■ It affects children and adults and is the most common inflammatory myopathy in children.
■It presents subacutely with muscle pain, weakness, and stiffness, and causes also a purple (heliotrope) discolorarion of the upper eyelids, edema around the eyes and mouth, facial erythema, and erythematous scaly papules over the knuckles and elbows (Gottron’s sign).
■ Contractures, subcutaneous calcification, intestinal ulceration, and other extramuscular manifestations are frequent in children.
#Pathology of dermatomyositis:
■ It includes inflammation, vasculitis, and perifascicular atrophy.
■The inflammatory cells are predominantly B-cells (with smaller numbers of CD4-positive T-cells) and are found around blood vessels, in the septa between muscle fascicles, and in fibroadipose tissue around muscle.
■ The key pathological change of dermatomyositis is a vasculitis, which involves endomysial and perimysial capillaries and arterioles.
■This vasculitis begins with endothelial swelling and is followed by endothelial necrosis and capillary loss.
■ Tubuloreticular cytoplasmic inclusions (TRIs) are often seen in endothelial cells. TRIs also occur in lupus and other collagen vascular diseases but are absent in polymyositis and inclusion body myositis.
■The vasculitis is thought to be caused by circulating anti-endothelial antibodies.
■ Interaction of these antibodies with vascular antigens activates complement, leading to formation of the membranolytic attack complex (MAC), which destroys endothelial cells.
■ A distinctive feature of dermatomyositis is atrophy and degeneration of myofibers at the periphery of fascicles (perifascicular atrophy-PFA), which occurs even in absence of inflammation.
■ It has been proposed that PFA is caused by ischemia from loss of the endomysial capillary bed.
■This affects more severely the distal portion of the vascular field, which is the periphery of each fascicle. In support of this hypothesis, ischemic infarction of muscle is seen in some cases of dermatomyositis. However, PFA is not seen in diabetic and other angiopathies.
■ Recent evidence indicates that the CD4-positive cells in dermatomyositis are plasmacytoid dendritic cells. These cells secrete type 1 interferons, which induce genes and molecular cascades that cause muscle injury.
■ A key interferon-induced pathology is deficiency of titin, an intramuscular protein that is a scaffold for contractile filaments. It is now thought that diffusion of interferons in the perifascicular areas causes the PFA.
■ Dermatomyositis and polymyositis (and less frequently inclusion body myositis) are associated with scleroderma, mixed connective tissue disease, and cancer.
■ The association with cancer is stronger with dermatomyositis. Patients with polymyositis and dermatomyositis may also have cardiac involvement leading to arrhythmia and heart failure, arthralgia, Raynaud’s phenomenon, interstitial pneumonitis, and renal involvement.
■Some of these extra-muscular manifestations are associated with circulating antibodies to anti-Jo-1 (an anti tRNA synthetase) autoantibodies.