Pediatric emergency medicine trisk 2965 2965
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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 2 to 5 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.
