{
 "topic": "Juvenile Dermatomyositis",
 "slug": "juvenile-dermatomyositis",
 "category_id": 15317,
 "summary": "Juvenile dermatomyositis: the Bohan-Peter diagnostic criteria, its vasculopathic pathophysiology distinguishing it from adult disease, MRI-first work-up, and extramuscular complications including ILD and calcinosis.",
 "written_by": "claude-sonnet",
 "references": [
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 ],
 "short": [
  {
   "title": "In short",
   "content": "- Juvenile dermatomyositis (JDM) is the most common pediatric inflammatory myopathy, accounting for more than 80% of childhood inflammatory myositis; overall prevalence is roughly 3.2 per million children in the US, with the whole group of idiopathic inflammatory myopathies occurring at about 1.5-5 per million.\n- Median age of onset is 7-11 years, with a bimodal peak at 3-7 years and again in early adolescence; girls are affected more often than boys.\n- Diagnostic criteria (Bohan and Peter): classic rash (heliotrope eyelid rash and/or Gottron papules) plus at least 3 of: symmetric proximal weakness, elevated muscle enzymes (CK, AST, LDH, aldolase), characteristic EMG changes, and muscle biopsy showing necrosis and inflammation.\n- Core pathophysiology is vasculopathy - endothelial injury of capillaries, venules, and small arteries in muscle, skin, and GI tract - distinguishing JDM from adult dermatomyositis, which is not typically vasculopathic in the same way and carries a worse prognosis in adults.\n- Unlike adult dermatomyositis, which is often a paraneoplastic syndrome, childhood dermatomyositis rarely predisposes to malignancy, so routine cancer screening is not indicated in children.\n- About 30% of JDM patients develop dystrophic calcinosis (soft tissue calcium deposits, 30-70% by another estimate), typically developing months to years after onset, usually periarticular; nailfold capillary changes (dilation with dropout) are common.\n- Severe extramuscular vasculopathic complications include intestinal ischemia/perforation, ulcerative skin disease, and interstitial lung disease (less common in children than adults); high KL-6, anti-MDA5, anti-Jo-1, and IL-18 levels are associated with rapidly progressive interstitial lung disease, which presents with increasing cough and progressive dyspnea (sometimes asymptomatic).\n- Juvenile polymyositis (JPM) is a rare (4-10% of childhood myositis) inflammatory myopathy with the same features as JDM but without skin involvement, often with more severe/distal weakness; treated the same as JDM.\n- Amyopathic JDM (JDM sine myositis) shows skin findings with little/no muscle involvement; up to 26% may develop overt myositis on follow-up beyond 4 years, but unlike the adult form, pediatric amyopathic dermatomyositis has not been linked to interstitial lung disease or internal malignancy, giving it a good prognosis; MRI is sensitive for detecting subclinical muscle involvement in this group.\n"
  }
 ],
 "long": [
  {
   "title": "Definition",
   "content": "Juvenile dermatomyositis (JDM) is a systemic autoimmune/connective tissue disorder of childhood characterized by chronic inflammation of skeletal muscle and skin, of unknown underlying cause. It is the most common clinical subset within the broader family of idiopathic inflammatory myopathies (IIMs), accounting for more than 80% (one source: about 85%) of childhood inflammatory myositis cases.\n"
  },
  {
   "title": "Epidemiology",
   "content": "JDM and the related idiopathic inflammatory myopathies are rare, occurring at a rate of about 1.5-5 per million children; JDM prevalence specifically is estimated at about 3.2 per million children in the United States. It affects girls more often than boys and shows a bimodal age pattern, with incidence peaking first at 3-7 years and again in early adolescence; median age of onset is cited as 7-11 years. JDM accounts for approximately 5% of all rheumatic disease in childhood. There is seasonal clustering of JDM onset, suggesting a viral or bacterial trigger for the illness.\n"
  },
  {
   "title": "Pathophysiology",
   "content": "JDM is fundamentally a vasculopathy: endothelial injury affecting the capillaries, venules, and small arteries of muscle, skin, and the gastrointestinal tract, involving both innate and adaptive (humoral and cell-mediated) immune mechanisms. This vasculopathic process, with intimal proliferation of small vessels, thrombosis, and sometimes infarction, produces the characteristic muscle inflammation/necrosis and skin findings and can lead to tissue damage such as permanent skin ulceration. This distinguishes pediatric from adult dermatomyositis: JDM is not associated with malignancy (unlike adult disease, which can be paraneoplastic) and tends to remit after several years, while adult dermatomyositis generally carries a worse prognosis.\n"
  },
  {
   "title": "Clinical Features",
   "content": "Patients typically present with a rash and proximal muscle weakness developing gradually over weeks to months. The classic rash includes a heliotrope rash of the eyelids and Gottron papules over the knuckles and elbows; rashes can range from subtle to severe, vasculitic, ulcerative lesions. Weakness is symmetric and proximal, often with tenderness and easy fatigability. Nailfold capillary changes - dilation with adjacent capillary dropout - are a common and useful examination finding. About 30% of JDM patients (up to 30-70% by another estimate) develop dystrophic calcinosis, usually periarticular, typically appearing months to years after disease onset. Extramuscular organ involvement can include the gastrointestinal tract (intestinal ischemia and perforation in severe vasculopathic disease), lungs (interstitial lung disease, less common in children than adults but presenting with increasing cough and progressive dyspnea, sometimes asymptomatic), and heart. Lipodystrophy with insulin-resistant diabetes is a recognized association. Juvenile polymyositis (JPM), representing about 4-10% of childhood myositis, shares JDM's clinical features (proximal, sometimes more distal, weakness; elevated CK and aldolase; muscle edema on MRI) but lacks the characteristic skin findings. A distinct subset, amyopathic dermatomyositis (JDM sine myositis), shows skin manifestations with little or no muscle involvement; up to 26% of these children develop overt myositis on follow-up beyond about 4 years, but unlike adult amyopathic dermatomyositis, the pediatric form has not been linked to interstitial lung disease or internal malignancy, giving this subgroup a good prognosis.\n"
  },
  {
   "title": "Diagnostics",
   "content": "The Bohan and Peter diagnostic criteria require the classic rash (heliotrope eyelid rash and/or Gottron papules) plus at least 3 of the following: symmetric proximal weakness; elevated muscle enzymes (creatine kinase, AST, LDH, aldolase); characteristic electromyographic changes (short, small polyphasic motor unit potentials, fibrillations, positive sharp waves, insertional irritability, bizarre high-frequency repetitive discharges); and muscle biopsy showing necrosis and inflammation. Serum muscle enzymes correlate with disease activity, though a normal creatine kinase does not exclude JDM. MRI (short-tau inversion recovery/STIR or T2 fat-saturated sequences) is a sensitive investigation of choice for muscle inflammation, typically showing symmetric involvement of proximal musculature (thighs more than pelvis more than shoulders, with vastus lateralis and vastus intermedius most commonly affected), and is also useful for detecting subclinical muscle involvement in amyopathic disease. Muscle biopsy shows perivascular and perifascicular inflammatory infiltrates with necrotic fibers, perifascicular atrophy, and regeneration; biopsy/EMG is used less commonly now and mainly reserved for diagnostically difficult cases. High serum KL-6, anti-MDA5 antibodies, anti-Jo-1 (histidyl tRNA synthetase) antibodies, and IL-18 are associated with rapidly progressive interstitial lung disease and should prompt closer pulmonary monitoring. Because childhood dermatomyositis rarely predisposes to malignancy (unlike the adult paraneoplastic form), routine malignancy screening is not indicated in children with JDM.\n"
  },
  {
   "title": "Differential Diagnosis",
   "content": "Juvenile polymyositis and other connective tissue diseases with myositis (SLE, mixed connective tissue disease, scleroderma) should be considered, along with muscular dystrophy, particularly when evaluating a child with proximal weakness but no rash (as in JPM).\n"
  },
  {
   "title": "Treatment",
   "content": "JDM is relatively responsive to immunosuppressive therapy, and rapid diagnosis with adequate treatment improves outcomes; juvenile polymyositis is treated the same way as JDM. Some patients go on to have difficult-to-control disease with persistent myositis over a long period.\n"
  },
  {
   "title": "Complications",
   "content": "Severe vasculopathic complications include intestinal ischemia and perforation, ulcerative skin disease, and interstitial lung disease; rapidly progressive interstitial lung disease is a particular concern in patients with high KL-6, anti-MDA5, anti-Jo-1, or IL-18 levels. Dystrophic calcinosis is a common longer-term complication affecting a substantial minority of patients.\n"
  }
 ],
 "clinical": [
  {
   "title": "Recognizing and Diagnosing JDM",
   "content": "Suspect JDM in a child (median onset 7-11 years, bimodal peak at 3-7 years and early adolescence, girls more often than boys) with gradually progressive, symmetric proximal muscle weakness together with a heliotrope eyelid rash and/or Gottron papules over the knuckles or elbows. Apply the Bohan and Peter criteria: with the classic rash present, confirm at least 3 of symmetric proximal weakness, elevated muscle enzymes (CK, AST, LDH, aldolase), characteristic EMG findings, or biopsy showing necrosis and inflammation - but remember a normal CK does not exclude the diagnosis. Order MRI (STIR/T2 fat-saturated) as the preferred first-line imaging study to demonstrate symmetric proximal muscle inflammation (thighs, especially vastus lateralis/intermedius, more than pelvis or shoulders), reserving muscle biopsy or EMG for diagnostically uncertain cases. Examine the nailfolds for capillary dilation with dropout, a useful supportive sign. In a child with proximal weakness but no rash, consider juvenile polymyositis or muscular dystrophy and pursue the same enzyme/MRI work-up, since JPM is managed identically to JDM once confirmed.\n"
  },
  {
   "title": "Screening for Complications and Directing Management",
   "content": "Do not pursue routine malignancy screening in a child with JDM - unlike adult dermatomyositis, childhood disease is not a paraneoplastic syndrome. Do screen for extramuscular vasculopathic involvement: ask about abdominal pain (intestinal ischemia risk), monitor skin for ulceration, and watch for cough or progressive dyspnea suggesting interstitial lung disease, checking KL-6, anti-MDA5, anti-Jo-1, and IL-18 when ILD is a concern, since elevated levels flag risk for a rapidly progressive course. Anticipate dystrophic calcinosis as a possible late finding (about 30% of patients), typically periarticular and appearing months to years after onset. In a child with skin findings alone and minimal muscle involvement (amyopathic JDM), reassure that the pediatric form, unlike the adult one, is not linked to malignancy or ILD, but arrange follow-up over several years since up to about a quarter go on to develop overt myositis. Refer early to pediatric rheumatology, since JDM is relatively responsive to immunosuppressive therapy and prompt, adequate treatment improves long-term outcomes.\n"
  }
 ]
}