Juvenile dermatomyositis (JDM) is the most common pediatric acquired inflammatory myopathy. This diffuse vasculopathy primarily involves inflammation in skin and striated muscle, which often leads to significant weakness and physical limitations. However, despite its nomenclature JDM is a multisystem disease. Characteristic cutaneous features, which classically include heliotrope rash and Gottron papules, aid in the diagnosis. The etiology of this condition is unknown, and has an estimated incidence of 1 to 3 in 1 million children,^1,2 with approximately a 2:1 female predominance. Affected children are typically between 4 and 10 years of age with an average age of onset of 7 years. Outcomes in JDM may vary, but historically there was significant morbidity; without adequate treatment, one-third of affected persons will have moderate to severe disability. Mortality attributable to the disease occurs in one-third of children.^3,4

PATHOGENESIS

Despite extensive research, the pathogenesis of JDM remains unknown. There is often seasonal clustering of new cases of JDM, suggesting an environmental or infectious trigger. Association with specific infectious processes raises the possibility of molecular mimicry as a mechanism in JDM,^5-7 but no single agent has been identified. There appears to be genetic susceptibility, with HLA associations HLA-DQA1*0501, HLA-DQA*0301, HLA-DRB1*0301, and HLA-B8 described in patients with JDM,^8-10 and TNF-α-308A allele has been associated with a prolonged disease course.¹¹

Abnormalities in humoral-mediated immunity, cell-mediated immunity, and immune complex disease have also been implicated in the pathogenesis, but these results have been varied.^12-17 Some research suggests a role of major histocompatibility complex (MHC)-I, where there is overexpression in JDM and myositis-associated antibodies. Elevated immunoglobulin levels and myositis-specific autoantibodies, which have been identified most patients with JDM,¹⁸ suggest a humoral component to the pathogenesis of JDM. Type 1 interferons have also been implicated and are thought to contribute to enhanced MHC-I expression, cell cytotoxicity, and activation of T cells. Additional research suggests a possible role of maternal microchimerism, where maternal cells trigger an immune response akin to graft-versus-host disease in the child.^19,20

Despite this broad range of possibilities in the pathogenesis of this condition, it is generally accepted that JDM occurs in a genetically susceptible patient in response to an environmental or infectious trigger, with a disruption in normal immune functioning.

Children with JDM are diagnosed at less than 18 years of age, and typically present with proximal muscle weakness and a pathognomonic rash. There is a heterogeneous presentation to JDM, which can vary from mild to severe skin involvement and mild to severe weakness, and the presentation may be acute or insidious. The formal diagnosis of JDM is based on the Bohan and Peter criteria²¹ (Table 149-1; www.imm.ki.se/biostatistics/calculators/iim). Weakness may have broad clinical presentations including nonspecific malaise, myalgia, decreased general stamina and endurance with activities, difficulty in climbing stairs and getting up out of bed, and difficulty with combing hair or brushing teeth.

The pathognomonic rash of JDM includes Gottron papules and heliotrope rash. Gottron papules are typically found over the extensor surfaces of the interphalangeal joints, elbows, knees, or ankles and are typically described as erythematous, raised papules (Figure 149-1). Heliotrope rash is a periorbital red-purplish discoloration associated with periorbital edema (Figure 149-2). Other rashes associated with JDM include periungual erythema and nail bed capillary abnormalities best seen with capillaroscopy (Figure 149-3), and a macular violaceous erythematous rash with distribution in the neck and upper chest (V-sign), nape of the neck, upper back, and posterior aspect of the shoulders (shawl sign), or lateral surfaces of the hips.

Other organ systems can be involved in JDM, such as the gastrointestinal tract, heart, lungs, joints, and central and peripheral nervous systems, which can be a source of significant morbidity and mortality and should be monitored for closely. Systemic features can include cardiac conduction abnormalities, decreases in ventilatory and lung diffusion capacity, esophageal and gastric dysmotility, and gastrointestinal ulcers, hemorrhage, and perforation. Ulcerative skin disease in JDM suggests a more aggressive course with extensive systemic vasculopathy. It is potentially life threatening and requires closer monitoring.

Based on the severity and duration of muscle weakness, patients may present with muscular atrophy due to disuse, and contractures around the joints of disuse that can also lead to irreversible physical limitations if untreated. In addition, more than half of children with JDM are reported to have associated arthritis.²²

Long-term cutaneous sequelae may include calcinosis and lipodystrophy. Calcinosis occurs in up to 40% of children with JDM; findings may include small subcutaneous nodules and large tumorous deposits.^23,24 Calcinosis typically occurs at sites of trauma (e.g. elbows, knees), especially in later phases of disease, severe cases, and cases where treatment is insufficient or delayed. Calcinosis may lead to physical limitations and contractures, as well as infections related to ulceration with significant cosmetic sequelae (Figure 149-4). Lipodystrophy occurs in up to 40% of children with JDM.^25,26 It is an acquired loss of subcutaneous fat in a localized or generalized distribution that is cosmetically disfiguring. Lipodystrophy commonly results in metabolic abnormalities such as insulin resistance and hyperlipidemia.

The diagnosis of “definite” JDM, based on the Bohan and Peter criteria, requires the pathognomonic rash of JDM and three other criteria, while “probable” JDM requires the classic rash and two other criteria (Table 149-1).