Juvenile dermatomyositis

Causes and mechanisms of development

The exact cause of juvenile dermatomyositis is unknown. The development of the pathology is viewed as the result of external triggers acting on a genetically predisposed organism.

Triggering factors include infectious agents (especially Coxsackie B enteroviruses, parvovirus B19, and streptococci), excessive sun exposure, vaccination, and certain medications. Breakdown of self-tolerance often occurs via the phenomenon of molecular mimicry. The genetic background is due to the carriage of the HLA-DQA1*0501 allele (in linkage with HLA-DR3), and mutations in the TNF-α and interleukin-1 genes. Interestingly, maternal chimeric cells (microchimerism), which are detected in the muscles and blood of almost all sick children, participate in the pathogenesis.

Pathogenesis is based on a combined disturbance of humoral and cellular immunity. Plasmacytoid dendritic cells excessively synthesize type 1 interferons (IFN-α and IFN-β). Blood vessels become the primary target of the attack. Vasculopathy leads to local ischemia and subsequent damage to muscle tissue. In response, the pathological expression of major histocompatibility complex antigens (MHC class I) increases in the muscles.

Clinical presentation: from skin to muscles

The disease most often debuts subacutely, beginning with a prodromal period (weakness, fever, weight loss). The main syndromes gradually unfold:

• Skin syndrome: Classic manifestations include heliotrope rash (purplish spots and marked edema of the upper eyelids — the "dermatomyositis goggles" sign) and Gottron's papules (erythematous-violaceous scaly plaques on the extensor surfaces of the joints). Partial lipodystrophy often develops in children.
• Muscle syndrome (myopathy): Weakness is symmetrical and affects the proximal and axial musculature. It is difficult for the child to get up from a chair, raise their arms, or lift their head from the pillow. Involvement of the bulbar muscles presents a particular danger, leading to dysphagia (choking) and dysphonia with a high risk of aspiration pneumonia.
• Vasculopathy: Manifests as Raynaud's phenomenon, livedo reticularis, and micronecroses. Severe vasculitis can lead to skin ulcerations.
• Visceral lesions: Interstitial lung fibrosis, myocarditis, and dilated cardiomyopathy may develop. In the gastrointestinal tract, vasculopathy provokes ulcers with the risk of profuse bleeding and peritonitis.

Calcinosis (in 20–40% of patients) is a specific complication. Calcium salt nodules form in the muscles and subcutaneous tissue and can be rejected through the skin as crumbly masses.

Laboratory and instrumental diagnostics

There are no strictly pathognomonic clinical signs; the diagnosis is based on diagnostic criteria (e.g., Tanimoto criteria). It includes a comprehensive evaluation:

1. Laboratory tests: In the active phase, markers of cytolysis (CK, LDH, aldolase, AST, ALT) and inflammation (ESR) are elevated. The immunogram reveals antinuclear antibodies (ANA) and myositis-specific antibodies (against aminoacyl-tRNA synthetases, such as anti-Jo-1, anti-PL-12, anti-PL-7). The presence of anti-Mi-2 antibodies indicates a milder course.
2. Instrumental methods: Electromyography (EMG) records a myopathic pattern (decreased motor unit potentials, fibrillations). Ultrasound shows muscle hyperechogenicity, and MRI allows for early detection of edema (a sign of inflammation) and calcifications.
3. Muscle biopsy: Necessary in complex cases. It reveals typical perifascicular atrophy, fiber necrosis, and perivascular infiltration with the presence of the membrane attack complex.

Principles of treatment and prognosis

Treatment is step-by-step. The main goal is to suppress disease activity and maintain sustained remission (for at least 2 years before attempting to discontinue therapy).

• Basic therapy: High-dose systemic glucocorticoids (prednisolone). Intravenous methylprednisolone pulse therapy is used for rapid suppression of high activity.
• Immunosuppressants: Methotrexate is the second-line drug of choice (prescribed strictly with folic acid). Cyclosporine or mycophenolate mofetil serve as alternatives.
• Biological therapy: For resistant forms, genetically engineered drugs are added: TNF-α inhibitors (infliximab) or anti-B-cell therapy (rituximab).

Non-pharmacological support plays a crucial role: mandatory and constant protection from ultraviolet radiation (sunscreens), since sun exposure provokes flares, and regular physical therapy to prevent contractures. With timely treatment, the prognosis is favorable — mortality does not exceed 2–3%, and complete functional recovery is possible.

Class and exam questions

What else to study on this topic

• Pathomorphological changes in muscle tissue (perifascicular atrophy)
• The role of maternal microchimerism and IFN-α/β in pathogenesis
• Diagnostic criteria for dermato- and polymyositis (according to Tanimoto K. et al., 1995)
• Differential diagnosis with juvenile idiopathic arthritis and muscular dystrophies
• Medical approaches to the treatment of severe muscular calcinosis

Other topics of «Chapter 9. Rheumatic Diseases»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.