Juvenile Dermatomyositis Workup
- Author: Ann M Reed, MD; Chief Editor: Lawrence K Jung, MD more...
Laboratory studies in the workup of juvenile dermatomyositis (JDM) include an erythrocyte sedimentation rate (ESR); muscle enzyme levels; lupus profile (ie, antinuclear antibody [ANA], extractable nuclear antigens [ENA]); and myositis-specific antibody assays such as antibodies against the aminoacyl t-RNA synthetases (ie, anti-Jo-1 antibody), antisignal recognition particle (anti-SRP antibody), and nuclear helicase (anti-Mi-2 antibody).
Nailfold capillary microscopy may show end-row loop capillary loss and formation of bushy loops representing capillary dilatation and branching.
A muscle biopsy is not usually performed to confirm the diagnosis of JDM, as it is for adult myositis. However, it is needed in the workup of juvenile polymyositis.
Magnetic resonance imaging (MRI) with T2-weighted fat suppression and short tau inversion recovery (STIR) is useful in the diagnostic workup because it reveals edema, a marker of muscle inflammation.
Muscle ultrasonography reveals increased muscle echogenicity, attenuation, and reduced bone surface echo. These changes are not specific for JDM, however, and this technique is not widely used.
Electromyography (EMG) reveals a reduction of the motor unit action potentials in the proximal muscles and fibrillation potentials suggestive of fiber splitting, necrosis, and vacuolization. However, the EMG findings may be normal in approximately 19% of children.
Tests of Inflammation and Autoimmunity
The ESR is commonly elevated in patients with JDM, but this finding is nonspecific.
levels of muscle enzymes such as aspartate aminotransferase, lactate dehydrogenase, creatine kinase, and aldolase may be elevated early in the disease course. Creatine kinase levels may initially be within the reference range in approximately 10-40% of patients and may return to normal within a few months of disease onset.[30, 66]
An elevated ANA level may be seen in approximately half of patients with JDM. Generally, the extractable nuclear antigens (SSA, SSB, Sm, RNP, DNA) are negative.
Approximately 10% of patients have myositis-specific autoantibodies that define clinical subsets and serve as prognostic indicators of disease severity.[1, 78] These subsets are associated with different sets of signs and symptoms and, in some cases, onset at different times of year.
Rarely, the aminoacyl-transfer RNA (tRNA) synthetases such as Jo-1 (histidyl-tRNA synthetase), PL-12 (alanyl-tRNA synthetase), and PL-7 (threonyl-tRNA synthetase) have been associated with a spring onset of myositis, with the following clinical manifestations[79, 80] :
Interstitial lung disease
The anti–signal recognition particle (SRP) autoantibodies are associated with an autumn onset and a severe, refractory myositis in less than 5% of JDM cases; in adults, the prognosis is poor, with a high mortality rate and cardiac involvement.[78, 81, 80] The anti-Mi-2 autoantibody may be seen in 5% of patients with mild to moderate JDM.
The p155/140 kDa doublet protein, a myositis-associated autoantibody, has been identified in 29% of patients with JDM and may correlate with more extensive cutaneous involvement, including ulcerations and edema.[82, 83]
Magnetic Resonance Imaging
MRI with T2-weighted fat suppression and short tau inversion recovery (STIR) is useful in the diagnostic workup because it reveals edema, which is a marker of muscle inflammation. STIR findings must be interpreted cautiously, however, because patients with muscle dystrophies may have inflammation as well.
The signal intensity from STIR may correlate with disease activity. Areas of calcinosis or intramuscular fluid collections may be revealed. T1-weighted images reveal atrophy and fatty replacement from chronic damage. MRI also helps in localizing an area for a potential muscle biopsy.
A muscle biopsy is not usually performed to confirm the diagnosis of JDM, as it is for adult myositis; however, it is needed in the workup of juvenile polymyositis. Emerging data has suggested that muscle biopsy findings may be useful in predicting the clinical course and prognosis of JDM based on extensive active myopathic and arteriopathic changes.[87, 88]
If muscle biopsy is performed, a moderately weak muscle should be selected with specimens frozen for cryostat sections to perform histologic and enzyme histochemical stains and immunocytochemistry for major histocompatibility complex (MHC) antigens, immunophenotyping of T cells, and detection of cytokines and complement.
Characteristic findings include perifascicular atrophy and a perivascular, mononuclear cell infiltrate with membrane attack complexes; swelling and occlusion of the capillary lumen; muscle degeneration; muscle regeneration; and infarcts. Necrosis of muscle fibers can occur in the fascicle periphery or in the center of the fascicle.[12, 18] CD4+ positive cells and B cells are seen in the perimysial and perivascular areas.
In contrast, in polymyositis, typical findings are invasion of non-necrotic fibers by CD8+ cytotoxic cells, activated macrophages with expression of major histocompatibility complex I, and inflammatory infiltrates in an endomysial distribution.
An international consensus group has developed a scoring system for muscle biopsy findings in JDM. The system is based on 4 domains of change (inflammatory, vascular, muscle fiber, and connective tissue) and includes the following :
CD3 + and CD 68 + endomysial, perimysial, and perivascular inflammation
Vascular changes, including capillary dropout, arterial findings, and infarction
Muscle fiber changes, such as MHC I overexpression, perifascicular atrophy, neonatal myosin, muscle degeneration, muscle regeneration, and necrosis
Endomysial and perimysial fibrosis
The consensus group suggests that if validated, the scoring system could be used in prospective studies to test which features of muscle pathology are prognostic of disease course or outcome.
Assessment of muscle strength, physical endurance, and function is the primary means for determining clinical status and predicting outcomes in JDM. Several clinical outcome measures in JDM patients have been developed over the last decade to facilitate assessment of clinical response to treatment.
Physical function is measured by the Childhood Health Assessment Questionnaire (CHAQ), a reliable and validated instrument that correlates well with disease activity and functional outcomes. The CHAQ is a 30-item questionnaire that measures proximal muscle strength by assessing 8 areas of physical function, including dressing, grooming, eating, walking, hygiene, and the use of assistive devices.
The Childhood Myositis Assessment Scale (CMAS) is a measure assessing muscle strength, functional capacity, and muscle endurance that has been developed and validated for JDM patients The 14-point scale measures activities such as timed neck flexion and sit-ups.
The Disease Activity Scale assesses skin and muscle involvement.
The cutaneous assessment tool (CAT), a 21-item test used to assess cutaneous manifestations in JDM, has been validated and correlated with skin disease damage scores.
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