What are features of dermatomyositis in adults & children?

Idiopathic inflammatory myopathies

The precise pathogenesis of idiopathic inflammatory myopathies is not well understood and involves many immunological pathways.^2,5

Idiopathic inflammatory myopathies are classified according to their patterns of presentation, age of onset, immunohistopathologic features, and treatment response.²

Identification of myositis-specific autoantibodies has improved the characterisation of myositis subtypes and associated clinical phenotypes – which vary in the severity of muscle involvement, extramuscular manifestations, and risk of malignancy.¹

Overlap myositis, which includes anti-synthetase syndrome, is the most common idiopathic inflammatory myopathy and accounts for up to half of cases⁴. It is defined as

• Myositis associated with another defined collagen vascular disease², e.g. systemic lupus erythematous, Sjögren syndrome, rheumatoid arthritis, or systemic sclerosis¹
• Presence of “overlap autoantibodies” including myositis-specific antibodies (MSA) or myositis-associated autoantibodies (MAA)²

Anti-synthetase syndrome may have similar clinical/pathological features to a number of idiopathic inflammatory myopathies (dermatomyositis, polymyositis or necrotising autoimmune myopathy), but clinically evident myositis may be absent.²

Dermatomyositis accounts for over one third of cases of idiopathic inflammatory myopathy.⁴

Overview Idiopathic inflammatory myopathies:

• Dermatomyositis (DM)
  Dermatomyositis typically occurs as proximal muscle weakness and/or a characteristic skin rash.²
• Polymyositis (PM)
  Polymyositis affects many different muscles, particularly around the neck, shoulders, back, hips and thighs.²
• Necrotising autoimmune myopathy (NAM)
  This subgroup is characterized by scattered necrotic muscle fibres and acute or subacute proximal weakness in arms and legs and very high creatine kinase levels.^2,4
• Sporadic-inclusion body myositis (sIBM)
  Inclusion body myositis (IBM) is the most common acquired myopathy after the age of 50.^8,16 Inclusion body myositis presents with progressive insidious weakness, often asymmetric, predominantly affecting the quadriceps and/ or finger flexors⁸. Dysphagia is seen in more than 50% of patients with inclusion body myositis.¹⁷

Features of dermatomyositis in comparison with other idiopathic inflammatory myopathies^1,2

Some key features of dermatomyositis in comparison with other idiopathic inflammatory myopathies include:

Rash: One of the most distinctive features of the autoimmune disease called dermatomyositis is a characteristic rash that appears on the skin, often on the face, neck, and upper chest. The rash may be purplish or reddish and may be accompanied by scaling.

Skin changes: In addition to the rash, people with dermatomyositis may also experience other skin changes, such as thinning of the skin, scarring, and development of calcifications.

Muscle weakness: Like other idiopathic inflammatory myopathies, dermatomyositis may cause muscle weakness, even though not all patients experience muscle symptoms. Muscle involvement mainly affects multiple proximal muscle groups, including those in the neck, shoulder, hip, and upper and lower extremities.

Find more details on the table below. Compare with other inflammatory myopathies by clicking the button on the right side:

	Dermatomyositis (DM)	Polymyositis (PM)	Necrotising autoimmune myopathy (NAM)	Sporadic-inclusion body myositis (sIBM)
Onset and disease course	Acute or insidiously progressive, symmetrical, proximal muscle weakness and/or a characteristic skin rash2 Usually painless2	Progressive onset of symmetrical, proximal muscle weakness2 Commonly painful2	Subacute onset, progressive symmetrical, proximal muscle weakness1,2 Weakness develops more rapidly than DM and PM, and is more severe2,4	Progresses over several years2 Unique versus other IIMs as it affects both the proximal and distal musculature in a symmetrical/asymmetrical pattern2
Age at onset	Children (Juvenile dermatomyositis)4 Adults4	Adults (usually)2	Adults2	Adults > 40 years old2
Symptoms, specific features	+/- dysphagia, specific skin and organ manifestation2,4 Rash may precede onset of muscular weakness1,2 Involvement of pulmonary system e.g. interstitial lung disease1,2,4 Malignancy in adults2,4 Heliotrope rash: reddish purple rash on or around the eyelids2 V-sign: Rash on anterior chest2 Gottron’s sign1, Gottron’s papules2 Shawl sign: rash on neck, back and shoulders2 Inverse Gottron’s papules on the volar aspect of hands, dilated capillary loops at the nail beds with periungual telangiectasias and thickened, cracked skin on the dorsal and ventral surfaces of the hands (mechanic’s hands)2	No rash2 Muscular and extramuscular organ involvement similar to DM2	No rash1,2 Myalgias, dysphagia may occur2 Creatinine kinase usually higher than in other IIMs1,2 Extramuscular manifestations include congestive heart failure2,4	No skin changes4 Weakness in flexor forearm muscles. Quadriceps weakness leads to falls/tripping2 Dysphagia very common2 Mild facial weakness common2
Muscle pathology	B cells and CD4+ T cells in perimysial and perivascular areas2 Perifascicular muscle fibre atrophy2 Membranolytic attack complex (MAC) deposition in microvasculature2	Invasion of endomysial cytotoxic CD8+ T cells in muscle fibres2 Macrophages invade non-necrotic muscle fibres2	Macrophage-mediated immune response2 Necrotic myofibres surrounded by sparse inflammatory infiltrate (predominantly lymphocytes)2 MAC deposition in microvasculature2	Similar to PM, invasion of muscle fibres by cytotoxic CD8+ T cells and macrophages2 Rimmed vacuoles characteristic of degenerative changes2 Amyloid deposits2
Treatment response	Usually responsive to immunotherapies2,4	Usually responsive to immunotherapies2,4	Usually responsive to immunotherapies2,4	Usually refractory to immunotherapies2
Histological section (images courtesy of Prof. Patrick Cherin)

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