JUVENILE DERMATOMYOSITIS
Dr. T.P. Yadav*
In charge Pediatric Rheumatology division Dr R M L Hospital New Delhi. *
Juvenile dermatomyositis (JDM), the commonest of the rare chronic juvenile inflammatory myositis; is a multisystem autoimmune disease characterized by non-suppurative inflammation of the skeletal muscle and skin.

JDM usually affects children between 2 and 15 years of age with an annual incidence of about 2 - 3 cases per million children per year. Females are usually affected more than boys; the ratio varied from 1:1 to 5:1 in different series. The diagnosis of JDM is based on Bohan and Peter criteria of 1975. They defined definite JDM by a characteristic rash and three of four other criteria. Probable JDM was defined by a rash and 2 of the 4 criteria.
Bohan and Peter Diagnostic Criteria
  1. Cutaneous features - heliotrope rash, Gottron's papules, malar erythema
  2. Symmetrical and progressive proximal muscle weakness
  3. Elevated serum levels of muscle derived enzymes e.g. creatinine phosphokinase, aspartate aminotransferase, lactate dehydrogenase, aldolase
  4. Electromyographic changes characteristic of myopathy and denervation
  5. Muscle biopsy findings of necrosis and inflammation

Some authors have defined a subset of JDM patients with characteristic rash of dermatomyositis without evidence of inflammatory myopathy - as amyopathic JDM. However other authors have challenged the existence of this entity, claiming that if these patients are followed up long enough, evidence of myositis will surface.

Estimation of serum levels of muscle enzymes is important for diagnosis and monitoring therapy. Sarcoplasmic muscle enzymes are released in the peripheral blood as a consequence of muscle damage. These are creatine kinase (CK), Lactic dehydrogenase (LDH), aldolase & aspartate transaminase (ASTor.SGPT). Serum levels of these enzymes have been reported to be raised in varying percent of patients of JDM in different series. Beside muscle enzymes, non-specific indicators of inflammation eg ESR and CRP are also elevated and tend to correlate with the degree of inflammation. They also help in differentiating from non-inflammatory myopathies eg muscular dystrophy or myotonia. Serum levels of von-Willebrand factor (reflecting endothelial cell damage) and neopterin (markers of monocytes and macrophage actvation) are elevated in patients of JDM. These tend to correlate better with disease activity. Autoantibodies like myositis-specific antibodies (eg Anti- synthetases, Anti Mi-2, Anti SRP) and myositis-associated antibodies (eg Anti PMScl, Anti UI RNP etc.) have been found occasionally in JDM.

Electromyography (EMG) occasionally is useful in confirming diagnosis of JDM. The characteristic EMG changes are those of myopathy and denervation and high frequency repetitive discharges. These include, increased insertional activity, fibrillations, positive sharp wave (denervation) decreased amplitude, short duration polyphasic (myopathic). Nerve conduction velocity and latencies are normal in JDM. EMG has been reported to be positive in 52-96% of patients.

Histologic evaluation of muscle biopsy specimens reveal peri fascicular atrophy, chronic inflammatory infiltrates, evidence of vasculitis, round cells inflammatory infiltrates extending through out all layers of medium sized blood vessels with swollen endothelium, associated with degeneration, necrosis and regeneration of myocytes which has been observed in 69.5% of patients.
Treatment
The aim of treatment is (i) Suppression of immunoinflammatory response (ii) Prevention of loss of muscle function and joints (iii) maintenance of general health, normal growth and development.

All over the world, corticosteroids are the mainstay of treatment. Methotrexate (MTX) was used as a second line agent for refractory JDM along with steroids. Recently in last few years, there has been a shift of approach. There is evidence that corticosteroids and early use of MTX improves outcome and reduces calcinosis.
Course and prognosis
Three courses have been described for JDM:
  1. Monocyclic (limited childhood dermatomyositis). These patients have limited disease, respond to steroids with no residual disease.
  2. Chronic ulcerative group of patients are those with cutaneous and GIT ulceration, active disease for years, do not respond to steroid, and have long term severe calcinosis and residual disability.
  3. Chronic non-ulcerative (polycyclic) are the ones who have progressive weakness, have good initial response, relapse and finally develop permanent disability, calcinosis and severe weakness.
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