introduction of potent immunomodulators earlier in the disease course has led to improved outcomes The mortality rate may now be as low as 1.5% in the United States, and drug- and disease-related morbidity are also improving Despite these advances, however, JDM remains a serious disease that requires the care of physicians experienced with its management While historically, Bohan and Peter’s criteria for DM/polymyositis (PM) in adults had been used in children, a revised diagnostic and classification system was introduced in 2017 The EULAR/ACR diagnostic and classification criteria are intended to diagnose inflammatory myopathies in both children and adults The system is made up of several variables related to skin and muscle inflammation with different scales depending on the presence or absence of muscle biopsy information ( Table 101.8 ) JDM differs from adult DM in several important features There is a more prominent degree of vascular inflammation, it less commonly involves detectable autoantibodies, and it rarely accompanies malignancies Further, in children, the appearance on MRI is essentially diagnostic because other causes of inflamed muscle and soft tissue are not seen in this age group ( Fig 101.4 ) Microscopically, the skin in JDM shows dermal atrophy, obliteration of appendages, and lymphocytic infiltration The muscle typically demonstrates a mixture of degenerating and regenerating muscle fibers, variations in muscle fiber size, perivascular lymphocytic infiltration, and perifascicular atrophy of muscle fibers Small arteries, venules, and capillaries of the skin, muscle, fat, and GI tract characteristically reveal angiopathy While the pathogenesis of JDM has not been clearly defined, there are several models to explain its development including environmental triggers like viral infections, particularly Coxsackie B; vasculitis caused by immune complex deposition; and cell-mediated cytotoxicity directed against muscle fibers Certain HLA haplotypes, especially HLA-B8/DR3, may predispose to the disease Synthesizing these findings, JDM is thought to represent an as-yet unexplained perpetuation of muscle inflammation in susceptible hosts following an exogenous trigger Clinical Considerations Clinical Recognition JDM has a wide clinical spectrum, from a mild form involving mainly the skin (JDM sine myositis or amyopathic JDM) to a severe vasculitic type with a fulminant course When untreated, the natural history of JDM is to pass through four overlapping phases that typically last for to years but may persist indefinitely: (i) A prodromal phase of nonspecific aches and pains, (ii) a phase of progressive muscle and skin inflammation characterized by weakness and rash, (iii) a phase of persistent active disease and cumulative tissue damage, and (iv) an indolent phase with development of contractures and calcinosis but minimal ongoing inflammation The goal of therapy is to abort this progression such that it ends before irreversible damage occurs The onset of JDM is often insidious, with aches and pains in the limbs, malaise, lowgrade fever, and edema of the hands, feet, and eyelids There may be a diffuse and nonspecific rash This prodromal stage evolves into the acute phase, when the characteristic features of JDM become evident Classical skin manifestations include a violaceous heliotrope rash in the periorbital region and occasionally on the forehead; dusky red or atrophic lesions over the extensor aspects of the knees, elbows, and knuckles (Gottron papules); and periungual erythema ( Fig 101.5 ) Skin findings may precede or follow the onset of muscle weakness Ulcerative skin lesions and anasarca are rare presenting manifestations of JDM associated with a particularly severe disease course Muscular involvement in JDM is characterized by symmetric proximal muscle weakness that may be accompanied by pain and tenderness Strength of the anterior neck flexors and abdominal muscles is particularly affected, while facial muscles are spared The disease may progress to involve the muscles of the palate and pharynx, resulting in regurgitation, nasal voice, and aspiration Weakness of the respiratory muscles can lead to a poor cough, pneumonia, and respiratory failure Risk of GI hemorrhage and perforation are increased at this stage The clinical course of JDM is variable; however, with the newer approaches to treatment, disease manifestations may be controlled within a few months in the vast majority of patients Children with ongoing muscle inflammation for more than to 12 months are at risk of developing late complications of JDM These include pronounced muscle wasting, contractures, lipodystrophy, and pigmentary changes of the skin The rash over the extremities often becomes dry, scaly, and atrophic Subcutaneous calcifications historically have occurred in up to 30% of children during this phase, although early aggressive treatment of inflammation dramatically reduces this rate Calcifications are most typically discrete nodules around large joints, but at times they may take the form of a diffuse encasement of the soft tissues, known as calcinosis universalis Occasionally, children pass through the early stages insidiously and come to the attention of physicians only when they develop contractures and calcinosis Clinical Assessment Although skin and striated muscle are the primary targets of the inflammatory process in JDM, typically other organ systems are also involved Up to one-third of children develop arthritis, which may be present at diagnosis or may develop months into the disease process The arthritis of JDM is generally nonerosive and often improves as the primary disease is treated, although some children require specific therapy for arthritis Neurologic manifestations of JDM are extremely rare, but peripheral polyneuropathy, seizures, psychosis, and one case of suspected brainstem vasculopathy have been reported TABLE 101.8 THE EULAR/ACR CLASSIFICATION CRITERIA FOR ADULT AND JUVENILE IDIOPATHIC INFLAMMATORY MYOPATHIES When no better explanation for the symptoms and signs exists these classification criteria can be used Score points Variable Age of onset Age of onset of first symptom assumed to be related to the disease ≥18 yrs and