polymyositis

Introduction

Inflammatory myopathy often grouped with dermatomyositis.

Etiology

• autoimmune
• associated with connective tissue disease
• associated with malignancy

Epidemiology

• adults < 50 years of age at onset*
• male: female ratio 1:1, female > male

* distinguishing feature from dermatomyositis

Pathology

• muscle biopsy
  • myofibril necrosis
  • cytotoxic T-cells (CD8+) * & macrophages infiltrating endomysial regions
  • vessels of the muscle are spared*
• serology, autoantibodies
  • NO activation of complement*
  • autoantibodies PMSCL1, PMSCL2 & Jo-1

* distinguishing features from dermatomyositis

Genetics

• no familial association

Clinical manifestations

• no rash*
• symmetric proximal muscle weakness
  • onset is acute or subacute
• muscle pain (25%)
• muscle weakness may involve the trunk & neck, but spares the face
• pharyngeal & respiratory muscle weakness
• dysphagia may develop from involvement of striated muscle of the pharynx
• anti-synthetase syndrome occurs in some patients with polymyositis
• hypoventilation secondary to proximal myopathy may be the most common cause of death

* distinguishing feature from dermatomyositis

Laboratory

• serum creatine kinase > 10-fold increase
• muscle biopsy:
  • gold-standard for diagnosis
  • inflammatory infiltrates*
  • negative (normal) in 10-30% of affected patients
• serology: (for classification, not diagnosis)
  • anti Jo-1 in serum (30%)
    • risk factor for interstitial lung disease
  • antoantibodies against EXOSC9, EXOSC10, RUVBL1, PMSCL1, PMSCL2
  • antinuclear antibody in serum
• always check serum TSH when evaluating myopathy

* distinguishing feature from dermatomyositis

Diagnostic procedures

• electromyography: myopathic pattern
• colonoscopy in elderly patients to evaluate for occult colorectal carcinoma
• pulmonary function testing

Radiology

• chest X-ray:evaluate for interstitial lung disease
• computed tomography
  • chest: evaluate for interstitial lung disease
  • thorax, abdomen, pelvis in elderly patients
    • evaluate for occult malignancy
• magnetic resonance imaging of muscle can identify sites for muscle biopsy, aid in the diagnosis & help assess response to treatment
• bone mineral density for patients treated long-term with glucocorticoids

Complications

• association with cancer not as strong as for dermatomyositis
  • modest increase in risk for some cancers
• lung disease may be most common cause of death
  • interstitial lung disease may precede muscle weakness
  • hypoventilation secondary to proximal myopathy
• other common causes of death
  • cancer, infections, cardiovascular events

Differential diagnosis

• dermatomyositis
• toxic myopathy
  • colchicine, zidovudine (AZT), statin, penicillamine, corticosteroids (steroid myopathy), alcohol
• metabolic myopathy
  • hypothyroidism: goiter, delayed reflexes, myalgias, weight gain
  • mitochondrial myopathy
  • glycogen storage disease
• myasthenia gravis: oculomotor muscle weakness, ptosis
• HIV
• polymyalgia rheumatica: proximal muscle tenderness
• vasculitis
• eosinophilia myalgia syndrome
• muscular dystrophies
• inclusion body myositis
• carnitine deficiency
• adenylate deaminase deficiency
• amyotrophic lateral sclerosis (ALS): muscle fasciculations
• myasthenia gravis
• polymalgia rheumatica
• peripheral neuropathy: muscle atrophy, hyporeflexia
• mixed connective tissue disease

Management

• combination of prednisone + other immunosuppressive agent
• response to glucocorticoids is variable*
• patients may respond to methotrexate or other immunosuppressive agent
• physical therapy to maintain muscle strength
• supportive therapy
  • calcium & vitamin D for patients treated long-term with glucocorticoids

More general terms

• myositis (inflammatory myopathy)
• autoimmune disease

Additional terms

• dermatomyositis

Patient information

polymyositis patient information