morbidity

Most patients with polymyositis respond favorably to immunosuppressive therapy but may require lifelong treatment. Five-year survival rates have been estimated at more than 80%. Causes of death include severe muscle weakness, pulmonary involvement, cardiac involvement, associated malignancy, and complications of immunosuppressive therapy, especially infection.

Race

In the United States, polymyositis is more common among blacks.

Sex

Polymyositis is more common in women than in men (2:1 ratio); inclusion body myositis is twice as common in men.

Age

Polymyositis usually affects adults older than 20 years, especially those aged 45-60 years. Polymyositis rarely affects children, unlike dermatomyositis.

• The age of onset of polymyositis with another collagen vascular disease is related to the associated condition.
• Eighty percent of patients with inclusion body myositis are older than 50 years at onset.

Clinical

History

Symptoms of polymyositis (PM) gradually develop over a period of 3-6 months. Diagnosis is usually delayed because, unlike in dermatomyositis, no associated rash occurs before the onset of muscle disease.

• Muscular manifestations
  • Patients with polymyositis usually present with symmetric proximal muscle weakness in the upper and lower extremities.
  • Weakness of neck flexors also occurs.
  • Initially, distal muscle weakness is rare but may occur late in the disease course.
  • Patients with polymyositis may report muscle pain and tenderness that may be confused with polymyalgia rheumatica.
  • Dysphagia secondary to oropharyngeal and esophageal involvement occurs in about one third of patients with polymyositis and is a poor prognostic sign. Dysphonia is also a poor prognostic sign but is much less common.
  • Ocular muscles are never involved in generalized polymyositis. However, isolated orbital myositis, an inflammatory disorder involving the extraocular muscles, is well described.
  • Facial and bulbar muscle weakness is extremely rare in individuals with polymyositis.
• Constitutional manifestations (Polymyositis is a systemic disease.)
  • Morning stiffness
  • Fatigue
  • Anorexia
  • Fever (associated with anti-synthetase antibodies such as anti-Jo-1)
  • Weight loss
• Pulmonary manifestations
  • Pharyngeal and esophageal weakness may lead to aspiration pneumonia.
  • Patients with polymyositis may experience exertional dyspnea secondary to weakness of chest wall muscles and diaphragmatic muscles.
  • Patients receiving immunosuppressants are at an increased risk of infections.
  • Interstitial lung disease occurs in 5-30% of patients with idiopathic inflammatory myopathy (associated with anti-synthetase antibodies, especially anti-Jo-1). Patients may be asymptomatic or present with exertional dyspnea, cough, and fever.²
  • Interstitial pneumonitis, bronchiolitis obliterans organizing pneumonia, and pulmonary capillaritis have all been described in conjunction with polymyositis.
• Cardiac manifestations
  • Cardiac involvement is unusual and, if present, portends a bad prognosis.
  • Rhythm disturbances, conduction defects, congestive heart failure, pericarditis, pulmonary hypertension, and myocarditis can occur.
• Joint involvement
  • Patients can present with arthralgias or arthritis.
  • Arthritis is usually symmetric and involves the knees, wrists, and hands (associated with antisynthetase antibodies).
  • A severe deforming arthropathy without erosions has been reported; erosive changes are very rare.
• Overlap syndromes
  • Polymyositis has been associated with other connective-tissue diseases such as systemic lupus erythematosus, rheumatoid arthritis, mixed connective-tissue disease, Sjögren syndrome, and scleroderma.
  • About 25% of patients with scleroderma have myositis; this phenomenon has been associated with anti-PM-Scl (anti-PM-1) antibody.
  • In Japan, anti-Ku antibody has been described with this condition.
• Gastrointestinal manifestations
  • Dysphagia
  • Odynophagia
  • Nasal regurgitation
  • Reflux esophagitis
  • Abdominal bloating
  • Constipation
• Renal manifestations: Intrinsic renal disease is rare in patients with polymyositis. Occasionally, severe rhabdomyolysis with myoglobinuria can result in acute tubular necrosis.
• Skin manifestations
  • Rash is absent in patients with polymyositis, unlike with dermatomyositis.
  • "Mechanic's hands" (associated with antisynthetase antibodies), ie, hyperkeratotic eruptions over the finger pads and lateral aspects of the fingers, have been reported.
  • Raynaud phenomenon has been described in patients with antisynthetase antibodies.
  • Rarely, periorbital edema may occur (best described in dermatomyositis).
  • Calcinosis occurs in approximately 5% of patients with polymyositis (in association with sclerodermalike illness).
  • Telangiectasias are uncommon.
• Inclusion body myositis: This condition is a slowly progressive idiopathic inflammatory myopathy that mostly affects men older than 50 years. Muscle involvement includes predominantly proximal muscles but may also include distal muscles (50%), and involvement may be asymmetric. Dysphagia is found in most patients (60%).

Physical

Physical examination findings of gout generally include symmetric proximal muscle weakness.

• Deep tendon reflexes are usually preserved, and sensory examination findings are normal.
• Muscle tenderness may be present.
• Dysphonia with nasal speech may be noted.
• Lung examination findings may include evidence of interstitial lung disease such as dry inspiratory crackles in the lung bases ("Velcro").
• Inclusion body myositis manifests as severe proximal muscle weakness with atrophy, often with distal muscle weakness. The weakness may be asymmetric. Deep tendon reflexes may be impaired or absent if weakness is severe.

Causes

The causes of polymyositis are still poorly understood, although the condition is believed to be an immune-mediated process triggered by environmental agents in genetically predisposed individuals. Recognition of other autoimmune diseases with polymyositis and the presence of circulating autoantibodies strongly favor an autoimmune etiology. An increased association of myositis has been found with HLA haplotypes A1, B8, and DR3, which also increases the risk for autoimmune diseases. Environmental triggers, especially infectious agents, have been suggested as etiologic agents, as follows:

• Infectious agents
  • Coxsackievirus B1
  • HIV
  • Human T-cell lymphotropic virus type 1 (HTLV-1)
  • Hepatitis B
  • Influenza
  • Echovirus
  • Adenovirus
• Noninfectious agents
  • Many drugs are known to cause myopathy. Most drugs, such as hydroxychloroquine and colchicine, cause a toxic or metabolic myopathy.
  • Several drugs, however, rarely induce an immune-mediated myopathy or myositis. Muscle biopsy shows chronic inflammatory changes consistent with polymyositis. Drugs such as D-penicillamine, hydralazine, procainamide, phenytoin, and angiotensin-converting enzyme (ACE) inhibitors have been associated with this type of inflammatory myopathy.
  • Statins occasionally cause severe muscle inflammation and rhabdomyolysis.

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