There is a lack of clear diagnostic criteria for idiopathic inflammatory myopathies.3
Additionally, multisystem involvement can complicate the diagnostic process, with wide variation in how the different subtypes manifest.3
Diagnosis of idiopathic inflammatory myopathies includes assessment of the clinical history, disease progression, and pattern of muscle involvement and/or cutaneous features.5
Except for some cases of dermatomyositis that have a typical presentation, diagnosis is usually not straightforward and requires a combination of tests.4
A number of diagnostic tests are available for patients where dermatomyositis is suspected, including serum muscle enzymes (e.g. creatine kinase), myositis antibodies, electrodiagnostic studies, muscle biopsy and muscle MRI.6
Muscle biopsy is the gold standard for diagnosis of dermatomyositis.2
Although the Bohan and Peter Criteria demonstrate a high degree of accuracy, they have limitations.3 For example, they do not include the use of myositis-specific autoantibodies to identify the specific idiopathic inflammatory myopathy subtype3, which are important for diagnosis.
More recently, EULAR/ACR classification criteria were developed for idiopathic inflammatory myopathies and these criteria have been partially validated and been shown to generally perform better than existing criteria.9
Bohan and Peter classification criteria for polymyositis and dermatomyositis3
|A||Proximal and symmetrical muscle weakness of the pelvic and scapular girdle, anterior flexors of the neck, progressing for weeks to months, with or without dysphagia or involvement of respiratory muscles|
|B||Elevation of the serum levels of skeletal muscle enzymes: creatine kinase, aspartate aminotransferase, Iactate dehydrogenase and aldolase|
|C||Electromyography characteristic of myopathy (short and small motor units, fibrillation, positive pointy waves, insertional irritability and repetitive high-frequency firing)|
|D||Muscle biopsy showing necrosis, phagocytosis, regeneration, perifascicular atrophy, perivascular inflammatory exudate|
|E||Typical cutaneous changes:|
Exclusion criteria: congenital muscular dystrophies, central or peripheral neurological disease, infectious myositis, metabolic/ endocrine myopathies and myasthenia gravis.
Tools to aid dermatomyositis diagnosis
Serum muscle enzymes
In dermatomyositis, serum creatine kinase (CK) levels are often elevated due to muscle membrane damage and muscle necrosis.6
However, in some patients CK levels are normal, and CK levels are not useful in monitoring disease progression or activity.
Other enzymes released from damaged skeletal muscle can also be increased in patients with dermatomyositis:
- Lactate dehydrogenase
- Alanine aminotransferase
- Aspartate aminotransferase
Electromyography (EMG) changes are non-specific and not useful for differentiating between myositis subtypes.4 EMG is useful to:
- Confirm a myopathic process6
- Rule out neurogenic conditions associated with proximal muscle weakness6
- Distinguish between acute changes in myositis and steroid myopathy weakness4
In patients with myositis, findings from nerve conduction studies are typically normal.6 However, when myositis-associated weakness is severe and diffuse, low-amplitude motor nerve responses can be seen.
- Muscle biopsy is the gold standard for diagnosing dermatomyositis2
- Perifascicular muscle fibre atrophy is a specific, histopathological feature of dermatomyositis6
- Inflammatory infiltrates are predominantly located in the perimysium and perivascular regions and include CD4+ T cells2,6, B cells2 and plasmacytoid dendritic cells6
- A reduced number of capillaries can be observed6
- Muscle fibres show evidence of necrosis and phagocytosis5
Histological section of healthy skeletal muscle
Histological section of dermatomyositis skeletal muscle
- Five known dermatomyositis-specific autoantibodies (Mi-2,TIF-1γ, NXP-2, MDA-5 and SAE) are associated with distinct clinical phenotypes and may be useful prognostic markers6
- Identification of myositis-specific autoantibodies can:
- Aid earlier diagnosis in myositis patients
- Guide clinical management based on autoantibody subtypes with the associated risks of malignancy, risks of extramuscular/organ involvement, and response to specific types of immunotherapy
Overview of dermatomyositis autoantibodies and their respective organ involvement6,10
|Anti-Mi2||Good prognosis, favourable response to steroids, relatively low malignancy risk|
|Anti-TIF1γ||Highly associated with malignancy in adults and severe skin manifestations|
|Anti-NXP2||Subcutaneous calcifications in juvenile and adult dermatomyositis, severe dermatomyositis, increased risk of malignancy in adults|
|Anti-MDA5||Minimal muscle involvement, rapidly progressive interstitial lung disease and poor prognosis, severe skin lesions|
|Anti-SAE||May present as amyopathic initially, severe rash, high incidence of dysphagia|
M = muscle / S = skin / L = lung / C = cancer
Muscle magnetic resonance imaging (MRI)
Non-invasive tool to:
- Assess muscle inflammation (manifested by e.g. oedema, atrophy or fatty replacement)6
- Assess the severity of muscle involvement, even at the subclinical level4
- Help localise affected muscles to provide guidance on where to perform a biopsy6
- Monitor a patient’s response to immunotherapy6
- Monitor for acute damage and current inflammation4
- Briani C et al. Autoimmunity 2006; 39:161-70.
- Malik A et al. Front Neurol 2016; 7:64.
- Oldroyd A & Chinoy H. Curr Opin Rheumatol 2018; 30:606-13.
- Schmidt J. J Neuromuscul Dis 2018; 5:109-29.
- Dalakas MC. N Eng J Med 2015; 372:1734-47.
- Goyal NA. Continuum (MinneapMinn) 2019; 25:1564-85.
- Bohan A & Peter JB. N Engl J Med 1975; 292:344-7.
- Bohan A & Peter JB. N Engl J Med 1975; 292:403-7.
- Lundberg IE et al. Ann Rheum Dis 2017; 76:1955-64.
- Ghirardello A et al. Auto Immun Highlights 2014; 5:69-75.
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