- Clinical science
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 (PM), (DM), and (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 , , and the . 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
- PM and DM: 2/100,000 per year
- In the US, PM disproportionately affects the black population.
- PM and DM: ♂ < ♀ (1:2)
- 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 )
General features of both PM and DM
- Proximal muscle weakness affecting both sides of the body (progresses within weeks to months)
- Muscle tenderness in approx. 33% of cases
Cutaneous manifestations of DM
- Gottron papules
- Heliotrope rash: erythematous rash on the upper eyelids, sometimes accompanied by edema
- Midfacial erythema
- Photosensitive poikiloderma:
- Juvenile DM
Inclusion body myositis (IBM)
- Progresses slowly over years
- Selective and asymmetric muscle involvement of both proximal and distal muscle groups
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.
- ↑ Muscle enzymes
- Inflammatory markers: ↑ ESR, ↑ CRP, leukocytosis, γ-globulin in protein electrophoresis
- 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 
- 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 fiber necrosis, degeneration, and regeneration
- PM: Cell-mediated inflammatory infiltrates that predominantly involve cytotoxic CD8+ T cells in the intrafascicular (within muscle fascicles) and endomysial (within the endothelial cell wall of endomysial capillaries) region
- DM: Antibody-mediated inflammatory infiltrates that predominantly involve CD4+ T cells, plasmacytoid dendritic cells, and B lymphocytes in the perifascicular and perimysial region that lead to perifascicular atrophy
- Skin biopsy (for suspected DM)
All patients diagnosed with DM should be tested for possible malignancies!
|Differential diagnoses of myopathies|
|Clinical features||ESR||CK||Muscle biopsy findings|
|Inflammatory myopathies||Dermatomyositis|| || |
|Polymyositis|| || |
| || |
| || || |
|Fibromyalgia|| || || |
|Hypothyroid myopathy|| || || || |
|Corticosteroid-induced myopathy|| || || |
| || || |
- : does not present 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 affected muscle
The differential diagnoses listed here are not exhaustive.