• 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.



  • 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


Epidemiological data refers to the US, unless otherwise specified.


  • Unknown origin, but most forms of IM seem to involve an autoimmune reaction that leads to muscle inflammation, particularly in genetically susceptible individuals; viral infections, malignancies, or connective-tissue disorders may play a role
  • Polymyositis (PM): cell–mediated cytotoxicity; against unidentified skeletal muscle antigens, chiefly affecting the endomysium
  • Dermatomyositis (DM): idiopathic or paraneoplastic antibody-mediated vasculopathy, associated with malignancies (non-Hodgkin lymphoma; lung, stomach, colorectal, or ovarian cancer )


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

Antisynthetase syndrome

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:
  • 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

Other procedures

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
  • ↑↑
  • ↑↑

Inclusion body


  • Peak incidence: > 50 years
  • >
  • Proximal and distal, asymmetrical muscle involvement
  • Weakness progresses slowly over years
  • No skin involvement
  • ↔︎ OR
  • Slightly ↑
Polymyalgia rheumatica
  • ↑↑
  • Normal
  • Not helpful in establishing diagnosis
  • Normal
  • Normal
  • Not helpful in establishing diagnosis
Hypothyroid myopathy
  • Muscle stiffness
  • Proximal muscle weakness affecting both sides of the body
  • Myoedema: localized muscle contraction that can be elicited by sharp blow with percussion hammer
  • 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


The differential diagnoses listed here are not exhaustive.




  • Respiratory failure
  • Myocarditis
  • Esophageal disease
  • Dystrophic calcinosis (common in juvenile dermatomyositis): Calcium deposits may be subcutaneous, intracutaneous, or intramuscular.


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