Idiopathic inflammatory myopathies

Last updated: July 27, 2023

Summarytoggle arrow icon

Idiopathic inflammatory myopathies (IIM) are a group of rare systemic diseases characterized by progressive weakness due to chronic skeletal muscle inflammation. They are classified based on clinicopathological findings into dermatomyositis (DM), overlap myositis (OM), immune-mediated necrotizing myopathy (IMNM), inclusion-body myositis (IBM), and polymyositis (PM). If onset occurs before the age of 18 years, it is called juvenile IIM, or JIIM. IIM typically manifests with proximal, symmetric weakness in appendicular muscles, which leads to difficulty reaching overhead, climbing the stairs, and/or standing up. The severity of muscle involvement can vary significantly between forms of IIM. In advanced disease, weakness may also affect oropharyngeal muscles (causing dysphagia and aspiration) and respiratory muscles (causing restrictive lung disease or even respiratory failure.) Patients may also have cutaneous features (including Gottron papules, heliotrope rash, and the shawl sign) and an increased risk of various malignancies (in DM, PM, and IMNM), and interstitial lung disease in some forms of DM, IMNM, and OM.

The diagnosis of suspected IIM is supported by laboratory tests, which show elevated muscle enzymes (e.g., creatine kinase, aldolase) as well as characteristic electromyogram (EMG) and biopsy findings. Management consists of supportive treatment (such as physical and occupational therapy) alongside immunosuppression with glucocorticoids and drugs such as methotrexate and azathioprine (except for IBM, for which treatment is supportive only).

Definitiontoggle arrow icon


Epidemiologytoggle arrow icon

Epidemiological data refers to the US, unless otherwise specified.

Etiologytoggle arrow icon


Clinical featurestoggle arrow icon

Clinical manifestations of IIM vary widely both between and within the subtypes. Symptoms can be grouped into three categories: muscle weakness, cutaneous features, and systemic manifestations. [2][6]

Muscle weakness

Cutaneous features [8]

Cutaneous features are characteristic of dermatomyositis but may be found in other subtypes.

Systemic manifestations

Characteristic presentations of idiopathic inflammatory myopathies (IIM) [2][6][15][16]
Muscle weakness Cutaneous features

Systemic features

Dermatomyositis (DM)
  • Progresses over weeks to months
  • Mild to moderately severe weakness
  • Typically present
Polymyositis (PM)
  • Absent
Antisynthetase syndrome
  • May be present depending on the subtype
Immune-mediated necrotizing myopathy (IMNM)
  • Severe
  • Progression over weeks to months
  • Rare
  • Rare
Inclusion body myositis (IBM)
  • Progresses over years
  • Symmetrical or asymmetrical
  • Distal muscles also affected
  • Absent
  • Rare
Juvenile IIM (JIIM)
  • Variable in severity; may be absent

Think HAIR, CHAIRS, and STAIRS for the problems in activities of daily living caused by inflammatory myopathy: Muscle weakness causes difficulty raising the arms, getting up from a seated position, and climbing stairs.

Diagnosticstoggle arrow icon

General principles [2][18]

  • There are different sets of diagnostic criteria for IIM; (e.g., the 2017 EULAR/ACR criteria) but none are universally accepted. [19]
  • Diagnosis is made based on the presence of characteristic clinical features and diagnostic studies. [6]
    • Suspect IIM in patients with classic clinical features and elevated levels of muscle enzymes.
    • Confirm the diagnosis with muscle biopsy.
    • Patients with DM, PM, and IMNM: Arrange cancer screening at the time of diagnosis.
  • The Peter and Bohan criteria are still widely referenced but have significant limitations. The criteria are as follows: [20][21]

Laboratory studies [3][22]



The choice of antibody tests depends on the type of inflammatory myopathy under investigation and whether other connective tissue disorders are suspected. Consult rheumatology prior to ordering.

Condition-specific antibody testing in IIM
Antisynthetase syndrome
Immune-mediated necrotizing myopathy (severe form)
  • First-line test: anti-signal recognition particle antibodies (anti-SRP)
    • Present in 5–10% of IIM patients
    • May also be seen in DM [25]
  • Consider also requesting anti-HMGCR antibodies.

Other procedures [2][22]

Evaluation for extramuscular features [3][22]

All patients diagnosed with DM, PM, and IMNM should be tested for malignancies.

Differential diagnosestoggle arrow icon

Differential diagnosis of myopathy [29]
Features ESR CK Muscle biopsy findings
Inflammatory myopathies Dermatomyositis
  • ↑↑
  • ↑↑
  • Intrafascicular infiltration of CD8+ T cells
  • Endomysial inflammation
  • Muscle atrophy (late finding in chronic disease)

Inclusion body myositis

  • Normal or mildly ↑
Polymyalgia rheumatica
  • ↑↑
  • Normal
  • 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
  • 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
  • Symptoms of hypothyroidism
  • Normal
  • ↑–↑↑
  • Not helpful in establishing diagnosis

Drug-induced myopathies

Corticosteroid-induced myopathy
  • Normal
  • ↑ in approx. 50% of cases
  • Not helpful in establishing diagnosis
Statin-induced myopathy [30]
  • History of statin intake
  • Muscle pain and weakness
  • Fatigue
  • Normal
  • Not helpful in establishing diagnosis
Zidovudine-induced myopathy [31]
  • History of zidovudine intake
  • Fatigue
  • Muscle pain and weakness that improves after discontinuing drug therapy
  • Normal
Alcohol-induced myopathy [32]
  • No information available
  • Not helpful in establishing diagnosis
  • Normal, ↑, or mildly ↑
Cocaine-induced myopathy [33]
  • Normal
  • ↑–↑↑↑
  • Not helpful in establishing diagnosis

The differential diagnoses listed here are not exhaustive.

Treatmenttoggle arrow icon

Approach [2][3][22]

  • Refer to a rheumatologist experienced in the management of IIM.
  • All patients: Start supportive therapies (physical, occupational, and/or speech therapy, as appropriate) as soon as possible.
  • Patients with any form of IIM except IBM: Start pharmacological therapy.
  • Advise patients with cutaneous manifestations to use photoprotective measures.
  • Educate patients on the possible increased risk of malignancy and the importance of attending screenings.

Pharmacologic therapy

Initial treatment [2][16]

Subsequent treatment [2][6]

Depending on the clinical presentation, additional medications may need to be added to the initial treatment regimen.

Complicationstoggle arrow icon

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

Referencestoggle arrow icon

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