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).
- Dermatomyositis (DM): : an inflammatory myopathy characterized by progressive symmetrical proximal muscle weakness and distinctive skin findings
- Overlap myositis (OM): any inflammatory myopathy that occurs in association with features of another autoimmune connective tissue disease. A classic form of OM is antisynthetase syndrome.
- Immune-mediated necrotizing myopathy (IMNM): an inflammatory myopathy affecting the proximal skeletal muscles that is particularly severe and associated with a markedly elevated CK level
- Inclusion body myositis (IBM): : an inflammatory myopathy affecting both the proximal and distal skeletal muscles
- Polymyositis (PM): : an inflammatory myopathy affecting the proximal skeletal muscles, with evidence of elevated CK and myositis on EMG and biopsy in the absence of any of the characteristic findings of the other IIM (a rare diagnosis of exclusion)
- Juvenile idiopathic inflammatory myopathy (JIIM): any inflammatory myopathy manifesting in an individual < 18 years old 
- PM and DM: 2/100,000 per year
- In the US, PM disproportionately affects the black population.
- Peak incidence
Epidemiological data refers to the US, unless otherwise specified.
- The origin is unknown, but most forms of IM seem to involve an autoimmune reaction that leads to muscle inflammation, particularly in genetically susceptible individuals. Viral infections, malignancies, and 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 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. 
- Symmetrical proximal muscle weakness and atrophy
- Commonly affects pelvic and shoulder girdle muscles, leading to difficulties combing hair, standing up from a sitting position, and climbing stairs
- Can also affect neck muscles (flexors more than extensors)
- May be accompanied by myalgia 
In inclusion body myositis, the weakness may be:
- Additionally affecting distal muscle groups, making it hard to grip objects
- Axial muscle weakness
Cutaneous features 
Cutaneous features are characteristic of dermatomyositis but may be found in other subtypes.
Symmetric erythematous rash on the: ; 
- Extensor surfaces of the hand joints; , elbows, and knees (Gottron sign); scaly papules; may form (Gottron papules)
- Upper eyelids (heliotrope rash); often associated with periorbital edema
- Mid-face 
- Upper back, posterior neck, and shoulders; (shawl sign)
- Upper chest and anterior neck (V sign)
- Hips and lateral thighs (Holster sign)
- may be seen in chronic disease. 
- Mechanic's hands: thickened and cracked skin on the sides of the fingers and palms (horizontal fissures may appear darkened or dirty, hence the name)
- Periungual telangiectasias
- Calcinosis cutis (in children)
- Interstitial lung disease
- Cardiac involvement (primarily in dermatomyositis and polymyositis), including: 
- Constitutional symptoms
- Increased risk of malignancy (DM, PM, IMNM)
- Raynaud phenomenon
- Gastrointestinal symptoms (e.g., abdominal pain, hematemesis, melena)
|Characteristic presentations of idiopathic inflammatory myopathies (IIM) |
|Muscle weakness||Cutaneous features|| |
|Dermatomyositis (DM)|| || |
|Polymyositis (PM)|| |
|Antisynthetase syndrome|| || |
|Immune-mediated necrotizing myopathy (IMNM)|| || || |
|Inclusion body myositis (IBM)|| || || |
|Juvenile IIM (JIIM)|| || |
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.
General principles 
- There are different sets of diagnostic criteria for IIM; (e.g., the 2017 EULAR/ACR criteria) but none are universally accepted. 
- Diagnosis is made based on the presence of characteristic clinical features and diagnostic studies. 
- The Peter and Bohan criteria are still widely referenced but have significant limitations. The criteria are as follows: 
Laboratory studies 
- CBC: may show leukocytosis 
- ESR and CRP: normal or mildly elevated
- ↑ Muscle enzymes 
- Exclusion of differential diagnoses
- Myositis-specific antibodies (MSAs): The choice of test depends on the suspected underlying condition. 
|Condition-specific antibody testing in IIM|
|Immune-mediated necrotizing myopathy (severe form)|
Other procedures 
Muscle biopsy: Gold standard for diagnosis of IIM ; 
- Perform on a muscle that is clinically affected but not wasted (MRI can be used to help select a site). 
Typical findings 
- Muscle fiber damage
- Inflammatory cellular infiltrates (except in IMNM)
- DM: antibody-mediated inflammatory infiltrates that predominantly involve CD4+ T cells, plasmacytoid dendritic cells, and B lymphocytes in the perimysium (the connective tissue surrounding the fascicle); can lead to perifascicular atrophy
- IBM and PM: cell-mediated inflammatory infiltrates that predominantly involve cytotoxic CD8+ T cells in the endomysium (the connective tissue surrounding each muscle fiber)
- Immunohistochemistry in PM and IBM: overexpression of MHC-I on the sarcolemma
- Electromyography: irritable myopathic pattern in all forms of IIM
Evaluation for extramuscular features 
- Evaluation for lung disease: baseline chest x-ray and pulmonary function tests
- Malignancy workup in patients with DM, PM, and IMNM 
All patients diagnosed with DM, PM, and IMNM should be tested for malignancies.
|Differential diagnosis of myopathy |
|Clinical features||ESR||CK||Muscle biopsy findings|
|Inflammatory myopathies||Dermatomyositis|| || |
|Polymyositis|| || |
| || |
| || || |
|Fibromyalgia|| || || |
|Hypothyroid myopathy|| || || || |
|Drug-induced myopathies||Corticosteroid-induced myopathy|| || || |
||| || || |
|Cocaine-induced myopathy || || || |
|Zidovudine-induced myopathy || || |
- : does not manifest with proximal muscle weakness
- Acute myopathy: results from a viral or bacterial infection (patients shows symptoms of infection)
- : distal muscle weakness with asymmetric onset
- ; : presents with normal muscle enzymes, anti-acetylcholine receptor antibodies, and facial paralysis
- : inflammatory infiltrate limited to the affected muscle
The differential diagnoses listed here are not exhaustive.
- 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 .
- Educate patients on the possible increased risk of malignancy and the importance of attending screenings.
Initial treatment 
- First-line: glucocorticoids
- AND (usually) a steroid-sparing immunosuppressive agent, e.g.: ; 
Subsequent treatment 
Depending on the clinical presentation, additional medications may need to be added to the initial treatment regimen.