• Clinical science

Inflammatory myopathies


Inflammatory myopathies (IM) are systemic muscle wasting diseases characterized by progressive weakness due to chronic inflammation of skeletal muscles. IMs are classified according to clinicopathological features and include polymyositis (PM), dermatomyositis (DM), and inclusion-body myositis (IBM). Patients with IM typically complain of muscle weakness with difficulties reaching overhead, climbing the stairs, and/or standing up. Advanced disease may present with dysphagia and aspiration because of oropharyngeal muscle involvement, or even respiratory failure if breathing muscles are affected. While IMs share many features, there are also some important differences between them: In contrast to PM and IBM, DM also affects the skin, resulting in lesions such as Gottron papules, heliotrope rash, and the shawl sign. Furthermore, DM is often associated with neoplasms (e.g., lung, ovary, lymphoma, GI). Unlike DM and PM, IBM mainly manifests in the elderly and progresses over years. Diagnosis of suspected IM is supported by laboratory tests, which show elevated muscle enzymes (e.g., creatine kinase, aldolase), as well as characteristic electromyogram (EMG) and biopsy findings. Treatment consists of immunosuppression with glucocorticoids or in some cases, with immunosuppressants such as methotrexate or azathioprine.


  • Polymyositis (PM): inflammatory myopathy affecting the proximal skeletal muscles
  • Dermatomyositis (DM): : inflammatory myopathy that presents similarly to polymyositis, with the addition of skin involvement
  • Inclusion-body myositis (IBM): : inflammatory myopathy affecting both the proximal and distal skeletal muscles


  • Incidence
    • PM and DM: 2/100,000 per year
    • In the US, PM disproportionately affects the black population.
  • Sex
    • PM and DM: > (2:1)
    • IBM: > (2:1)
  • Peak incidence
    • PM: > 20 years
    • DM: bimodal (5–10 years and ∼ 50 years)
    • IBM: > 50 years


Epidemiological data refers to the US, unless otherwise specified.



Clinical features

General features of both PM and DM

  • Proximal muscle weakness affecting both sides of the body (progresses within weeks to months)
    • The pelvic and shoulder girdle muscles are most commonly affected. Other muscles, e.g., the neck flexors, may also be affected.
    • Leads to difficulties combing hair, standing up, and climbing stairs
    • Dysphagia is present in approx. 30% of patients because of esophageal muscle involvement.
  • Muscle tenderness in approx. 33% of cases

Cutaneous manifestations of DM

Inclusion-body myositis (IBM)

  • Progresses slowly over years
  • Selective and asymmetric muscle involvement of both proximal and distal muscle groups
    • Quadriceps muscle weakness: knees lack support → frequent falling
    • Finger flexors: difficulties gripping, e.g., shopping bags or a briefcase



Diagnostic criteria

Diagnosis of PM and DM is based on clinical presentation, laboratory results, and pathology findings.

  • PM is diagnosed if the following criteria are fulfilled:
    • Proximal muscle involvement
    • Positive laboratory findings
    • Electromyogram (EMG) suggestive of inflammatory myopathy
    • Typical biopsy findings
  • DM is diagnosed if additional cutaneous manifestations are present.
  • If two or more of the criteria are fulfilled, PM or DM is considered a possible diagnosis.

Laboratory tests

  • ↑ Muscle enzymes
    • ↑ CK and aldolase
    • Additional elevated enzymes: myoglobin, LDH, AST, ALT
  • Inflammatory markers: ESR, CRP, leukocytosis, γ-globulin in protein electrophoresis
  • Antibodies
    • Antinuclear antibody (ANA)
    • Myositis-specific antibodies (MSAs)
      • Anti-Jo-1 antibodies (∼ 5% DM, 30% PM); : especially in patients with interstitial lung disease; responds poorly to treatment
      • Anti-Mi-2 antibodies (∼ 10% of cases): more favorable prognosis
      • Anti-signal recognition particle antibodies (anti-SRP): associated with a severe treatment-resistant necrotizing myopathy; typically in patients with PM [12]

Other procedures

  • Electromyography: abnormal in 90% of cases
  • Imaging: An MRI may be helpful in identifying inflammation and a potential biopsy site, although it is not always recommended.
  • Muscle biopsy
  • Skin biopsy (for suspected DM)

All patients diagnosed with DM should be tested for possible malignancies!


Differential diagnoses

Differential diagnoses of myopathies
Clinical features ESR CK Muscle biopsy findings
Inflammatory myopathies Dermatomyositis
  • Peak incidence: 5–10 years and ∼ 50 years
  • >
  • Proximal and symmetrical muscle weakness (esp. pelvic and shoulder girdle)
  • Weakness progresses within weeks to months
  • Skin is involved (i.e., heliotrope rash, Gottron's papules)
  • Associated with malignancies
  • ↑↑
  • Peak incidence: > 20 years
  • >
  • Proximal and symmetrical muscle weakness (esp. pelvic and shoulder girdle)
  • Weakness progresses within weeks to months
  • No skin involvement
  • ↑↑



  • Peak incidence: > 50 years
  • >
  • Proximal and distal, asymmetrical muscle involvement
  • Weakness progresses slowly over years
  • No skin involvement
  • ↔︎ OR
  • Slightly ↑
  • Endomysial inflammation
  • Intramuscular vacuoles resembling inclusion bodies of aggregated proteins
Polymyalgia rheumatica
  • Elderly women
  • Fatigue and malaise
  • Often rapid-onset: symmetric pain on shoulder, pelvic girdle, and neck (proximal muscles)
  • Morning stiffness (> 45 min)
  • Associated with giant cell arteritis
  • ↑↑
  • Normal
  • Not helpful in establishing diagnosis
  • Peak incidence: 20–50 years
  • Tender points over multiple areas of the body with no signs of inflammation
  • Chronic fatigue, sleep disturbances
  • Morning stiffness
  • Normal
  • Normal
  • Not helpful in establishing diagnosis
Hypothyroid myopathy
  • Muscle stiffness
  • Proximal muscle weakness affecting both sides of the body
  • Exercise intolerance
  • Further symptoms of hypothyroidism
  • Normal
  • Not helpful in establishing diagnosis
Corticosteroid-induced myopathy
  • Normal
  • Not helpful in establishing diagnosis
Statin-associated myopathy
  • History of statin intake
  • Fatigue
  • Muscle pain and weakness
  • Normal
  • Not helpful in establishing diagnosis
  • SLE: does not present with proximal muscle weakness
  • Acute myopathy: results from a viral or bacterial infection (patients shows symptoms of infection)
  • ALS: distal muscle weakness with asymmetric onset
  • Myasthenia gravis; : presents with normal muscle enzymes, anti-acetylcholine receptor antibodies, and facial paralysis
  • Muscular dystrophy: inflammatory infiltrate limited to affected muscle


The differential diagnoses listed here are not exhaustive.





We list the most important complications. The selection is not exhaustive.