Shah M, Mamyrova G, Targoff IN, et al.; Childhood Myositis Heterogeneity Collaborative Study Group. The clinical phenotypes of the juvenile idiopathic inflammatory myopathies. Medicine (Baltimore). 2013 Jan;92(1):25-41.
“The juvenile idiopathic inflammatory myopathies (JIIM) are systemic autoimmune diseases characterized by skeletal muscle weakness, characteristic rashes, and other systemic features. Although juvenile dermatomyositis (JDM), the most common form of JIIM, has been well studied, the other major clinical subgroups of JIIM, including juvenile polymyositis (JPM) and juvenile myositis overlapping with another autoimmune or connective tissue disease (JCTM), have not been well characterized, and their similarity to the adult clinical subgroups is unknown. JDM was characterized by typical rashes, including Gottron papules, heliotrope rash, malar rash, periungual capillary changes, and other photosensitive and vasculopathic skin rashes. JPM was characterized by more severe weakness, higher creatine kinase levels, falling episodes, and more frequent cardiac disease. JCTM had more frequent interstitial lung disease, Raynaud phenomenon, arthralgia, and malar rash. Differences in autoantibody frequency were also evident, with anti-p155/140, anti-MJ, and anti-Mi-2 seen more frequently in patients with JDM, anti-signal recognition particle and anti-Jo-1 in JPM, and anti-U1-RNP, PM-Scl, and other myositis-associated autoantibodies more commonly present in JCTM. Mortality was highest in patients with JCTM, whereas hospitalizations and wheelchair use were highest in JPM patients. Several demographic and clinical features were shared between juvenile and adult IIM subgroups. However, JDM and JPM patients had a lower frequency of interstitial lung disease, Raynaud phenomenon, “mechanic’s hands” and carpal tunnel syndrome, and lower mortality than their adult counterparts. We conclude that juvenile myositis is a heterogeneous group of illnesses with distinct clinical subgroups, defined by varying clinical and demographic characteristics, laboratory features, and outcomes.”
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Huber AM. Idiopathic inflammatory myopathies in childhood: current concepts. Pediatr Clin North Am. 2012 Apr;59(2):365-80.
“The JIIM are rare and serious chronic illnesses. Awareness and careful clinical evaluation are necessary to recognize these illnesses and to distinguish them from other muscle problems, such as dystrophies and myopathies, and from other mimicking illnesses. Clinical features, such as proximal muscle weakness, Gottron lesions, and heliotrope rash can provide important diagnostic clues. In patients with atypical presentations, additional investigations, including muscle biopsy, are warranted. Current treatment includes high-dose corticosteroids and methotrexate, which in most patients results in good clinical outcomes, although significant morbidities are not uncommon. Given the rarity and complexity of these illnesses, it is recommended that patients with suspected JIIM are referred for multidisciplinary specialist care to maximize the likelihood of the best outcomes possible.”
Feldman BM, Rider LG, Reed AM, Pachman LM. Juvenile dermatomyositis and other idiopathic inflammatory myopathies of childhood. Lancet. 2008 Jun 28;371(9631):2201-12.
“Childhood idiopathic inflammatory myopathies can be divided into more homogeneous clinicopathological or serological subsets with distinctive epidemiology, and clinical, pathological, or prognostic features ( ).  Juvenile dermatomyositis is the most common subset, representing up to 85% of childhood idiopathic inflammatory myopathies. [48,53] The two other major subsets of idiopathic inflammatory myopathies are juvenile polymyositis, in which the characteristic rashes are absent, and overlap myositis—ie, polymyositis or juvenile dermatomyositis patients who also meet the criteria for another autoimmune disease. The incidence of polymyositis is much lower than that of juvenile dermatomyositis, [6,75] and the prevalence of polymyositis is 2–8% of childhood idiopathic inflammatory myopathies. [48,53] Weakness in patients with juvenile polymyositis is often both proximal and distal, and muscle atrophy is frequent. Such patients have similar degrees of dysphagia, arthritis, and contractures, as do patients with juvenile dermatomyositis. ”
Pilkington CA, Wedderburn LR. Paediatric idiopathic inflammatory muscle disease: recognition and management. Drugs. 2005;65(10):1355-65.
Criteria for diagnosing paediatric idiopathic inflammatory myopathies:
“The four criteria are:
- progressive muscle weakness that is symmetrical and affects the proximal muscles including the limb-girdle and neck flexor muscles;
- an increase in serum muscle enzymes such as creatine kinase (CK), lactate dehydrogenase (LDH), AST or aldolase;
- a muscle biopsy consistent with myositis (perifascicular atrophy, perivascular inflammatory infiltrates, internal myonuclei and necrosis of muscle fibres);
- an electromyogram (EMG) showing myopathy and denervation
These criteria are also used for polymyositis. This is defined as dermatomyositis without a rash, and it is extremely rare in childhood. It is seen more commonly in adult patients, and has different features on muscle biopsy, with inflammatory infiltrates, in particular of CD8+ T cells, being widespread within the muscle.”
More PubMed results for JPM.