Background/Purpose: Juvenile dermatomyositis (JDM) is a chronic inflammatory disorder primarily involving the skin and striated muscle.  Classic JDM presents with rash, proximal muscle weakness, and objective evidence of muscle inflammation.  A subset of patients presenting with cutaneous manifestations in the absence of muscle weakness have been variably termed dermatomyositis sine myositis or amyopathic dermatomyositis.  The prevalence in adult populations has been reported up to 20% while the prevalence in children is unclear.  The extent of the evaluation for myositis in pediatric patients varies between individual practitioners.  Given the variability in evaluation, we hypothesized that truly amyopathic JDM is rare when a comprehensive evaluation for myopathy is performed. 

Methods: A chart review of the initial evaluation was performed on all patients with the diagnosis code for dermatomyositis (ICD-9 code 710.3) seen at the Children’s Hospital of Wisconsin between January 2000 and April 2013.  Patients with disease onset after age 18 years or those previously treated with systemic anti-inflammatory therapy were excluded.  Data collected included patient demographics, presenting symptoms and exam findings, muscle enzyme panel (AST, ALT, LDH, CK, and aldolase), muscle biopsy, electromyography (EMG), and magnetic resonance imaging (MRI). 

Results: Forty-six patients were evaluated for JDM and met study criteria.  All presented with typical cutaneous features of JDM.  Twenty-six patients (57%) reported weakness as a symptom, while thirty-six (78%) demonstrated weakness on exam.  Therefore, ten patients (22%) were classified as clinically amyopathic.   The sensitivity of detecting myositis in the entire study population was 95% when all five enzymes were checked.   All ten clinically amyopathic patients had a full muscle enzyme panel, nine had an MRI of the thigh to assess for myositis, one had an EMG, and one had a muscle biopsy.  Six amyopathic patients had at least one abnormal muscle enzyme.  Of the four amyopathic patients with normal enzymes, two had an abnormal MRI consistent with myositis while the other two were normal.  The two with normal MRIs and enzymes did not have any additional testing and were labeled as amyopathic based on these studies alone.  The one muscle biopsy was done in a patient with abnormal MRI and elevated enzymes; however it did not demonstrate pathologic features consistent with JDM.  The EMG was performed on a patient with elevated enzymes and normal MRI; the EMG was normal.

Conclusion: In children, true clinically amyopathic dermatomyositis is rare when a full panel of muscle enzymes and other ancillary studies are performed.  In our series, EMG and muscle biopsy were not consistently done, and it is not clear how much these studies contribute to the diagnosis.  In patients with negative evaluation (enzymes and MRI), a muscle biopsy should be considered to confirm that the disease is truly amyopathic.  Further research is necessary to define the natural history of true clinical amyopathic patients to determine if such a comprehensive evaluation is necessary and to help identify the appropriate therapy to initiate at time of diagnosis.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/how-often-are-clinically-amyopathic-dermatomyositis-patients-truly-amyopathic/